General Question: WCS Accuracy After Power Cycle and Homing

I'm curious what kind of errors you see in your WCS's after turning a VMC off for the night or weekend and have to rehome. I know in theory there shouldn't be any but it's not what I'm experiencing. I'm usually off .001-.002, which is fine for a first op WCS but not if I have to leave a part on the machine till the next day to run the second op.

Jay Fleming said:

I'm curious what kind of errors you see in your WCS's after turning a VMC off for the night or weekend and have to rehome. I know in theory there shouldn't be any but it's not what I'm experiencing. I'm usually off .001-.002, which is fine for a first op WCS but not if I have to leave a part on the machine till the next day to run the second op.

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as machine warms up it changes size. grid shift or zero return changing .001" or more is quite common as machine warms up
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quite common to program 5 minute delay with spindle turning to get spindle to warm up and mill <.0003" literally you can have a 10 foot path and go in a circle facemilling and by the time it gets to end of circle its .0005" lower cause spindle and tool holder and tool grew longer. or other use a 2nd float pass repeating the tool path. it cuts the high spot the 2nd time thats highest cause spindle was not fully warmed up at beginning of path
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then there is oiling the ways. dry machine slides change as oil gets between the slides. .0003" changes are common. another reason people use warm up cycle moving full travel on the slides its to oil the ways that have dried over night

Jay Fleming said:

I'm curious what kind of errors you see in your WCS's after turning a VMC off for the night or weekend and have to rehome. I know in theory there shouldn't be any but it's not what I'm experiencing. I'm usually off .001-.002, which is fine for a first op WCS but not if I have to leave a part on the machine till the next day to run the second op.

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You know I haven’t really paid attention. My parts usually come out good still so I’d say I’m within .002. I have a fadal so maybe that makes a difference because of the way it finds it’s home.

i use a ring gage bolted to center tee slot at side of table checked with .001 feeler gage
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then probing program will measure position from zero return. run probing 2nd time see a big .001 change(machine warming up), run 3rd time see maybe .0005 change 4th and 5 th probing repeats maybe to .0003

probe back jaw of kurt vise. clamp part then reprobe back jaw of kurt vise. normally clamping pressure springs back jaw over .001
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probe part sticking up in kurt vise only in jaws 0.100 depth and lightly clamped. reprobe with vise jaws tight. you will measure the part being squeezed or compressed

Jay Fleming said:

I'm curious what kind of errors you see in your WCS's after turning a VMC off for the night or weekend and have to rehome. I know in theory there shouldn't be any but it's not what I'm experiencing. I'm usually off .001-.002, which is fine for a first op WCS but not if I have to leave a part on the machine till the next day to run the second op.

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Are you talking about the machine warming up / cooling down? Or are your work offset numbers actually changing?

the distance from zero return to say corner of kurt vise jaw at center of machine is always changing
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just turn coolant on thats 5F different temperature and it will change in a few minutes
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holding .0002" tolerance i often have to decide turn coolant on or not and for how long. depends on if coolant acting warm or cold

Matt@RFR said:

Are you talking about the machine warming up / cooling down? Or are your work offset numbers actually changing?

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Basically, you setup a cold machine (hasn't been run that day) and probe your parts. Come back the next day after shutdown and homing and re-probe your part. How far off is it?

Jay Fleming said:

I'm curious what kind of errors you see in your WCS's after turning a VMC off for the night or weekend and have to rehome. I know in theory there shouldn't be any but it's not what I'm experiencing. I'm usually off .001-.002, which is fine for a first op WCS but not if I have to leave a part on the machine till the next day to run the second op.

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cast iron 40" with 10F temp change is .0024" change in length. better machines both the coolant and the lubricating oil is temperature controlled. better machines have heaters on the slides and ball screws to maintain same temperature through out the day
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even with all that temperature control things still can vary .001"