Automatic Operation of Half-nuts for Threading [prototype works] - Page 1
Forum section: electronics / general

author: jpanhalt
author_rank: Super Contributor
timestamp: October 24, 2024, 02:30:19 pm
content: 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/#msg5171796The 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.

author: Ian.M
author_rank: Super Contributor
timestamp: October 24, 2024, 03:32:21 pm
content: 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 24, 2024, 04:02:05 pm
content: 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 24, 2024, 04:50:15 pm
content: 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.

author: reboots
author_rank: Regular Contributor
timestamp: October 24, 2024, 04:53:09 pm
content: 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 24, 2024, 06:04:38 pm
content: @Andy WatsonA dog-clutch on the leadscrew with the spindle turning would make it a nightmare to line up for the next cut. My spindle actually has a dog-clutch to engage the flat-belt drive to the spindle. That's a possibility to look at and would be better that half-nuts, if it worked.The machine shop at my first job had several Hardinge HLVH's. They are a dream to thread with. In 2001, I almost got a good one in Cleveland that had an early S/N. The price was only $3k to $4k. That was so low, I wanted to check the S/N and went home to check. By the time I called back, it was gone.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 24, 2024, 06:15:01 pm
content: @reboots,Yes, it is a Smart & Brown A Model of about that capacity that was sold originally to the RAF in 1965. I have the accessory threading gears too, but only 2 collets. It runs quietly and is solid, but everything is Whitworth. It has a few quirks compared to American lathes, but you get used to it.The 3-phase VFD is hidden under the panel (done last year) on the left and the most left knob on the switch is a pot to control spindle speed.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 26, 2024, 01:24:50 pm
content: Why not go a bit further in automation?If you control the spindle with a (stepper or servo) motor, then you can also start and stop it accurately, so you engage the half nut when the main shaft is turning, but the spindle is standing still, then press a start button, and a threading run is engaged, with the threading spindle stopping at the end. , then you turn back the depth of cut to get clearance, and either let the motor turn the saddle back without disengaging the half nut (it can spin really fast here), and then you set a new depth of cut and press the start button again.Another advantage of this method is that you do not have to cut a relief upfront. Because the tool stops at the exact same positon each time, it can cut it's own reliefI think "Not an Engineer" is working in implementing this, but I have not seen his video yet.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 26, 2024, 03:58:23 pm
content: I am definitely not an engineer and do not plan to redesign the lathe. I have read that engineers figure out how to solve problems with what they are given. I have attached two images (Backgear and Headstock) to further illustrate the problem. Both are from a S&B site.The backgear and threading leadscrew are geared to the spindle. The dog clutch connects the spindle to the belt drive. Unfortunately, the image for that was taken from the S&B site of a rather beat-up version of my lathe and seems to be missing that clutch. On my lathe, it is a typical dog that engages the left-most side of the flat drive pulley. "Out" is regular spindle; "In" engages the backgear and reduces spindle and leadscrew speeds considerably. It is a little stiff, but if one turns it to the left (or halfway to the right) the belt drive freewheels and the spindle/leadscrew stop. If I take that approach to stopping the lathe, I will need to figure a way to allow re-engagement easily. It's definitely not like a 9" Southbend, if you are familiar with that backgear, and will only re-engage with the flat belt pulley in one position.I mentioned other quirks earlier. To the left of the backgear control is the control for the feed direction whether turning or threading. It's labeled "Left" and "Right." Now here's the question, which way does the saddle move in the "Left" position? Don't cheat. Left and right refer to the threads one would make, so even when not threading and just using power feeds, "Left" moves the saddle to the right. You either get used to that or switch to a Southbend.Back on topic, if I go the half-nut approach, a servo or stepper would probably work. I am concerned a solenoid would be more mechanically complex to install and might lack the power. Which would you recommend? If I go the dog clutch approach, same question. Although a better approach, it might be a lot harder to implement mechanically.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 03, 2025, 03:29:37 pm
content: UPDATEAfter many unplanned delays, I have been able to work on this project lately. Disengaging the dog clutch on the spindle was a option, but it would be a PITA to re-engage it frequently as is needed in threading. It would also be subject to coasting after disengagement. Opening the half-nuts was probably the better option with that lathe, but mechanically it would be quite involved. I then focused on simply interrupting the "On" signal that is NC in a "three-wire" circuit from the 3-phase VFD. I confirmed it was 12V DC and probably low current from the VFD. My concern was coasting as with the dog clutch, but visually and without any load from threading, the chuck seemed to stop faster by interrupting the circuit than when using the dog clutch.I added an AQV212 PhotoMOS (Panasonic) to the circuit and code set a stop point, calculate a difference, and stop when zero was passed. Attached pictures show my very first results from 03/30/25. Chuck speed was about 240 rpm and thread gearing was set to 8 tpi (0.125" / revolution).Of note in those pictures , the difference is not negative. The Z-axis result is actual and the difference between that and the stop point of 1.0000" is overrun. It's generally between 35 and 50 thousandths at such a coarse thread and speed. That is probably acceptable, and a large part of that is unavoidable because the Mitutoyo scale cannot be read more frequently than once ever 45 to 50 msec. 50 msec at 240 rpm and 8 tpi is 0.0250". The Diff shown in the picture is the difference before the stop condition was met.I updated the code so the difference when the stop condition is met is displayed. That was confirmed today to be negative (as 10's complement). In the one case I bothered to calculate the reading was 9.9834 or -0.0166". Of course, that overrun will vary depending on the uncertainty of the measurement time.As a side benefit, a calculation of rate of change in the Z-axis and tpi can give me rpm, but I don't plan to do that.

author: johansen
author_rank: Super Contributor
timestamp: April 03, 2025, 04:10:46 pm
content: a solenoid triggering a spring loaded mechanism to release the half nuts should be relatively easy to build. you then don't need a particularly large solenoid, a handful of rubber bands can provided the force to open the half nuts.if you're not going to automate the cross slide, you will end up with a continuous groove at the bottom of the threads.. which means you can cut the groove to full depth first, and then run the lathe in reverse and thread away from the chuck, with the cutter upside down.

author: jpanhalt
author_rank: Super Contributor
timestamp: April 03, 2025, 04:36:06 pm
content: Quote from: johansen on April 03, 2025, 04:10:46 pmIt's a threaded spindle typical of lathes at that time. Even Hardinge's had them. I don't have the cajones to do that while threading 1-3/4 x 8 tpi. Working the half-nuts is conceptually easy. Attaching it to that carriage would be the complicated part (the threading dial is incorporated in the control).As for the groove, there will be a small one, but it's internal threads, and visibility plus slowed reflexes is why I didn't want to do that.EDIT: The crossfeed will not be automated, but I am adding a coolant-proof Mitutoyo DRO to it. BTW, that's just a practice encoder. The one I will actually use will go full length of the bed.