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Dimensional Layout Errors


Jim Howe

Since reading some of the fine articles being submitted I decided to really bore the readers with one more article on measuring?
This article was written for my own benefit when I was suddenly assigned to perform reverse engineering layouts. You understand, I am sure, that its one thing to perform dimensional layout when you have a drawing in front of you and you know what the dimension should be. It is quite another when you have no drawing and no clue what it should be.
To reconsile the logic in my "brainbox" and minimise errors the article was written. I warn you it is quite boring! :caution:
Jim Howe



Jim Howe

I want to attempt to remove the article from theory and show a practical application. Please view attached photo and note that we would be measuring the distance between the two bores in both the x, y and z axis. (red arrow represents the z-axis) :nopity:


Tim Folkerts

Super Moderator

Thanks for submitting an article. OK, I'll agree with you that it isn't the hippest topic in Quality, but it was well-writen and useful for those looking to measure offsets of holes.

A couple more specific notes...

Stop reading now if you don't want to have the plot spoiled

1) If you just used the names "high pin" and "low pin" instead of "Pin 1" & "Pin 2", you could simplify the two rules on the top of page 3 to a single rule something like "The center to center distance is the surface to surface distance PLUS the radius of the lower pin MINUS the radius of the higher pin."

It makes one rule to remember instead of 2.

2) I think you are slightly overestimating the potential error due to being off-center. Consider the example you give of the measuring instrument being up to 10 degrees off-center from the top of the pin, with the larger pin higher. Look at four cases:
  1. Both measurements are 0 degrees off. This gives the true center-to-center distance (CC).
  2. The larger pin is 10 degrees off. This will decrease the measured CC by the 0.015 you calculated.
  3. The smaller pin is 10 degrees off. This will increase the measured CC by 0.030.
  4. Both are off. This will increase the CC by +0.030-0.015 = 0.015.
The worst you can be off is the error caused by the smaller pin. The addition of an error in the larger pin will actually decrease the overall error in CC! This would be true for whichever pin is higher.

Or think about two identical sized holes. If the measuring instrument is off-center the same amount for both, you will still get the right answer! You don't have to add the errors for each.

Tim F
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