Lathe Threading Auto Stop

[T Bredehoft]

I'm no gunsmith, so triggers aren't part of my repertoire. I got to thinking about this, came up with a lever on the engagement handle, operated by a push rod (with roller bearing on the end) to disengage the feed on my MK2. It took me all weekend to whittle it out. I'd planned on eventually adding a threaded ring adjustment, may yet do that. If you look close you can see the aluminum collar I applied to the engagement handle, held on by 5 5-40 set screws.


It ought to work for threading and feed. It seems to stop within a couple of thou.

 

[hman]

Reading this and other threads, as well as (who was it)'s excellent thread about the gear box technique (also written up in HSM or MW), I've been wondering if there weren't a simpler method of achieving a quick stop when threading. After having installed a 3 phase motor and VFD with braking resistor on my 12x24 lathe, I've been impressed by how quickly it can come to a stop. I have the stop time set to ½ second, and could probably go even lower. Couple this with a measurement I did quite a number of years ago, indicating that a microswitch's operating point is reproducible within about .001" to .002", I offer the following "wild hair" for your consideration:

Attach a microswitch to something like a feed stop bracket, that can be positioned along the ways and locked in place. Wire the microswitch into the VFD controls, so that it activates the stop. With the lathe off, run the carriage to where the threading tool is in the relief groove. Position the microswitch to activate at this point. Now back off the cross slide, run the carriage to the start point of the thread, advance the cross and/or compound as required, and start hit the start button on the VFD.

If my wild hair is valid, the lathe will make a threading cut and stop when the tool is in the groove. Then back off the cross, reposition the carriage, reset the tool, and repeat the process.

Consider this a target on the wall. You're all invited to start throwing darts at it! I really don't want to try this unless it passes the smell test.

 

[homemadetools]

Nice build! We featured it in a recent newsletter, properly credited to you and Hobby-Machinist.com of course. Looks like you guys just reached over 100 homemade tools that we've indexed. I hope we've been able to send you lots of visitors. Well done.

 

[Canobi]

Reading this and other threads, as well as (who was it)'s excellent thread about the gear box technique (also written up in HSM or MW), I've been wondering if there weren't a simpler method of achieving a quick stop when threading. After having installed a 3 phase motor and VFD with braking resistor on my 12x24 lathe, I've been impressed by how quickly it can come to a stop. I have the stop time set to ½ second, and could probably go even lower. Couple this with a measurement I did quite a number of years ago, indicating that a microswitch's operating point is reproducible within about .001" to .002", I offer the following "wild hair" for your consideration:

Attach a microswitch to something like a feed stop bracket, that can be positioned along the ways and locked in place. Wire the microswitch into the VFD controls, so that it activates the stop. With the lathe off, run the carriage to where the threading tool is in the relief groove. Position the microswitch to activate at this point. Now back off the cross slide, run the carriage to the start point of the thread, advance the cross and/or compound as required, and start hit the start button on the VFD.

If my wild hair is valid, the lathe will make a threading cut and stop when the tool is in the groove. Then back off the cross, reposition the carriage, reset the tool, and repeat the process.

Consider this a target on the wall. You're all invited to start throwing darts at it! I really don't want to try this unless it passes the smell test.

I have an old Grayson lathe which is very shy on features so I've been thinking along the same lines as I have a bunch of inverters (I work at Invertek Drives UK). I'm new to machining but have an almost lifelong electronics background so my plan is to add at least five switches and a trim pot for analogue speed control.

Essentially I'm just going to get creative with a bunch of paddle, momentary and latching switches to use with the manual start/stop/forward/reverse to control the carriage.

Once I get it all figured out I'll make a new thread and post easy to follow wiring diagrams and make a short vid of the system in action. In all honesty though, I can't say when this will happen exactly as I'm in the middle of fixing up a Mk1 Dore Westbury mill I recently acquired but it will happen...

 

[Canobi]

I finally got time to concentrate on the VFD saddle control, so the thread is now up:
http://www.hobby-machinist.com/threads/fun-with-a-vfd-lathe-stops-and-auto-traverse.63072/

I must say though, as much as the VFD will come into play for turning and threading, a half nut disengagement mechanism is definitely on my hit list of things to make, though there's not much real estate on my lathe for fixing things to, so I'm also looking into using a long throw solonoid actuator as it will allow for a much more compact mechanism.

 

[tertiaryjim]

I'm no gunsmith, so triggers aren't part of my repertoire. I got to thinking about this, came up with a lever on the engagement handle, operated by a push rod (with roller bearing on the end) to disengage the feed on my MK2. It took me all weekend to whittle it out. I'd planned on eventually adding a threaded ring adjustment, may yet do that. If you look close you can see the aluminum collar I applied to the engagement handle, held on by 5 5-40 set screws.
View attachment 226287

It ought to work for threading and feed. It seems to stop within a couple of thou.

Tom
The half nut lever closes or spreads the half nut so if it's opened slowly there will be a variance in the thread pitch.
Thats why a trigger is used to open it quickly at the end of a run.

 

[T Bredehoft]

Thats why a trigger is used

I threaded a dozen aluminum pieces,it stopped close enough to the same place on each piece that I couldn't visually see a difference. It may not work as well in steel.

 

[tertiaryjim]

Sorry Tom
I guess I didn't make myself clear.
While the engagement will release at the same point the graduall release will
allow the the threads of half nut and lead screw to retard .
They will engage on a different point because the thread is following the thread face
like slipping down a wedge. This will cause a varience in the lead of the thread.

 

[T Bredehoft]

This will cause a varience in the lead of the thread.

Gotcha, Jim. For my purpose it works well. The thread at the end of the pieces I'm working (this job) are not required to be correct, the piece gets a plastic cap screwed on. If I need it I can put a groove there for it to stop in.

 

[jarhead]

James, W----WOW, thanks for sharing!

Your lathe saddle and my grizzly G0750G are exactly the same. I am going to make one of these thanks to your efforts/info!

Have you made any updates over the years since first posting this?
Thanks again, Joe