Setting an Axis with a 5mm diameter locator

[djayne]

Our engineers are asking us to set up a part axis using a 5mm diameter machined locator. This part has an 80mm bore with a goose-neck sticking out of it. The locator is on the side of the goose-neck about 230mm away from the datum bore. I have tried to space the hits as far apart as possible on the 5mm plane to level an axis to but, I am getting a ton of variation. I keep telling the engineers that the plane is just too small and too far from the part origin to pick up an axis accurately. Does it seem reasonable to measure a part with this method? I believe that it is just too small of a plane over to great of a distance.


RH Honda Accord.JPG

[Tested]

I'd agree with you. Any variation in that surface or devation from perfect will mutiply alot over that distance.

With so many good things to setup on why do they want to go out there?

[djayne]

This locator is machined in the first operation along with the bore. The second operation locates on the bore and clocks to the machined locator. It sounds great in theory to do this but, it almost physically impossible. Now they are wanting to spend a bunch of money to buy a TP200 and module and I believe we'll get the same results.

[Tested]

What do they set on to machine the 5mm locator and first bore?

[dwade]

Ask them to pick a target point on this surface, then construct your line and align the CS from that based on the trigged rotation.

[djayne]

Ask them to pick a target point on this surface, then construct your line and align the CS from that based on the trigged rotation.

I did just this thing for them a couple of weeks ago and repeatability was great. When the 2nd shift programmer came in (who is on this project team), they made him change it back - no explaination given as to why. Anyway they're convinced that using that little plane over that big distance is the only way to go and there must be something wrong with the CMM or the fixture. It's like talking to a brick wall with these guys.

[rerun_lives]

Ask them which they believe more:
A. The likley hood that that part is that bad using CNC control or
B. the methodology of the measurement setup must be flawed. If the CMM can measure some other known standard correctly ie. a tooling ball, or the Brown and Sharpe Practice part. Then it is not the machine. Get it?

As far as your coworkers changing the setup back to where it was now "that is a problem"! Whenever you do not trust your numbers Verify, Verify, Verify with another method. Best wishes sorting that out!

[rerun_lives]

What does your print describe as the Primary, Secondary, and Tertiary, Datums? Is the Axis you are describing the Z Axis in your Graphic? Is this pad Offset to the Bore in the X Axis?

[Scott Brown]

Offset Alignment

Looks like an offset alignment to me. Level Y axis to the bore (cylinder).Switch working plane to Y-plus, construct an offset line from the bore to the net pad (use the offset value from the bore to the net pad in the X direction). Final alignment would look something like Level Y axis to bore, rotate Z axis to offset line, bore is the X and Z origin and Y origin may be the top of the bore or whatever the print asks for. This should work, give it a shot.....

Scott

[AJHVW93]

Not sure how well this would work? If they insist on going off the 5mm plane .How about butting a precision plate against surf and measure off of larger plate . Exageration comes into effect though just a thought

[djayne]

Not sure how well this would work? If they insist on going off the 5mm plane .How about butting a precision plate against surf and measure off of larger plate . Exageration comes into effect though just a thought

Never thought of that - that sounds like a great idea - I'll see if I can work some sort of floating hardened stop to the fixture. The engineers may just go for that. Thanks for the suggestion AJHVW93!

[cmmguy]

Our engineers are asking us to set up a part axis using a 5mm diameter machined locator. This part has an 80mm bore with a goose-neck sticking out of it. The locator is on the side of the goose-neck about 230mm away from the datum bore. I have tried to space the hits as far apart as possible on the 5mm plane to level an axis to but, I am getting a ton of variation. I keep telling the engineers that the plane is just too small and too far from the part origin to pick up an axis accurately. Does it seem reasonable to measure a part with this method? I believe that it is just too small of a plane over to great of a distance.

Front Steering Knuckle:
I have done parts like this before. That locater works in conjunction with another locater. It is a single point and you need to align this single point with another point - maybe another pad like it or the centerline of the 88 mm bore.

Usually a part like this is located in a fixture for checking.