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 : ) ?
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.
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.
Glass scales work great if your using machines made of glass to machine
glass else you're keeping everything at 68F
http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion#Thermal_expans
ion_coefficients_for_some_common_materials
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.
Then I take it back. I was exceptionally bitchy this morning when I
wrote that, sorry Sam.
I have seen glass scales minimize Z axis repeatability from .002 to
.0002. At least you can expect the same accuracy in Z as you can with X
and Y. In most application that is significant enough to justify the
price of the option.
Yes they do very well at mitigating error due to worn thrust bearings and
ballscrews.
And no apology needed--I'm an exceptionally bitchy person as a general rule
and so I'd suggest the best thing is to try and not take it personally.
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)
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.
<snip>
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).
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
<snip>
You'd have to see his shop and the shithole town it is in to fully
understand why he is so bitter.
I walked through and asked for a job (that I would never take), but he is
too arrogant to know who is around him.
Ignorant means you havent been taught; stupid means you are proud of your
ignorance.
This guy is stupid.
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