Best practices? Using workshift or wear offsets to compensate for thermal growth

[torinwalker]

Gentlemen,

I start up my machine cold at around 16 degrees Celsius and over the course of eight hours, the temperature of my shop (and the machine) rises to 23C. Needless to say, this results in thermal growth that I must compensate for. I do not have the luxury of running the machine continuously. I originally solved this by setting a variable equal to whatever offset I felt is needed and added that to certain critical cuts, but I'm running many more parts now and the drift is being noticed in other cuts.

So I've decided to write a macro that iterates over all 32 tool wear offsets to set an X wear offset to adjust for the thermal growth. I don't use wear offsets otherwise, so it's more convenient than adjusting the tool offsets.

Before I go about doing this though, I wonder if anyone else has done this, and if so, how did you do it?

Another possibility is to use the X workshift offset, but the Hardinge manual STRONGLY advises leaving X set to ZERO. Using one variable (X offset) seems like an ideal way of compensating for the overall X thermal growth, but I'm not sure if this would screw anything else up.

Suggestions?


Torin...

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[rcs60]

you are not specific on what machine or control that you have with that in mind i will assume that it is a lathe by the Hardinge name and just x offsets affected
if it is thermal growth you should have growth in the z axis also this might not be as important if running bar stock
you mention that it is over 8 hours but most of the growth should be in the first hour or so and then it would typically lessen the amount of thermal growth until it stabilized
i understand Hardinge not recommending changing the x work offset as this is also affecting the location of id stations or a gross error inputting information could cause a crash

two things you could do

1) run a warm up program that runs the spindle thru various rpms and moves the x and z this would eliminate the worst of the thermal growth in the morning usually they are about 30 minute long
this would warm up both spindle and ball screws



2) you could write variables to compensate for the thermal growth/ tool wear

I would set the geometry values so that the wear offsets would start the wear values at 0.0000
The caution would be to rest the wear values back to the original numbers in the morning if it is strictly thermal growth
this would make 10 pcs. then make the adjustment.
if in the morning it was every pc. the set #101 to 1 and it will do every pc.
by changing #101 this controls the time frame by the # of pcs. before the adjustment

#100=0
#101=10
IF[#100 EQ #101]GOTO1000(EVERY TEN PCS. ADJUST TOOL OFFSETS)
N10
PROGRAM
M30


(GUESSING AT THE TOOL OFFSETS for your machine)
N1000(TOOL OFFSET COMP)
#2001=#2001+.0001(WEAR COMP X tool 1 )
#2101=#2101+.0001(WEAR COMP Z tool 1)

ect

#100=#100+1(adds 1 to count)

GOTO10(RETURN TO PROGRAM )

[torinwalker]

Thank you for your reply.

The machine is a Hardinge T42 (not the "super-precision", and therefore no temp comp) with Fanuc 18T control.

The Hardinge T42SP accomplishes growth compensation with a glass rod installed in some strategic location on the machine and calls a macro O9027 (as often as the operator wishes) to measure and then ~somehow~ adjusts the control to compensate. [edit] I have the O9027 macro, but it isn't obvious to me how they take in temperature (or glass rod measurement), calculate, then adjust. Anyone know how the SP glass-rod temp compensation works?

For now, I wrote something very similar to the code you suggested:

I created a macro that takes a single argument, A, and call it with G65 P1000 Ax.xxxx from MDI. A represents the offset in X that I wish to apply to all tools. The macro simply iterates over all X values in the wear offsets and sets (not adds) this compensation value.


O1000 (X AXIS WEAR COMP)
#100 = 1
WHILE [ #100 LE 32 ] DO1
G10 P#100 X#1
#100=#100+1
END1
M99


Where;
#100 iterates from 1 to 32, and P#100 are the wear offsets 1 ~ 32, respectively.
X#1 is the parameter A being passed through the G65 call which sets each X wear offset.

The disadvantage to this technique is I can't use wear offsets, but then I don't use wear offsets anyway.

I measure parts as they come off the lathe and average the values, usually, over three or four parts. One can follow the drift - usually a tenth in diameter per 5-ish parts (or a tenth per hour, roughly.)

To take this one step further, I am looking into installing a thermocouple somewhere on the machine, possibly on the ball-screw nut, or one of the ends, or perhaps even on the base of the turret close to the ball screw nut. Ideally, the thermocouple would be fed into one of the the control's free ADC ports. read by the macro program, then turned into a value using a lookup table to adjust the wear offsets. Alternatively, I could make a subroutine that positions the turret against a tenths indicator (or a touch-off tool). Either the control's position, or the indicator itself will identify the error which can then be fed back into the aforementioned macro. Not sure if either of these are ideas are worth the trouble though. May just measure with a mic and keep adjusting manually.

Thoughts, please, if you have them.


Torin...