These may be made by following the instructions given for plain hollow mills, except that the mill must be split, Fig. 168, to allow for alteration in size.

Mill with a Strong Tooth.

Fig. 167. Mill with a Strong Tooth.

Fig. 168. Adjustable Mil!.

Methods Of Adjustment

There are two methods of adjusting the mill. In one the outside of the cutting end of the mill is tapered, and a collar having a corresponding taper hole is forced on the mill. The collar closes it, and causes it to make a smaller cut. The other method is to turn the outside of the hollow mill straight, and close by means of a clamp collar, Fig. 169.

Cutting Teeth

As adjustable hollow mills are generally used for finishing cuts, and not when taking heavy cuts, the teeth may be made finer than those of solid mills used for roughing. The teeth, being nearer together, will finish a cylindrical piece more accurately than if the teeth were cut farther apart. It is customary to give adjustable hollow mills which are to be used for finishing, from six to eight teeth. The cutting edges should be radial for most work. Better results will be obtained if the hole in the cutting end of the mill is left .005 inch small, and ground to size after the mill is hardened.

Hardening And Grinding Hollow Mills

The hollow mill, whether it be solid or adjustable, should be hardened a trifle farther up than the length of the teeth, and drawn to a straw color. The mill is sharpened by grinding on the ends of the teeth.

Clamp Collar.

Fig. 169. Clamp Collar.

Hollow Mill with Removable Blades.

Fig. 170. Hollow Mill with Removable Blades.

Hollow Mills With Inserted Blades

For large work, hollow mills are made with inserted blades. The type shown in Fig. 170 does good service on rough work. The blades of this mill may be made of selfhardening steel and inserted in a machine-steel body; the grooves in the body, to receive the blades, should be milled with a cutter whose thickness corresponds to the size of the steel to be used for the blades. The grooves are cut somewhat deeper at the front end of the holder, in order that the blades may have clearance to prevent binding. The edge of the slot corresponding to the cutting edge of the blade should be radial.

Two collars should be made of machine steel, with holes sufficiently large to allow their being placed on the mill when the blades are in the slots. Each collar should be provided with the same number of set screws as there are blades in the mill. One collar holds the blades in the holder, while the other is placed nearly at the ends of the blades to support them while cutting. This form of mill is used on cuts not exceeding one inch in length, as the blades must project beyond the holder to the length of the cut.

The size of cut may be changed somewhat by setting the cutters back or ahead in the slots, or paper may be placed in the slots under the blades to increase the diameter of the cut. The blades are set to an even length by bringing them against a surface perpendicular to the axis of the body of the tool.

Hollow Mills With Pilot

It is often desirable to mill the outside of a projection central with a hole passing through it. This may be done very satisfactorily with a hollow mill having a pilot, as shown in Fig. 171. It is advisable to hold the pilot in place by means of a set screw. In order to give clearance to the teeth to prevent the mill binding when cutting, the hole may be bored tapering, .010 inch in 1/2 inch of length, making it largest at the back end.

When hardening a mill of this description, it is advisable to dip it into the bath with the cutting end uppermost, working it up and down rapidly. After being hardened, it should be drawn to a straw color. The pilot should be turned .010 inch above finish size, hardened, drawn to a brown color, and ground to the desired dimensions. At times it is necessary, or desirable, to use a hollow mill as a counterbore; that is, it is necessary to enlarge a hole all the way through a piece of stock. As the core removed would bind and stick in the hole in the mill, the hole is made eccentric, Fig. 172. The pilot is concentric with the outside, and should not be a tight fit in the hole to be enlarged. The core removed will be smaller than the hole in the mill, and consequently will not bind.

Fig. 171. Hollow Milt with Pilot.

Hollow Mill with Eccentric Pilot.

Fig. 172. Hollow Mill with Eccentric Pilot.