07-11-2010, 04:47 PM
Here's where we have a situation commonly known as "tolerance build up" - when each dimension in a succession of dimensions is within the plus-or-minus tolerance, but maybe in this case all are on the "plus" end of the tolerance scale. Now we introduce a part that is on the "minus" end of the tolerance scale, that fits into a hole at the far end of that succession of dimensions, all which we know are within tolerance. But even though all parts are within tolerance ... horrors ... the new part doesn't fit! 
The solution to that particular pitfall in part design is a practice know as "datum dimensioning," or "dimensional tolerancing," where all subsequent dimensions after the initial one are measured from a "zero datum" or initial measuring point. This system eliminates the potential for "tolerance build-up." Of course, the same scenario could also present itself in the opposite tolerancing direction, in which case it would be "Tolerance Shrinkage," but the result would be the same ... a part that doesn't fit and a production tool that has instantly become a piece of very expensive tool steel junk!
I hope everyone was paying attention and that those with weak memories were taking notes, as there will be a quiz at the next class meeting!
Edit: The class instructor cleaned up syntax, grammar and spelling errors. Grading of the referenced test will be lenient due to the aforementioned errors in lesson delivery.
The solution to that particular pitfall in part design is a practice know as "datum dimensioning," or "dimensional tolerancing," where all subsequent dimensions after the initial one are measured from a "zero datum" or initial measuring point. This system eliminates the potential for "tolerance build-up." Of course, the same scenario could also present itself in the opposite tolerancing direction, in which case it would be "Tolerance Shrinkage," but the result would be the same ... a part that doesn't fit and a production tool that has instantly become a piece of very expensive tool steel junk!
I hope everyone was paying attention and that those with weak memories were taking notes, as there will be a quiz at the next class meeting!
Edit: The class instructor cleaned up syntax, grammar and spelling errors. Grading of the referenced test will be lenient due to the aforementioned errors in lesson delivery.
biL
Lehigh Susquehanna & WesternÂ
"America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves." ~~Abraham Lincoln
Lehigh Susquehanna & WesternÂ
"America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves." ~~Abraham Lincoln