Electronics > Mechanical & Automation Engineering

Automatic Operation of Half-nuts for Threading

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jpanhalt:
There has been some progress in the past month with my project for adding NC control for threading to a 1965 toolroom lathe.
Here are some related threads in this subsection:
https://www.eevblog.com/forum/mechanical-engineering/mitutoyo-acu-rite-dro-zero-setting/
https://www.eevblog.com/forum/microcontrollers/stymied-why-are-my-data-sometimes-doubled/
https://www.eevblog.com/forum/mechanical-engineering/mitutoyo-982-537-1-dro-display-with-series-572-linear-scales/msg5171796/#msg5171796

The last of those links is now up and working.  I like the ST7525 GLCD.  The keypad, not so much.  I am now giving thought to the last critical step of how to control the half-nuts for threading.  See attached images.

In the "open" version, lever A controls the half nuts.  Lever B controls power feeds (both crossfeed and Z-axis).   That will continue to be manual for the time being.  The image labeled "closed" has the half-nuts engaged.  I estimate it at about 45° to 55° of motion.

The half-nut lever is fixed to its shaft with a drive pin.  My initial plan is to make an extension for that shaft.  Mount the drive gear (?) nearest the saddle and the current lever distal to that.  I want to maintain manual control.  Consider doing an 8 tpi thread at 100 rpm.  That's 0.125"/600ms or 0.010" per 48ms.  My goal is to control the stop point, which will be toward the spindle, to within 0.010".  Manually, I can do about 0.100" with a little lower rpm, but if I lose concentration, all hell breaks loose.  The "Test Plug" is something I did last year.  The "safety zone" is more than 0.1".  BTW, the lathe has a threaded spindle, and I would not want to risk trying to thread something like that running in reverse.

I am considering a belt drive with stepper or servo motor as mounting to the lathe saddle would be quite easy.  Attaching a solenoid would seem to be more complicated.  I am open to any suggestions.

Ian.M:
I don't have specific knowledge of your lathe, but beware of how much the half-nut lever can load up when taking a heavy threading cut at a coarse pitch.  If your mechanism cant provide enough force and it fails to disengage, all hell breaks loose, with greater energy.   Maybe when opening, if there is a mismatch between the lever position and its commanded position for longer than it normally takes to open, then arrange to trigger the E-stop.

jpanhalt:
I am aware of threading dangers and had a very small number (maybe 2) of crashes years ago.  Getting it stopped in time is always been a concern.  As my reflexes have gotten worse (age 81), and I need to do internal 8 tpi for a new chuck, I figured it was time to try to find a solution for myself and perhaps others.  I have a much smaller Prazi 3" lathe that doesn't even have half-nuts or a threading dial.  I set up a DC motor and just start and stop that.  Of course, I don't do anything like the plug shown, but it did OK for making some C-mount threads for optical work.  Doing 32 tpi is a lot different than 8 tpi.

As for safety, I will also be rigging up a relay/mosfet switch to stop the lathe motor (it has a VFD drive) past the assigned endpoint and will keep my hand on the half-nut lever at all times.  I never turn my back on any machine tool that has any manner of motorized feed engaged.  Eventually, I may add a similar device to engage the crossfeed at the end of a thread and pull the cutter away from the work.  The neat thing about crossfeed, unlike threading, on my lathe is it doesn't take much resistance to make it disengage.  A simple mechanical stop works.

Andy Watson:
That's a nice looking lathe!

Have you considered stopping the leadscrew drive with a dod-clutch.Better still if you can arrange for the dog-clutch to be on the spindle - as with Hardinge or the more recent designs of Graham Meek - you wouldn't need to worry about maintaining half-nut engagement for those non-integer threads. The knock-off is mechanical in these designs but I see no reason why it could not be electromagnetic.

reboots:
This is a cool project. Is your lathe the Smart & Brown 9x18?

Can I ask why you chose to use the existing threading mechanism, rather than doing an electronic lead screw conversion?

I think a motorized servo actuator is the way to go, over a solenoid. You may end up wanting a programmable torque threshold, possibly different settings for engaging and disengaging, to keep from jamming or breaking the levers.

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