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Posted by D Murphy on April 2, 2006, 5:25 pm
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> Hi and thanks for all your responses to my question yesterday about
> how to cut a 4-start thread on a small lathe. I'm going to try the dog
> approach since it seems the easiest to set up and probably gives good
> results.
>
> My second question is about the other half of the equation: the nut
> that the 4-start 1/4"-20 screw uses needs to obviously have the same
> threaded type to work properly. The nut is about 1/2" thick. Because
> the hole is small (for a 1/4"-20 thread), other than having a custom
> 4-start tap made (expensive?), are there any tricks to cut this 1/2"
> long 4-start 1/4"-20 internal thread? I know that my current internal
> threading tool for the lathe can't do a hole this small.
Well your choices are to buy a tap, make a tool to single point the thread,
or make your own tap.
Remember if you make your own tool to single point, you need to calculate
the helix angle of your thread and have that many degrees plus a couple
more ground into your tool for clearance. I'm coming up with about 16.4
degrees, do you would need about twenty degrees ground into the leading
edge of your threading tool.
You don't need any clearance at all on the trailing edge. In fact your tool
can look like this [//] just so long as the trailing edge is less than 16.4
degrees. For the ID thread you'll probably need to grind this way for
strength. Just don't stop the feed in the cut, as the tool has no clearance
when the tool is not moving at the thread pitch.
--
Dan
Quid Aere Perennius
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> how to cut a 4-start thread on a small lathe. I'm going to try the dog
> approach since it seems the easiest to set up and probably gives good
> results.
>
> My second question is about the other half of the equation: the nut
> that the 4-start 1/4"-20 screw uses needs to obviously have the same
> threaded type to work properly. The nut is about 1/2" thick. Because
> the hole is small (for a 1/4"-20 thread), other than having a custom
> 4-start tap made (expensive?), are there any tricks to cut this 1/2"
> long 4-start 1/4"-20 internal thread? I know that my current internal
> threading tool for the lathe can't do a hole this small.