HELP Old MAZAK Fanuc 6T Zero return problem

My old MAZAK M4 with Fanuc 6T control has an intermittent zero return problem. Only on the Z axis when I start up, the Z zero return gives an error message 090. "Distance from start to zero too small or speed too slow". Some weeks I don't have any problem, then for 3 or 4 days the machine won't normally zero return on Z. I jog backwards and forwards, trying to zero from both sides, and then it zero's with no apparent change in procedure . Sometimes it takes 10 minutes of trying, sometimes hours!! There is no interference with the limit switch or the stop dog. I have tried power off and on with P & CANCEL held down. I have moved the stop dog, tried to do a new "origin" point on Z, tried a g28 return in MDI. I have a really nice big job on the machine and have lost 1 1/2 days now (of the last 3 days) not being able to zero. Today for the first time the Z zero return refused while the machine was running, ie without a power down in between. I am at my wit's end.

My shut down procedure is simply, move off X & Z zero position, press in E Stop button, power off switch on control box, then power off main switch.

I hope there is a way to sort this out. My best source of information locally is talking about replacing bubble memory at R20K to R30K cost. I simply cannot afford to pay that now, and he doesn't sound confident that it will solve the problem.

Any help will be GREATLY appreciated.
PS Please remember that I am a novice machinist and even more novice on CNC.

Regards
Bach

When was the last time the brushes were out for checking? There is a limit of wear specified by Fanuc, depends on motor model.
Are the ways being lubed properly?
With the power off, how difficult is it to turn the motor shaft by hand?
Minder.

Thanks for your prompt response Minder. I have had the machine for a little over 3 months now, and I haven't even thought of looking at brushes. Lube to the rear slideway is a little slow, I have to manually increase the lube pump pressure to get oil to it. Could that be the problem, insufficient lube??

I can turn the chuck by hand, how hard depends on what gear range was last selected, but even in low range I can turn it without toooo much effort.

Any further explanation of your thoughts would be most welcome.

Best regards and thanks

Bach

If it is the Z axis that has the problem, this where you should be looking for the cause, the spindle does not really come into it at this point, if you just got the machine, the first thing to do is a comprehensive maintenance inspection, all axis brushes being one of them, brushes are about $35.00 a set and a motor is $k's.
The other thing that is probably original is the metering points on the end of the lube lines, this machine is most likely 80's origin and if metering has not been changed, most could be blocked.
You could try some manual lube for now.
The Z axis is the one to try to turn by hand for rough spots or overly tight to turn.
On another note, make sure you have all the parameters recorded.
Minder.

Thanks again Minder,
It has been in my mind to check the metering units. I will open up the covers, with ref to the maintenance manual, as soon as this job is finished. I have no idea where to try to turn the z axis by hand, I suppose when I open up it could be obvious.

Best regards

Bach

Hey Bach,

When push comes to shove, just leave the machine on overnight. This way, you don't have to worry about zero-returning Z after startup.

The Mazak won't use much electricity at idle during the night. Also, it will use more lube oil as it sets, but this is a good thing for an older machine: getting more lube oil out to the moving parts.

Also, you need to be pretty far away from the home positions before you attempt to zero the machine out. You may be too close to the Z-axis home when you are getting the alarm.

good luck,

greg

Do you have a manual on it? Somewhere in my foggy brain I remember something about Mazaks w/Fanuc having 2 (Two) Z Zero positions. One was a "working 0" and one was a "home zero." Boss had to call the rep out on it when it messed up. He went through lolng explanation that I don't remember all of now. Long a\go and far away. I'm now retired and try not to think too much about Mazaks 'cuz it aggravates my bleeding ulcer. If I remember more and/or come across my notes I'll post more here.

Thanks Cnc and toolman,

Whilst this job is on I have left the machine on, so thats Ok, but, the main hydraulic motor runs continuously and gets quite hot. Our ambient overnight temperatures are around 20 deg C, and in my small shop things get a bit warm. I don't think it's so bad though, have a friend who has a IR temp probe, will get him to come and do a reading for me.

I have heard that these old Fanuc screens can "burn" if left on, how do I shut down the screen only??

I am giving the machine up to 300mm to return, and even then, WHEN I have the problem, it doesn't help.

Toolman, I have read in the manual about a soft limit as opposed to the home zero, haven't quite worked out what to use it for though. Is that what you mean? I have a feeling that Minder has it right with the lube to the ways though. Will open it up as soon as I've finished the 3 shafts for the job I'm doing now. Will post pics now of the BIG form tools I completed at 3am Monday morning, on my thread Homeshop in SA on the members and shop forum

Thanks for your interest and help.

Best regards
Bach

Hi Bach,

The problem lies with the encoder, i have just had a similar problem on one of my machines, an electronics mate of mine repaired it in 10 minutes.

On the encoder you have three pertentuometers(not sure of the spelling) they are small rotary switches that are fitted on the boards inside the encoder cover, basicly it acts like a limit switch to change from normaly on to normaly off.

The one that needs the adjustment is the "z" one , there are two others "A" and "B" leave them only adjust the "Z" one about 1/3 - 1/2 a turn in an anticlockwise direction. If you are not to sure get an Electronics guy in to check them.

Hope this helps

Roadworriar,
Thanks for that info. I am still having the problem, but intermittently, which makes it worse to deal with. I will have an electronic tech around soon for something else and will discuss this course of action with him.

Thanks again and best regards

Bach