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Harold Hall

Workshop Processes

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Why 1.045" diameter?

If the viewer chooses to cut the internal thread using a basic single point tool this will produce a thread without a radius on its tips which will then foul with the root of its mating thread. When using this form of cutter the internal bore must always be larger than the theoretical value so as to avoid the problem. Similarly, with an external thread the outer diameter should be undersize to prevent a similar situation. The bore diameter given therefor produces a thread depth of 75% a situation that only very slightly reduces the strength of the joint made.

 

For me, making the thread with a tap, the tips of the thread produced would have the designed radius on their tips. However, as there is very little reduction in strength, it speeds up the process as there is less depth to cut. As cutting an internal thread is a slow process reducing the number of passes that the cutter has to make is very worthwhile and would suggest that whatever method of producing the thread you decide to use that for internal threads a depth of between 65 and 75% should be the norm.

 

Ready for the next stage

With the parallel portion and the thread now complete the workpiece can at last be tested against the mandrel nose that should be, if the above process has been worked to, just a formality. You will now be ready to proceed with whatever it is you are attaching to the mandrel, chuck backplate, faceplate or collet chuck, the accuracy of which will benefit from the attention to detail taken in the initial stages of its manufacture.

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Drawing