136 MACHINE SHOP WORK  • loose collars on the arbor; third, by being threaded and screwed on the arbor; and fourth, when the cutter is quite small and the work light, by a large-headed screw, slotted for the screwdriver, and tapped into the end of the arbor. In the latter case, the thread must be  Fig. 203. Screw-Slot Cutter  Fig. 204. Slitting Saw  right- or left-handed, according to the direction of revolution, so that the torsional strain of the work will tend to keep the cutter screwed tightly against the shoulder. Usually cutters are made right-handed; that is, if held so that the side which goes against the collar on the arbor is toward the eye, the cutter should turn in the same direction as the hands of a clock. Locating Position of Cutter. To locate the cutter in the proper position on the arbor to suit the work to be done, loose collars of various thicknesses are used on the arbor, placing as many on each  Fig. 205. Plain Milling Cutter  Fig. 206. Spiral Cutter with Nicked Teeth for Heavy Cuts Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts  side of the cutter as are necessary to fill the space between the fixed collar D, Fig. 202, and the clamping nut F. The cutter and loose collars must have smooth, true, and parallel faces; otherwise the  MACHINE SHOP WORK 137  arbor will be sprung when the clamping nut is screwed up, and will not run true. Plain Milling Cutters. Screw-slotting cutters, Fig. 203, and slitting saws, Fig. 204, are saws of a special type. The true milling cutter, Fig. 205, has a face much wider in proportion to its diameter than the common slitting saw. It is for the production of surfaces, rather than for a thin saw kerf in separating pieces of metal. These plain cutters are made in a large number of diameters and lengths, and are all designed for the generation of plane surfaces. Spiral Cutters with Solid or Nicked Teeth. As we have seen in the case of reamers, heavy cuts can be taken more easily when the  Fig. 207. Side Milling Cutters Mounted as a Heading or Straddle Mill  Fig. 208. Interlocking Cutter with Four Teeth Cut Away Courtesy of Union Twist Drill Company, Athol, Massachusetts  chip is broken up in small pieces; therefore, in milling cutters designed for roughing, it is customary to nick the teeth, Fig. 206, in such a way that the stock left by one tooth may be taken out by the following tooth. This makes the cutting easier. A plain cutter of any considerable length, with teeth formed by straight grooves, will not often make a smooth surface because of the varying pressure of the cutter as one tooth after another leaves the work. To avoid this springing tendency, cutters are made with spiral teeth, Fig. 201, either right- or left-hand, so that there is practically a uniform dis-tribution of pressure at all points during the cut. Side Milling Cutters. When it is desired to mill the side of a piece, it is necessary that there should be teeth on the side of the

Above: Lincoln-Style-Pratt-and-2-Whitney-Milling Machine like the one in my collection. Lincoln Milling Machine Pic Courtesy of Pratt and Whitney Company,       Hartford,  Connecticut : https//antiquemachinery.com/Smithsonian-My-Lincoln-Mill
 

pg

pg-4 
MILLING-MACHINE WORK. § 13

pg-132-133

132 MACHINE SHOP WORK;
mounted in a gang on an arbor, and perform operations which it would be hard to duplicate on the shaper or planer. Even in the present age of special machines for milling, a great deal of work of this character is still performed by the method indicated. 

Fig. 198. Sawing Flat Stock Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island 
One of the great advantages of milling is the certainty of exact duplication—a feature of prime importance in the manufacture of interchangeable work. About the first machine built exclusively for milling was the so-called Lincoln miller, Fig. 199, which consists essentially of a bed 
MACHINE SHOP AVORK 
carrying the equivalent of the headstock and tailstock of a lathe, with means for rotating the cutter arbor, which is carried directly by the headstock spindle, and steadied and supported by the tail-stock. There is also provided a table upon which the work can be fastened either directly or by means of a vise ; and an automatic feed 

Fig. 199. Lincoln Milling Machine Courtesy of Pratt and Whitney Company, Hartford, Connecticut 
across the machine at right angles to, and below, the cutter arbor. This type of machine in various designs is much used in modern manufacturing. 
MILLING CUTTERS Classification. As the type of cutter used determines, in a large measure, the design of the machine itself, it will be better at this point to take up a description of some of the different cutters, 
pg-133

134 MACHINE SHOP WORK, milling cutters
in order that the adaptation of the machine to the cutter may be clearly seen. Cutters are classified according to their form or the use to which they are put, some of the more common types of these devices being as follows: 
1. Slitting 2. Grooving 3. Fluting 4. Straight 5. Angle 6. Double-angle 7. Straight mill 8. Spiral mill 9. Nicked-tooth spiral mill 10. Side mill 
11. Straddle mill 12. Straight end mill 13. Spiral end mill 14. T-slot mill 15. Formed mill 16. Inserted blade 17. Inserted-tooth facing 18. Inserted-tooth surfacing 19. Shell mill 20. Fly or single-tooth cutter 
This classification does not include any of the cutters used in cutting gears, racks, spirals, helical gears, ratchets, sprocket-wheels, 

Fig. 200. Details of Ordinary Milling Cutter 
and similar work, which is usually considered as gear-cutting work. However, ratchet teeth may be cut with an angle cutter; brass gears, with a single-tooth or fly cutter, properly formed; and some others may be applied to a variety of uses, the cutter, in fact, not infre-quently displaying a remarkable adaptability to the varying conditions of work and material. 
MACHINE SHOP WORK 

pg 134 -135    Fundamental Characteristics. The several details of an ordinary milling cutter are shown in Fig. 200. A is the outside diameter; B, the thickness (or in mills such as shown in Fig. 201, the length) ; C, the diameter of the hole; D, the width of keyway; E, the depth of keyway; F, the pitch of the teeth; G, the top of the teeth or land; H, the backing-off or clearance, either on the lands or on the side of the cutter; J, the depth of the teeth; K, the face of the teeth; L, the relieving recess made for the purpose of reducing the surface to be ground; and M, the hub. The direction of revolu-tion is indicated by the arrow. Cutter Arbor. Fig. 202 shows the usual form of cutter arbor, in which A is the taper shank fitting the taper-reamed hole in the milling-machine spindle; B is the flattened portion or tang fitting in the cross-slot and preventing the arbor from turning; C is a nut used in withdrawing the arbor from the hole when it has been forced tightly into it; D is a collar formed upon the arbor, against which loose collars or the cutter itself are forced when placed upon the arbor at E and confined by the clamping nut F. The end G is finished as a 

Fig. 201. Milling Cutter with Spiral Teeth Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island 

Fig. 202. Ordinary Form of Cutter Arbor 
journal or bearing for an outer support attached to or forming a part of the overhanging arm of the milling machine. In the outer end is drilled and reamed a center hole for a similar purpose. Fastening Cutter in Arbor. Cutters are prevented from turning upon the arbor in any one of four ways—namely, first, by a key in the keyway DE, Fig. 200; second, by being clamped between 

