Troubleshooting A Drilling Inconsistency

Professionals diagnose a case of inaccurate drilling. April 10, 2005

We are having a problem with our Bussellato Point to Point Machine. When drilling top and bottom holes on our gables we are finding that within the series of holes, some can vary +- 0.5mm compared to the other holes. We have been trying to eliminate some variables but are having no luck. Can someone help give some insight as to where this variation could be coming from?

Forum Responses
(CNC Forum)
From contributor A:
There could be backlash in one of the axes. We just replaced the y axis lead screw on one of our Morbidelli's. An easy way to check it is to drill a series of holes with one tool. An example would be drill the first hole at x 50 y 10, then y 20, y 30, y 40, y 50, then x 60 y 50, y 40, y 30, y 20, y 10, then x 70 y10, y 20, y 30 etc. Check the distance between the holes where the axis has changed direction (in this case the y axis). If there is a difference between holes at the direction change, it's most likely a mechanical problem - lead screw or whatever. The numbers in the example are in millimeters.

From contributor B:
An easier way to check the backslash on your machine and accurate - use a dial indicator. Set it 0 jogging one way move 1mm same way then move back 1mm and see the dif. it should be 0 again if not you know now how much is the backslash. You can test on both directions just move 1, 0 the dial move 1 same direction, move back 1.

You have to do this for each axis - most controllers handle this problem. The error you find, you have to set in a parameter, then try again the test to make sure the compensation was correct. To know if your NC has this feature, call the manufacturer. Morbidelli with NUM controllers have this feature.

From contributor C:
Do you mean that the center to center distance of holes that were gang-drilled varies from 32 mm, or that the depth of the holes varies? Please clarify and I can help with possible solutions.

From the original questioner:
I mean the centre to centre distance. The drill series sits on the y-axis. The variance is being found in the x direction within the series from centre to the edge of the gable

From contributor C:
So you are getting variation in the distance of holes from the edge of the panel? Are you cutting that edge on the machine? If not, then the pins may be out of alignment, making the edge of the part not parallel with the drills in Y. Check for this problem.

From contributor D:
We had a similar problem on an old Morbidelli U-13 some years ago. The problem was loose set screws which held the encoder in place. The screws were hidden by a rubber dust protector which didnít allow us to see them. The encoder seemed to be tight when we pulled at it, but it slipped just slightly on startup, throwing the drilling out (in this case on the X-axis).

Two months of testing, trying, and replacing were solved with a phone call (in the middle of the night) to Morbidelli support in Italy. The tech zeroed in on the problem in two sentences and it was fixed the next day in a few minutes. I'm not familiar with Bussellato machines but I'm sure there are similarities.

From the original questioner:
After some testing we have narrowed down our cause of variation to the drill head. We have found horizontal play in certain drill bits sitting in the head.