Tool Rest

this, a special grinding machine may be employed for the finish grinding of this and other small tools. A machine suitable for this work, comprising an electric fan-motor fitted with a small India wheel, was described in Chapter II.

Scribers. If a scriber is to operate effectively and mark a clean-cut fine line, its point must be really sharp ; moreover, if heavy downward pressure has to be applied to make the scriber cut, it is quite possible that the guide rule will be displaced and a faulty marking will result.

As will be seen in the drawing in Fig. 52, the blade of the scriber tapers gradually throughout its length, and this taper culminates in an acute-angled working point.

If the point requires to be ground, this is carried out in a manner similar to that described in the case of the centre punch, but, owing to the small mass of metal present, additional care must be exercised to prevent overheating of the steel and too rapid abrasion of the tip.

For these reasons, it is better to do any grinding necessary on a fine India wheel, as has already been described in the case of the centre punch.

Light sharpening of the point is best carried out on a hard oilstone such as an Arkansas.

Dividers. These tools, which are used for scribing circles and marking-off dimensions set from a rule, are easily sharpened, but it is better to give them periodic attention rather than to allow the points to become really blunt.

Except in the event of accidental damage, grinding of the points should be unnecessary, and all that is required to produce a fine sharp point is to rub the sides of the tip with a oilstone slip, as indicated in Fig. 53 ; examination with a magnifying glass will show the progress of the work and will reveal any irregularities of the point requiring attention.

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