Reply with QuoteI replaced mine with an optical homing system. I have repeatability to +/- .0001". I still have the OEM limit switches with them set about .02" past the optical but, with the soft limits in PathPilot, I never contact them any more.
R J
Anybody using non-OEM limit switches?
The X-axis limit switch on my Series II 1100 bit the dust in an interesting fashion (I'll post pics later). Before ordering another one from Tormach and remounting it I was just curious if anyone else has adapted another solution that has proven reliable and at least as accurate as the model that Tormach has sourced. It sure was nice just being able to tick off disable homing in Path Pilot and continue chugging away, but I do want to have everything functional again soon. Thanks!
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Re: Anybody using non-OEM limit switches?
I replaced mine with an optical homing system. I have repeatability to +/- .0001". I still have the OEM limit switches with them set about .02" past the optical but, with the soft limits in PathPilot, I never contact them any more.
R J
Re: Anybody using non-OEM limit switches?
Interesting concept. Did you post a thread on that here? If so I must have missed it in my search or read it and went brain-dead (the latter is happening more and more these days). I will agree that optical sensing is a much more precise way of detection than a mechanical contact method. I'd surely have to do some tweaking, shielding and venting design to keep them alive, but the notion is logical and makes perfect sense. Thanks for letting me know about your success!
Re: Anybody using non-OEM limit switches?
Hello Bob
I remembered that I questioned the temperature stability of your design and you posted the following results:Originally Posted by CountrySmith
I was sure I'd posted another response but apparently not! As I said in a preceding post I don't believe these opto devices are intended for such accuracy in the linear region. A repeatability of .0001" could be achieved with a more complex system but the benefits would be questionable.
@ Pickled: the original thread is in the PathPilot forum:
http://www.cnczone.com/forums/tormac...ml#post1813392
Step
Re: Anybody using non-OEM limit switches?
Hi Step
What can I say? I have consistently experienced sub thousandths repeatability over a period of several years. I have also experienced variation of as much as +/- .003" with the OEM limit switches.
The sensitivity of of my optointerupter is about 100mv/mil and the gain of the comparator is 200v/mv so there is more than ample sensitivity for precision trigger points. Of bigger concern is stability of the electronics. I have no hard data on the stability of the electronics other than my empirical observations. I also have to add that I am operating under fairly tight temperature conditions. My ambient temperature is 64 - 72ºF. I suspect that were I operating in a garage or outdoor shop where temperatures can vary by as much as 100ºF, the results might be quite different.
If I had it all to do over again, I would though.
R J
Re: Anybody using non-OEM limit switches?
Pickled, Step posted the link to the original post. To make the optical homing system perform well, shielding from stray light and contamination from coolant and/or swarf is indeed a consideration. I had designed the electronic enclosures to be hermetically sealed and provided sealed protective4 covering for the wiring runs.
The X axis sensor was not a problem as the drip tray provides a fair measure of protection. I did add a gasket to the interface with the table and machined plugs for the mounting screw access holes. There is a secondary shield over the sensor and all three sensor housings were designed to minimize stray light and contamination.
the Z axis sensor is located on the control housing just below the Z axis limit switch and is fairly immune to any contamination.
The Y axis was most problematic as on my setup, it is located on the left just forward of the rear Y axis bellows and there tends to be quite a bit of coolant and swarf at that location. I have a drainage system for the table which helps but heavy coolant use still sees a fair amount of coolant. I installed a shield above the sensor housing which helps to deflect contaminants. A better location would have been about 5" forward so that it was under the X axis motor housing.
I uses oil resistant PVC tubing for running the wiring through with custom brass fittings and NEMA rated enclosures for the electronics. The wiring runs through the bottom of the column and into the control cabinet to a distribution PC board. 12 volts for running the circuitry was pulled off the controller board. Connections were made using ferrules as Tormach did and can quickly be reverted to the OEM configuration should there be a component failure.
R J
Re: Anybody using non-OEM limit switches?
Wow thanks for the great information Gentlemen! I am going to throw an OEM on the X for now and re-pot the Y. I will be looking to replicate a similar design to CountrySmith's in the not to distant future though. My temps do fluctuate substantially in my building. This is especially true in the winter months here in MI. I'm thinking that I could incorporate homing switch data and a few other added sensor values into a series of discrete machine "health-checking" values. I'll noodle over it a bit more before I pull the trigger. Thanks again guys!
Re: Anybody using non-OEM limit switches?
Hi Bob
What can I say? They were your values
If you remember I predicted that the self heating of the emitter diode would be a significant contributor to the thermal drift - regardless of how constant your workshop temperature is maintained. In my opinion the sensitivity of the comparator doesn't help if the threshold wanders. I'd expect any variations due to the LM317 and LM311 over the temperature range of your workshop to be negligible.
By adding a slotted disk to the ballscrew (inside the coupler housing) with another optical switch and AND-ing the outputs the sensitivity to travel could be increased by about a factor of 50. Temperature issues should then be eliminated.
Step
Re: Anybody using non-OEM limit switches?
Hi Step,Hi Bob
What can I say? They were your values
If you remember I predicted that the self heating of the emitter diode would be a significant contributor to the thermal drift - regardless of how constant your workshop temperature is maintained. In my opinion the sensitivity of the comparator doesn't help if the threshold wanders. I'd expect any variations due to the LM317 and LM311 over the temperature range of your workshop to be negligible.
By adding a slotted disk to the ballscrew (inside the coupler housing) with another optical switch and AND-ing the outputs the sensitivity to travel could be increased by about a factor of 50. Temperature issues should then be eliminated.
Step
I understand what you are saying about thermal drift affecting the home position. The Emitter LED is drawing 34 mw. of power. I don't know what the thermal resistivity of the LED and case is but it is closely surrounded by what amounts to a very effective heat sink.
I redid the drift measurements today over a period of several hours. I homed the machine immediately on power up and zeroed DRO using my digital test indicator on a vise mounted 1-2-3 block. I then rehomed and remeasured at 2, 5, 10, 20, 30, 60, 90, 120, and 180 min. after power up. This test was repeated twice. The worst case drift was .0002" with most readings being .0001" or less.
IMO, there is no concern with any self heating effect. Certainly, the errors due to backlash, lost motion , and thermal growth greatly exceed the the homing system errors.
For critical work, I will rezero on the work itself . One of the early jobs a ran had no good way to zero on the workpiece so I have drilled and reamed 12 3/16" dowel pin holes in my table at precise locations. I have a .6000" dia. cylinder which I can drop into any of those holes for a reference position. I then zero one of the work offsets and record the offset from G54. The biggest problem with that methodology was that corrections had to be made manually unlike what occurs when homing. This was one of the motivations for building the optical homing system.
I would be concerned about adding additional variables into the measurement chain with your suggestion of the rotary encoder. It also would add considerable complexity to the system. One thought would be to monitor the LED forward voltage drop as a measure of the LED temperature and use it to correct for the drop in luminous flux with increasing temperature..
Thanks for your input.
R J
