The advantage of forming the table in dioptres in place of radii directly is that the tabular differences are small at all parts of the table so that interpolation can be readily done and this is not the case in tables which give the radii directly.
If upon measuring the radius of the tool or lap being turned in the sphere turning machine, [Fig. 4], with this spherometer, the tool is found to be in error by an amount Δp this may be corrected by changing the position of the cutting tool by an amount 20 (Δp ⁄ p2).
Note 2—Cross Section of the Sphere Turning Rest.
Fig. 7
In [Fig. 7] is shown a cross section of the sphere turning rest further illustrated in [Fig. 4]. In machining this the following suggestions should be followed. The piece M should be cast with a lug projecting from the face PQ to chuck it by and all the turning done at one chucking. It should be made a close fit to R and bolted tight against DG and ED´ with the bolts S3 and S4, clearance being given along the line HF. To compensate for ware the face DG and ED´ can be releaved from time to time with a file. The base N, should be planed along AB, where it fastens to the cross slide of the carriage, then bolted to a face plate of the lathe and finished, care being used to leave the setting of the compound rest unchanged between machining the faces CD and C´D´ of the pieces M and N. The dove tail on R should be first planed and then this bolted to a face plate and the boss GHFE and the faces KG and EL turned at one setting. If these directions are followed almost no hand work will be needed. W is a brass worm wheel held by screws not shown and J is the sliding tool post clamped at X with the tool at K´.