Thermal Expansion

I am cutting a family of parts with +/-.001 Z depth tolerance for
multiple dimensions. About three to five hours into the run (20-30 min
cycle time) we experience a .0005" to .0008" shift in the depth. No
problems with cutter comp in X/Y tolerances. I have spoken with
several machinists running a variety of machines and this is
consistent across VMC brands. Our solution is to 100% inspect the
critical dimensions and adjust the work offset accordingly as
necessary. I am running Haas VMCs. Any comments or opinions?
Anybody want to buy a watch : ) ?
Reply to
JL
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JL:
If it's various pocket depths made with different tools, you can sometimes use the SAME tool to do all the finish passes, even if it's small.
You could document the "time to Z depth" change and manually change the Z offset a couple of tenths at the start of certain cycles.
You could make a macro that uses a counter to change the Z depth automatically after so many part cycles.
Or you might leave the machine running the program night and day - even if there is no part being machined to keep it thermally consistent.
Reply to
BottleBob
I don't know if Haas has them, but most machine builders have an option to use glass scales to measure the expansion and compensate for it. Most machines will repeat within .0002 in X and Y. But Z will lose >.001 due to thermal expansion.
Reply to
Bill
why not check the offset after every other part ? it shouldn't take more than a minute or two with a guage block
Reply to
raamman
Are other folks using a height touch tool to zero tools on their Haas? Or are people using feeler gauges off the top of the work piece?
Touching off a tool and using the "next tool" button on the Haas you can zero out the tools used in a typical program to a couple .0001" in a couple minutes off a height offset tool. This is a spring loaded $100 lightup gauge and is brain dead simple to use. You use the difference between the touch-off height and program zero in your z offset.
So, you could either zero out your carousel a few times during the day (only takes a few minutes) or you could periodically touch off the tool and subtract the measured distance from the H offset distance in the control and subtract that from your work offset.
Am I missing something? (probably)
Reply to
Polymer Man
This is short run production over seven similar part numbers.
We are currently inspecting offline with a Mitutoyo digmatic and getting some fairly good data from the SPC. I had not considered a macro, if we can predict the spike consistently it would be a good way to go.
We are using the same tools on multi-depths where ever possible. The blending and potential for ledges kicking a part out are too great.
We do not have anything in the way of Renishaws on the machine to verify online. Cost to profit just not there to set it up, yet.
Leaving the spindle and coolant may be an option. I am also considering offsetting our hours so that we can run 12-16 hours consistently. No chiller, I will look into the cost of an add on.
We use a led touch off gage. I occasionally still use a .100 gage block. Feeler gages are usually to close to miss a button and jam a tool into the part. I only used paper once, and then had the chipped tooth ground out. I train all the tools to a dedicated pin/toolholder. Touch off the pin to establish the work offset, then all the tool offsets are the difference between the pin height and the tool. Going from part to part the pin gets touched off, the new work offset is entered and the tools are theoretically set. Minor adjustments get made as usual. We are using the Haas G52 global offset to make the . 0005 or so adjustments so that original touch off is retained. Start of the day "cold" it is right there and we can get back to it easily. Anybody interested in a favorite touch off techniques post?
I don't have a newsreader at work. I am tinkering with my newsreader at home, said no carriers in their service for this group. My test posts bombed so far. May have to change services. Will figure it out and make the jump.
Thanks all for the input.
Reply to
JL
Just a thought, don't know how practical this is, but as the goal is consistency of temperature rather than any particular temperature, how about plumbing in a water heater element in the coolant line with a thermostat to keep the coolant at say 80 degrees or a few degrees hotter than the equilibrium temp.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
Reply to
F. George McDuffee
Since it's a Haas machine, you're lucky the shift isn't 10 times what you're seeing.
The reason this is consistent across most VMC's is that the bodies of the machines are "C" shaped. Most machines sag (head droops toward the operator) when they're cold, and then gradually stand up straight as they get warm. The front of the column tends to get warm from slideway action, and heat transfered from the spindle; but the back of the column doesn't. So the front expands and the back sits still, making the whole thing change shape just slightly. This means that depths DECREASE as the machine warms up. In really bad cases, Y moves too, for the same reason.
At the same time, machines with less than optimal bearing arrangments or cooling characteristics tend to grow their spindles downward as they warm up. This means depths INCREASE with heat; but Y isn't affected.
The combination of the two major kinds of movement can be tricky. If you're seeing what appears to be a sudden shift, as opposed to gradual drifting that eventually stops, then you probably have the column standing up (decreasing depths) and the spindle growing downward (increasing depths) at the same time. One of them is getting stable before the other, and the two effects can stop cancelling each other pretty suddenly.
One of the simplest ways to minimize the problem is to check the level of the machine - not just whether it's sitting horizontally; but whether all its feet are carrying the proper amound of weight. The heaviest, and most heavily stressed, part of a "C" frame VMC is at the bottom front of the column. If the screws under this area aren't down tight on the floor, and carrying their proper share of the weight and stress, the machine can become very flexible. Most modest size VMC's have 6 leveling screws. I like to start with the middle screw on each side completely off the floor. Then I get the table Earth level using just the four corners. Then, I gradually, simultaneously, turn the middle screws until they just lift the back of the table slightly. That ensures that the middle of the machine is well supported, and least likely to squirm around.
To see if this is what you're machine needs, don't use depths, tool setters, or other length-based indications. These will always include too many variables to be reliable. Let the machine sit overnight, and tram it when it's cold with an indicator swinging in the spindle. You'll probably find a difference between 12:00 (Y+) and 6:00 (Y-). Then run the machine till it's good and warm, and tram again. You'll probably see a major difference. That's how much the machine is standing up or bending from the heat. This information, the leveling procedure above, and some experience with repeated tramming an testing, can let you find the optimal adjustment of the leveling screws to make the machine most stable.
After that's done, the spindle growth can be the only issue. And that's usually a fairly brief, gradual growth that stabilizes, and can be dealt with using a warmup program.
Hope this helps. Please let us know.
KG
Reply to
Kirk Gordon
The nice thing about linear scales is you don't have to do anything but make parts. Haas does have them.
Reply to
Bill
JL wrote in news:7832b929-ded5-4022-a0bd- snipped-for-privacy@k36g2000pri.googlegroups.com:
You can use a Bluhm laser set on the table to check the tool at speed before finishing at final Z-depths or between parts.
Reply to
D Murphy
Another question would be how much wear your z axis might have in the travel range you are working in. After running many long hours in a couple of inch range, like in making patterns, the z axis "wear" could possibly be eccentric. I don't have a Haas, but with a Cincy vertical mill there is a z axis counter balance cylinder that could be screwing up. There could be a ton of reasons for seeing less that a thou shift.
Reply to
Michael
The probe may not register the expansion if it shifts with the head. I get that on my mill. Jerry
Reply to
Jerry
Too bad you don't have the Renishaw probe setup, you can integrate the macros right into the program and tool-offset all the tools in a few seconds at the beginning of every program.
My suggestion would be to run the machine full-time or at least start it running 4-6 hours before you need to run parts. Adjusting your shift schedule sounds like a good idea in this case.
Reply to
Woodworker88
Glass scales work great if your using machines made of glass to machine glass else you're keeping everything at 68F
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Otherwise the scales are just going to cause you more errors.
I thought everyone already knew this but I guess there's a Haas born every minute.
Reply to
Bipolar Bear
*argumentative knucklehead alert* *argumentative knucklehead alert* *argumentative knucklehead alert*
Reply to
Bill

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