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Beginning and end to internal thread - how would you do it?

fusker

Cast Iron
Joined
Apr 13, 2006
Location
Denmark
Making a new back plate to my 40 inch lathe. Managed the internal thread (68 mm thread on spindle, that is 2.68 inches dia.), had luck, seems to fit.
But I need good ideas on how to put a nice beginning and end on the thread. Don't have a Bridgeport, only a shaper - and I don't fancy using the shaper for this, although it presumably just might be done. Screw-up expectancy factor too high.
Thinking of using a cold chisel, after all no-one will see the result when the back plate is in place. But I will know it's there and that it will be ugly as h***.
Any ideas, fellers?
Regards, fusker
 
You could clean it up with a die grinder and a burr. Mount the grinder in the toolpost, fasten some kind of leverage to the chuck (with backplate mounted concentric), feed the burr in to depth and slowly rotate the work until you get rid of the skinny part of the thread. If you want to be really fancy, you can do this with the halfnuts engaged as per threading the backplate, so this will feed the grinder inwards/outwards as you rotate the spindle. Work the backlash out of the leadscrew gear chain first before you begin.
 
Use a die grinder or Dremel (or Aldi/Lidl/Netto equivalent) to create a "Higbee cut". Grind away any partial thread until thread starts with a full section, with the end reasonably square/radial. This will clean the mating thread on the lathe mandrel and make starting the thread more easy. Can be done quite well by hand.
 
I'd second (or third) the recommendation of the Higbee cut. In case you don't have a rotary tool (die grinder, Dremel), I've in the past used an endmill to clean up the last bit of thread.

It would be tricky on a lathe (with the endmill held in a collet or chuck) but certainly could be done with time and patience.

Henry
 
Tool post or hand held grinder

Thanks, all. I do have a primitive home-made tool post grinder, not very efficient or easy to use so haven't been eager for it. Will try it now.
A Dremel is rather expensive but would be nice to own. Didn't know Aldi or Lidl types were any good but will accept the recommendation and maybe buy one.
Have considered using a small cutter but building a tool post mill will take some time. An interesting project though.
Regards, fusker
 
Another option would be a key woodruff key seat cutter to cut the "higbee". This would work great with a rotary table on a mill.
 
The end of the backplate threads that bear on the spindle face needs to have the
correct register diameter cut into it as a counterbore. I've always
found that once this is done, that end of the thread doesn't need any other
treatment to speak of aside from a cleanup and de-burring with a file.

The outboard end (which will be hidden once the chuck is bolted on) is typically
just left as-cut. Maybe a light 45 chamfer on it.

Jim
 








 
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