 

page 136-137    136 MACHINE SHOP WORK 
• loose collars on the arbor; third, by being threaded and screwed on the arbor; and fourth, when the cutter is quite small and the work light, by a large-headed screw, slotted for the screwdriver, and tapped into the end of the arbor. In the latter case, the thread must be 

Fig. 203. Screw-Slot Cutter 

Fig. 204. Slitting Saw 
right- or left-handed, according to the direction of revolution, so that the torsional strain of the work will tend to keep the cutter screwed tightly against the shoulder. Usually cutters are made right-handed; that is, if held so that the side which goes against the collar on the arbor is toward the eye, the cutter should turn in the same direction as the hands of a clock. Locating Position of Cutter. To locate the cutter in the proper position on the arbor to suit the work to be done, loose collars of various thicknesses are used on the arbor, placing as many on each 

Fig. 205. Plain Milling Cutter 

Fig. 206. Spiral Cutter with Nicked Teeth for Heavy Cuts Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts 
side of the cutter as are necessary to fill the space between the fixed collar D, Fig. 202, and the clamping nut F. The cutter and loose collars must have smooth, true, and parallel faces; otherwise the 
MACHINE SHOP WORK 137 
arbor will be sprung when the clamping nut is screwed up, and will not run true. Plain Milling Cutters. Screw-slotting cutters, Fig. 203, and slitting saws, Fig. 204, are saws of a special type. The true milling cutter, Fig. 205, has a face much wider in proportion to its diameter than the common slitting saw. It is for the production of surfaces, rather than for a thin saw kerf in separating pieces of metal. These plain cutters are made in a large number of diameters and lengths, and are all designed for the generation of plane surfaces. Spiral Cutters with Solid or Nicked Teeth. As we have seen in the case of reamers, heavy cuts can be taken more easily when the 

Fig. 207. Side Milling Cutters Mounted as a Heading or Straddle Mill 

Fig. 208. Interlocking Cutter with Four Teeth Cut Away Courtesy of Union Twist Drill Company, Athol, Massachusetts 
chip is broken up in small pieces; therefore, in milling cutters designed for roughing, it is customary to nick the teeth, Fig. 206, in such a way that the stock left by one tooth may be taken out by the following tooth. This makes the cutting easier. A plain cutter of any considerable length, with teeth formed by straight grooves, will not often make a smooth surface because of the varying pressure of the cutter as one tooth after another leaves the work. To avoid this springing tendency, cutters are made with spiral teeth, Fig. 201, either right- or left-hand, so that there is practically a uniform dis-tribution of pressure at all points during the cut. Side Milling Cutters. When it is desired to mill the side of a piece, it is necessary that there should be teeth on the side of the 

pg-136-137
carry the cutting tools. Each spindle, and hence each cut-ting tool, is usually made to be independently adjustable in relation to the work. In most machines of this class, the work can be moved in a straight line in one direction only. Multispindle milling machines are intended for finishing several surfaces simultaneously, and are usually en-iployed for heavy work only. 1 6. Special milling machines may take any con-ceivable form that will adapt them for the class of work for which they are designed, but no matter in what manner they are constructed, the principles of operation will be the same as those of any regular milling machines.

 
CONSTRUCTION OF A Milling MACHINE. ESSENTIAL PARTS. 
17. A milling machine consists of certain essential parts, which in some form or other must exist in any of its numerous modifications. The essential parts are the frame, the spindle, the table, the feed-mechanism, and the cutting tool. The function of the frame is the supporting of the spindle, table, and feed-mechanism. The spindle, which by suitable means is revolved in bearings provided for it in the frame, carries the cutting tool. The function of the table is to serve as a support for the work, which may be attached either directly to the table or to holding devices carried by it. The feed-mechanism serves to move the work past the cutting tool ; it may operate directly upon the table, or upon the spindle, or upon both. The function of the cut-ting tool is self-explanatory. 
CONSTRUCTION. 
1 8. The universal milling machine is the most advanced form for general work, and embodies all the features found in other types. For this reason it is here selected and 
§ 13   MILLING-MACHINE WORK. 
described. As far as the universal machines of various makes are concerned, their general arrangement is similar to that of the machine illustrated in Fig. 1; they differ only in the design of the details, which are modified in accordance 

pg 134 MACHINE SHOP WORK  Cutters and Arbors:

in order that the adaptation of the machine to the cutter may be clearly seen. Cutters are classified according to their form or the use to which they are put, some of the more common types of these devices being as follows: 
1. Slitting 2. Grooving 3. Fluting 4. Straight 5. Angle 6. Double-angle 7. Straight mill 8. Spiral mill 9. Nicked-tooth spiral mill 10. Side mill 
11. Straddle mill 12. Straight end mill 13. Spiral end mill 14. T-slot mill 15. Formed mill 16. Inserted blade 17. Inserted-tooth facing 18. Inserted-tooth surfacing 19. Shell mill 20. Fly or single-tooth cutter 
This classification does not include any of the cutters used in cutting gears, racks, spirals, helical gears, ratchets, sprocket-wheels, 

Fig. 200. Details of Ordinary Milling Cutter 
and similar work, which is usually considered as gear-cutting work. However, ratchet teeth may be cut with an angle cutter; brass gears, with a single-tooth or fly cutter, properly formed; and some others may be applied to a variety of uses, the cutter, in fact, not infrequently displaying a remarkable adaptability to the varying conditions of work and material. 

MACHINE SHOP WORK 135 

Fundamental Characteristics. The several details of an ordinary milling cutter are shown in Fig. 200.

A is the outside diameter;

B, the thickness (or in mills such as shown in Fig. 201, the length) ;

C, the diameter of the hole;

D, the width of keyway;

E, the depth of keyway;

F, the pitch of the teeth;

G, the top of the teeth or land;

H, the backing-off or clearance, either on the lands or on the side of the cutter;

J, the depth of the teeth;

K, the face of the teeth;

L, the relieving recess made for the purpose of reducing the surface to be ground;

M, the hub.

