Jon Elson wrote: > 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" >> >> > Well, if the drawing showed metal to be there, and there was no metal in > that position, > then the part did not match the drawing. How can they argue with that? > > Now, if the dimensions were in some way wrong, so you cut one side, flip > it, cut the > other side and there's nothing left, because the drawing instructs them > to mill more than > half the thickness from both sides, that's an inconsistent drawing, and > should have been > caught before machining, but the drawing is wrong. > > If the problem was due to tolerance stackup or the relief of stresses in > the stock as material > was removed, and competent machinist SHOULD have been aware of the > problem just from > examining the drawing. If tolerance stackup, then a fixture should have > been made so the part > could have been machined with fewer setups (preferably just one or > two). If part warpage, > then the whole machining process was flawed, either due to wrong > material selection, wrong > approach, wrong fixturing or whatever. > > Some shops would be offended if you tell them how to machine a part, > they OUGHT to know > better how to do it with their machines and materials. But any shop > that complains that the part > doesn't match the drawing and it is YOUR fault for making it "hard to > machine" sounds like a > bunch of idiots. Button pushers, not machininsts. Ie, they went > straight from a drawing, to CAD, > to CAM, with no understanding of materials and machining processes. If > they complained about this, > it is actually FUNNY, because they were revealing their own ignorance in > a VERY embarrassing way! > > Jon > > ------------------------------------------------------------------------------ > 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 > > I may be all wet, but it seems to me there aren't any standards much now. Mostly mayhem in machining. At one time there was Henry Ford and his people writing automotive standards and they were an authority. Seems like professors are doing the same thing in teaching, in a lot of areas but there seems little agreement between them. I guess this is because we are in transition in manufacturing. The fact that there is misunderstanding over you drawing is proof of that. I believe mechanical design is mostly bedlam for now. More and more it doesn't matter though the machine just follows the codes given it. It has no knowledge of making a part it is simply following code. Doug
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