Motion control - Achieving 'N' spindle revolutions in forward and reverse with smooth ramping.

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
I'm assessing the practicality of building an auxiliary motor drive for my manual lathe for the purpose of easy thread cutting. Instead of the traditional method of engaging and disengaging the threading half-nuts with critical timing (one mistake or slightly off on any of the dozen passes and it's a crash), the idea is to leave the half-nuts engaged throughout the entire operation. Usually this is done by means of a hand crank to turn the spindle through the cut and then in reverse to reset for the next pass.

Example

In lieu of the hand crank, I am wondering how difficult/practical it would be to set up a gearhead motor with an encoder and some sort of motion controller to smoothly ramp up the spindle, count out 'N' revolutions and ramp back down to stop right at 'N' - and the same in reverse. Push a button in to feed forward, back the tool out of the cut with my other hand, release the button to feed backwards, reset and advance the tool for the next cut. I've got some old 24VDC wheelchair motors lying around with mounting provisions for encoders of some description. I've also got a 240VDC gearhead motor but I would have to use a separate encoder with that one. Power transmission will probably be via a timing belt drive. What sort of motion control hardware should I be looking at for this application? I'm relatively familiar with PLCs but have never dealt with motion control systems before. The motors are somewhere in the range of 24VDC, 10-20ish amps and 240VDC, 2-3ish amps off the top of my head. A pushbutton/touchscreen HMI from Automation Direct would be ideal for setting revolution counts, configuring spindle speed and ramping rate, manually jogging left and right, setting zero, etc. A torque (current) limit would be very handy for detecting crashes and preventing damage to my spindle bore, since power transmission will be via a tapered wedge expander inside the spindle.

If it ends up being more trouble than it's worth I'll just stick with the hand crank, but I figure I should at least see how difficult/expensive automation would be first. Save myself having to crank e.g. 576 revolutions for 12 passes over 1" of 24TPI threads.

Or it might be just as easy to use forward and reverse pushbuttons and V-belting to run the gear motor manually if a way to achieve smooth, predictable starting and stopping can be devised. (Starting choke and a modest brake resistor?) Using the lathe's own underneath motor drive for that is impractical because the constant starting and stopping would strain the paper start capacitor and it simply has too much inertia to stop in any predictable fashion. (Coast-down time at any speed is about 5-6 seconds.)

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MaxHeadRoom

Joined Jul 18, 2013
22,630
There is the free Mach3 which is a PC based CNC controller that does it.
In a Industrial type control, the X axis servo is geared off of the spindle encoder.
With the simpler Mach3 et-al it just uses a limit switch to start the X axis motion at the same point every time.
Not sure if you want to go to that extent?
More info on it can be found on the CNCzone & Mach3 forums.
Max..
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
I'm not looking to do a full CNC conversion on a 1946 machine... just adding a convenience feature without performing any actual modifications. Now that I think about it more, I would be plenty happy just being able to run the spindle back and forth under power in a controllable fashion with the half nuts engaged. A simple bi-directional speed controller with soft starting (and ideally dynamic braking) utilizing a three wire forward-reverse control arrangement would be perfect if I could find a kit or schematic.
 

LesJones

Joined Jan 8, 2017
3,073
When I replaced the single phase motor on my lathe with a three phase motor and VFD I added a similar option in the control circuit. As well as the normal start and stop buttons and a speed control pot I added a jog function. This is controlled by a three position switch which has an off position, a normal position and a jog position. In the jog position a separate speed control pot is selected. (This gives a lower maximum speed than the normal speed control pot.) For jog there is a center biased three position toggle switch to jog it forward and reverse. The VFD has settings to ramp the speed up and down so the starting and stopping is fairly gentle. One thing you have to remember is to withdraw the tool before reversing as the backlash on the leadscrew and gearing will be in the opposite direction to moving forward. Also reverse past the end of the thread far enough for all the backlash to be taken up when moving forward again before it actually starts cutting. I have attached the schematic of the control circuit. The relay is only very small as it just provides the stop / start latching in normal mode. The connection MVI on the right of the schematic is the speed control voltage from the pots to the VFD.

