opinions on feed idea

[shipto]

I have been thinking about thread cutting on my lathe for some time but the gears work out really expensive (and almost as rare as dragons teeth for my drummond)
so I thought tonight about a electronic feed system, basically you would have a bolt on sensor for the main shaft and a motor for the feed screw.
the sensor would send pulses to a control circuit which would then send pulses to the feed motor (the same as a usual cnc feed)
so in effect you could just program the feed to give you whatever feed ratio you needed, no more being restricted to the gear available.
does such a system exist already, if so links please and what are your thoughts on how accurate it would be?

[Weston Bye]

I did it. See this thread:
http://bbs.homeshopmachinist.net/showthread.php?t=42769

Absolutely dead-nuts accurate.

[shipto]

excellent work I never even concidered that it would be able to combine with milling as in #14.
May be contacting you for advice in the near future if you dont mind.

[macona]

http://www.autoartisans.com/ELS/

[Weston Bye]

... May be contacting you for advice in the near future if you dont mind.

I don't mind, drop me a PM when you're ready..

[djc]

Another really cheap option is TurboCNC. It will run on a DOS-based PC ($free); it costs $60 to register it. Unless you had the parts to hand, I don't think you could build Weston's system for this money.

[Weston Bye]

I don't think you could build Weston's system for this money.

Going back to the Original Post, I don't know that he would build "Weston's system" with the auxillary gearbox. His lathe probably has a geared head or at least back gears. The encoder would probably go somewhere upstream, rather than at the spindle.

So lets break it down:
Stepper motor, drive and power supply - the same for all systems.
Spindle zero sensor - same for all systems.

Other systems - breakout board or such.
My system - a handful of TTL chips, sockets, perfboard, terminals, thumbwheels. You have to put all these together - could be a big negative - but the money is about the same as a good quality breakout board.

My system - Encoder, about 60 bucks, or more, or less - it will depend on your installation.

Yeah, my system might be more costly, but not that much more.

Remember, my system is an electronic alternative to manual threading, nothing more - ever. If you want to add bells and whistles or migrate to CNC, take another path.

[shipto]

And of course with your system theres no need to have a pc attached.
Although I have been thinking things over and thought of upgrade for this, Again I am not sure if it already exists but here goes.
instead of one encoder and one stepper I am thinking two stepper motors with encoders built in (yes I know they are extremely expensive) one attached to the spindle and the other to the feed screw. this way you would have the option on smaller threads to just feed the tool in and have the spindle turn to suit the position.
Going as far as this however it may be easier to just go full cnc and have done with it.

[djc]

...Again I am not sure if it already exists but here goes.
instead of one encoder and one stepper I am thinking two stepper motors with encoders built in....

I am not sure it would work with steppers but it can and has been done with servos. See over in the main forum for a post on gear hobbing by Texas Tornado. Macona, I think, posted a youtube video of the concept.

[Weston Bye]

See this thread:
http://bbs.homeshopmachinist.net/showthread.php?t=32574
Look particularly at the second youtube video.
Steppers on everything except the tool post grinder.

[macona]

There is a video on youtube where they use EMC2 to couple a lathe spindle to the feedscrew. One advantage of EMC is its HAL allows close coupling of spindles to the screw where Mach can not. Mach just figures out the speed of the spindle and sets the feed to match. Usually it works. I have been having real bad luck with it. Art found he screwed up big time in the code but Brian has no interest in fixing it.

So, emc is one way to do it. The other way, like the hobber Lazlo and I helped Texas Turnado get going, is using the internal electronic gearing in modern servo drives. You can set just about any ratio of encoder pulses in to motion out.

Putting encoders on a stepper will do you little good unless you want to create system to close the loop.

[shipto]

See this thread:
http://bbs.homeshopmachinist.net/showthread.php?t=32574
Look particularly at the second youtube video.
Steppers on everything except the tool post grinder.
I really like that looks very expensive though :eek:

[KEJR]

There is a video on youtube where they use EMC2 to couple a lathe spindle to the feedscrew. One advantage of EMC is its HAL allows close coupling of spindles to the screw where Mach can not. Mach just figures out the speed of the spindle and sets the feed to match. Usually it works. I have been having real bad luck with it. Art found he screwed up big time in the code but Brian has no interest in fixing it.

