machined part imbalance 2

[luckydog]

i work in an automotive production facility. there is a particular part that we machine completely, but are producing "flyers" with regard to a balance spec of .25 ounce/inch. the current rate is about 1.5%-2%. it is a round flange, splined, and has 8 tapped holes, and 2 location holes. we are currently dynamically checking the balance 100% due to an external audit non-comformance for this issue, and it's severely hampering production. there are no provisions for introducing an actual corrective balance to the part. furthermore, one would be inclined to think that since all subject parts pass all gaging and are within blueprint limits, that an out of balance condition would be eliminated by complete machining. such is not the case. Any suggestions? Repeatability of the balancer is a known concern.

 

[Ron]

You do not mention whether this part is cast, forged, or machined from a rolled billet. If cast or forged, variations in the density of the material can cause an imbalance. You might want to do a metallurgical cross section to see if there are internal discontinuities that might be causing this.

Secondly, check your measuring devices and balancer for variations.

Ron

 

[luckydog]

ron,
the part is a casting. nodular iron. bhn of 241-302.
i have pondered the density possibility, actually, it was one of the first avenues of pursuit i wanted to take. yes, it could be possible to narrow it down to a poor cavity at the foundry, or irregularities in the foundry's pour. the data collected so far (in my opinion) points in this direction. if it were a machining process problem, all or a vast majority of the parts would be out of balance. since the part is machined completely, i'm beginning to doubt that the machining is introducing an imbalance to only a small percentage of the total on any given day.

bob

 

[arunmrao]

As you have mentioned that the alloy is nodular iron there could be microcavities caused by perhaps high pouring temperature or a phenomenon called mold dilation(soft molds). Hence the observation.