Les.
 

Attachments

shortbus

Joined Sep 30, 2009
8,429
Or you could just take the time to learn how to cut threads. It amazes me, as an old time machinist, who started doing this stuff in 1965, how many skills have been lost. And with every Youtube video and CNC machine even more skills are being lost.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
Or you could just take the time to learn how to cut threads. It amazes me, as an old time machinist, who started doing this stuff in 1965, how many skills have been lost. And with every Youtube video and CNC machine even more skills are being lost.
There are many ways to cut threads. Which ones are you referring to? Engaging and disengaging the half nuts with the spindle running forward? Jogging the spindle back and forth with the half nuts permenently engaged? Cutting away from the spindle with the machine running in reverse and the tool mounted upside-down or behind the workpiece? Starting a threading die onto the part then jogging the spindle with the handle resting atop the saddle? Sticking a tap in the tailstock and lightly feeding?

I don't see how you can draw the conclusion that I am refusing to learn how to cut threads by way of choosing to pursue a particular technique of cutting threads. The fact that this sort of technique is not seen in a production environment does not in any way invalidate it for a hobby application. The end goal here is therapy and relaxation, not squeezing every last ounce of productivity out of a 70 year old antique just so I can make a buck. Take your elitist nose-upturning elsewhere unless you have something of value to add. While you're at it, maybe take a hint from those fancy quotes you've pasted into your signature and try being a little less closed-minded.
 
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MaxHeadRoom

Joined Jul 18, 2013
22,630
Well the only way I know that simulates the electronic gearing method of industrial CNC, is to use a 1 pulse/rev encoder/sensor on the spindle and have a servo-like driven Z axis, that returns to the start position each time and when the servo 'sees' the pulse it starts the thread cycle, at the end of the thread, pulls out and returns to the start to await the next pulse.
Max.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
I might just go with a transformer-rectifier power supply to run the motor with a current-limiting choke in series on the AC side to soften the starting impulse. The drag from the brushes and the gearhead might even be enough to stop it in a timely fashion. If not, I'll add a brake resistor. Some rabbit V-belt pulleys, control stations w/ 800T pushbuttons, whatever beefy transformers I have lying around and a pile of bulletin 700 contactors hiding somewhere should just about cover the bill of materials. Add a sliding resistor to the forward circuit for speed control and voilà.
 
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strantor

Joined Oct 3, 2010
5,505
Now that I think about it more, I would be plenty happy just being able to run the spindle back and forth under power in a controllable fashion with the half nuts engaged. A simple bi-directional speed controller with soft starting (and ideally dynamic braking)
You're barking up the right tree with this one IMO. what you described at first seemed a bit too Rube Goldberg to me. However if you change your mind and decide that's the direction you want to go, check out ELS (Electronic Lead Screw) - it's a kit for doing something close to what you first asked about. Just Google it and you'll find all the videos and links.

But yeah, forward and reverse with dynamic braking is plenty good enough for threading without opening the half nut. Is your chuck a thread-on? If so, this can be dangerous. If your braking is too aggressive you can unthread the chuck with inertia while it's still spinning.

You didn't say what kind of motor you're running currently. Your explanation of dynamic braking didn't make much sense to me. Brushes? Current limiting choke on the AC side? I feel like I missed something. Is this not an AC motor?

I would recommend replacing the spindle motor with a 3ph model and using a VFD with a braking resistor. That's what I use on my lathe and it's such an improvement across the board, not only for threading. You'll wonder how (or why) you ever went without it.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
The motor in the underneath motor drive is the (essentially) original 1HP Century cap start induction motor. A 50's model I think. Big, heavy monster twice the size and mass of a modern replacement. Real soft starting which is wonderful on a lathe. Gives a guy a chance to figure out something's wrong *before* the thing reaches mach 3. The machine was originally ordered with a 3 phase Westinghouse. Why that's the case or where it disappeared to I'm not sure. As for the one I'd be supplying for this project, either a 24VDC or 240VDC brushed gearmotor of about 1/8~1/6 HP. Since they're brushed PM motors, connecting a resistor across them will act as a brake.

