Relief

ground to the same equivalent angles for use on mild steel, but for turning brass alloys it is usual to give no top-rake and, also, to reduce the clearance angles ; however, it may be found that in some instances better work results when a small amount of top-rake is provided, if at the same time chatter and digging-in can be avoided.

Although only a few standard angles have been instanced, it may be found that better results can be obtained with individual lathes, and when machining different materials, if these angles are varied somewhat, but in all cases experiment will be found the best guide.

Tool Grinding. When the tool angles have been decided on, the actual grinding should be carried out with the aid of one of the adjustable rests described in the previous chapter, and, as a general principle, less material should be removed as the operation proceeds from the rough to the finished stage, in order to lessen the work and prevent overheating of the tool. Thus, if the final angle honed by the oilstone is io deg., then an angle of 12 deg. should be ground on the coarse wheel, followed by one of 11 deg. on the fine wheel.

To facilitate setting the angular rest, templates of the form shown in Fig. 22, and made of plastic material or sheet-metal can be used with advantage ; whilst to maintain the template in a vertical position on the work table it should be mounted in a rule holder or supported against a metal block.

For- the sake of convenience, a set of templates should be made to cover the range of angles required for the various tool forms used.

Although these templates are in every way suitable for setting the angular grinding rest correctly in relation to the side faces of the wheel, some difficulty may be experienced when they are used to adjust the height of a rest relative to the periphery of the wheel. In the latter case, as has been pointed out, the setting of the rest will vary with the thickness of the tool and with the diameter of th wheel when a constant clearance angle is required.

It follows, therefore, that if tools of three different

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