The direction of revolu-tion is indicated by the arrow. Cutter Arbor. Fig. 202 shows the usual form of cutter arbor, in which A is the taper shank fitting the taper-reamed hole in the milling-machine spindle; B is the flattened portion or tang fitting in the cross-slot and preventing the arbor from turning; C is a nut used in withdrawing the arbor from the hole when it has been forced tightly into it; D is a collar formed upon the arbor, against which loose collars or the cutter itself are forced when placed upon the arbor at E and confined by the clamping nut F. The end G is finished as a 

Fig. 201. Milling Cutter with Spiral Teeth Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island Fig. 202. Ordinary Form of Cutter Arbor 
journal or bearing for an outer support attached to or forming a part of the overhanging arm of the milling machine. In the outer end is drilled and reamed a center hole for a similar purpose. Fastening Cutter in Arbor. Cutters are prevented from turning upon the arbor in any one of four ways—namely, first, by a key in the keyway DE, Fig. 200; second, by being clamped between  

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page  138-139   MACHINE SHOP WORK 

of the cutter; but when using the end of the cutter, it means that the teeth can have no rake, and must scrape rather than cut the work. In order to use a leading spiral on the cutter, the shank must be held positively in the spindle. This usually is accomplished by inserting in a threaded hole at the rear end of the shank, a rod which extends through the hollow spindle and brings up against a collar on the out-side. This can be set up solidly, and all danger of loosening-up of the cutter shank will be avoided. When the cutter is small, as compared with the diameter of the spindle taper, a screw collet may be used, as the friction of the collet will be greater than the tendency of the leading spiral to move the cutter from the spindle. These screw collets are commonly made of machine steel, while the end mills are made from tool steel. The short, steep taper and threaded end are shorter than the long taper shank, resulting in a cheaper cutter. One of the best means for hold-ing small end mills with straight teeth is by the use of spring collets, Fig. 221, which can firmly grasp the straight shank of the cutter. When cutters are to be changed frequently, 

Fig. 221. Typical Spring Collets 
this is a particularly satisfactory method, although it will not answer for roughing cuts where cutters of large diameter are used, as the torque will be too great for the jaws of the collet to prevent turning. An ordinary drill chuck can be held in the spindle by means of a taper shank, and furnish a means of holding straight-shank drills and other small straight-shank tools. A very convenient method of holding certain tools consists in fitting a three-jawed universal lathe-chuck to the threaded nose of the spindle, thus enabling straight-shank tools of large size to be held firmly and accurately. Cutters of any kind are rarely held in chucks on the milling machine, but a large number of other small tools can be held advantageously. 

MACHINE SHOP WORK pg-139 

 TYPES OF MILLING MACHINES Bench Miller. In taking up the subject of machines devoted especially to milling, it is well to consider that the transition from 

Fig. 222. Rivett Lathe with Milling Attachment Courtesy of Rivett Lathe Manufacturing Company, Boston, Massachusetts 
milling in the lathe to the special milling ma-chine was bridged by an attachment to the lathe by which the functions of the milling machine are well served. This is especially noticeable in the milling attachment attached to bench lathes, Fig. 222, said attach-ment being mounted on the bed of the lathe and the spindle provided with a milling cutter. This arrangement is 

used for simple milling operations. 
Fig. 223. Bench Miller [Looks-like a Sloan & Chance]
Such devices led to the intro-duction of the bench miller, Fig. 223, which is naturally intended for small work only, and therefore is not provided with automatic feeds, hand-feeding by means of levers being used. 

138-139

pg 140-141 --Properly locating each tooth as it is presented to the wheel. The usual arrangement is a finger adjustable to the proper height to produce the required amount of clearance, which is about 3 degrees, as shown at B, Fig. 190. With this amount of clearance, the cutter works freely and retains its edge; if more clear-ance is given, the cutter is likely to chatter, and the edges of the teeth will become dull rapidly. Fig. 197 shows a cutter in position for grinding the teeth; it will readily be seen that the tooth being ground rests on the centering gage E, which can be adjusted to give any desired amount of clearance to the tooth. For grinding the teeth on the side of a milling cutter, a small emery wheel may be used in order to get the necessary amount of clearance with-out touching the tooth next to the one being ground. If a grinder is used which will take a cup wheel, Fig. 198, and whose table can be turned to bring the cutter in the position shown in Fig. 199, a form of clearance is given which is more satisfactory than a clearance ground with a small wheel. With the cup wheel the line of clearance is straight, while with the small plain wheel it is hollowed out, and as a consequence the cutting edge is weak. 

Fig. 198. Section of Cup Grinding Wheel 

Fig. 199. Grinding Milling Cutter with Cup Wheel Courtesy of Cincinnati Milling Machine Company, Cincinnati, Ohio 
Side Milling Cutter. Cutting Teeth. The form of cutter shown in Fig. 190 is known as a side milling cutter. When cutting teeth on the sides, it is necessary to put the cutter on a plug whose upper 

    pg 151  TOOL-MAKING
end does not Project much above the top face of the cutter; this plug may be made straight and held in the chuck on the end of spindle in the spiral head. Such a plug is shown in Fig. 200, inserted in the cutter. If many cutters are made with teeth on the sides, it is advis-able to make an expanding arbor. Fig. 201. whose shank fits the taper hole in the spin-dle of the spiral head. When milling the teeth on the sides, the index head must be in-clined a little so that the side of the mill will stand at a small angle from the hori-zontal, in order that the lands of the teeth may be of equal width at each end. The amount of this inclination can-not readily be computed. It is formed by cutting first one tooth, leav-ing the cut somewhat shallow, then turning to the next tooth. After cutting the second tooth, the change in inclination will be apparent. Hardening. When the teeth are cut and the burrs removed, the diameter and length of the cutters may be stamped as shown in Fig. 190. The cutter is now ready for hardening. To harden success-fully, it is necessary to have a low, uniform red heat; the teeth must be no hotter than the portion between the hole and the bottom of the teeth. If held toward the light, there should be no trace of black in the interior of the cutter. When a uniform heat, no higher than is necessary to harden the steel, has been obtained, the cutter should 

Fig. 200. Milling Cutter Mounted on Plug 

Fig. 201. Typical Expanding Arbor 

be plunged into brine from which the chill has been removed, and worked around rapidly in the bath until the singing has ceased. It should then be removed from the brine and immediately plunged into oil and allowed to remain there until cold. When cold, the 

   

 

M

MACHINE SHOP WORK         pg 144-145--

The Becker Milling Machine.     The Van Norman Duplex milling machine. 
Such machines are provided with the feed motions of the horizontal type, and also with a rotating table by which circular work can be done. A large amount of work formerly done in lathes 

Fig. 230. Vertical Milling Machine with Working Parts Shown in Ghost Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts 
is now being done in vertical spindle machines, as well as many pieces formerly machined on planers and shapers. Duplex Milling Machines. The duplex milling machine, Fig. 231, has both the horizontal and vertical spindles combined in one, which allows the spindle to be placed at any angle from horizontal to vertical, and combines all the good points of both machines. The 
■ 
 pg 153- 
head of the duplex miller can be moved out over the table so as greatly to increase the range of the machine; and this head is also provided with a drilling attachment whereby holes may be drilled at any angle. 

