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Author Topic: Homing operation does not pulls back enough to get away from switch?  (Read 1329 times)

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #10 on: April 04, 2018, 06:47:22 PM »
Hi,
this is not really about combining limits and softlimits its about combining 'home switches' with 'limit switches'

I have always argued they should be separate. If you decide you need limit switches by all means put them there
but separately, ie they should not try to be home switches as well.

Craig
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Offline Tweakie.CNC

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #11 on: April 05, 2018, 10:37:48 AM »
Quote
Has anyone else experienced "sudden unexplained" shifts ? 

Hi Philippe,

Are you using a genuine Mach3 license that is registered in your name ?

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

Offline Filou

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #12 on: April 05, 2018, 11:41:10 AM »
Good evening Tweakie, yes, my license is registered in my name, no cheating. I suppose you are asking about the "strange shift".

I'm trying to reproduce this unique (but scary) event, but have failed thus far.  The job ROI was well confined within range defined by the soft limits. I was not next to the rig when it happened, but I heard the "crash" in one of the supports. I ran to the machine and saw one support holding the work piece pulled off, and the machine nicely going further  with a lateral shift of about 200 mm. On the next direction change (surfacing job) it passed soft limits and was stopped by HW limits that I have wired SEPARATELY from the homing devices, as Colleague "joeaverage" advertises vehemently.

Enjoy your day/evening, Philippe   

Offline Filou

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #13 on: April 06, 2018, 08:38:55 AM »

this is today the second event of Mach3 driving BEYOND one of the soft limits. Crash, minor damage. Thank you hardware limits, mounted and wired separately from the homing devices, etc. Driving axis off of limit, cleaning up the mess, and jogging to the home of this particular axis shows a shift of dY = 180 mm, in MACHINE COORDINATES and a minor
dX of 23 mm.

Now, if for some reason, the machine coordinate value of one axis shifts by a certain amount, the soft limit will trigger before or after, depending on the sign of the shift.

Which mechanism can produce such a shift ?
Am I the only fellow with this issue ?

Two events of that sort in one week after more than a year of smooth operation is a good statistics ... waiting for a shift in Z !!

Comments are welcome.

Philippe

Offline ger21

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #14 on: April 06, 2018, 09:16:14 AM »

Unless you are running geared reduction or high reduction belt drives, i.e. an unusual setup, the steppers do not have the power to do too much damage if they run away or you jog into the end stop.

There's a bit more to it then that. You have to consider inertia. Think of a battering ram. It's not the speed it's moving at, but the weight that's moving. The moving mass has far more power than the stepper motors.

If you jog an axis up against the stops, and then jog into the stops, the force is only what the motors can deliver. Now run into it at high speed, and the force is much higher.

Sort of like placing a brick on a glass table, vs dropping it on the glass table from 10 ft up. Big difference.
Gerry

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Offline Filou

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Re: Homing operation does not pulls back enough to get away from switch?
« Reply #15 on: April 06, 2018, 10:58:48 AM »
Gerry, you are absolutely right, its called kinetic energy, the exact amount of which must be dissipated by the "stop" or some sort of "brake".