These attachments differ somewhat in their construction and arrangement but the principle of their operation is similar. [Fig. 38] shows a Hendey relieving attachment applied to a lathe. A bracket carrying the gearing A through which the attachment is driven is mounted upon the main gear box of the lathe, and the special slide B, which is used when relieving, is placed on the cross-slide after removing the regular compound rest. The gears at A are changed to suit the number of flutes or gashes in the cutter, tap or whatever is to be relieved. If we assume that the work is a formed milling cutter having nine teeth, then with this particular attachment, a gear having 90 teeth would be placed on the “stud” and a 40-tooth gear on the cam-shaft, the two gears being connected by a 60-tooth intermediate gear. With this combination of gearing, the toolslide would move in and out nine times for each revolution of the work, so that the tool could back off the top of each tooth. (The gearing to use for various numbers of flutes is shown by an index plate on the attachment.) The amount of relief is varied to suit the work being done, by means of a toothed coupling which makes it possible to change the relative position between the eccentric which actuates the toolslide and the cam lever, thereby lengthening or shortening the reciprocating travel of the tool.
Fig. 39. Relieving a Formed Cutter
Application of Relieving Attachment.—Some typical examples of the kind of work for which the relieving attachment is used are shown in [Figs. 39] to [42], inclusive. [Fig. 39] shows how a formed milling cutter is relieved. The toolslide is set at right angles to the axis of the work, and the tool moves in as each tooth passes, and out while crossing the spaces or flutes between the teeth. As the result of this movement, the tops of the teeth are backed off eccentrically but the form or shape is the same from the front to the back of the tooth; hence, a cutter that has been relieved in this way can be ground repeatedly without changing the profile of the teeth, provided the faces are ground so as to lie in a radial plane.
When relieving, the cutting speed should be much less than when turning in order to give the toolslide time to operate properly. A maximum of 180 teeth per minute is recommended, and, if wide forming tools are used, it might be advisable to reduce the speed so low that only 8 teeth per minute would be relieved. It is also essential to use a tool having a keen edge, and the toolslide should work freely but be closely adjusted to the dovetail of the lower slide. Before beginning to back off the teeth, it is a good plan to color the work either by heating it or dipping into a strong solution of copper sulphate. This will enable one to see plainly the cutting action of the tool in order to stop relieving at the proper time.
Fig. 40. Relieving Side of Angular Milling Cutter