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Author Topic: Stop Mach from ignoring limits when homing  (Read 7935 times)

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Stop Mach from ignoring limits when homing
« on: March 23, 2009, 08:07:09 PM »
My lathe has separate home switches from limit switches on both axes so I don't need (or want) Mach to ignore the limit switches when doing its homing procedure.  I had a home switch fail today and the machine ran to the mechanical stop since Mach ignored the fact that the limit switch had actually been tripped.  Is there a way I can tell Mach not to ignore the limits when homing.  Or, alternatively, can I write a brain that does the homing for me in which the limits aren't ignored?

Thanks,
-Ryan
Re: Stop Mach from ignoring limits when homing
« Reply #1 on: March 23, 2009, 09:23:22 PM »
Mabe wire your limit switches to the E-Stop input and leave the Home as they are.
Then, hitting a limit would be the same as hitting the E-Stop and would not be ignored.
But getting off of the switch would be a prob.

RC

EDIT:  Then you could hardwire a momentary push button to the E-Stop to act as an OVERRIDE to then back off of the limit.
Just a thought,
RC
« Last Edit: March 23, 2009, 09:34:12 PM by Overloaded »
Re: Stop Mach from ignoring limits when homing
« Reply #2 on: March 23, 2009, 09:26:12 PM »
Also, all of the switches should be wired normally closed. Less likely then for a failed switch to cause a problem.
RC
Re: Stop Mach from ignoring limits when homing
« Reply #3 on: March 23, 2009, 10:07:23 PM »
My limit switches arent mechanical switches, it's a hardinge CHNC lathe so they used fancy hall effect switches.  Wiring the estop string through them would require some significant changes to the overall machine wiring. 

My other concern about Mach ignoring the limits is, what happens when you tell the machine to home and you're on the wrong side of the home switch...the axis will drive to the mechanical stop since Mach won't trip the limit.  On this lathe, it's easy to be on the wrong side of the home switch, trust me, I hit the limit switch many times when attempting to home when it had its original control on it.

Is there a way to overcome (through software) Mach's ignoring of the limits?

Offline Hood

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Re: Stop Mach from ignoring limits when homing
« Reply #4 on: March 24, 2009, 03:30:03 AM »
Write yourself a Brain that looks at the limit switches and if it sees them does a Stop.
Hood

BClemens

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Re: Stop Mach from ignoring limits when homing
« Reply #5 on: March 24, 2009, 05:24:04 AM »
Looks like if the limit switches on the machine are not wired into the operating system as intended, then the interface to the lathe is incomplete. You could add switches or go ahead and wire those hall switches so that they are doing as intended - prevent damage to the machine. Or do as Hood suggests; do up a Brain, which is probably the easiest way to go. The original operating system for the machine must have had the luxury of the use of those limit switches.....

Nice lathe!

Bill C.
Re: Stop Mach from ignoring limits when homing
« Reply #6 on: March 24, 2009, 06:56:08 AM »
Other than being sure to jog to the proper side of the home switch before homing, how can the machine know which direction to look for home ?
It will only seek home in the direction specified.
If it is set to Home +, I would think you must ensure that the axis is - of the switch before beginning the homing.
RC

Offline Hood

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Re: Stop Mach from ignoring limits when homing
« Reply #7 on: March 24, 2009, 07:26:45 AM »
RC
 Its actually quite easy to press the Ref All when its the wrong side of the switch, I know you should always make sure its the right side but from experience I know its a mistake that can be made ;) and that is where having the limits able to trigger an error is a good thing.
With my lathe I now have the drives themselves doing the homing so Mach knows nothing about it until the Drives tell Mach it has just been homed, the good thing about my drives is they can look at the state of the Home switch and if its active then they will back off it first before starting the Homing procedure. I altered the trigger on my switches so after they are active if I am past the home position so I dont have to be so careful nowadays :)

Hood

Offline Greolt

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Re: Stop Mach from ignoring limits when homing
« Reply #8 on: March 24, 2009, 07:37:22 AM »
If a home switch is not at the end of axis travel then it should be set up in such
a way that it is active all the way to one extent of travel.

If this is done then Mach will know which way to seek the home switch.

When not active Mach will seek home in one direction.  Towards the home switch.

If already active it will seek it in the other direction.  Also towards the home switch.

This is easier to set up with non contact type home switches.  EG optical or proximity switches.

Greg

Offline Hood

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Re: Stop Mach from ignoring limits when homing
« Reply #9 on: March 24, 2009, 08:11:49 AM »
Greg, thats the way I have the home switches now, didnt know that Mach would move the opposite direction if it was active, however that may not give you an accurate home unless it actually moves the wrong way until the switch is inactive then goes back onto it for the actual homing move, does it do that? My homing is now done internally in the drive and it just uses the switch as a signal for the drive to then look for the index pulse from the encoder so its not really important to me how Mach does it now but still usefull to know to for others.
Hood