huangyhg

current state of model based definition
there have been several threads both here and in other forums on use of mbd (model based dimensioning). when i'd previously looked at it i'd decided it would be more effort than it would be worth for most of our stuff, however, have implemented it on some castings and moldings.
we're being pushed to decrease development time cycles and my boss wants to look at mbd as a way of achieving this.
so how many people are doing mbd and have any major developments in this field come along since it was last discussed in depth?
i'm especially interested in those out side of the 'managed supply chain' of aerospace/defense and automotive where it seems to have been achieved largely by the oem's dictating what cad system will be used and using their leverage to enforce this on their suppliers.
we are a smaller/independent player in the technology field who outsource all our machining etc. with only assembly done in house (even some of this is outsourced) and i'm wondering how other smaller players have dealt with it. do you:
1. only deal with suppliers with the same (or at least compatible) cad/cam system(s) so that tolerances etc. in the model are carried across?
2. use a lightweight format and does it correctly handle mbd.?
3. use hybrid drawing/model and if so how do you handle tolerancing of features not explicitly defined on the drawing?
4. have extensive "rules of use" in addition to/in place of the above or just rely on asme y14.41 or equivalent?
i have a copy of 14.41 and looked through it a while back but i'm not completely familiar with it.
models are generally considered basic except where tolerances are added. to fully define a part, you'll still need a gd&t scheme, often supplemented by traditionally dimensioning and tolerancing where needed. the difference is that this now has to be done within the model itself.
additionally, any information that would've appeared on the drawings also now has to appear within the model itself. so, shortcutting the drawing step doesn't mean you get to not bother with all the information you would've included on the drawing. it just means all of that now needs to appear in the model.
with that said, asme y14.41 supposedly standardizes this effort. in my opinion (and yes i've read it and "own" a copy), it is drasically lacking right now.
if you do go mbd, just make sure everyone understands that the model is now the drawing and that means it will need to be as actually detailed as the drawing would've been.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
now, to address your 4 questions:
1. only deal with suppliers with the same (or at least compatible) cad/cam system(s) so that tolerances etc. in the model are carried across?
a: in my opinion, this is not practical unless you own your suppliers. the variety of packages and versions for each package increases each year. it is good to analyze what is common in your industry, but that's not the end of this.
2. use a lightweight format and does it correctly handle mbd.?
a: don't know. this sounds like this question is particular to one particular 3d cad application.
3. use hybrid drawing/model and if so how do you handle tolerancing of features not explicitly defined on the drawing?
a: my company does this. we still drive critically from the drawing, and use the model as the completion of the part definition. i've seen some companies callout a general profile tol based on the model for all surfaces not defined by the drawing. i've seen others simply use the model as part of the direct specification (model is not basic, but to be measured from directly with a previously established traditional tol)
4. have extensive "rules of use" in addition to/in place of the above or just rely on asme y14.41 or equivalent?
a: given my own opinion about y14.41, i would suggest that your additional "rules of use" would have to be extentive indeed.
these are just my opinions...take them only as far as you can throw me.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
1. we have a wide base of suppliers that own all kinds of different 3d software so we usually transfer the drawings in pdf and the model in step and or parasolid format. not many suppliers use i-deas anymore ):
2?
3.we call out all critical dimensions with applicaple tolerances and gd&t. we attach the following block to our raw material drawings. see attached
thanks

i've seen several efforts at moving to a model based enterprise, and all have had issues.
there fortunately are well accepted neutral model formats (particularly popular are step and parasolid), but they are "dumb" solids; i.e. no parametrics and no imbedded data. these are ok with cam packages that look at the model surface, but don't consider the math behind the surfacing. this isn't the most efficient method when doing complex surfacing though, so a compatible cam package that can use your native cad model format is better in those situations.
from what i've seen so far, none of the cam packages recognize the gd&t data whether annotated or imbedded into the model. that's probably because the cad packages do a rather poor job of imbedding the gd&t into the model in the first place ... tried it ... don't like it.
what i've found works best under the situations i've been involved in is a hybrid system. send the supplier a neutral format (parasolid is my preference for comparatively simple geometries, but step works better with complex surfacing) to program their cam packages with, and an annotated gd&t model file that they can read the annotations from. there are supposed to be some cmm packages out there that can read the imbedded gd&t data from the models of certain cad packages, but i've not seen proof of this when challenged. what i've seen usually happen is that the gd&t is read from the print or model by a cmm pogrammer, then manually applied to the model again in the cmm software. rather cumbersome, and prone to transcription errors.
keep in mind that y14.41 was written so that the cad/cam/cmm software developers could have a common starting point and set of functional requirements. also, y14.41 doesn't just apply to cad data, but essentially defines pdm (product data management) structure and requirements. now that y14.5-2009 is released, the y14.41 committee will be starting on the next revision. now is the time for anyone with comments & suggestions for y14.41 to post them to asme for consideration.
jim sykes, p.eng, gdtp-s
mechnorth
"and an annotated gd&t model file that they can read the annotations from"
what do you mean by this? are you sending both a step/parasolid type 'dumb' model and a lightweight file type, something like jt, that supposedly carries across mbd annotation etc? why not just the jt type file?
kenat,
isn't mbd solely to replace drawings, not to be used for cam?
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
kenat,
i've sent e-drawings with the gd&t annotations on the model, and solidworks files which can be viewed with a viewer program but can't be used otherwise without the sw program. i haven't tried a 3-d pdf, but i recall someone telling me that they'd had success with it for sending "dumb" annotated models.
i would send the native annotated cad model if the supplier could use it directly, but they can't always.
jim sykes, p.eng, gdtp-s
ctopher,
i don't know if there is an "official" definition of mbd. i take it to mean that the model is the basis of the design rather than a print. as a result, the model can be used directly for manufacturing and inspection. most of the automated (cnc) shops that i've dealt with have been switching over to model-based cam and cmm programming; those shops range in size from two machine operators to several dozen or more. manual shops often go back to generating a drawing themselves if they receive a model ... of course gd&t isn't usually much of a consideration for them anyway.
jim sykes, p.eng, gdtp-s
i may be mistaken, but nx offers embedded gd&t which can be read by valisys cmm software, as well as by nx cam.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

i did a search here at my company. mbd files are also used for cam and include all gd&t definitions. they are the same as if they are a regular drawing.
on some programs, a separate file is used for cam from the mbd file.
when a file is imported from another company that uses a different 3d cad application, they have to follow certain guidelines and be pre-approved how those files are presented. i don't think they follow their own guidelines because i never had to be pre-approved in the past at other companies.
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5

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