Triac loosing Z datum

Roy · Sat 14 Jul , 2007 21:17 pm

A small problem sometimes occurs with my '86 Triac in that it looses its Z datum occasionally this happened once last Friday whilst milling some slots and twice yesterday whilst drilling and profiling.

What appears to happen is when it moves in the + direction away from the job it looses its zero, so next time it go's in the - direction it is deeper than intended. When milling some slots last week it came up from the job in rapid to what was supposed to be Z20, but when it plunged into the next slot it was about 3mm deeper and machined into the fixture plate

Any ideas why this loss or movement of the Z datum is occuring? could it be the drive board??

Thanks, Roy

Martin · Sun 15 Jul , 2007 14:04 pm

It may be worth adjusting the "gib strip" on the Z axis.
If it is slack in may allow the Z to stall

Mon 16 Jul , 2007 12:13 pm

Or how about lubrication ?

The Triac Z is fairly prone to stalling because of its weight and inertia.
If you are running the same programs with the same drive & control and only had problems recently, then its likely something mechanical that has changed.

Roy · Mon 16 Jul , 2007 12:52 pm

Will Have a look at the Gib tonight, although I think this should be ok as I checked everything when I got the Machine in March.

I don't think its a Lubrication problem as there is plenty of oil getting to the slide and ballscrew.

With regard to only having problems recently, I have only recently being using the machine reguarly (Since last month), So could it be now that this is showing up some electrical probs? Just a thought?

Will advise outcome of Gib, Thanks Roy

Steve · Mon 16 Jul , 2007 13:33 pm

I would doubt it is an electrical problem. It is probably stalling as it tries to retract. What control is on the machine?

It may be possible to reduce the Z rapid speed in the parameters.

The problem sounds mechanical!

Roy · Mon 16 Jul , 2007 15:48 pm

Hi Steve,

The Control is the old PNC 3. The Rapid speed is 1000 mm/min.

Is it possible to alter this parameter on this control, and if so how?

Cheers, Roy

Steve · Mon 16 Jul , 2007 16:13 pm

Hi,

Sorry I don't know how you change the speed or if its possible on the PNC 3. I am sure someone else will know though.

If it had been running on VR Milling then the parameters are easy to change.

I would still look to the mechanics though as it will have run at that speed when new so something must have changed.

Good Luck.

Matt · Mon 16 Jul , 2007 19:28 pm

Hi

Z sounds very fast. I think I use 50mm/min!

G1Z-2.000FZ50L

Matt

Roy · Mon 16 Jul , 2007 22:04 pm

Matt, 1000 mm/min is the rapid speed (G00), not feedrate!

I have investigated the Gib adjustment and this seems fine.
I wrote a little programme just moving the z up and down in rapid to try and observe whats happening. I notice when it starts to move in the Z+ direction there is a little 'knock' sound. This gets me thinking perhaps there is some backlash in the ballscrew, so I will try and look into this tommorow night.

Any tips on this one?

Cheers Roy

Triac whizz · Tue 17 Jul , 2007 11:53 am

Hmm, any backlash wouldn't acumulate. something loose maybe but still won't give 3mm of error - other than a pulley loose, also check the tightness/state of the belt & while you're there make a note of the size for future reference.

Roy · Tue 17 Jul , 2007 21:30 pm

Checked out the backlash tonight and all seems ok, belt is fine also.
Can't seem to find anything a miss, so guess i will just have to run the m/c as normal and just see if my 'playing about' with everything has made any difference!

Roy

Wed 18 Jul , 2007 9:54 am

I agree with Matt about the feedrate - I Know its supposed to be rapid, but its still 1000mm/min.
When I have played around with older triacs with older drives, 600mm/min was around the maximum reliable feed for the Z axis.

Maybe you can reduce the rapid feedrate for Z somehow ?

Roy · Wed 18 Jul , 2007 17:18 pm

I would like to try and reduce the rapid on the Z to see if this helps, but as I said before is there anyway of doing this? Because obviously I don't want to change all my Z rapids for G01 s!

Roy

Triac whizz · Wed 18 Jul , 2007 17:39 pm

Have you tried with a DTI? do a prog
G00 Z0
Z-100
Z0
Z-100...etc

then finish with Z-101 so that it kisses the DTI (which you've previously setto zero.
If it don't work at rapids try feedrates just to see if you can make it work.

I'm sure I've heard that those drives do that when they start going duff, but don't quote me on it

Roy · Wed 18 Jul , 2007 17:49 pm

Hi Whizz,

I have had a prog similar running for the last two nights, But do you think I can make it misread ?

Nope seems just fine at the moment, I expect it will start playing up as soon as I start machining again

Triac whizz · Wed 18 Jul , 2007 17:54 pm

perhaps it's your code

Thu 19 Jul , 2007 9:05 am

This may be a long shot, but it could be electrical noise from the spindle.
I've have seen it where step pulses are "added" to the drive step input purely by noise coming from the spindle drive / motor wires - especially if the earthing and shielding isn't right.

Try run your test with ths spindle running, and try put load on it somehow (hold a plank of wood against it - at your own risk of course ! - I didn't recommend it - don't blame me etc....)

Steve · Fri 20 Jul , 2007 8:18 am

If you take the program with the problem and run it first without the spindle running. (delete the M03's) and check if there is any loss of position.

If not add the M03's back in and try again. If still no problem then try while cutting.

If you have coolant it may be a noise problem with the coolant switching on and off so try with and without coolant.

You could put a clock on the spindle and then just start and stop the spindle. If there is a problem with noise it will occur on the accel and decel of the spindle which is the period when the current is highest.

You need to find the point where you have a stall and then work out what else is happening at that point.

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