Author Topic: Homing - 0.050" Move  (Read 1010 times)

beezerlm

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Homing - 0.050" Move
« on: September 19, 2016, 03:45:49 PM »
I am trying to set up homing on my machine and from what I can tell the PMDX-424 wants to travel 0.050" past the point where it sees the home switch.  My axis only has .060" of travel because it is a very fine grinding wheel outfeed. This causes the machine to bury the axis before homing is complete. Is there a way to change this setting?

Steve Stallings

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Re: Homing - 0.050" Move
« Reply #1 on: September 19, 2016, 03:55:59 PM »
Currently this is hard coded in the plug-in. We were expecting some people to want more,
but not less.  8-(

Is your home switch a precision device that will operate properly with less overtravel?

Steve Stallings
www.PMDX.com

beezerlm

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Re: Homing - 0.050" Move
« Reply #2 on: September 19, 2016, 04:41:20 PM »
Well the home switch on this machine is not the traditional setup. The switch is not on the actual axis because it is not practical on this machine.  There is a large shaft that turns the feed screw, the shaft has a protruding finger that hits the home switch when it is in the right spot.  It has a decent length of engagement on the switch, but it is probably only 0.001" - 0.002" (just a guess) of travel on the axis.  It has a very fine feed screw.  I am not sure the model of the switch,  but they look like high end switches.  They are from the original control unit made by GE.
« Last Edit: September 19, 2016, 04:44:06 PM by beezerlm »

Steve Stallings

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Re: Homing - 0.050" Move
« Reply #3 on: September 19, 2016, 09:07:01 PM »
Well, I know this sounds like a kludge, but could you scale the numbers in your
G code such that 0.001 == 0.0001 thus making the backoff 0.005" in the real
world?
Steve Stallings
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beezerlm

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Re: Homing - 0.050" Move
« Reply #4 on: September 19, 2016, 09:30:23 PM »
That would probably work, but I would hate for mistakes to be made on a final grinding operation over a number translation issue.
« Last Edit: September 19, 2016, 09:33:56 PM by beezerlm »

beezerlm

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Re: Homing - 0.050" Move
« Reply #5 on: September 20, 2016, 11:40:50 AM »
    I will just manually home the machine for now.  Will there be an option to change this value lower in a future release?  Attached is a pic of the existing switch.

Steve Stallings

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Re: Homing - 0.050" Move
« Reply #6 on: September 20, 2016, 12:01:12 PM »
The style of mechanical switch that you show likely has significant
hysteresis and may not work without overtravel. Have you proven
manually that it functions properly on both the initial trigger and on
the backoff with less than 0.050" travel?
Steve Stallings
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beezerlm

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Re: Homing - 0.050" Move
« Reply #7 on: September 20, 2016, 01:07:27 PM »
The problem is that the axis movement and shaft rotation are not 1:1.  The shaft is about 4" diameter.  It has to rotate 37.8947? to move the axis 0.0010".  If you calculate the linear distance the shaft moves per thousandth move in the axis, it comes out to about 1.3228".  So in other words,  .0010" on the axis translates to 1.3228" along the switch - or - for the shaft move along the switch .050" The axis would have to move .000037"

Attached is a drawing of a similar machine.  Item 120 is the home switch.
« Last Edit: September 20, 2016, 01:14:46 PM by beezerlm »

Steve Stallings

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Re: Homing - 0.050" Move
« Reply #8 on: September 20, 2016, 01:30:40 PM »
OK, got it. Will put this on the to-do list, but the list is already long so it may
take a month or more to get there.
Steve Stallings
www.PMDX.com

beezerlm

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Re: Homing - 0.050" Move
« Reply #9 on: September 20, 2016, 01:58:05 PM »
Thank you!

Bob at PMDX

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Re: Homing - 0.050" Move
« Reply #10 on: September 21, 2016, 08:47:09 AM »
Out of curiosiity, and for my use when I get to making the homing params user settable, what is the "counts per unit" (i.e. steps per inch) setting for that axis.  It has got to be something pretty huge, I would think.

Bob
Engineering Hell: Everything's right and nothing works.
Bob's Corollary: If everything's right and nothing works, double check your assumptions.

beezerlm

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Re: Homing - 0.050" Move
« Reply #11 on: October 21, 2016, 08:17:41 AM »
Sorry for the late reply, I must not have email notifications set up.

Yes it is over 1 million counts per inch.

beezerlm

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Re: Homing - 0.050" Move
« Reply #12 on: January 04, 2017, 05:08:29 PM »
Hi, I noticed there was an update in December.  Is the homing move adjustable in this new version?

Steve Stallings

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Re: Homing - 0.050" Move
« Reply #13 on: January 04, 2017, 05:20:21 PM »
Not there yet. That update was primarily about getting lathe threading working.

We have not forgotten the backoff enhancement, but the list of things to do is long
and we don't have it scheduled yet.

Steve Stallings
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