Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: garyhlucas 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.
-
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
:)
-
dang ... forgot the pics.
I saved em' .... might as well post em'. ;D
-
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?
-
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 (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
-
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.
-
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
-
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
-
Hi RT,
I home at 20%, my machine has axis gearing and is quite slow, 1200mm/min. So homing happens at 240mm/min.
Craig
-
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
-
Hi Gordon,
thats a good idea.
You may have seen the photo I posted earlier in the thread. With these roller switches you don't have to put them
at the end of travel. You may also have noted that the aluminum ramp which activates the roller is on a slotted
mount which allows you to adjust it somewhat. The repeatability that they offer would provide a good way to confirm
your axis calibration.
The real question is why do you need to check the calibration? I set (steps per unit) mine four years ago and have not
touched it since. There are other maintainence/development items which would improve my machine rather than checking
calibration which has not changed in the life of the machine.
Craig
-
I use roller switches similar to Craig's - maybe even identical - for both homing and over-travel end stops. Have not had any issues with them yet.
I wrote a homing routine to bump into the roller switches, then back off until I saw the "Z" signal on the optical encoder on the servo motor shaft. Nice routine, I made the code real pretty, it was real accurate, but over the years I went back to just using the roller switch as it was close 'nuff, AND I could hear the switch "click" and I knew I was good to go.
Only time it is a problem is when i get junk on the roller. But that is a problem with any switch.
____
Possibly a little OT, I set my limit switch close to the end of travel, and make the LS's target keep the switch plunger pushed in all the way until the over-travel is hit. That way I know when i start to "home" which direction i need to move the axis.
-
Hi MadDog,
luv the name...
Possibly a little OT, I set my limit switch close to the end of travel, and make the LS's target keep the switch plunger pushed in all the way until the over-travel is hit. That way I know when i start to "home" which direction i need to move the axis.
That's a good idea. I have my home switches about 3mm inboard from the end of travel. Under almost any conceivable circumstances I only ever need to start homing
in one direction. Has worked for three years without me crashing it and I can crash just about anything!
Your right about the repeatability of the switches, I was going to go for index homing as well, and now that I'm using Mach4 its built in and trivially simple but I
realize I don't really need it. I get 0.02mm or better...that's enough. There are other ways I can spend time and money without trying to track down 0.01mm that I don't
need.
Craig