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Chuck insert wrong thread ?

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Max Power

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I've just received a 1 3/8" x 6 insert for a Vicmark chuck to fit my Wadkin RS and although the diameter looks right and its got the same number of threads per 1/2" it doesn't fit.
The threads on the insert are slightly flat at the top could this be all that's different meaning I could get them re-cut to match mine or will I have to get a new insert made somewhere ?
 

TFrench

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Do you have a whitworth thread gauge to double check the new one is the correct thread form?
 

CHJ

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Does sound as though you have a mix of whitworth and UNC thread form.

The same problem can occur with the Poolwood 28-30/40 lathes, the early mechanical variable speed machines having a different 1-1/2" x 6 thread form to the later and electronic control versions.
 

Duncan A

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Both UNC and BSW have 1 3/8" x 6tpi threads which (bad practice) are sometimes interchangeable despite differences in the thread angle.
Sounds like your insert is UNC because the thread tops are flat. If so, Included angle will be 60 degrees.
BSW threads should have a rounded top and an included angle of 55 degrees.
This may help you to identify your lathe spindle thread if you can't get hold of a thread gauge. It may be worth putting engineers blue ( or some other marker) on the spindle thread to see which surfaces are binding up.
Duncan
 

TFrench

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The wadkin side is definitely 1 3/8" BSW - I know, because I've had my own struggles with getting chucks for mine!
 
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