The spindle nose is threaded, but a very gentle bump when spinning it on sticks it in place quite hard. You have to *want* the chucks to come off on this particular machine. Pretty hard to crack them loose without a sharp jerk on the chuck key, even when they're threaded on gently.

I get the whole VFD thing but part of the charm for this machine is keeping it as original and authentic as I can aside from little convenience accessories like this which are not permanent fixtures and do not actually alter the machine. It is extremely quick and easy to change speeds with a flat belt drive and the four steps plus back-gears in this machine offer a good range to chose from in intiutively sized steps from 40-850RPM.
 
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strantor

Joined Oct 3, 2010
5,505
The motor in the underneath motor drive is the (essentially) original 1HP Century cap start induction motor. A 50's model I think. Big, heavy monster twice the size and mass of a modern replacement. Real soft starting which is wonderful on a lathe. Gives a guy a chance to figure out something's wrong *before* the thing reaches mach 3. The machine was originally ordered with a 3 phase Westinghouse. Why that's the case or where it disappeared to I'm not sure. As for the one I'd be supplying for this project, either a 24VDC or 240VDC brushed gearmotor of about 1/8~1/6 HP. Since they're brushed PM motors, connecting a resistor across them will act as a brake.

The spindle nose is threaded, but a very gentle bump when spinning it on sticks it in place quite hard. You have to *want* the chucks to come off on this particular machine. Pretty hard to crack them loose without a sharp jerk on the chuck key, even when they're threaded on gently.

I get the whole VFD thing but part of the charm for this machine is keeping it as original and authentic as I can aside from little convenience accessories like this which are not permanent fixtures and do not actually alter the machine. It is extremely quick and easy to change speeds with a flat belt drive and the four steps plus back-gears in this machine offer a good range to chose from in intiutively sized steps from 40-850RPM.
So you want to attach an additional (gear)motor to it? How does that fit in with the desire to keep it looking original and authentic as possible? It's going to look like an amputee with one too many prosthetic appendages.

Your spindle motor is already not original. If merely being old is good enough to fit its style, then you could get an old 3ph motor. One that looks like it belongs there. And hide the VFD somewhere out of sight. Use an old school F-N-R switch and an old timey speed pot. You could easily make it look legit.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
This is just going to be a V-belt pulley mounted on an expanding insert that fits inside the spindle bore. The gear motor will sit somewhere on the ground or cantilevered off of the rear vee-way that's hidden behind the headstock casting. Quick and simple to install & remove just like changing from a 4-jaw chuck to a collet drawbar and closer. Being a flat-belt driven machine it's just a matter of releasing the half-turn belt tension crank to completely disengage the underneath motor drive.
 

strantor

Joined Oct 3, 2010
5,505
This is just going to be a V-belt pulley mounted on an expanding insert that fits inside the spindle bore. The gear motor will sit somewhere on the ground or cantilevered off of the rear vee-way that's hidden behind the headstock casting. Quick and simple to install & remove just like changing from a 4-jaw chuck to a collet drawbar and closer. Being a flat-belt driven machine it's just a matter of releasing the half-turn belt tension crank to completely disengage the underneath motor drive.
Ok, sounds cool. Please post pictures once you get it working!
 