Fig. 231. Duplex Milling Machine Set for Cutting Spirals Courtesy of Van Norman Machine Tool Company, Springfield, Massachusetts 
MILLING OPERATIONS Classification. These may be classified in a manner similar to the cutters themselves, whose names will suggest the kind of work for which they are adapted. Plane Milling or Surface Milling. This is the machining of plain, flat, horizontal surfaces by means of cylindrical mills whose length is usually much greater than their diameters, the larger kinds being constructed with inserted blades or teeth. Side Milling or Face Milling. This operation is the machining of vertical surfaces, or surfaces at right angles to the axis of the milling cutter. 

  Fig. 204. Spiral Cutter with Nicked Teeth Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts 

  

         

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-2

 

pg 146-147--------MACHINIST

Such machines are provided with the feed motions of the horizontal type, and also with a rotating table by which circular work can be done. A large amount of work formerly done in lathes 

Fig. 230. Vertical Milling Machine with Working Parts Shown in Ghost Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts 
is now being done in vertical spindle machines, as well as many pieces formerly machined on planers and shapers. Duplex Milling Machines. The duplex milling machine, Fig. 231, has both the horizontal and vertical spindles combined in one, which allows the spindle to be placed at any angle from horizontal to vertical, and combines all the good points of both machines. The 
■ 
 pg 153- 
head of the duplex miller can be moved out over the table so as greatly to increase the range of the machine; and this head is also provided with a drilling attachment whereby holes may be drilled at any angle. 

Fig. 231. Duplex Milling Machine Set for Cutting Spirals Courtesy of Van Norman Machine Tool Company, Springfield, Massachusetts 
MILLING OPERATIONS Classification. These may be classified in a manner similar to the cutters themselves, whose names will suggest the kind of work for which they are adapted. Plane Milling or Surface Milling. This is the machining of plain, flat, horizontal surfaces by means of cylindrical mills whose length is usually much greater than their diameters, the larger kinds being constructed with inserted blades or teeth. Side Milling or Face Milling. This operation is the machining of vertical surfaces, or surfaces at right angles to the axis of the milling cutter. 

 page 150-151 Machine-Shop-Work Milling-Machines.
     

p ppg=150 MACHINE SHOP WORK 
a total working angular movement of 90 degrees, 45 degrees on either side of the normal position. While the milling machine developed from the lathe, through the Lincoln miller, to the standard horizontal universal machine, its development for work on which heavy cuts are necessary took an opposite course. Planer Type Milling Machines. The slabbing miller, Fig, 225, is of the planer type, the cross-rail carrying a rigidly supported cutter, while the table has the comparatively slow feed required for milling. This type of machine is especially valuable where broad surfaces are to be machined on pieces of work which are of such shape that they can be readily and uniformly supported to withstand the cut. Another milling machine of the planer type, having four spindles, is shown in Fig. 226. It is designed for very heavy work. Especial Care Neces-sary to Keep Work True. In order to produce true work by heavy milling, it is not only necessary that the work shall be supported as already outlined, but also that the cut be nearly uni-form in depth and width. If the section of the cut varies greatly, or, even with uniform cut, if the work is irregularly supported, the metal will spring under the influence of the cutter, and it will be found that the work is not true. Therefore, work of a character that from its shape is especially liable to be distorted by the process of milling, may be machined to better advantage by the process of planing. 

Fig. 228. End Milling Attachment on Planer 
Milling Attachments for Planer. It is often desirable, from the point of view of economy of time, to combine the operations of milling and planing, and, with this end in view, milling attachments are made for the planer in a single machine, Fig. 227, and attached to the cross rail. The changes required from the planer drive, are an extra belt to rotate the cutter, and a special countershaft to slow down the move-

MACHINE SHOP WORK 
pg -151 
ment of the table. This attachment can carry a slabbing, gang, or formed cutter on an arbor for horizontal milling; or it can carry end mills, Fig. 228, by turning the attached head through 90 degrees, thus bringing the spindle to a vertical position. This last arrange-ment of the spindle is of great utility, as it allows cutters to reach down into places which would be inaccessible by any other means. Vertical Milling Machines. Vertical Head on Horizontal Machines. The advantages of the vertical milling spindle are so 

Fig. 229. Vertical Milling Head Attached to Horizontal Milling Machine Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island 
evident that nearly all makers of horizontal machines furnish what is called a vertical head, Fig. 229. This vertical head is very rigidly supported on the column by means of the overhanging arm, so that cuts can be taken of as great depth as with the horizontal spindle. The vertical spindle can also be turned in the vertical plane, so that an end mill can be used at any angle with the table. Vertical Spindles Only. There are several machines made in which the vertical spindle alone is employed, Fig. 230, there being no provision for a horizontal spindle.

 page 152-153 Machine-Shop-Work Milling-Machines.
     

152 MACHINE SHOP WORK 
Such machines are provided with the feed motions of the horizontal type, and also with a rotating table by which circular work can be done. A large amount of work formerly done in lathes 

1.300tiot 

6111%*0111" etNoulleanloolfiglionek., 

Fig. 230. Vertical Milling Machine with Working Parts Shown in Ghost Courtesy of Becker Milling Machine Company, Hyde Park, Massachusetts 
is now being done in vertical spindle machines, as well as many pieces formerly machined on planers and shapers. Duplex Milling Machines. The duplex milling machine, Fig. 231, has both the horizontal and vertical spindles combined in one, which allows the spindle to be placed at any angle from horizontal to vertical, and combines all the good points of both machines. The 
■ 
MACHINE SHOP WORK 153' 
head of the duplex miller can be moved out over the table so as greatly to increase the range of the machine; and this head is also provided with a drilling attachment whereby holes may be drilled at any angle. 

Fig. 231. Duplex Milling Machine Set for Cutting Spirals Courtesy of Van Norman Machine Tool Company, Springfield, Massachusetts 
MILLING OPERATIONS Classification. These may be classified in a manner similar to the cutters themselves, whose names will suggest the kind of work for which they are adapted. Plane Milling or Surface Milling. This is the machining of plain, flat, horizontal surfaces by means of cylindrical mills whose length is usually much greater than their diameters, the larger kinds being constructed with inserted blades or teeth. Side Milling or Face Milling. This operation is the machining of vertical surfaces, or surfaces at right angles to the axis of the milling cutter. 

