【转帖】flatness in assemblies

yang686526

flatness in assemblies
(ref:
designmr,
   i sure hope that is .187 inches.  
   this is a question for your welder.  the guys i deal with claim they can weld to +/-1/16".  
   for weldments, you need to stare at the thing and figure out exactly what you need.  what exactly needs to be accurate?  can you design in a three point mount and grind faces to specification?  can you make a bracket adjustable?  
   i hope your three-place dimensions do not mean +/-.005".
                       jhg
yes, it is .187 inches.....and our format has +/- .010.  our shop supervisor just laughed when i told him, and laughed louder when he realized some of the frames might get welded by outside vendors....
this whole frame has to be welded, maybe had they thought about this problem over a year ago, that design feature might have been added, now it is too late.
thanks for your input
designmr,
   where i work, most of our stuff is machined.  a whole bunch of people prepare drawings showing .125" holes located to +/-.005" for 4-40unc (.112") screws.  we are consistently able to assemble the parts.
   if you do the math, you will see that the parts would not assemble if the machinists took advantage of the .005" per axis errors they are being allowed.  machinists can cover up all sorts of bad drafting practise.  
   you can prepare drawings for machined components in brain-dead mode.  when you get into sheet metal, welding and castings, you need to pay attention to tolerances, and feed that back into your design and your intended assembly procedure.
   when you tell a welder that he should work to +/-.010", he has two choices.  he can take you seriously, and figure out the jigging and clean-up machining, and write up the resulting astronomical quote.  he can assume you are an idiot, quote his normal price and use his standard fabrication process.  yet another option is to promise a "best effort", leaving you no control over quality.
   your .187" tolerances sound reasonable to me.  i would open up all those .010" tolerances to .062", or greater.
   definitely, chat with your welder.  there are some very good contractors out there.
                     jhg
thanks drawoh....