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