***********************************************************************************************************************

pg-154   MACHINE SHOP WORK 
Angle Milling. As the name suggests, this is the machining of a surface at some other than a right angle to the axis of the milling cutter. Form Milling. The machining of some special cross-section generally composed of straight lines and curves, or wholly of curves, is called form milling. Profiling. This operation is usually considered as machining the vertical edges of pieces of irregular contour, and is generally done with an end mill mounted in a vertical spindle. The exact form is generally determined by a templet or profile attached to the piece or to the fixture supporting it. Care of Milling Cutters. This is a matter of much importance, since a worn or dull cutter will never produce good work, and a good cutter is soon spoiled by improper use or lack of care in handling. The cutting edge should always be sharp and keen; but it is of still greater importance that each edge should be exactly the same distance from the axis of rotation—or, in other words, that the cutters should run true. When this condition does not exist, the greater part of the work will fall upon two or three of the teeth, and these will be speedily ruined, while the others do little or no work. Care should be taken to have the arbor run true; otherwise a cutter that is ground true will not run so. Therefore, cutter arbors should be examined and tested frequently to see that the portion upon which the cutter or loose collars rest runs true and is smooth, and not defaced by bruises. from rough handling. Grinding Milling Cutters. A good cutter-grinding machine is absolutely essential. It should have a well fitted and true spindle, and such attachments for holding cutters of various kinds as to be able to grind all the usual forms without important changes of mechanism. The centers for supporting arbors, and the devices for holding cutters not on arbors, should be well fitted and true. The machine should be equipped with such graduated circles as will enable the operator readily to set it for grinding all the usually required angles. Fig. 232 shows a regular machine for this purpose. It is so arranged that various forms of cutters can be ground either when mounted upon cutter arbors or held in the machine fixture provided; and it has a number of well-designed attachments by which a con-siderab'_ e amount of general grinding can be successfully done. 
MACHINE SHOP WORK 
pg -155 
In keeping milling cutters in order, they should be ground as soon as they become dulled, whether wanted for immediate use or not. It is more economical to have them always ready, as the emergency is likely to occur at a time when a cutter is wanted at once, and when there is not time to grind it properly. Cutters should be kept sharp. A dull cutter will not only wear away more rapidly than a sharp one, but it will also do poor work; 

Fig. 232. "Cincinnati" No. 2 Universal Cutter and Tool Grinder Courtesy of Cincinnati Milling Machine Company, Cincinnati, Ohio 
it will take a great deal more power to drive it, and the milling machine will be more rapidly worn out.

Care should be taken, in grinding angular cutters, that the points are not heated so as to draw the temper. This very easily happens if considerable care is not used, the cutting edges becoming so softened as to be rapidly worn away and the cutter spoiled by use. Formed cutters are frequently affected in a similar manner. The excessive friction of a dull cutter will frequently generate a sufficient amount of heat to draw the temper of the teeth at the cutting edge.  

ppg-156-157 MACHINE SHOP WORK 
 

In making the grinding machine ready to grind a cutter, it is necessary to see that the emery wheel runs perfectly true; and if it does not, it should be trued up before any grinding work is done. If the cutter is to be sharpened upon an arbor, the latter should be tested to ascertain if it runs true before putting the cutter on it. In grinding the cutter, light grinding cuts should be taken, and the cutter moved rapidly across the face of the wheel. The wheel should be the proper grade of emery, not finer than 90, nor coarser than 56. The coarser and softer the wheel, the higher may be the speed. It is not advisable to make the speed over 4,500 feet per minute at the outer edge of the wheel. The cutting edge of the wheel need not be over an eighth of an inch thick, in any case. Preparing the Milling Machine for Work. The taper shank of the arbor and the hole in the spindle should be wiped clean and free from oil or grit. Should the outer end of the arbor be sup-ported by a pointed center or a bushing, it will not be difficult to keep it in place; but if not so supported, it must be driven tightly into the spindle, using care that the flattened end or tang fits perfectly into the slot provided for it. If it is noticed that the arbor does not fit fairly into the spindle, it should be removed and examined to see that there are no dents or bruises on it, and that the tang is not too long or too thick, or the shoulders not cut back far enough to permit it to fit properly. When arbors work loose, it is on account of some one of these causes. If the arbor does not run true when the cutter is mounted and the clamp-nut screwed up, the nut and collars should be removed and examined. Fine chips or dirt are likely to be found between the collars, or between them and the cutter, causing the arbor to spring when the clamp nut is screwed up. The parts should be cleaned and again put in place. Cutting Speeds. Conditions Governing Speed. There are no hard and fast rules that will properly govern a majority of cases of the continually varying conditions of milling cutters, machines, and the material to be machined. In any case, much must be left to the judgment of the foreman and the operator. Prominent among the conditions that tend to vary the cutting speed are the following: The cutter may be newly ground, keen, and sharp; or it may have been considerably dulled by use. While not dull enough to require grinding, it will 

MACHINE SHOP WORK 157 
not be safe to run it up to the speed of a sharp cutter. The teeth may be worn thin from long use and re-grinding, and not strong enough to stand the strain of maximum speed. The cutter may be of such a form—as a double-angle cutter—that the teeth will not bear the strain of full speed. The machine may be well designed and built, and free from vibration; or it may be directly the reverse, a fast speed producing so much chattering as to spoil both work and cutter. The arbor may be large and stiff, or small and slender. In one case, a fast speed may be maintained; and in the other, both work and cutter would suffer. The driving gearing may be well designed and its teeth fit accurately with no backlash; or it may be poorly designed and made, or much worn, and cause much chattering on a fast speed. There are many other similar conditions. The material may be of varying degrees of hardness and toughness, and of a great variety of forms. Some iron castings will be more severe on a cutter than tool steel would be. The scale on cast metal is very hard to cut through, and dulls the teeth of a cutter quickly. The varying hardness of steel, from that ordinarily found in the bar to that properly annealed, is great. The amount of carbon in steel is always a varying condition for which it is difficult to formu-late rules. Therefore it is only possible to give rules that will meet a fair average of conditions. 
In order to accommodate different sizes of cutters, maintain a uniform cutting speed, and also allow for difference in hardness of the material being worked, it is necessary that the milling machine should be supplied with several speeds. In the ordinary miller we usually have a four-step cone with back gears, which gives eight speeds with a single overhead belt. The countershafts for these machines are of the friction type, and are supplied with two driving pulleys driving in the same direction, but at different speeds, giving a total, including the back gears, of sixteen speeds for each machine. Form of Cutter as Affecting Its Speed and Feed. A slitting cutter (practically a saw) may be run much faster than one of broad face. A cutter of small diameter will cut faster than a large one, as the arc of action is much less. Angle cutters must be run at lower relative speeds so as not to break off the slender points of the teeth. The speed may sometimes be profitably increased without changing the rate of feed. Again, the speed should be decreased according to the conditions of the work. There is no direct and constant ratio between speed and feed. Conditions may vary either one without changing the other. A roughing cut will often work better with a moderate speed and a coarse feed. The smoothness of the work is not so important as 

