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Y axis homing problem

Posted: Sat 22 Nov , 2014 16:44 pm
by bishbosh
1990 TRIAC VMC running Fanuc OM-C control.

All was working fine until one afternoon getting her to home on the Y axis was temperamental but after a few attempts she did home after which she functioned normally.

That was last weekend. Now however she will not home on Y at all. X & Z home normally. The machine hasn't been moved or messed with so I have no idea what has caused this to happen.

When homing Y the axis sets off at rapids speed until it gets close to home and then slows as you would expect. However it continues straight through zero and out the other side until the axis drive cut off switch is triggered. I don't get an over travel alarm, the servo drive relay (I presume) cuts out and I get the "Not Ready" until I use the axis override button and jog back within limits.

I have taken the front bellows off and manually triggered the two switches and diagnostic 0017 reacts changing bit 6 from 1 to zero when the proximity switch is depressed. Similarly, diagnostic parameter 0000 changes bit 6 to zero when the limit switch is triggered.

Out of curiosity / desperation I flicked through the pages of diagnostic parameters and happened to notice that 51 58, 315, 317, 320, 322, 520 & 540 are all constantly changing value with some or all bits rapidly and apparently randomly changing. No idea if this is relevant or not.

I have tried turning the control on with P and CAN depressed but it has made no difference.

Any clues what to check / test / swear at next?



Re: Y axis homing problem

Posted: Sun 23 Nov , 2014 7:38 am
by Martin
The Fanuc does work strange when being homed. It may be worth checking the distance between the 2 switches using the control. It needs up to a full turn of the motor (10mm I think) between the home switch & the over travel switch.
It may also be worth checking the home pulse on the Fanuc axis motor.Try pressing the the home switch earlier then wait 20mm & release it. It should stop in less than a full turn.

Re: Y axis homing problem

Posted: Sun 23 Nov , 2014 16:12 pm
by bishbosh
Thanks for the reply Martin.

I am not sure I quite understand what you want me to do when you say "checking the distance between the 2 switches using the control." The switches are approximately 50mm apart I would estimate (not in front of the mill right now to check) but certainly significantly more than 10mm.

Also, if the machine was homing fine a week ago, what could have changed to make it fail to do so now?

I shall try the 20mm hold on the home switch and report back - I have tried pressing the home switch momentarily with the y axis a long way off which resulted in the axis slowing from rapids but not stopping until it hits the overtravel switch the other side of zero.

Am I right in saying that there is no dog on the Y axis and the switches use the casting for the drive nut as their contact surface?

Re: Y axis homing problem

Posted: Sun 23 Nov , 2014 19:56 pm
by Martin
Not sure about how the switches are actuated. We did several variations over the years. In order to check the distance you need to find to the diagnostic for the home switch. (It's on the first page I think) then jog on to the switch. It should stay in for at least 20mm. Then when it drops off the switch make sure there is at least 10mm more movement until it hits the overtravel.

Jogs towards the switch.
Hits the switch & slows down(Must be over switch. for at least 20mm)
Comes off the switch & looks for the next marker pulse from the servo motor. (Could be up to a full revolution of the motor)

Re: Y axis homing problem

Posted: Sun 30 Nov , 2014 19:15 pm
by bishbosh

Turns out to simply be a gummed up proximity switch - if I had looked at the diagnostic screen when the machine was homing I would have seen that the switch wasn't disengaging all the way to the over travel switch.

A quick blast of contact cleaner and some manual cycling of the switch and all is well!

Thanks for your help. :)