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The radius of the arc would here again be determined by the distance of the tool point from the centre of rotation of the pivot, or, what would be the same thing, from that of the tool post.

The ordinary tool post is placed in this extra rest.

Hence the capacity to use octagon steel and keep it level while adjusting its height, added to the fact that the tool is supported clear to the edge of the tool rest, and the tool post is so blocked as to virtually become a part of the rest, constitute a very important advantage.

The tool is here supported along the full length of the gib, and there is no set-screw at the top of the tool post, which enables a much more unobstructed view of the tool.

Both these rests are provided with tool post clamps, to hold tools made of round wire, such clamps being shown in position in figure 657.