【转帖】ight tolerancing and fabrication

yang686526

tight tolerancing and fabrication
for practice i am applying gd&t to an assembly that has already been made. the design is filled with extremely tight fits (for example a fixed fastener dia of .112 with a clearance hole dia of .114). the positional tolerances i'm calculating have been from .0005 and up.
what throws me off is that this assembly has been fabricated based on these plans. since the plans didn't have any gd&t at the time of fabrication, can i assume that many of these clearance holes were made larger? that the fabricators fudged the hole sizes to make everything work?
if i were to accept this and actually tolerance these things at .0005, or zero for that matter, would that even be possible? (putting cost aside).

eng-tips forums is member supported.
are these tight tolerances used on the assembly for some hardware attachment, or for the parts themselves?
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
jlang17,
   have you inspected the parts and verified for example that the ?.114" hole is indeed ?.114"?
   were the parts fabricated separately, or were pieces drilled together?
               jhg
surely that depends on how the parts are manufactured? if the holes are put in with one set up on a cnc mill or pierced on a press tool then it would not be unreasonable to expect them to be within those limits to each other.
i have always thought it was good design practice to provide the maximum allowable tolerances for parts. i would question what drives this design to such a tight tolerance.  not just a question of "is it achiievable".
some engineers like to design assemblies so that the fasteners themselves do the alignment.  in those cases, i've seen ridiculously tight tols on holes (often sqr tols, not even geotols).  this should be avoided if the device even will need maintenance (which most do).  it's poor practice to use threaded fasteners to align parts.  this should be done with fits, grooves, and/or alignment pins.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
the better question that you should ask is how hard is it to assembly.  go and ask the guy who is assembling and then inspect a few parts.  in the end the guy who is doing the assembly is the one that is going to be living with the problems of the assembly.
chris
"in this house, we obey the laws of thermodynamics." homer simpson
the parts might meet spec but the assembler may be filing out the holes so that things fit without you knowing it. he wont say anything without you asking because he's got to deburr anyway and besides he is getting paid by the hour...   
david
thanks for the replies, the designer will be in next week so i have a few things to discuss with him.  i'm not sure if i'll be able to dis-assemble the device to inspect, but i'm sure he'll have some insightful feedback.
jlang17,
   you also want to talk to the guy who built it.  the designer may be surprised at how it actually got fabricated and assembled.
               jhg
do tolerance stack to double check your dims and tolerances.  if you do proper tolerance stacks then this makes sure your assembly will go together