Center method

[AliThePro]

Can somebody describe the Alternative "Center Method" for MMC or LMC in plain English? I am not sure how it works.

Thanks

 

[markn82]

I assume that you are referring to applying material bonus to your datum(s). As your datum departs from MMC, or LMC, as the case may be, you get a zone in which your datum centerline may move. If your departure from MMC were 0.060mm, you would get a zone that is 0.060 wide in which your datum can float, .030, to either side of the actual C/L of the datum feature. If the datum is for a group of features, then the new (shifted) datum will be used for all of the features in that group. Some people will refer to that as a pattern shift. To throw a little monkey wrench into it though, if there is a callout that states that all of the features can be checked individually, or if it is just one feature, you could simply apply the datum bonus to the feature just like adding material bonus from the feature itself.
Hope that doesn't cloud it up any worse for you. Without being able to use graphics, it's hard to explain.

 

[alexit]

Try:

http://www.tec-ease.com/tips/november-98.htm

and see if that clears it for you.

 

[markn82]

Very good example. Take note of the text towards the center of the page.

"This means the four-hole pattern may shift out of position in one direction while the datum feature shifts in the other direction as shown. A very common error made at inspection is to use this allowed datum shift as a bonus on the features being controlled. This would allow the features to be out of position to each other more as the datum feature departs from MMC.

I have seen a lot of cases where people working at various inspection houses mis-apply this by adding the datum bonus directly to the feature. Doing so can allow the features to move away from each other, while looking good on paper, it destroys the integrity of the pattern itself.