shortbus

Joined Sep 30, 2009
8,429
I don't see how you can draw the conclusion that I am refusing to learn how to cut threads by way of choosing to pursue a particular technique of cutting threads. The fact that this sort of technique is not seen in a production environment does not in any way invalidate it for a hobby application. The end goal here is therapy and relaxation, not squeezing every last ounce of productivity out of a 70 year old antique just so I can make a buck. Take your elitist nose-upturning elsewhere unless you have something of value to add. While you're at it, maybe take a hint from those fancy quotes you've pasted into your signature and try being a little less closed-minded.
And explaining why you can't do it is a stain on you. I learned on and most place I worked at had similar equipment. Old and worn out many of them Southbends from before WW2. Thread cutting is an art, and it takes time to learn. Grinding the tool correctly lets you get up the a shoulder on the work. Under cutting the area at the end of the thread, to the minor thread diameter and 1 1/2 times the pitch, at the shoulder lets you just concentrate on unlatching the half nut. Back gear is what allows you to cut threads in tricky situations. Choosing the same number every time on the thread dial eliminates any wear problems in the half nut, forget the old odd/even number thing the books tell you. I machine at home, and one of the lathes is an old Logan from the 20s or 30s, an old overhead flat belt model that was originally run by a overhead shaft that powered a whole shop. So don't talk to me like I'm an elitist, I'm just a competent machinist that learned the right way from guys that were my age now, in their 70's.
 

shortbus

Joined Sep 30, 2009
8,429
Here is a link to a servo system that I think comes close to doing what you require.
http://www.uhu-servo.de/servo_en/index.htm

Les.
But doing that or anything similar doesn't make threading any easier. Hardinge Tool Room lathes do it for threading, you never release the half nut, just hit reverse(three phase motor so instant reverse) BUT they also have a lever on the lathe compound to retract the tool ~.100" before doing it.

I understand the TS may just want to do it his way, but why limit yourself to not learning how to do it? Then if and when there is an uprade of machines in his life or the chance to run another lathe some where he knows how to do threading, anywhere? The time spent making and developing this "attachment" would be better spent in learning to thread.
 

LesJones

Joined Jan 8, 2017
3,073
I think the TS's requirement is a reasonable one. The thing I can't understand is the desire to keep the lathe in it's original condition. I thought it was only some Myford owners who had to paint them exactly the right colour, would not drill any holes in the to mount things such as scales for digital readout and must use the original DEWHURST switch for reversing.

Les.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
187
why limit yourself to not learning how to do it? Then if and when there is an uprade of machines in his life or the chance to run another lathe some where he knows how to do threading, anywhere? The time spent making and developing this "attachment" would be better spent in learning to thread.
There you go making that assumption again. I *know* how to cut threads the traditional way. It's not hard. Watch the threading dial, engage the half-nuts at exactly the right time, watch the tool until it reaches the undercut, jam the half-nuts back out, back the tool off, rewind the carriage to start, reset the cross-slide to zero, advance the compound a few thousandths depending, all in time for the thread dial to come around to the same mark again. Wash, rinse, repeat a dozen or so times depending on the material, sharpness of the tool, rigidity of the work, size and profile of the threads, how deep the tool is, etc. and hope you don't miss a step along the way. Cosine of 30 is 0.866.

As I touched on before, I'm exploring this technique as a matter of convenience, not neccesity. If it's easier and less stressful for me to push a button, back the cross-slide out, push another button, then reset for the next pass without any consideration for timing - why wouldn't I look into it? I have no boss telling me to get it done, no time constraints to meet, no parts quota that needs to get shipped out, no reason at all to do it the stressful way if I don't feel like it. I'm a creature of convenience when I'm having fun on my own time. If it gives me a fun project to work on which yields a tangible benefit at the end of the day and I can pull it off with little or no cost out of pocket - then why not? And if it gives me exposure to new processes along the way like cutting a new #9 B&S arbor for my horizontal milling machine so I can learn how to run a slitting saw then that's even more reason for me to go for it. And if it just so happens that I end up going over to a friend or family member's place and need to run one of their machines - then so be it... because I *can* and *have* cut threads the old fashioned way on their machines just fine.

As for the originality of the machine, I don't know. Something about drilling and tapping holes to mount digital readout scales, etc. on a beautiful old 1946 machine with remarkably little wear for it's age seems like a sin. To anyone else maybe it's just a machine tool to be used and abused, but I see a piece of American iron with a unique and fascinating history behind it.
 
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