Rehoming Machine After Startup

[backyardengineer]

Hi all,

I have a simple question that perhaps I can get a quick answer to on my 1998 HAAS VF3.
I recently machined a big part about 12" x 14" x 5" and had to call it a night and leave the shop. I stopped the machining progress at a good point and turned off the machine.
The next day, I powered the equipment up and did the normal start-up checks even running spindle warm-up. After finishing the job, it was evident that something shifted and the last three operations were off by like 1/32"

The question is, is it possible that this could've happened, after tuning the machine on and homing a 1/32" error is possible?
If so what are some mitigations, keeping the equipment on overnight?

any help is appreciated.

thanks,
FG

 

[nihilistic]

Well sure keeping it on would maintain live accuracy. Checking your homing stops/encoders to see if they are loose. I would recheck the origin and then figure out what "like 1/32" actually means then I would know where to look. I would also want to see if it is repeatable in a consistent manner then you could compensate. Do you do an axis-warmup as well? That could have an effect over time if it is drifting as it heats/cools.

 

[SeymourDumore]

I've only had this happen on an '07 vintage machine where the X-axis encoder cable was damaged by chafing and chips.
No error at all, just every once in a while after a power up the machine was off by some amount, always in X only.
Then, one day the cable finally gave up the ghost completely.
After replacement, never any issue.

Again, I did not have any errors or ever a missed motion.
We just got into the habit of checking the X-zero after every power up.
Then one day the power-up solidly errored out with a cable fault during any motion of the X-axis.

 

[1032screw]

The machine should home to within tenths of previous position or something has gone wrong. I could see a work offset drifting by thousandths if it was set with the machine at drastically different temperature then when homed but being off by up to 1/32 indicates something has gone wrong.

On the Haas and probably most cnc's without scales the homing process moves the axis until a limit or home switch is hit, the control then starts counting encoder pulses and watching for the encoder index signal which happens once per servo revolution. The machine stops on the encoder index pulse so in theory homes to an accuracy equal to the servo resolution plus minus a bit for thermal changes. The machine is counting pulses between limit switch activation and the index mark to essentially detect errors when homing. There is a parameter for each axis that specifies how many pulses should be counted between the limit switch signal and the index mark. If the control counts more or less encoder pulses than the parameter it will give an error. There is some allowable deviation from the value to account for the limited accuracy of the limit switch but counts outside the allowable range indicate a sticky or malfunctioning limit switch or a servo to ball screw coupling that has slipped.

 

[coyoinu]

if its old enough to have microswitches instead of hall effect sensors for limit switches, id imagine theres a good deal of hysteresis possible between power ups.

recently i saw something similar on NGC controls, but it was only a few thou, and only once in like 20 tries.