pg 158-159

pg-158 MACHINE SHOP WORK 
taking off the surplus stock. With a finishing cut, the conditions are reversed and a fine feed is necessary. Cutters with inserted blades will not usually stand as high a speed as solid cutters, particularly when the blades have a large cutting surface. This condition is emphasized when cutting rather hard and tough material. If there is a comparatively small space for chips between the teeth of the cutter, a light cut must be taken, or a slower feed used, so that the chips will not clog the cutter. Speed Used on Particular Work or Material. The speed used on any par-ticular work depends, as before stated, on the diameter of the cutter and the character of the work. Thus, with car-bon steel cutters, the cutting speed will be 30 to 60 feet per minute. With high-speed steel cutters, double these speeds may be maintained if the drive of the machine is strong enough to pull the cut. When using very small cutters, the machine itself will not usually give a speed which is high enough to suit the diameter of the cutter. For such work, a high-speed attachment, Fig. 233, is furnished, by which the small, light cutters may be driven at a suitable rate. Of equal importance with the correct speed for the cutter, is the maximum feed or table speed, which is reckoned in inches per minute. A more logical method of designating the feeds, and one which has been adopted by several makers, is to give the advance of the table in thousandths of an inch for every turn of the spindle. Based upon the use of the ordinary carbon steel cutters, the Brown and Sharpe Manufacturing Company have prepared the following statements regarding the speed of cutters: It is impossible to give definite rules for the speed and feed of mills. The judgment of the foreman or man in charge of the machine should determine what is best in each instance. As usually the highest possible speed and feed are desirable, it pays to increase them both until it is seen that something will break or burn, and then 

Fig. 233. High-Speed Attachment for Milling Machine 
MACHINE SHOP WORK 
TABLE IV Speeds and Feeds for Milling Cutters* 
pg-159 
MATERIAL SPEED (ft. per min.) FEED (in. per min.) Soft cast iron 60 — _ .1N 0,1.44.-4,4.1N.I.mi..1.0.1N,51.0 ,--4 ,--1 ,—.1 C\1 C\1 r-4 cz, Hard cast steel 40 Wrought iron 45 Soft machine steel 36 Hard machine steel 24 Tool steel, annealed 30 Tool steel, not annealed 20 Soft brass 120 Hard brass 100 Bronze 80 Bronze, gun metal 60 Vulcanized fiber (gray and red) 60 
reduce to a speed and feed of safety. Sometimes the speed must be reduced, and yet the feed need not be changed. The average speed on wrought iron and annealed steel, using carbon steel cutters, is perhaps 40 feet per minute, which gives about sixty turns per minute with mills 21 inches in diameter. The feed of the work for this surface speed of the mill can be about 12 inches per minute, and the depth of the cut about .2-6- inch. In cast iron, a mill can have a surface speed of about 50 feet a minute while the feed is n inches per minute and the cut16 inch deep. In tough brass, the speed may be 80 feet, the feed the same as in cast iron, and the chip -22- inch. As small mills cut faster than large ones, an end mill, for example, inch in diameter, can be run about 400 revolutions per minute with a feed of 4 inches. Addy, an English authority, gives as a safe speed for cutters of 6 inches diameter and upward : Steel, 36 ft. per min., with a feed of 1 in. per min. Wrought iron, 48 ft. per min., with a feed of 1 in. per min. Cast iron, 60 ft. per min., with a feed of 11 in. per min. Brass, 120 feet per min., with a feed of 21 in. per min. He also gives a simple rule for obtaining the speed : The number of revolutions which the cutter should make when working on cast iron equals 240 divided by the diameter in inches. In Table IV are given the average speeds in feet per minute of the periphery of the cutter, and the rate of feed in inches per minute for various materials. Tables V, VI, VII, and VIII, have been prepared by the Brown and Sharpe Manufacturing Company, to give the speed, feed, and depth of cut that can be obtained with a machine similar to that illustrated in Fig. 224. It is understood that these speeds *Attention is called to the seemingly slow speed and fast feed for vulcanized fiber. Practice, however, proves it to be correct. 

pg 160-161  MACHINE SHOP WORK 

TABLE V Surface Milling of Cast Iron 
DIAMETER OF MILL (i..) REVOLUTIONS PER MINUTE SPEED OF CUTTER PER MINUTE (ft.) DEPTH OF CUT (ln.) WIDTH OF CUT (in.) FEED PER MINUTE In Scale of Cast Iron (in.) Under Scale of Cast Iron (in.) 42 34 -1'4 -1- -1`4-l- -1- 42 34 42 34 42 34 42 34 42 34 42 40 00 di co 42 40 42 50 42 50 42 50 42 50 42 50 
TABLE VI Surface Milling of Soft Machinery Steel 
DIAMETEROF MILL (in.) REVOLUTIONS PER MINUTE SPEED OF ,-, CUTTER PER MINUTE (ft.) DEPTH OF CUT (in.) WIDTH OF CUT (in.) FEED PER MINUTE In Scale  of S.M.S. (in.) . Under Scale of .M.S S (in.) . 38 30 t-loo .K4 .loo Hoz) .lao .lei r-loo C0 1-4 c 1-1 7-4 1-4 Cq 38 30 38 30 3 38 30 38 30 38 30 38 35 00 tiia 31 38 35 25 30 25 30 41 25 30 25 30 25 30 
and feeds are those used when the milling cutters are made from a good grade of carbon tool steel. If the cutters used are made from the newer high-speed steels, these figures can be decidedly increased. 
MACHINE SHOP WORK 
TABLE VII 
End or Face Milling of Cast Iron 
161 
DIAMETER OF MILL on.) REVOLUTIONS PER MINUTE SPEED OF CUTTER PER MINUTE (ft.) DEPTH OF CUT (in.) WIDTH OF CUT (in.) FEED PER MINUTE In Scale of Cast Iron (in.) Under Scale of Cast Iron (in.) 382 50 .1cgi r—I r-4 r—I tr) 1-1 23 35 382 50 7 11 191 50 30 40 191 50 3 51 109 50 .loo 17 23 109 50 31 4i 42 55 21 48 10 45 i 1 
TABLE VIII 
Face Milling of Soft Machinery Steel 
DoIFA(iMMnE.I)TLELR REVOLUTIONS PER MINUTE SPEED OF CUTTER PER MINUTE (ft.) DEPTH OF CUT (in.) WIDTH OF CUT (in.) FEES PER MINUTE In Scale of S. M. S. (in.) Under Scale of S. M S. (in.) 267 35 Midr-IINMId 267 35 152 40 152 40 r—I 87 40 87 40 
Use of Oil on Machines and Work. The milling machine, and in fact all the machines of the shop can do efficient work only when they are well cared for. An important element is that they should be frequently cleaned and well oiled. Great care should be exercised that chips do not get into the tapered holes in the spindles or between the arbor collars. When at work on steel, the milling cutter is kept flooded with oil or a solution of sal soda, as already specified for lathe work.  

