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Author Topic: Hard Limits & Home positions  (Read 2443 times)

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Hard Limits & Home positions
« on: March 25, 2013, 10:19:17 PM »
I am building a machine with 2 NC limit switches on each axis.  The plan was to simply connect them all in series with the E-stop button to prevent a crash against the end stops.  To get jog away from a limit switch would require holding in the reset button until you've cleared the limit.  My question is, would it be possible to also use those same switches as home positions if I home each axis separately?  I'm thinking I'd just hold in the reset button during homing, so it wouldn't go into E-stop when the limit is hit. However I don't believe the switch resolution is going to be good enough to home the machine and then use that to locate a fixture, so this may all be a waste.

In the past I ran a pair of Fadal machining centers that had home positions, and you had to home the machine for the tool changer and such to work properly.  I also ran a Spindle Wizard CNC knee mill, but on that machine I never bothered homing it. I just picked up the work location with an edge finder and set tool lengths to the top of part and away we go. If there were lots of operations and lost position would really screw me up, I'd just write down on a post-it the dial readings for each axis when at 0,0,0 of the work location.

My tool holders are repeatable for length.  So my plan is to keep an edge finder permanently set in one holder, and use that to pick up all work locations. I am also considering bolting down a small block to the table that I can call home by picking up the top and two sides, then set all tools off of it.  That way on a long job where a power failure would lose my positions I could reset the machine by picking up the home block with the edge finder again. This machine does not have dials so I can't use the post it method!


Gary H. Lucas

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Re: Hard Limits & Home positions
« Reply #1 on: March 25, 2013, 10:55:14 PM »
Lots of ways to skin this cat Gary.  I like the idea of haveing the limit switches normally closed in the estop loop so that no matter what, all motion and spindles stop if they are broken.  But, you will want some good quality switches for repeatability in homeing so, why not keep the limit switches in the estop loop for your limits and add one homing switch to each axis for your homing?  You can use a single input for all home and limit switches to Mach.  In the homing routine it sends the axis home in order until the state of that pin changes, backs off and repeats for next axis.  When not in homing a change of state on that pin throws a limit alarm and stops Mach.  If the machine can't tear itself apart if it overtravels, useing a single pin for limits and homes is a neat cheap way to go.  If it has the ability to tear it's self apart, I would consider it mandatory to add the limits to the estop loop.

;D If you could see the things I have in my head, you would be laughing too. ;D

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Re: Hard Limits & Home positions
« Reply #2 on: March 26, 2013, 08:18:36 PM »
800 oz/in steppers, 80 volt drivers @ 68 volts, 20mm x 5mm pitch ball screws, moving about 100 lbs on THK ball slides with ball spacers.  Yeah, I think a crash could break something!  Hence the reason I will not try running it without end limits, and they are going to be set far from the ends until this thing is completely debugged.

Gary H. Lucas