Practical Treatise on Milling and Milling Machines/Chapter 6
CHAPTER VI
Cutters
The development of the manufacture of milling cutters, and a better understanding of their care and use, have resulted in a rapid growth in the number and variety of milling operations, and a corre- sponding increase in the sizes and varieties of cutters. It is evident, therefore, that the selection, care and use of milling cutters are points of utmost importance in attaining success in the process of milling. The failure to obtain commercial results may often be attributed to the fact that the wrong cutter has been used on a certain job, or even if the right cutter has been chosen, the work has not been done under the most favorable conditions.
Either the operator or the person in charge of the job should be proficient in the selection and care of cutters, and capable of determin- ing the correct speeds and feeds at which to operate them. No theoretical knowledge of the design and manufacture of cutters is necessary to aid in this work, although a general understanding of these points is of material help. While we are able to give in the following pages such information as applies in common to the running of milling cutters, the most valuable experience will come only through actual work at the milling machine.
Carbon and High Speed Steel. Milling cutters are made from either of two varieties of steel, known as Carbon Steel and High Speed Steel. Those made from High Speed Steel can be subjected to more severe service than those made from Carbon Steel, and they are especially desirable where large amounts of metal must be removed rapidly, as in roughing out pieces of work. Cutter manufacturers can usually furnish all styles and sizes in either steel. No fixed rules can be given for their choice. The requirements of each job and experience in the use of cutters must determine which steel is more economical and will give the most satisfactory results.
Plain Milling Cutter. This is a common type of cutter found in every shop, and may be described as a cylinder having teeth on the periphery only and producing a flat surface parallel to its axis. It is manufactured in a large variety of diameters and widths to meet
 Coarse Tooth Plain Milling Cutter With Spiral Teeth  Plain Milling Cutter  Coarse Tooth Side Milling Cutter  Coarse Tooth Shell End Mill with Spiral Teeth |
 Coarse Tooth Milling Cutter with Spiral Nicked Teeth  End Mill with Straight Teeth  End Mill with Spiral Teeth  Coarse Tooth End Mill with Spiral Teeth  Centre Cut End Mill  Two-Lipped Slotting End Mill  T Slot Cutter |
Inserted Tooth Face Milling Cutter with Cutter Driver and Drawing-in Bolt.[1]
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Convex and Concave Cutters with Teeth that can be sharpened without changing Form | 
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| Metal Slitting Saw |
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Angular Cutters
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Convex and Concave Cutters with Plain Milling Cutter Type of Teeth | 
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Formed Cutter, Teeth can be sharpened without changing Contour | 
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Saws for slitting metal and slotting screws are essentially plain mill- ing cutters, although rarely regarded as such on account of their extreme thinness.
Plain milling cutters 34" or less in width are usually made with straight teeth, while those above that width have teeth of a spiral form. The object of the spiral is to give a shearing cut, reducing the stress upon the teeth, and preventing a distinct shock when each tooth engages the work as is the case with straight teeth. Consequently, a spiral tooth cutter on wide surfaces produces much smoother results
Fig. 47
than a straight tooth cutter. It requires less power to operate, and, in relieving the cutter of strain, the tendency to vibrate or chatter is reduced.
The teeth of cutters, especially those of a wide face, often have notches or nicks cut in them, the nicks following each other alter- nately. Cutters made in this manner can be run at coarser feeds than those with plain teeth, for the nicks break up the chips, and help to keep the cutters cool.
Side Milling Cutter. This type of cutter is like a plain milling cutter with the addition of teeth on both sides.
Side milling cutters are employed on a large variety of work, being used often in pairs with a space between, as shown in Fig. 47. When so used, they are known as "straddle mills." In work that has to be 
Fig. 48
milled on two parallel sides at once, as milling the heads
of bolts, nuts, tongues, etc., straddle mills can be used
most advantageously.
These cutters are also made with interlocking side teeth for milling slots to standard width. The teeth interlock, as shown in Fig. 48, and the standard width of the slot is maintained by packing washers between the cutters.
Face Milling Cutter. This cutter may be likened to a disk with teeth on the periphery and on one face. It is fastened at the end of the machine spindle, and the teeth on the flat face come in full contact with the work, Fig. 48 w hile only a small length of the teeth on the periphery act on the piece. There are cutters of this type made which have no teeth on the periphery; an example of one is shown in Fig. 49.
End Mill. This type of cutter, like the face milling cutter, has teeth on the periphery and at the end.