So, emc is one way to do it. The other way, like the hobber Lazlo and I helped Texas Turnado get going, is using the internal electronic gearing in modern servo drives. You can set just about any ratio of encoder pulses in to motion out.

Putting encoders on a stepper will do you little good unless you want to create system to close the loop.

EMC uses a RTOS, so it can be as fast as the parallel port. I think this tops out at 30-50Khz or some such. That equates to maybe 10 pulses per rev at 3000 RPM, but who would thread that fast on a home brew system using a parallel port anyway? I am thinking that at reasonable feed speeds that you could get away with a low res encoder unless your spindle speed varied alot. I know in theory you want to track the actual position of your spindle, but realistically how much does your spindle speed change due to a light threading cut? I could be wrong but I can't imagine much at all on a decently sized machine, or at least not in a sudden way due to inertia.

Have you tried EMC yet macona? I'm looking to CNC my lathe at some point down the road and this could be a deciding factor. I've geeked with linux alot in the past so it doesn't scare me off. I was hoping to use Mach so I could use my windows CAD though. I guess going into the next room to do drawings and CAM isn't too bad.

Do you happen to know the details of the limitations with Mach with regards to spindle tracking?

KEJR

[macona]

Im putting off EMC. I got good info from a guy with a strong linux background it is still a pain in the rear. Im waiting for a new board to come out that hopefully will support doing rigid tap in Mach.

Mach, in mill mode, it just does not watch the spindle other than for feedback for speed. Even in lathe mode threading is problematic at best. I have been having issues big time. But apparently Brian at Mach thinks it good enough so nothing is going to happen on this front.

Linux just annoys me. I avoid it like the plague.

[skunkworks]

moonna - that is too bad.. I think you would get more bang for your buck going the emc route.

Here is a decent writeup with emc replacing an analam control.

http://code.google.com/p/sector67-sandbox/wiki/ProjectSheetCake

sam

[macona]

Cant see that. I know for sure getting my pendant to work will be a pain. it uses modbus to talk to the control. Although EMC is supposed to support mod bus now, I would have to write a plugin and I have no interest in that.

I have been in touch with Sergey at KSI Labs. He has a product called the Centipede. Looks real promising, a PCI card that handles all the IO for Mach. Since it is a PLD it can also be used as a motion controller and he plans on incorporating stuff like Rigid tapping. And since it is a PCI card there are no lag issues like with the USB controllers.

http://www.ksilabs.com/

[KEJR]

I've been talking to the smooth stepper guy and their product handles the spindle tracking in hardware now with 1 pulse per rev. He thinks that it may be possible to update the carriage position 8 or 16 times a rev with the current hardware, which would be independent of mach3. I'm just throwing that out there for guys who might go the smoothstepper route anyway. I wouldn't hold them to that, they didn't promise anything.

If you REALLY need tight control from your Axes I'd use high end servos properly matched to the hardware in conjunction with a professional motion controller from Delta Tau or Galil, etc. I don't think this is needed for most hobby work, but for professional (or tricky) applications it would be a wise choice. You can do the drive Master-slave thing as mentioned, but you have to configure the drive for each thread pitch and kick it back to regular command mode for "regular" cnc use. There might be a clever way to do this reading some general purpose switch inputs of a drive and an onboard program in the drive but at that point I think I'd look into one of the other solutions.

KEJR

Edit: Many smart drives support serial port input,s o that might be another interface option to kick between "1.5 axis thread" and "CNC" mode for doing a master/slave thing.

[skunkworks]

Here is emc threading with a spindle quadrature spindle encoder + index...

http://www.youtube.com/watch?v=TDUv0kKAfH0

This is with a caveat. The threading cycle in emc doesn't back up while synced. so - if the person above would turn the spindle backwards - the axis would not move. It would start moving again if you ran the spindle forward and got to the point where you had started turning it backwards. (continuing on with the thread) This is different than rigid tapping - which tracks the spindle in and out. (although I do not know what would happen if you spun the spindle manually each way...)

actual gearing can be done with emc by creating the connections within hal to do it... Like

http://www.youtube.com/watch?v=ZhICrb0Tbn4

you can see him grab the spindle and the rotory axis tracks both ways. (emc is very flexable that way)

sam