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Harold Hall

Workshop Processes

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My advice would be to cut the thread gauge with a chaser or a single point tool that produces a precise thread form, Photograph 2, as using a tool that does not produce the radius on the outside of the thread will complicate the issue in terms of its diameter,for this I would recommend +0.02 to + 0.05mm. However, using a basic single point tool would be acceptable when eventually cutting the internal thread. I will assume the viewer is conversant with cutting threads and will not elaborate on the subject. If guidance is though required then seek out further reading on the subject, such as "Screw cutting in the Lathe" Workshop Practice book number 3

When making the parallel gauge do make a short length, say the first 3mm, 0.05mm under size as this will give an indication that the bore is close to the required size and at which point considerable care has then to be taken when proceeding.

 

Attempting a tolerance of only + 0.002mm, which is easy to write down but far from easy using normal turning techniques to achieve, how then is this accomplished? The lathe's cross feed in likely to be calibrated in 0.02mm or even 0.025mm divisions in which case we are attempting to reliably work to a depth of cut of less than 1/20 of one interval on the dial, virtually impossible using the cross slide!

 

Achieving precise diameters.

Fortunately, the solution is relatively easy to adopt and is to set the top slide to 0.6° as this gives a ratio of 100:1 between the axial and radial movements of the cutting edge. My method of achieving the 0.6° angle is briefly as follows and can be done quite quickly if the required accessories are available. First, set the top slide precisely parallel with the lathe's axis using a DTI and testing along the length of a "between centres test bar" Sk. 1-1. Next, place a piece of steel under the top slide clamp and, with its edge firmly against the test bar, clamp in place, Sk. 1-2. Finally, loosen the top slide to enable it to rotate and at a point 100mm from the left hand end of the piece of steel place a 1mm diameter drill shank between the piece of steel and the test bar with the other end against the bar. With that done clamp the top slide in this position   Sk.1-3,  see also Photograph 3.

 

For less demanding situations an angle of 6° gives a ratio of 10:1 between axial and radial movement with the lathe's calibration for setting the angle of the top slide sufficiently accurate for this requirement.

Screw Cutting on the lathe
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 2

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 3