In this paper, the depth of surface layer formed by superfinishing on the test pieces of carbon steel (0.1%C, 0.3%C, 0.9%C) and gauge steel, was measured by a newly modified microhardness method.
The depth of surface layer by this method was nearly equal to that by etching method, and the values were 35 μ (0. 1%C), 35 μ (0. 3%C), 24 μ (0. 9%C), 1.5 2 μ (gauge) respectively.

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Following up our primary studies on lathes, the dynamical characteristics of lathes which cut the work actually were studied from the motion of the work. In the present research the effect of the form of cutter tips was principally considered.

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By observing the chip-deformation in the zone adjacent to the tool face, followings were found out about the strain and the strain rate in the zone :
1. As a chip flows along the tool face, the strain increases at first rapidly and then slowly, reaching a constant value at the point where the chip leaves from the tool face.
2. The strain rate is maximum at the cutting edge, and decreases to zero amost linearly.
3. With an increase of the cutting speed, the strain decreases and the strain rateincreases.
4. With an increase of the depth of cut, both the strain and the strain rate increase.
5. With an increase of the rake angle of the cutting tool, the strain apparently decreases and the strain rate decreases to some extent.
6. Considering the distributions of the temperature and the strain rate, the distribution of the shear stress on the tool face was presumed.
7. The strain rate (logarithmic mean value) in the shear zone is 10 to 100 times as high as the strain rate on the tool face.

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The conventional mechanical method of diamond machining is not efficient. Therefore, in order to improve the machining efficiency, we applied the electric spark machining method developed at the National Bureau of Standards in U. S. A. to the manufacturing of diamond dies.
By an electron microscope, we also studied the surface structures of diamonds machined through this method.
The results of our experiments are as follows :
(1) This method is very effective in comparison with the conventional methods.
(2) By electric spark machining, diamond cleaves but does not change to graphite.
(3) It is difficult to lap the diamond surface by this method.

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