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Offline Tweakie.CNC

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Re: limit switch accuracy
« Reply #10 on: September 24, 2011, 01:18:10 AM »
 ;D ;D ;D

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Hood

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Re: limit switch accuracy
« Reply #11 on: September 24, 2011, 02:43:34 AM »
Optical switches are cheap and very accurate so if you can keep them dirt and coolant free they work very well. Housings can be made with a rod sticking out either side. The rod has a flag in the middle that interrupts light and is held in position with a spring either side. When the rod gets pushed the flag will move away from the switch and let the light through and your switch will send the input to Mach.
I got my original ones from I think Industrial Hobbies but the dont do them now since the original guy sold the business, shame really as they were reasonably priced.
 I did tests when I first got mine, I had a glass scale and external DRO on the mill and I sent the axis to a position and zeroed the external DRO, I then jogged away and then sent it back to the initial position and the DRO read zero. I did this about 100 times and only once or twice did the DRO not read zero and even then it was only 0.005mm. This was on a clapped out old Bridgeport that was my first conversion and I would guess the few times it showed as not zero was due more to the slop in the axis rather than the switch.

Hood
Re: limit switch accuracy
« Reply #12 on: September 24, 2011, 07:27:49 AM »
I guess that is where I am concerned.  It seems like the switches I am using are pretty good.
When I repeatedly probe my grinding wheel on the mechanical switch I am using, I get the same value
each time as long as I do a homing move first to machine zero.  But when I turn the machine off
and start up again, the value tends to be different.  I am wondering where the difference in coming
from so trying to investigate other sources of backlash.

Thanks
Keith

Offline Hood

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Re: limit switch accuracy
« Reply #13 on: September 24, 2011, 12:00:54 PM »
When yo turn off the machine your axis will likely move a bit, for example with a stepper you may switch off on a microstep but then the motor would tend to move itself to the nearest full step so your axis will be different to where it was. You should always home your axis after starting Mach and after any E-Stop or drive fault etc.
Hood
Re: limit switch accuracy
« Reply #14 on: September 24, 2011, 12:19:24 PM »
Hood, that is what I have been doing but even after starting up the machine and rehoming the machine,
the next time I believe the probing of the wheel is different but I need to double check it to see if
it is repeatable.  I will check to see if a timing pulley might be slipping or ball screw nut not secure.

Thanks,
Keith
Re: limit switch accuracy
« Reply #15 on: September 25, 2011, 05:06:34 AM »
RS sell the My-Com range.
This one with 1um accuracy is about £63.00
http://uk.rs-online.com/web/p/precision-position-switches/0341418/

It says the hysterisis is 2um..but I recon we can live with that amount of slop  ;D

ATB
Derek
You can "chop it off" but can't "chop it on"
Re: limit switch accuracy
« Reply #16 on: September 25, 2011, 09:06:39 AM »
When yo turn off the machine your axis will likely move a bit, for example with a stepper you may switch off on a microstep but then the motor would tend to move itself to the nearest full step so your axis will be different to where it was. You should always home your axis after starting Mach and after any E-Stop or drive fault etc.
Hood

It's actually worse than that - Stepper drivers will typically start up in the same full step position each time, as they have no way of knowing what position the motor is in.  So, when re-powered, the motor could end up several full steps off its last position.

Regards,
Ray L.
Regards,
Ray L.

Offline Jeff_Birt

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Re: limit switch accuracy
« Reply #17 on: September 25, 2011, 12:23:00 PM »
Quote
This one with 1um accuracy is about £63.00

You have to be very careful when reading a data sheet, it does not say  '1um accuracy', it says 'switching point sensitivity' (which is what?).

I've attached the mechanical specs of the switch below. Its repeatability is shown as +- 0.001mm with hysteresis of <= 0.002mm. While these are still impressive sounding numbers the typical well adjusted machine might have backlash as low as of 0.01mm, add in less than perfectly clean activating surfaces, temperature changes in teh machine ,etc and the 'real world accuracy' of even a precise switch like this starts becoming less clear.

I'm not wanting to be a negitive nellie but rather want everyone to get the 'whole picture'. There are many factors involved and you can't just fixate on impressive sounding numbers from one sensor/switch and ignore the rest of the system.
Happy machining , Jeff Birt
 
Re: limit switch accuracy
« Reply #18 on: September 25, 2011, 12:52:25 PM »
Quote
This one with 1um accuracy is about £63.00
I'm not wanting to be a negitive nellie but rather want everyone to get the 'whole picture'. There are many factors involved and you can't just fixate on impressive sounding numbers from one sensor/switch and ignore the rest of the system.

No one is fixated on the numbers or ignoring the rest of the system- rather the price to performance which appears to be rather good.
I don't expect a My-Com switch to eliminate all the inaccuracies from a machine / machining process, or anyone elses for that matter - that would be silly :-)...but it looks to be a good starting point for anyone looking for a accurate home switch.
You can "chop it off" but can't "chop it on"
Re: limit switch accuracy
« Reply #19 on: September 25, 2011, 01:01:13 PM »
I appreciate the discussion and the sources for more accurate switches.

Thanks!
Keith