End mills are used for a large variety of light milling operations, such as milling cuts on the periphery of pieces, cutting slots, and facing narrow surfaces. They are made in four distinct styles, the ordinary solid end mill, with either straight or spiral teeth, the end mill with centre cut, the slotting end mill with two lips, and the shell end mill with either straight or spiral teeth.
Fig. 49
The ordinary solid end mill has its teeth cut on the same piece of steel that forms its shank; in reality, the space where the teeth are cut is only a continua- tion of the shank. The shell end mill has a hole through the centre so it can be mounted on the end of an arbor. This type should be used whenever Fig. 49 possible, because it is cheaper to replace when worn out or broken than the solid mill. End mills with centre cut differ from the others in that the end teeth are designed to cut at the inner ends, while these teeth in ordinary end mills have no cutting edge at the centre. Centre cut end mills are used for milling shallow recesses in a surface where there has been no hole previously bored for starting the cut, for milling squares on the ends of round shafts, and other similar work. This form of mill has fewer teeth, and is, therefore, better adapted to taking heavy cuts than the regular solid or shell end mills. Slotting end mills with two lips, or cutting edges, are especially adaptable to fast milling of deep slots from the solid where there has been no hole previously drilled for starting the cut. In fact, these mills embody both the principles of a drill and end mill. A depth of cut equal to one-half the diameter of the mill can usualy be taken from solid stock. The best results are obtained by maintaining a high surface speed.
End mills with right-hand teeth usually have a left-hand spiral, and those with left-hand teeth have a right-hand spiral. By having the direction of spiral opposite to the faces of the teeth the thrust of the spiral tends to force the shank of the mill solidly into the spindle, although there is little danger of pulling out the mill when the teeth and spiral are of the same hand.
T Slot Cutter. The T slot cutter has teeth upon its periphery, and alternating teeth on the sides. The teeth are neither parallel nor perpendicular to the axis of the cutte,r but are at some oblique angle. The cutter may have more than one angle.
These cutters can be mployed on a variety of work, as cutting the edge of a piece to a required angle and milling teeth of cutters and reamers. Where the nature of the work is such, as in dovetailing a piece, that the cutter cannot be fastened to the arbor with a nut, the cutters are furnished with threaded holes, or made solid on a taper shank.
Formed Cutters. Formed Cutters constitute and important group, their cutting edge usually being an irregular outline. These cutters have teeth that are relieved so that they may be resharpened repeatedly or until the teeth are too slender to permit further grinding, without changing the original form as long as the teeth are ground radially on their faces. Illustrations of this type are shown on page 91, and Figs. 50 and 51 show the extent to which they can be ground without changing the form of the teeth. Formed cutters with teeth relieved so that they may be ground on the faces without changing the contour, should be employed wherever the requirements of work demand that the original form of the cutter be maintained, as in manufacturing duplicate irregular pieces.
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| Fig. 50 | Fig. 51 |
With this style of cutter, exact duplicate pieces of irregular outline can be produced far more cheaply than by any other method. In fact, no invention has so revolutionized the manufacturing of small parts of machinery and tools.
Concave and convex cutters, cutters for grooving taps, corner rounding cutters, gear cutters, etc., are made with teeth relieved so that they may be sharpened repeatedly without changing the contour. Cutters for producing irregular outlines are also made with plain milling cutter type of teeth, but it is necessary to have special grinding machines for them, and the concave cutters have to be made interlocking to preserve the size of circle. Cutters of this type are shown on page 91.
Fly Cutter. The most simple cutter for producing a form is the fly cutter, shown in Fig. 52. This cutter is very similar to a planer tool but is held in an arbor and rotated instead of being clamped in a tool head. It can hardly be classed with the cutters previously mentioned, 
Fig. 52
but there it fills an important place. As it has only one cutting edge, it mills accurately to its own shape, but it does not cut so fast or wear as long as cutters with a number of teeth. It can be formed very exactly to any desired shape at a comparatively small expense, and thus may be used for many operations that otherwise would not bear the cost of special cutters, as, for example, when one or two teeth of special form are wanted in experimental work. The outlines of several possible shapes are shown in connection with the figure.
Fig. 54
Right and Left-Hand Cutters. Cutters or end mills with taper shanks and those which have end teeth, may be either right or left- hand, according to the direction in which the cutting edges of the teeth point. Taking an end mill for example, a right-hand mill is one which, held in the hand with the teeth away from you, presents the cutting edges of the teeth when revolved to the right or clock-wise. A left- hand mill is one that, similarly held, presents the cutting edges of its teeth when revolved to the left. Milling cutters having straight holes can be used either right or left-hand as desired.
