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Author Topic: Switches for Home position  (Read 1861 times)

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Switches for Home position
« on: January 04, 2018, 12:29:48 PM »
For those of you that want to home your machine very accurately so that you can resume a job after a shutdown for any reason I see that Automation Direct is now offering precision limit switches down to as low as 3 microns repeatability.  That should do it for most of us!  I have sloppy limit switches to prevent crashing into the ends of axis travels and can home to them but the repeatability is very poor. I might add a couple of these so I can home it for position.  These switches might be good for building an accurate tool touch off plate to.
Re: Switches for Home position
« Reply #1 on: January 05, 2018, 03:53:45 PM »
Hi Garyhlucas, I saw those as well.
They actually have 0.5 micron accuracy switches ... one specifically for a tool setter.
But, there is no hysteresis built into these.
They are listed as "No Movement Differential" which might make them somewhat problematic if used as a home switch the way basic  Mach3 implements its homing routine.
Dandy for a tool setter though.
I use a lot of their stuff. Have had very good luck with it ... so far.

Russ
 :)

Re: Switches for Home position
« Reply #2 on: January 05, 2018, 03:57:51 PM »
dang ... forgot the pics.
I saved em' .... might as well post em'.   ;D
Re: Switches for Home position
« Reply #3 on: January 08, 2018, 11:17:12 PM »
I was thinking of using some of their limit switches on my CNC router for accurate homing so after turning off I get zero perfectly.

I was looking at this model https://www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders/Limit_Switches/Precision_Limit_Switches/Precision_Touch_(3_micron_repeat_accuracy)/PT5M3WA

Would that work with Mach 4 for accuate homing? I know the model you posted you said wouldn't work, but would the one I linked?

What is hysteresis and why is it necessary to have?
Re: Switches for Home position
« Reply #4 on: January 13, 2018, 01:43:25 AM »
Hi,
I've use these and they work well.
http://nz.element14.com/omron-industrial-automation/z-15gq22b/microswitch-z-roller-plunger-spdt/dp/1181279

The prices are NZD. I got some equally good Honeywell units for $13USD. Pretty good value. With a ramp actuator I get 0.02mm repeatability.

With any snap action switch you get hysteresis. When pushing downwards to cause the switch to activate, call it z=0, but as you release it upwards it will go past
the point where the switch made, say z=.05mm. Hysteresis is not critical, all it really does is place a larger gap between where the switch makes and where
Mach backs up and the switch unmakes.

One spec which you should think about is 'overun'. It is the distance the button or plunger can travel after the switch activation without banging into an internal
stop and damaging itself. You may notice that the roller plunger types I have used (they are commonly and widely used by OEMs and CNCers) will not be
damaged if the axis overruns, the ramp will at worst depress the roller a couple of mm even with a runaway axis and the switch has a 5mm overrun tolerance.

I can stop a job on my machine, de-power and come back tomorrow, rehome the machine and continue from where I left off within 0.02mm. I now use Mach4 which
has index homing support for even closer homing but I've found I don't really need it, these switches are goo enough on their own.

Craig
My wife left with my best friend...
     and I miss him!
Re: Switches for Home position
« Reply #5 on: January 13, 2018, 10:28:06 AM »
Craig,
That is pretty good for the typical home shop machine application, about +-0.0007". I might give these a try on the travel ends where home occurs.  Extremely easy to mount on my machine.
Re: Switches for Home position
« Reply #6 on: January 13, 2018, 02:09:18 PM »
Hi Gary,
because of the ramp actuation you don't have to mount the switches right at the end of the axis and so avoid fouling with limit switches. Very convenient.
Also ramp actuation means no overrun problems.

Prior to fitting these switches I had no home switches. Would manually reference the machine at start-up but inevitably would not be perfect and as a consequence
soft limits were worse than useless. Suffered quite a few crashes and misaligned cuts on jobs that were in the machine over several days. After fitting these switches
I can home reliably, easily and quickly. Now soft limits are effective and at 0.02mm repeatability jobs lasting several sessions are no longer daunting that they used
to be. Good home switches are the simplest and yet the best means of reducing crashes.

Craig
My wife left with my best friend...
     and I miss him!
Re: Switches for Home position
« Reply #7 on: January 18, 2018, 04:08:11 PM »
Craig,

What speed do you home at to get your switch to have that accuracy.  I just ordered one so any info would be helpful.

RT
Re: Switches for Home position
« Reply #8 on: January 18, 2018, 04:41:21 PM »
Hi RT,
I home at 20%, my machine has axis gearing and is quite slow, 1200mm/min. So homing happens at 240mm/min.

Craig
My wife left with my best friend...
     and I miss him!
Re: Switches for Home position
« Reply #9 on: February 01, 2018, 06:21:33 PM »

Hi All,

I was thinking of adding a second endstop on my z axis and use the two to calibrate my steppers.  Anyone have experience with this technique?  I think the hysteresis of the end stops would come into play. 

Thanks

Gordon