【转帖】gdt and manufacturing process

yang686526

gd&t and manufacturing process
hi all
i m working on a typical problem regarding identification of the relationship between the gd&t and different manufacturing processes.
i would like to know that if there is any formulas or guidelines available for assigning different feature tolerances like circularity,cylindricity,parallism for different machining processes like planning ,milling etc?
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a good book to have a copy at your desk is the "machinery's handbook". in general, you dimension your part and add gd&t for fit/form/function, not for process or machining. you generally don't know how the part will be made in the end.
chris
solidworks/pdmworks 08 3.1
autocad 06/08
dhavalvp,
   you need to chat with your fabricator.   there is a chart in the machinery's handbook showing the iso tolerance classes that can be achieved with a given manufacturing process.  search around the limits and fits pages.
   you are not supposed to provide manufacturing instructions on your fabrication drawings, but i have rarely proceduced fabrication drawing where it was not obvious what fabrication process would be used.  when you know this, you should know what tolerances can be reliably specified on your drawings.
                  jhg
agree with above, function should drive the tolerancing & you don't normally define the manufacturing process on drawing unless it materially affects performance.  
however, you don't want to end up with tolerances that can't be met except by expensive processes unless you really have to.  sometimes a change in design can allow looser tolerances and this is good practice, to know when to do this you need some idea of the process capabilities.
if you have your own shop, or one/few external shop/s that does/do most of your work you may be best off talking to them.
the chart in machineries is probably based on iso2768.  this is a dogfff"> of a standard that should not be invoked in its entirety by any self respecting engineer/designer (see recent posts on the subject by myself and others).  however using it as guidance on what are typically achievable tolerances may have merit.  extracts of this standard are available on the net if you do a google search.  i may have even linked them in my earlier posts on this standard.
also, while i'm not sure it has exactly what you need, ctopher posted a few months ago a pretty good link to a machine shops website with information on what causes them problems/make it easier for them.  take some of it with a pinch of salt as it's biased heavily to making the machinists job easy without much consideration for function but it has some good stuff.
finally i seem to recall drawoh having a website with a little such info, or was it dingy, one of them.
kenat, probably the least qualified checker you'll ever meet...
thsnk you all
actually you all have make my problem easy,but while surfing about iso2768 i found another standard i.e.iso 2768-2:1989. it is something about "general tolerances -- part 2: geometrical tolerances for features without individual tolerance indications." but on some of the sites it is written that it is highly recomended that u should not use this standard! can anyone expalin about this standard and about the notification?
iso 2768 is a 2 part standard.  part one gives +- dims for various size features and various 'qualities' of machine shop.  part 2 gives information for geometric tolerances.
as to why this standard is a @#%%$^@#$% @#$%$@%!@# @#$%$# piece of #$%@#$5234; i'd have hoped that looking at my and others earlier posts on the subject would have answered that.  take a look at these links then feel free to draw your own conclusion.
dhavalvp,
to expand on what kenat stated, 2768-1 (when invoked) is a general tolerance for untoleranced dimensions. if you like to rely on a general "title block" tolerance to define an entire drawing, this standard may cause problems. (see kenat's quote).  since the standard only applies to untoleranced dimensions, you should add a specific tolerance to any dimension that you don't want the standard invoked on.
joe
sw office 2006 sp5.1
p4 3.0ghz 1gb
ati firegl x1
i suppose the other reason i don't like it is that different people seem to interpret it differently.  the way the german supplier i worked with used it didn't match what i understood the spec said or what anyone else i could find thought it meant.
kenat, probably the least qualified checker you'll ever meet...
if you would really like to guide the vendor to a particular manufacturing process, you can do so with surface finish specifications.
hope this helps.
thanks kenat
thanks for the links that u have given.
i have another doubt regarding gd&t,while refering to the ansi y14.5 standard i came to know about the formulas like the fixed fastner and the floating fastner that are used to allocate the postional tolerances.
is there any other such formulas avilable for allocation of different geometric feature tolerances ?
coaxial features when using position is in 14.5, just after fixed fasteners.
i can't think of anything else off the top of my head.
kenat, probably the least qualified checker you'll ever meet...