Gentlemen, As loooonnnngggg as opinions are being proffered. When we encounter a print that the 'nominals' will not make the part as drawn we wonder just 'who the hell drew this?'. A license from a crackerjack box does not make a draftsman. With the advent of CAD this discovery is waning. A solid model is a wonderful thing. The only time you should have to 'fudge' the numbers is when you are making parts from forgings or casting. Tolerance stackage, warpage, die slippage and a host of other 'ages' can cause the features to shift. Most of the time a forging or casting will have a sweet spot. Find the sweet spot prior to making the first cut and you are home free. Our aerospace customers want the parts as close to nominal as possible. They would like all parts to be 'dead on' so the parts weigh the expected amount. No more/ no less. They allow tolerances because perfection is not attainable but they wish for you to get closer with each succeeding part. thanks for reading my rant Stuart
On Sat, Jun 13, 2009 at 5:36 PM, Matt Shaver<[email protected]> wrote: > On Sat, 2009-06-13 at 11:27 +0100, Andy Pugh wrote: >> Most of the dimensions for the general geometry were +/- 0.2mm except >> for the flexural element, which was 0.2mm +/- 0.05mm dimensioned from >> a face with a stacked-up positional tolerance of about 0.4mm. >> The machinist set up his CNC mill to the centre value of each >> tolerance starting from a part edge and pressed "go". When the program >> finished the flexural element was not even there. >> >> Who was at fault? I argued that the wider tolerances elsewhere in the >> geometry were specifically so that they could get the flexure right, >> they said "You always work to mid-tolerance, and the drawing should >> assume that" > > The machinist. I used to do job shop work in my shop. The feature that > you described should be inspect-able, and the from dimension you quote > it should have measured (by whatever method is appropriate) > between .15mm and .25mm. Since it was not there, we can assume that it > would measure 0.00mm and is therefore out of tolerance. > > I have heard this "middle of the tolerance" argument many times and it > is as wrong today as it ever has been. And I was on the side > (machinists) that tried to use it to our advantage! > > If the stack-up of multiple tolerances actually prevent the part from > being made such that each individual tolerance limit can be observed, > then the drawing should go back to drafting with that explanation. If > you actually make the part, then it must pass inspection, even if some > features must be created near their tolerance limits to allow other > features to exist within their own tolerance limits. The "middle" of the > tolerance band is no more valid or important than any other point within > the tolerance limits. > > Wow, who would have thought I was this opinionated? :) > > Thanks, > Matt > > > > ------------------------------------------------------------------------------ > Crystal Reports - New Free Runtime and 30 Day Trial > Check out the new simplified licensing option that enables unlimited > royalty-free distribution of the report engine for externally facing > server and web deployment. > http://p.sf.net/sfu/businessobjects > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > > -- you can lead a person to knowledge but you cannot make him think ------------------------------------------------------------------------------ Crystal Reports - New Free Runtime and 30 Day Trial Check out the new simplified licensing option that enables unlimited royalty-free distribution of the report engine for externally facing server and web deployment. http://p.sf.net/sfu/businessobjects _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