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   pg-162-163 TOOL-MAKING

 162 MACHINE SHOP WORK 
Oil is used in milling to obtain smoother work, to make the cutters last longer, and, where the nature of the work requires, to wash the chips from the work or from the teeth of the cutters. Some lubricant is generally used in milling steel, wrought iron, malleable iron, or tough bronze. Frequently, when only a few pieces are to be milled, it is not used, and some steel castings are milled without a lubricant; also in cutting cast iron it is not used. For light, flat cuts it is often put on with a brush, giving the work a thin covering like a varnish. For heavy cuts it should be led to the mill from the drip can that is usually sent with each machine; or it should be pumped upon or across the mill when cutting deep grooves, milling several grooves at one time, or, indeed, in milling any work where, if the chips should stick, they might catch between the teeth and sides of the grooves, and scratch or bend the work. The Brown and Sharpe Manufacturing Company recommend the use of lard oil in milling. Any animal or fish oil, however, may be used, and then separated from the chips by the use of a centrifugal separator or by dumping into a tank of water. In the latter method, the chips fall to the bottom and the oil rises to the top, whence it may be drawn off with but little waste. Laying Out and Drilling Holes. One of the operations for which the miller is particularly adapted is in locating and drilling holes which require accurate placing. The graduated feeds of the mill-ing machine allow the distances to be set off as closely as .00025 inch, and holes can also be drilled to a given depth with equal accu-racy. In starting holes, it is best to use a spotting drill, Fig. 234, which is extremely rigid and per-fectly true. The spot made should be of slightly greater diameter than the drill to be used. The drill should be what is known as reamer size—that is, inch below the standard—and the hole may then be reamed, either in one operation, using a standard reamer, or by first using a machine reamer which is about .005 inch under size, to be followed by the standard reamer. It is evident that holes thus drilled and reamed will be parallel, and, by using the vertical 

Fig. 234. Spotting Drill 

MACHINE SHOP WORK 
163 
head, holes can be drilled at right angles in like manner. When extreme accuracy in holes is demanded, a boring bar may be used in the spindle after the drill, in order to correct any error due to the running of the drill itself. Splinting Shafts. An-other operation suited to the milling machine, although sometimes performed on the shaper or planer, is that of splining shafts. The slots in the table give the proper alignment to the shaft; the cutter can be set with cor-rect relation to the axis without difficulty, and the spline cut full depth at one operation. The only objec-tion to this form of spline is the curve at the end due to the shape of the cutter. An end mill in the vertical head can be used to remove this objectionable feature; and some splining machines are made, Fig. 235, which per-manently carry both cut-ters, so that the work can be quickly shifted from one to the other. Making Dovetails. The operation of making dove-tails, which is a delicate and expensive job on a shaper, i3 readily performed on the milling machine, especially of the vertical type, Fig. 236, the cutter being a form of end mill suited to the size and angle of the dovetail. 
Fig. 235. 
Splining Arrangement with End Mill in Vertical Head 
Fig. 236. Dovetail Cutter on Vertical Milling Machine 

pg-164-165

164 MACHINE SHOP WORK 
T-slots are cut in a 
similar manner, either directly from the solid, or by following a groove made with a plain cutter. Fluting Taps and Reamers. One of the common operations per-formed between centers is the fluting of taps and reamers, Fig. 237, which is done by the special cutters already referred to in Fig. 214. It will be noticed that the cutter should be set in such a way that the cutting edge of the tap or reamer will be radial. If left as an obtuse angle, the tool will simply scrape and not cut; while, if the tooth is undercut to any extent to correct this, it will often be so weakened as to be liable to break. The flutes in twist drills and in spiral fluted reamers may also be cut between centers; but, if the cutter is carried directly by the spin-dle, the operation requires a universal machine. If the cutter be carried by a vertical or sub-head of any kind, a plain machine will answer for the purpose. The angle to which the table or vertical head must be set for spiral cutting, Figs. 238 and 239, is the angle between the axis and the development of the determined by the following 

Fig. 237. Fluting Taper Reamer Courtesy of Van Norman Machine Tool Company, Springfield, Massachusetts 
I.

Fig. 238. Milling Spirals with Table at Angle 
spiral. This angle can be closely 
graphical method: 

MACHINE SHOP WORK 165 
Construct a right-angled triangle having a base equal to the axial distance represented by one full turn of the spiral (this is the lead of the spiral), and a perpendicular equal to the circumference of the work, Fig. 240. Draw the hypothenuse of this triangle. If the construction has been carefully done, the 

Fig. 239. Cutting Spiral with Milling Machine Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island 
angle between the base and the hypothenuse may be closely determined by the use of a protractor, and will be the angle to which the table or head must be set. This angle can be more closely and quickly determined by a very simple problem in plane trigonometry—namely, finding the tangent of the angle. To do this, divide the perpendicular of this triangle by its base, and obtain the value of the angle from a table of tangents. Spirals. The cutting of spirals requires another operation which differs from ordinary work. In addition to the angular setting, the 

fingle For Table Length One Turn  

ICir.curn—ference lof Work 
Fig, 240. Graphical Method of Determining Angle for Cutting Spirals 
work must be rotated in order to produce the spiral, as well as fed forward to the cutter. This rotation of the work must be positive, which means geared; and one rotation of the work will, of course, 

PG-166-167

166 MACHINE SHOP WORK 
equal the lead of the spiral, which is usually expressed as one turn in n, inches. After cutting one spiral groove, the work is turned and indexed the same as in plain milling. Cams. Both open and closed cams can be readily cut on a plain milling machine by the use of the cam-cutting attachment, Fig. 241, which nearly all makers are able to furnish. The outline of the cam is first laid out and worked down by hand on a plain disc, or male leader, as it is termed. This leader and a suitable blank are mounted, 

Fig. 241. Cam-Cutting Attachment for Milling Machine Courtesy of Brown, and Sharpe Manufacturing Company, Providence, Rhode Island 
with their outlines coinciding, on the spindle of the cam-cutting attachment. A cam roll of the size to be used is mounted on a stationary roll stud; and an end mill of the same diameter, or enough larger for clearance, is mounted in the milling machine spindle directly opposite the cam roll. The spindle of the cam-cutting attachment is mounted on a carriage, which, by means of a weight over a pulley at the end of the milling machine table, is always kept with the leader in contact with the cam roll. A worm and worm gear are used for rotating the attachment, and thus the spindle approaches 
 

MACHINE SHOP WORK 
167 
or recedes from the cam roll according to the shape of the leader. When cutting closed cams, it is sometimes desirable to use the hand-made male leader as a form from which to make a closed or female leader. This female leader will surround the cam roll in such a way 

Fig. 242. Cutting Spur Gear on Milling Machine Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island 
that, even if the weight should fail to act, no serious damage can be done to the blank. The cutting of face cams differs from the above method only in that the spindle of the attachment is at right angles to the spindle of the milling machine, instead of parallel to it. The leader and cam roll are used in the same manner as before.

pg-68 MACHINE SHOP WORK 

Gears. The cutting of gears of all descriptions was formerly done on some type of milling machine, although now each type of gear may have its special and, in many cases, automatic machine. 

