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Posted by: Allen1 ® 05/26/2005, 11:30:57 Author Profile Mail author Edit |
Some co-workers of mine are doing tolerancing of mechanical parts (mostly sheet metal fabrication) by asking a supplier what tolerance they can meet in general. They then apply this tolerance to every dimension on the part. My thought is that they should be determining the tolerances for the different dimensions based on the actual requirements of the design. This way many of the tolerances would be much looser. The problem I see with the method they are using is that a different supplier may have a looser (but perfectly functional) tolerance, but we can't use them even if the cost is lower. Also, we end up rejecting parts at receiving inspection that would function perfectly. Can someone point me to a respected source (book, web site, etc.) on tolerancing methods to help me convince them that there is a better way to do it? |
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Posted by: Kelly_Bramble ® 05/26/2005, 15:08:32 Author Profile Mail author Edit |
I think that you are right in saying that "dimensions based on the actual requirements of the design", however if one can design a part / assembly by using recommended easy manufacturing tolerances, and achieve assembly interchangeability, they are doing everybody a favor. Blindly assigning manufacturer recommended tolerances, is by all measures, not a good idea. Most manufacturers, will give extremely loose tolerances which may result in parts which will not fit into the end item assembly. Scrape parts and time wasted attempting to assemble parts is not in any organizations interest.
Unfortunately, many designers and engineers design exclusively to perceived requirements. Normally, this results in very expensive and challenging parts to manufacture, which drive up costs. Understanding the easy tolerances to manufacture, and then working within those design guidelines is the best approach. Modified by Kelly_Bramble at Fri, May 27, 2005, 08:35:50 |
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Posted by: traingdt ® 05/27/2005, 09:34:37 Author Profile Mail author Edit |
I agree with Kelly...
There are basically two approaches: The "top-down" method is when we design based solely on functional requirements, and filter those numbers down, and whatever manufacturing gets is what they get. The "bottom-up" approach is what you referred to; the manufacturer dictates what can be held, and those tolerances are plugged in to work up to the final requirements. As you might guess, a middle-ground approach is the best way. Yes, tolerances should be initially based on functionality -- but manufacturing should be consulted to see if those numbers make sense for the machine or process being used! There are standard tolerance guides available for different materials and processes, but those should be used only if you have no idea what to plug in for the first tolerance. From there, GD&T formulas such as stack calculations, virtual condition, and fixed/floating fastener formulas will help. |
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