Cutter-heads are often provided with two sets of cutters, one set being used for roughing and the other for finishing. It is a good plan to make these cutters so that the ends _e_ (Fig. 6) will rest against the bar or bottom of the slot, when the cutting edge is set to the required radius. The cutters can then be easily set for boring duplicate work. One method of making cutters in sets is to clamp the annealed stock in the cutter-head and then turn the ends to the required radius by placing the head in the lathe. After both sets of cutters have been turned in this way, they are ground to shape and then hardened.
[Illustration: Fig. 7. Cutter-heads equipped with Adjustable Tools]
Boring cutters intended for roughing and finishing cuts are shown in the detail view Fig. 8 at _A_ and _B_, respectively. The side of the roughing cutter _A_ is ground to a slight angle _c_ to provide clearance for the cutting edge, and the front has a backward slope _s_ to give the tool keenness. This tool is a good form to use for roughing cuts in cast iron. The finishing tool at _B_ has a broad flat edge _e_ and it is intended for coarse feeds and light cuts in cast iron. If a round cutting edge is used for finishing, a comparatively fine feed is required in order to obtain a smooth surface. The corners of tool _B_ are rounded and they should be ground to slope inward as shown in the plan view. The top or ends _d_ of both of these tools are "backed off" slightly to provide clearance. This clearance should be just enough to prevent the surface back of the cutting edge from dragging over the work. Excessive end clearance not only weakens the cutting edge, but tends to cause chattering. As a finishing tool cuts on the upper end instead of on the side, the front should slope backward as shown in the side view, rather than sidewise as with a roughing cutter. The angle of the slope should be somewhat greater for steel than cast iron, unless the steel is quite hard, thus requiring a strong blunt tool.
[Illustration: Fig. 8. Boring Tools for Roughing and Finishing Cuts]
=Cylinder Boring.=--Fig. 9 illustrates the use of a cutter-head for cylinder boring. After the cylinder casting is set on the platen of the machine, the boring-bar with the cutter-head mounted on it is inserted in the spindle. The bar _B_ has a taper shank and a driving tang similar to a drill shank, which fits a taper hole in the end of the spindle. The cutter-head _C_ is fastened to the bar so that it will be in the position shown when the spindle is shifted to the right, as the feeding movement (with this particular machine) is to be in the opposite direction. The casting _A_ should be set central with the bar by adjusting the work-table vertically and laterally, if necessary, and the outer support _F_ should be moved close to the work, to make the bar as rigid as possible.