【转帖】old vs. new- model vs. drawing

yang686526

old vs. new- model vs. drawing
i work in the lighting industry- pedestrian scale light fixtures- and the majority of our manufactured parts are die and sand cast. before i came to this company the process was to design the parts in 3d cad, create a technical drawing with only critical fit and function dimensions (mostly for inspection purposes) and only qualify vendors who would make the part from the model. the main reason for this was each part only has maybe 10-15 critical dimensions, but a
considerable amount of cosmetic surfaces that are difficult and timely to dimension.
now, however, because of quality issues (hello outsourcing to china!), we are starting to go back to full dimensioning so that we have back-up when the vendor produces a sub-par part. i think that having a written clause in the purchase order as well as a note defining the part is to be made from the model and must be within given tolerances
should be enough.
so my question: how does your company deal with this process? are there any good solutions you have come up with? do we really just have to go back to the days of full dimensioning because of vendor quality? i feel like going backwards is a huge mistake- but i need a good argument as i will be a lone 23 year old female engineer launching this against an engineer who would rather computers didn't exist. i know ansi y14.41 is starting to make some inroads but i think it's usefulness is further down the road.
thank-you! and sorry for the novel.
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"...written clause in the purchase order as well as a note defining the part is to be made from the model and must be within given tolerances" is your way out.  we do this for aerospace components everyday.  the model is the master definition and any undimensioned features are covered under a note stating such, as well as notes defining the applicable tolerances.  if your vendor can't meet these requirements, you should find another vendor.  there is no excuse in their claiming that they can't interrogate the model if it is in a neutral format such as .x_t, .stp or .igs.
noraono,
being clear on the drawing is all one needs (the note you mentioned), but it helps to also mention it on the p.o.  warning about the note on the drawing: be cautious when using a generic note to define tolerance.  many industries have their own set of tolerances they will use based on the processes used.  be sure to be fully aware of these when creating the components and specifying a general model tolerance note.  for example, it may be impossible to keep a tolerance of +/-.010 on a cast part over 12 inches; there may be a per-inch tolerance applied.
best of luck!
matt
cad engineer/ecn analyst
silicon valley, ca
we do "out sourcing" to china through a "trusted" broker for some of our high volume molds.  i have recently run into a problem with the broker not using my drawings but instead only using the 3d model.  i had no idea this was happening until i got the first run of parts.  i did a first article on the parts then marked all the dimensions that didn't meet print.  the next e-mail i got back from them was we didn't know they were critical features.  well read the print next time before burning the mold.
i can honestly tell you regardless if you have mdd or full dimensioned drawings you will fight this quality issue.  sand casting in china is even worse then injection molding or die casting.  it's best to work through a broker who is located in the usa otherwise you will have a cultural fight on your hands.  trust me.....i'm in the fifth round and getting ready for a tko
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
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(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
why doesnt your print match the model?  why use the model for manufacture at all if it isn't correct?  this seems to me like your asking for mistakes.
one of the biggest obstacles we encountered with this type of issues was with the introduction of gd&t for our drawings.  while a good idea from a standardization point of view, the infrastructure was not in place to support it either internally (we are getting a bit better) or externally (again we are improving that area as well).  if you are going to utilize the model as master geometry, it is even more important to be confident that your supplier can open and read the model (not modify!)fff"> and keep communication going on changes.  we also have a quality clause in our purchase orders regarding overall quality as well as esthetics as appropriate.
regards
heckler,
   i am headed into casting territory, although, i hope, not to outsourcing to china.
   my first pass at a casting has been to use geometric forms i can apply tolerances to.  i am assuming that i can create section views with gd&t profile tolerances, and that i can use these to create inspection tools.  even if the geometry is very weird, i should be able to insert a go gauge, and verify that there is not excessive clearance around it.
   my problem at the moment is that all my design to date has been machined parts, sheet metal and some weldments, where drawings are used and are straightforward to generate.  our foundry guy used our 3d model to create rapid prototypes.  i am not sure i am speaking their language yet.
   definitely, i am in favour of 2d drawings.  the problems all look solvable to me.
                          jhg