Fig. 243. Gear Cutter with Divided Head 
Forms of Cutters. The cutters for milling spur and bevel gears are of two types, producing both the cycloidal and the involute tooth. For each pitch, the cycloidal system requires twenty-four cutters, while eight cutters usually suffice for the involute system. These cutters are plainly marked with the style of tooth, pitch, and number of teeth for which they are suitable. Some cutters are also marked with the full depth of the tooth expressed in thousandths of an inch, Fig. 279. The gear blanks, having been very carefully turned as to outside diameter, are mounted on an arbor be-tween centers, and the cutter placed so that its central plane passes through, and is parallel to, the axis of the arbor. Clamp the saddle in this position; raise the table knee until the cutter, when rotating, just touches the outside of the blank. Using the table screw, move from under the cutter; using the graduated dial, raise the knee an amount equal to the whole depth of the gear tooth. With the exception of the indexing, the gear blank is now ready to be cut, Fig. 242. 

Fig. 244. Cutting a Bevel Gear 
MACHINE SHOP WORK 
169 
Use of Dividing Head. In order that the gear may be accurately and quickly set for cutting each tooth, a dividing head is used, which is shown in Fig. 243. The mandrel upon which the gear blank is mounted is held by the centers AA, and firmly dogged to the face-plate B. The index plate C is geared to the head spindle that carries 

Fig. 245. Hobbing Teeth in Worm Wheel Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island the faceplate B; the index plate is provided with a number of holes. These holes are arranged in circles, each circle having a different number of holes, and these holes are accurately spaced at equal distances apart. The arm D carries a stem E, having a knurled head - at one end and a pin at the other. The pin is held in one of the holes 

170 • MACHINE SHOP WORK 
of the index plate by a spring. The arm D can be moved to any desired position relative to the index plate, and there fastened. When a gear is to be cut, the arm D is shifted so that the pin is opposite a row of holes the number of which is the same as the number of teeth to be cut, or a multiple of that number. Thus, 

Fig. 246. Rack-Cutting Attachment on Milling Machine Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rh )de Island suppose a gear with 45 teeth is to be made. The pin may be set opposite the circle of 90 holes. Assuming that the ratio of revolution between D and B is 40 to 1; th of a revolution at B requires 12- of a revolution at D. The pin E must, therefore, be moved 12- of 90 holes, or 80 holes, for each tooth cut. 

 
MACHINE SHOP WORK 170 
Bevel Gears. Bevel gears are held on a taper-shank arbor in the dividing head, which is swung up to bring the bottom of the tooth parallel with the table, Fig. 244. As all parts of the tooth of a bevel gear are elements of a cone, it is evident that both the tooth and the space should vanish at the apex of the cone. No solid cutter, there-fore, can do more than give an approximately correct shape to the tooth; for this reason two cuts are made in order more nearly to approach the desired contour. Spiral Gears. Spiral gears are cut in the same manner as any other spiral—that is, by using the angular setting of the head or table with positive rotation of the work. Worm Gears. Worm gears can be hobbed out by two different methods. A common method is to gash the blank with a stocking cutter; then mount it on an arbor held freely between centers, so that the hob, when sunk in the gashes, will rotate the blank. The blank is raised slowly against therotating hob until the hob reaches the proper tooth depth. A more accurate method is by means of a train of gearing to rotate the blank positively at a speed corresponding to the pitch of the hob, and raise the rotating blank against the rotating hob until the proper tooth depth is obtained. This method requires no preliminary gashing, Fig. 245. Rack Cutting. Rack cutting requires a special attachment, Fig. 246, so that the cutter spindle may be carried at right angles to the length of the table. 

pg-171-GRINDING MACHINE Value of Grinding as Finishing Process. When greater accuracy than that obtainable on the milling machine or the lathe is required, recourse is had to grinding. This operation depends upon the abrasive or cutting qualities of emery, corundum, and carborundum. With work properly held to a solid grinding wheel, it is not difficult to attain great accuracy. By means of the grinding machine, parts may be economically finished, even in hardened steel that could not possibly be machined on such shop tools as the lathe, planer, or shaper. One type of machine used for this purpose is shown in Fig. 247. With such a machine, round surfaces may be ground so that the variation from the nominal diameter is less than .0001 inch. 

170 • MACHINE SHOP WORK 
of the index plate by a spring. The arm D can be moved to any desired position relative to the index plate, and there fastened. When a gear is to be cut, the arm D is shifted so that the pin is opposite a row of holes the number of which is the same as the number of teeth to be cut, or a multiple of that number. Thus, 

Fig. 246. Rack-Cutting Attachment on Milling Machine Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rh )de Island suppose a gear with 45 teeth is to be made. The pin may be set opposite the circle of 90 holes. Assuming that the ratio of revolution between D and B is 40 to 1; th of a revolution at B requires 12- of a revolution at D. The pin E must, therefore, be moved 12- of 90 holes, or 80 holes, for each tooth cut. 
put \inn! Inn 
—clt) Of 
MACHINE SHOP WORK 171 
Bevel Gears. Bevel gears are held on a taper-shank arbor in the dividing head, which is swung up to bring the bottom of the tooth parallel with the table, Fig. 244. As all parts of the tooth of a bevel gear are elements of a cone, it is evident that both the tooth and the space should vanish at the apex of the cone. No solid cutter, there-fore, can do more than give an approximately correct shape to the tooth; for this reason two cuts are made in order more nearly to approach the desired contour. Spiral Gears. Spiral gears are cut in the same manner as any other spiral—that is, by using the angular setting of the head or table with positive rotation of the work. Worm Gears. Worm gears can be hobbed out by two different methods. A common method is to gash the blank with a stocking cutter; then mount it on an arbor held freely between centers, so that the hob, when sunk in the gashes, will rotate the blank. The blank is raised slowly against the rotating hob until the hob reaches the proper tooth depth. A more accurate method is by means of a train of gearing to rotate the blank positively at a speed corresponding to the pitch of the hob, and raise the rotating blank against the rotating hob until the proper tooth depth is obtained. This method requires no preliminary gashing, Fig. 245. Rack Cutting. Rack cutting requires a special attachment, Fig. 246, so that the cutter spindle may be carried at right angles to the length of the table. 

GRINDING MACHINE Value of Grinding as Finishing Process. When greater accuracy than that obtainable on the milling machine or the lathe is required, recourse is had to grinding. This operation depends upon the abrasive or cutting qualities of emery, corundum, and carborundum. With work properly held to a solid grinding wheel, it is not difficult to attain great accuracy. By means of the grinding machine, parts may be economically finished, even in hardened steel that could not possibly be machined on such shop tools as the lathe, planer, or shaper. One type of machine used for this purpose is shown in Fig. 247. With such a machine, round surfaces may be ground so that the variation from the nominal diameter is less than .0001 inch. 

The End