Gallery, Projects and General > How to's

Single point threading on the lathe

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Darren:
I was thinking about your calculator trick the other day while out shopping of all things....yes I was indeed waiting for the other half yet again and find this is a time the mind tends to wander....


I was wondering if a calc with some "calcked" number could count the revolutions on an imperial machine to give a metric reading. of course backlash would still be an issue unlike a proper DRO.

John Hill:
Darren, I do not think backlash would be relevant when trying to find the point at which to re-engage the half nuts, I have a couple of ideas on the subject but the sun is shining today so not 'shop playtime until chores are done!

I will do a short topic in the calculator thingio.

HENNEGANOL:
When screwcutting in the lathe I was taught to set the cross slide to half the included angle of the thread ie., 30 degrees for metric or unified threads and 22.5 degrees for imperial threads.  The tool is set up using a gauge as described earlier, then the  feed is put on with the cross slide only.  Which results in the left face of the tool doing all the cutting.

When I served my time all our machines were calibrated in imperial units, all the lathes were supplied with a 127 tooth gear for use when cutting metric threads!

In order to cut metric threads on my Myford lathe fitted with a quick change gearbox it was necessary to purchase an additional set of gears and quadrant.  Even then you must not disengage the nut after each cut, the saddle has to be wound back by reversing the motor.

Oops, I should have said set the top slide to half the included angle

bogstandard:

--- Quote from: HENNEGANOL on August 25, 2009, 04:05:48 AM ---When screwcutting in the lathe I was taught to set the cross slide to half the included angle of the thread ie., 30 degrees for metric or unified threads and 22.5 degrees for imperial threads.
--- End quote ---

A couple of mistakes there HENNEGANOL, but I understand what you mean.
 
Actually, this is a much better method of cutting threads, using the compound (topslide) set to half the included thread angle, and using that for putting the cut on when leaving the leadscrew permanently engaged, and was the way I was always taught how to cut single point threads. Only using the cross slide for the final clean up.

This is due to the fact your cross slide will always be at the same setting when retracting it and putting it back to its former position, as you should do when winding back on the leadscrew, rather than trying to remember the setting you were at on the last cut, otherwise if you don't retract, backlash will kill the threads by taking extra metal off. Just use 0 for your cross slide setting, a rather easy number to remember.

When, eventually, I get my retracting topslide made, everything will be taken care of automatically.


Bogs

arnoldb:
HENNEGANOL, Bogs gave good advice. May I just add that the angle for the topslide for cutting a 55 degree thread using this method is 27.5 degrees and not 22.5.
Not being picky - just to make sure someone does not accidentally fall into a trap and use the wrong setting  :D

Hmm... less computer and off to cut that 12 tpi internal thread - never done that; hope it comes out OK  :scratch:....
 :beer: Arnold

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