and these are available in wide variety of shapes. Sometimes clamps are also provided which bridge
the vee to secure the work. These are also supplied as matched pairs in which case two V-blocks
are of similar size and of the same grade of accuracy. In the case of V-blocks the working surfaces
are flanks of vees, base end faces, top and side faces. (Refer Fig. 2.45)
Depending upon the accuracy, IS : 2949—1964 specifies the V-blocks into two grades, viz.
grade A and grade B. As in the case of surface plates, here also the tolerance on flatness is defined
as the maximum permissible distance separating two imaginary parallel planes, within which the
surface under consideration can just be enclosed. Similarly the tolerance on squareness is defined
as the maximum permissible distance separating two imaginary parallel planes, (which are
perpendicular to the datum face of the part in question) within which the surface under considera-
tion can just be enclosed.
V-blocks are furnished by hand scraping process till they have a bearing area of not less than
20%. Any departure from flatness in case of vees should be a convexity and on other surfaces it
should be a concavity. In a direction parallel to the axis of the vee the departure from flatness should
be a concavity and not convexity.