Gallery, Projects and General > How to's
Single point threading on the lathe
bogstandard:
Nick,
I think you are getting confused because of the different threads that are required to be cut.
I will try to explain the reasons behind what we are on about.
Imperial Lathe.
These are almost always fitted with an imperial leadscrew, and because the imperial system uses TPI (threads per inch), then by using the drop in dial that is matched to that leadscrew, and with the correct change gears fitted, imperial threads can be cut directly by dropping in the correct numbers of the screwcutting dial.
When metric threads are being cut on an imperial machine, a transposing gear is usually fitted into the gear train to allow the leadscrew to turn at the correct rate for metric threads. Unfortunately, because metric threads use a pitch system instead of a TPI one, you cannot use the drop in dial, but instead you have to leave the half nuts engaged, and reverse the machine backwards to wind the carriage back to the beginning, to carry out the next cut.
Usually, metric threads are not true pitch when cut on an imperial machine, they are an approximation of the pitch, but usually close enough for a normally cut metric nut or bolt to fit. The problems arise when you cut a long metric thread that has to be used with a long normally cut metric hole or bolt thread. They can very easily bind up solid because of the missmatch.
Metric Lathe.
These are normally fitted with a metric pitch system of leadscrew. But unlike the imperial system, you also have to compensate on the drop in dial for the different pitches being cut. Usually this is done by having change gears on the bottom of the dial that pick up the readings from the leadscrew. There are 3 on my machine, and these little posts I did might explain what they look like.
http://madmodder.net/index.php?topic=870.0
http://madmodder.net/index.php?topic=973.0
Unfortunately, even with the compensating gears, some metric threads still have to be cut by leaving the half nuts engaged, and setting up the gear train for the correct pitch.
For cutting imperial threads on a metric machine, it is basically the same as cutting metric threads on an imperial machine. A transposing gear is fitted, the gear ratios are changed to give what TPI you require and the thread is cut with the half nuts permanently engaged. The same problem with the fit also rears it's ugly head, they are only very close approximations of the thread required.
Some very expensive lathes have all these facilities built in, just select on the gearbox. But on the sorts of lathes we normally use, we have to compromise.
I hope this has made it a little clearer.
Bogs
Darren:
Tim,
I have a 7x12 that I cut threads with which I believe is the same as your lathe?
I can't say I have the issues you describe. So thinking about it the play can't be in the half nuts/leadscrew because they are clamped together. So the problem must lay in the leadscrew mountings at each end. They are to some degree adjustable and I believe some others have made adjustable adaptors at the tail end for this very reason.
spuddevans:
--- Quote from: Darren on August 29, 2009, 03:50:16 PM ---I can't say I have the issues you describe. So thinking about it the play can't be in the half nuts/leadscrew because they are clamped together. So the problem must lay in the leadscrew mountings at each end. They are to some degree adjustable and I believe some others have made adjustable adaptors at the tail end for this very reason.
--- End quote ---
You could be right are most likely right about adjusting the leadscrew, :thumbup: I have done very little to the lathe since getting it and de-greasing it. After I finish the Paddleducks build I intend to start on a whole stream of mods to both the lathe and mill. Thinking about it, there is a lot of play in the leadscrew on mine. That's why I have to withdraw the tool before reversing the lathe when threading.
Tim
John Hill:
After a long, and rather exciting, exchange on another board I have concluded (actually it was pointed out to me) that when using a metric lathe to cut imperial threads the drop in position repeats at 127 revolutions of the lead screw for all (whole number) imperial pitches. (127 is an even mulitple of 25.4 which is the ratio of millimetres to the inch).
SO....having a turns counter on the lead screw that repeats at 127 you can always withdraw the tool, wind the carriage back and re-start without phutzing about telegraphing the engine room to run astern!
I have seen one design for a 127 tooth indicator gear but if that is to engage with a 3mm pitch lead screw it would be about 5 inches in diameter. Other suggestions have been a sprocket on the lead screw running a loop of light chain long enough to repeat every 127 turns, the chain of course has a link or so painted to indicate the drop in point. Neither of these is impossible but both rather cumbersom.
The 4020 (there are others) CMOS integrated circuit can drive LEDs directly and has a reset pin. It should be easy enough to mount one of these in a little plastic box complete with a reed switch which suitably position would be triggered by a magnet on the lead screw. You would then have a little display with a number of flashing LEDs in an easily recognised pattern and when it gets to the end of the sequence thats the time to drop in the half nuts. I confidently expect Darren will make one of these in short order.
Darren:
So glad to inspire confidence John....... :lol:
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