Inserted Teeth. Plain milling cutters above 8 inches diameter, side milling cutters above 6 inches diameter, and face milling cutters, are usually made with inserted teeth. The body of the cutter is of steel, the teeth being held securely in place by various means. We employ a bushing and screw for this purpose, as shown in Fig. 54.
The introduction of cutters of this style has done more for heavy milling than any other improvement in the cutter line, for with them the heaviest and fastest cuts can be taken, and shouldany of the teeth become broken, it is not a question of a new cutter, but simply that of replacing the broken teeth. The economy of this is of considerable importance to a shop.
If, for any reason, it becomes necessary to replace the full set of blades, or teeth, the new ones are clamped securely in position, and afterwards sharpened to correct any slight difference in height.
Teeth are released by removing the screw and inserting an extractor that threads into the bushing, and has a long end that reaches to the bottom of the hole in. the cutter body. This extractor is shown in position in Fig. 54. As the extractor is turned by means of a wrench, the bushing is forced out and the tooth can then be removed.
Another type of inserted tooth face milling cutter that can be easily made in any shop is shown in Fig. 49. The teeth in this case are simply round pieces of steel inserted in holes made in the cast iron body of the cutter, and held in place by set screws. Sometimes two sets of teeth are put in these cutters. With this arrangement on heavy work that is not wider than the diameter of the inner circle of teeth, and which does not require close limits, the outer circle of teeth can be set to take a roughing cut, and the inner circle to take the finishing cut; thus work can be finished milled at one traverse of the table. Or if an exceptionally heavy roughing cut is to be taken off, the stress can be divided between the two circles of teeth.
Method of Holding Face Milling Cutters. Face Milling Cutters
Fig. 55
are drawn directly onto the taper nose of the spindle
by a cutter driver and drawing-in bolt.
driver fits into a slot in the face
of the cutter and a recess in end
of spindle. The shank of the
cutter driver is threaded in the
end to receive drawing-in bolt
by which the cutter is drawn Fig- 55
onto the spindle with the aid of a wrench. Cutter, cutter
driver and drawing-in bolt are shown at top of page 91.
This method of attaching face milling cutters is simple and convenient and assures a positive drive. All possibility of "freezing" is eliminated, the removal of the cutter being as easily accomplished as its placement. Diagram of section through spindle and cutter, Fig. 55, shows cutter driver in place.
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| Lowering Spindle into Cutter | Cutter in Place |
Face milling cutters are attached to spindle of Vertical Spindle Milling Machines as follows: Cutter is placed on table with cutter driver in place (wood block is used to avoid damage to table or cutter teeth). Spindle is lowered, the nose of spindle entering hole in cutter, and cutter driver entering recess in spindle, where it is securely held by drawing-in bolt.
An additional advantage is found in this cutter (page 91), in the increased available working space. There is no long hub, as the cutter is held close to the spindle. The body of each cutter is made of steel, and the blades of high speed steel.
On earlier machines, the spindle nose was threaded and a different style of face milling cutter was used. Face milling cutters designed for use on taper-nose spindles can be used on threaded-nose spindles by the use of an Adapter Outfit. This outfit consists of taper sleeve with threaded hole, cutter driver and drawing-in bolt.
Fig. 56
The taper sleeve fits over threaded nose of spindle and, being of the same taper as hole in face milling cutter, allows cutter to be drawn onto it by use of the cutter driver and drawing-in bolt; cutter driver fitting recess in spindle and slot in cutter.
Diagram, Fig. 56, shows sleeve and cutter driver in place. Number of Teeth in Cutters. This subject has been discussed at some length by various writers in books and technical papers. Standard cutters have been found satisfactory for the majority of work, and practically indispensable on some work of the lighter class, but cutters having wide spaced teeth have a marked advantage over the standard type in their ability to remove a considerably greater quantity of metal in a given time without distressing the cutter or overloading the machine.
The free cutting action of these coarse tooth cutters is largely due to the fact that less cutting is actually required to remove a given amount of metal, each tooth taking a large, deep chip. This results in a considerable decrease in the tendency to slide over the surface and spring the cutter arbor. The rake and increased spiral of the teeth give a more nearly perfect shearing, rather than a pushing or dragging action. Accordingly there is less friction generated for a given cut, leaving the teeth much cooler and causing them to do considerably more work between grindings.
