Posted by: yldhare ® 09/28/2004, 21:55:17 Author Profile Mail author Edit

I have two parts, each have 51 holes in them (mirror parts) with 8 holes having a flagnote "mate with opposite part" and "maximum permissable mismatch .00393" These particular holes have a true position requirement of .00393 to datums AB with MMC. What is the requirement here? are the parts to be machined together for those particular 8 holes or do you have to satisfy the true position requirements only and pin together after completion? GREG

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Re: MATING PARTS?
Re: MATING PARTS? -- yldhare	Post Reply	Top of thread	Forum

Posted by: astrotnt ® 09/29/2004, 04:17:55 Author Profile Mail author Edit

Although "maximum permissable mismatch .00393" is a bit confusing requirement that I've never seen, it appears that you need to satisfy the true position and mismatch simultaneously. In this case, since amount of true position and mismatch tolerances are same, if you satisfy true position first, then mismatch will be automatically met as long as you make hole sizes identical. Now because it is not possible to make all the hole sizes identical, you'd have to control the relationship of hole size and true position tolerance to automatically satisfy the mismatch allowance. For instance, while you want to make holes as large as possible to take advantage of MMC, if you can control the hole size within .002, then plan to have true position of  .00293(.00393T.P-.002/2) plus additional MMC tolerance. I think this can be easily done without having to machine together. If you ever have to machine two parts together, it could also be done. However, in that case, you want to make sure two parts never move during the machining and I have experienced this is not easy. One of the tip I can offer is give several tag welds alongside the edges of two parts and remove them after the machining by subsequent machining. Obviously in your case, you will have to machine datum A and B first by picking the holes and then grind off all the remained welds. Hope this helps you. Good luck!!

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