DMF_TomB
Diamond
- Joined
- Dec 13, 2008
- Location
- Rochester, NY, USA
had a job yesterday i had to recut some rail surfaces .00035" off with a twist. the casting
near as i can tell was twisted when the opposite side was machinned similar to what cold rolled steel does when machinned.
.
part is about 40" long (200lbs) and supported on 3 points. since i usually use .001" and .0015"
shim stock or feeler gage material i had to think about how do i shim at support points
0f .00035, .0003, and .00025" ??
.
i have plastic onion skin of about .0004" but it is flimsy and hard to put into a small gap.
so i figure i would try 3 each .001" shims at each of the 3 supports and a screw jack at 4th
corner. i would vary hold down bolt tension and use screw jack to squeeze the .00025 to
.00035" difference. obviously the more shims used the more they can vary when tightened
due to small amounts of oil and dirt still on shim material even if only .0001" on each shim.
.
it worked in that i squeezed over a .0002" by varying bolt torque and using a screw jack.
i was just wondering how others shim parts with less than .0004" difference between spots ?
obviously if i had a larger amount of shims they could squeeze too much when they are
tightened and be unstable. as it was the shims i used, it did vary .0002" after 10 minutes
and i had to measure and readjust just before machinning part.
.
i have heard of the make a crayon mark and put part on and measure and the crayon mark
having thickness does act as a shim but i did not want to keep lifting a 200 lbs casting
up to keep making multiple crayon marks and measuring so many times. i use a .0001"
test indicator and magnetic base stuck to the spindle of a big cnc gantry mill for
measurements. as it is the cnc machine servos moved back and forth .0003" and take
some time to stop moving after a Z axis adjustment to get a stable reading.
.
i am just asking, does anybody have a better way of shimming heavy castings of less than
a .0004" difference other that use multiple shims and varying tightening tension ???
near as i can tell was twisted when the opposite side was machinned similar to what cold rolled steel does when machinned.
.
part is about 40" long (200lbs) and supported on 3 points. since i usually use .001" and .0015"
shim stock or feeler gage material i had to think about how do i shim at support points
0f .00035, .0003, and .00025" ??
.
i have plastic onion skin of about .0004" but it is flimsy and hard to put into a small gap.
so i figure i would try 3 each .001" shims at each of the 3 supports and a screw jack at 4th
corner. i would vary hold down bolt tension and use screw jack to squeeze the .00025 to
.00035" difference. obviously the more shims used the more they can vary when tightened
due to small amounts of oil and dirt still on shim material even if only .0001" on each shim.
.
it worked in that i squeezed over a .0002" by varying bolt torque and using a screw jack.
i was just wondering how others shim parts with less than .0004" difference between spots ?
obviously if i had a larger amount of shims they could squeeze too much when they are
tightened and be unstable. as it was the shims i used, it did vary .0002" after 10 minutes
and i had to measure and readjust just before machinning part.
.
i have heard of the make a crayon mark and put part on and measure and the crayon mark
having thickness does act as a shim but i did not want to keep lifting a 200 lbs casting
up to keep making multiple crayon marks and measuring so many times. i use a .0001"
test indicator and magnetic base stuck to the spindle of a big cnc gantry mill for
measurements. as it is the cnc machine servos moved back and forth .0003" and take
some time to stop moving after a Z axis adjustment to get a stable reading.
.
i am just asking, does anybody have a better way of shimming heavy castings of less than
a .0004" difference other that use multiple shims and varying tightening tension ???