A marked advantage arising from the free cutting action is the consumption of less power, as might be expected from the fact that there is less friction and heating.
The wide spaces between the teeth allow the cutting edges to be well backed up, which was not always possible with closely spaced teeth. This increase in the strength of the teeth is much greater in proportion than the increase in work done by each tooth in removing the larger chips. Therefore the cutters are well prepared to handle deep and rapid cuts without danger of failing.
In developing the line of Brown & Sharpe Coarse Tooth Milling Cutters, particular attention has been given to the angle of rake and the lead of the spiral of the teeth. After a long series of practical experiments we have adopted a type with steep spiral and consider- able angle of rake as the most economical and practical form, this type also being adapted to a large range of work which is not of the heavier class.
Angle of Tooth Face. Single point tools such as those used on the lathe and planer are usually given a slight rake; that is, the face of the tool is undercut a few degrees from a radial line. A similar practice is followed in setting the teeth in the body of large inserted tooth cutters so that they have a certain amount of rake. A smoother cut is gained and less power is consumed than would be with radial teeth. For other cutters, however, it will be found that satisfactory results as to finish are gained with cutters whose tooth faces are perfectly radial. Practically all ordinary stock cutters with the above noted exception have radial teeth.
The clearance or angle of the teeth back of the cutting edge is also of considerable importance, and it will be taken up later in con- nection with sharpening cutters.
Diameter of Cutters. It is well to use cutters as small in diameter as the strength will admit. The reason is shown by Fig. 57. Suppose the piece I D C J E is to be cut from I J to D E. If the large mill A is used, it will strike the piece first at I when its centre is at K, and will finish its cut when the centre is at M. The line G shows how far the work must travel to cut off the stock I J D E. If the small mill B is used, however, it will strike the piece when its centre is at L and the work travels only the length of the line H.
Small mills are also preferable because they can do more and better work than larger ones, as there is less possibility of their chat- tering. Furthermore, they require less power and are not as expensive as large mills. The advantage of small mills has been illustrated in our own works, where a difference of 12 an inch in the mills has made a difference of 10% in the cost of the work.
Temper of Cutters. A cutter is not necessarily too soft because it can be scratched with a file. On the other hand, care should be taken that cutters are not too hard or brittle, for trouble will quickly arise from the teeth breaking. If there is any question as to the temper of a cutter, it is better policy to consult with the cutter manu- facturers than to attempt to correct it by drawing the temper, or re- tempering.
Gang Milling. Gang Milling receives its name from the fact that two or more cutters are placed together on an arbor and used at one time. Sometimes plain milling cutters are so combined in order to cover a wider space than the longest stock cutter. Again, formed cutters are used either with or without plain or side milling cutters. The use of formed cutters and plain milling cutters together should be avoided whenever possible, on account of the difficulty of main- taining relative diameters in sharpening the gang.
The value of gang milling is found in the fact that it reduces the cost of production and insures accurate duplication of parts, in that several operations can be performed simultaneously, and with one setting.
It should be kept in mind that in this kind of milling, cutters of the largest diameter, or those that take the heaviest cuts, should, if possible, be used nearest the nose of the spindle, thereby reducing the strain on the arbor. If several of the cutters are plain milling cutters, it is well to use both right-hand and left-hand spirals in order to equalize the end thrust of the arbor. When, in gang milling, the cutters vary considerably in diameter, the inequality of the peripheral speeds may be overcome by having the cutters of large diameter made of high speed steel, and those of small diameter made of the ordinary carbon steel.
Fig. 57
when considered in connection with the life and efficiency of a cutter and volume of output. Little can be said, however, in the matter of general rules to follow in determining correct speeds and feeds, owing to the different conditions that exist in different shops, and, in fact, in the same shop, where one set of rules will not always hold on like jobs. The amount of power and rigidity in different machines, kind of material, width and depth of cut, quality of finish required, and many other factors, all enter into the question, and prevent the establishing of any definite rules. Sometimes the speed must be reduced, yet the feed not changed, and vice versa; again both speed and feed must be reduced or increased, as the case may be. Often the rate of feed depends almost wholly upon the degree of accuracy and quality of finish required. In general, work of a delicate character, requiring an accurate finish, demands light cuts and fine feeds, and work of a heavy character, where the principal object is to remove metal rapidly, requires deep cuts and coarse feeds. On work that permits of heavy roughing cuts, the finishing cuts should usually be light. The feed, inasmuch as it governs the output of work, is of greater importance than the speed of a cutter, and it is generally a safe rule to follow, that the speed should be as fast as the cutter will stand, and the feed as coarse as is consistent with good work. Much must be left to the judgment of the operator as to the correct speed and feed to use for the work in hand, and many cases will require repeated experiments before the best results are obtained. When any difficulty is encountered in obtaining the right combination of speed and feed, it is well to seek the advice of the foreman in charge of the job, or that of a widely experienced milling machine operator.
