REGRINDING.

It cannot be too strongly insisted that it is very wasteful to use a dull cutter. It is as hopeless to mill successfully without adequate grinding arrangements as it would be to turn satisfactorily with only the door-step to sharpen the tools on. When a cutter is changed in time, the sharpening should only occupy a very few minutes for most small sizes. If run too long, the grinding becomes a serious operation, which causes the grinder to lose his temper, and to draw the temper of the cutter.

When the resharpening cannot be accomplished by two or three passes over the emery wheel, the cutter should be mounted on a mandrel and ground whilst revolving until the worn part has all been removed; and the tooth-by-tooth grinding should be reserved for backing off to give the cutting edge. Not only is this much the quicker way, but there is no risk of drawing the temper if ordinary care be exercised.

It must always be remembered that however good a cutter is, the cutting-edge may be so damaged by a little carelessness in grinding as to receive any degree of injury up to the point of being ruined. It is well to touch the cutting-edge with an oilstone after grinding.

As the teeth are usually reground on a dry wheel, it is important that arrangement should be made for exhausting the dust produced. Dry grinding is now recognized as a dangerous occupation, causing lung diseases. The operation is not capable of imparting consumption itself, but it so irritates the throat and lungs as to keep them in an unhealthy condition and render them susceptible to consumption germs. For this reason the emery wheel should be enclosed, as far as possible, in a hood, and a good exhaust provided by a fan or other suitable means.

QUALITY OF STEEL TO USE FOR MILLING-
CUTTERS.

The all-important question of the quality of steel to be used Self-evident as it is, the fact may yet be overlooked that two cutters, one made of the best steel and

is too often ignored.

one of the worst, may be identical in appearance, and the difference will only become apparent in use.

In small or complicated cutters, in which the cost of steel is only a small proportion of the total cost, the amount saved by using cheap steel is slight.

In large cutters of simple forms with little machining on them, where the cost of steel is perhaps one-third or even onehalf the cost of the finished cutter, the saving effected by using a poorer quality of steel amounts to a great deal, and may reconcile the user to an inferior cutting edge. Good steel may be recut, and after the hardening the cutter should not be perceptibly inferior to a new one.

SELECTING A SET OF CUTTERS FOR A MILLINGMACHINE.

A person buying a milling-machine for general use, who has not had previous experience, is immediately confronted with the problem of cutters, and the questions are frequently asked, "What should I buy for a starter?" and "What is likely to be required for my work?" It is to this class that these suggestions are offered rather than to those who by years of experience and study are prepared to give counsel and are not in need of what I have to offer.

To begin with, do not under any circumstances buy up a lot of second-hand cutters because they can be had at a bargain, as they are liable to prove very expensive in the end for many reaThey may be unsuited for the work, out of date in design, and will unconsciously be copied in the new cutters that are made, or they may be worn away so that further grinding is impossible and consequently useless.

sons.

AN ASSORTMENT OF MILLING-CUTTERS.

The assortment of cutters shown in Fig. 263 makes a good set to put with the new milling machine. A wide range of work can be done with them, including the making of new cutters of almost any style or size. This set consists of two of No. 6 and one mill arbor, suitable for shell-end mills from 2 to 5 inches

in diameter, and No. 7 illustrates an end-mill 24 inches in diameter to fit it. The arbor has a threaded collar with tongues to fit in the slots milled in the back end of the cutter for driving it.

The screw tapped into front end of the arbor drops into the counterbore in the cutter, thus keeping out of the way of the

chips and holding the cutter in place. Figs. 264 and 265 show two other styles of end-mills and arbors, each having something to recommend them. The cutters shown in the group at the right are tapped standard, and have a slot milled across the back end to fit the loose collar, which is used to force off the cutter and serve no other purpose. If desired, the cutter itself could be extended and milled to fit a wrench, the only objection being that the cutter would be slightly more expensive.

The arbor shown with cutters to fit in Fig. 264 has No. 10 B. & S. taper to fit in the machine, No. 4 Morse taper in front

to fit the cutters, and Woodruff key to do the driving. It has a nut to force the cutter off and a screw to hold it on, the same as the screw in No. 1 of Fig. 263.

These three styles of arbors and cutters are excellent and any one of them will give good results. The threaded cutter is the

cheapest because it does not require internal grinding or lapping. The taper-arbor and its cutter are perhaps slightly more expensive to make, because it is necessary that the cutter be ground internally to fit the taper. This is to be recommended when the most accurate work is required.

SHELL END-MILLS.

Shell end-mills are very useful cutters and will be largely used wherever a milling machine is supplied with them.

Small end-mills should be made solid, perferably with tapershanks (Nos. 3 and 4, Fig. 263), as the most accurate and satisfactory way to hold them.

SPINDLE SURFACE-MILLS.

The spindle surface-mill (No. 5, Fig. 263) is 24 inches in diameter, 3 inches face, and is one of a great variety listed by the cutter manufacturers whose practice is to make with straight

teeth where the face is less than 3-inch wide. This style of cutter, in widths to suit, is commonly used for key-seating.

Cutters with side teeth (No. 6) could be used for key-seating, but it is obvious that they would fall below size much sooner than the cutter with outside teeth.

Teeth milled spiral will do better work on wide cuts than

when milled straight, on account of the shearing out, and for heavy roughing the teeth should be nicked by cutting a coarse thread around the blank before milling the teeth.

The side-cutter is most useful in pairs for milling both sides of a piece at once, like squaring a tap-shank; the cutters operating on opposite sides of the piece take away any tendency to spring and produce accurate work rapidly.

GANG-MILLS AND INTERLOCKING CUTTERS.

A gang of spiral surface-cutters with side teeth, the inner pair made interlocking, is shown in Fig. 266. The teeth are cut spiral, right and left hand alternately, to balance any side-thrust and to give top rake to the side teeth doing the cutting. The inner pair are made with clutch teeth to interlock; the bearingfaces being scooped out to allow the clutch teeth to engage. Paper is used to extend the cutter as the sides are ground away, maintaining a constant size and insuring interchangeability. The same cutters can also be used for roughing and finishing by taking out some of the packing while roughing, and restoring the cutters to the proper width before taking the finishing cut.

Fig. 266 shows a group of common forms. Care should be taken in grinding to have the face of the teeth radial; the tendency is to grind the point more than the base of the tooth, which places the cutting-edge at a great disadvantage.

Generally it is more economical to buy standard cutters from the maker, and in many instances special ones also, but it is at times desirable to do some of this work at home, being cheaper if the tool-room is properly equipped and organized, and the educational advantage of such work has a distinct value.

MAKING CUTTERS.

For making cutters, Nos. 10, 11, and 12 of Fig. 263 provide a good outfit. The first two have sixty degree angles, one right and one left hand, and will suffice for most straight tooth work. No. 12 is for milling spiral cutters and has twelve degree angles on one side and forty degree on the other.

Practice has shown that it is best to make cutters with radial