The following surface speeds will serve to give an idea, or basis, to work from. They may be varied slightly to suit the requirements of the work in hand. Using carbon steel cutters: For brass, 80 feet to 100 feet per minute; for cast iron, 40 feet to 60 feet per minute; for machinery steel, 30 feet to 40 feet per minute; and for annealed tool steel, 20 feet to 30 feet per minute, have been found satisfactory. With high speed steel cutters for the same materials, the following speeds are advocated: For brass, 150 feet to 200 feet per minute; for cast iron, 80 feet to 100. feet per minute; for machinery steel, 80 feet to 100 feet per minute; and for annealed tool steel, 60 feet to 80 feet per minute. Useful tables for determining the number of revolutions per minute to obtain the more common surface speeds of cutters of different diameters, will be found on pages 327 and 328.
Sharpening Cutters. The importance of keeping all kinds of milling cutters well sharpened must not be overlooked. It might be supposed upon first thought that better economy in cutter wear would be gained by regrinding no oftener than positively necessary. This is not the case, however, as experience has shown that a dull cutter wears more rapidly than a sharp one, and consequently one that is kept in good condition by frequent regrinding will invariably outlast one that is not so cared for. Besides, a dull cutter not only consumes more power, but cannot be operated as rapidly or take as heavy cuts as a sharp one, and the quality of the work is never as good. Too frequently in shops today, the efficiency of milling machines is impaired by the use of dull cutters, for no other reason than carelessness and negligence on the part of the operator. Milling is never a complete success where such conditions exist, and with the improved grinding machines and convenient means of removing and replacing cutters, there is no reason for limiting the capabilities of a machine by using dull cutters. Grinding a cutter takes only a short time, and the good results that are obtained, together with the economy assured, more than compensate for the time expended in grinding. Whenever possible, it is a good plan to have two sets of cutters, so that one set can be reground while the other is in use; the milling machine then need only be stopped long enough to change the cutters.
Plain milling cutters, side milling cutters, end mills, etc., are sharpened upon the tops of the teeth, while formed cutters of all kinds are sharpened upon the faces of the teeth. Modern cutter grinding machines are necessary where many cutters are employed, and are advantageous, even where there are only a few cutters used, for it is nearly impossible to properly resharpen cutters, except with a machine especially designed for that purpose. We illustrate at the back of the book the cutter grinding machines we build that are very suitable for use in connection with milling machines.
It is impossible to treat in detail the many points about resharpening cutters without going to great length, but we issue a book and booklet[2] devoted exclusively to the subject, one of which is furnished with each of the machines mentioned above.
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| Beveling the Corners of a Coarse Tooth Shell End Mill | Grinding a Formed Cutter on Index Centres |
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| Re-Nicking a Coarse Tooth Milling Cutter | Grinding the End Teeth of a Coarse Tooth End Mill |
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| Grinding the Teeth of an Angular Cutter | Grinding the Teeth of a Small Saw |
is the amount of material removed from the top of the teeth back of the cutting edge to permit the teeth to clear the stock and not scrape over it after the cutting edge has done its work. On formed cutters, the clearance does not have to be considered in resharpening. This is because the teeth are so formed that when ground on the faces, the clearance remains the same.
The angle of clearance depends upon the diameter of the cutters, and must be greater for small cutters than for larger ones. The clearance on the teeth of plain milling cutters should be 4° for cutters over 3 inches in diameter, and 6° for those under 3 inches diameter. The clearance of the end teeth of end mills should be about 2°, and it is well to have the teeth a little hollowing, making them .001 or .002 inch lower near the centre than at the outside, so that the inner ends of the teeth will not drag on the work. This can be done by setting the swivel on the cutter grinder slightly away from 90°.
Vibration of Cutters. If the clearance of a cutter is too great, vibrations are likely to occur in operation, and this should be corrected by regrinding the teeth. "Chattering" is a serious drawback to successful milling, as it impairs the quality of the work, limits the capacity and injures a machine, and reduces the life and efficiency of a cutter. While it is impossible in many cases to eliminate it, every precaution should be taken to reduce it to a minimum.