unilateral tolerancing 5

[mmolt]

Friends,

In a meeting today one of our manufacturing engineers stated he was going to call another meeting to state "we should never use unilateral tolerancing". As a design engineer, I am against forbidding use of unilateral tolerances as I feel it is removing a communication tool for use on our drawings. He claims we should always model the parts so that the nominal dimension can utilize a bilateral tolerance. He is saying this because they are using the CAD models to generate CNC programs, and then they have to go back the CNC programs and edit the numbers so that they are in the middle of the allowable tolerance. I hope that makes sense.

What are your thoughts?

Thanks in advance for your input,

-Mike

 

[BARM]

To TheTick, I hate to burst your bubble but in my automotive experience "CAD is GOD" as the saying goes.

For example, we receive the CAD (math data) and the GD&T(paper drawing) but the two don't jive. Changes have been made to the math data but often the drawing(s) are far behind. So... in this case the customer says "go by the CAD data".

It appears to me that the fast and furious pace in the auto industry has lead to this state. I'd had always been in your current school of thought until landing in the auto industry.

Just to let you know.

---SolidWorks 2008 SP3.0---

 

[TheTick]

Sayings are not the same as standards. It also has little (nothing?) to do with whether it is appropriate to mandate a single tolerance scheme on a print.

In my automotive experience, nothing got made for a customer until they signed a drawing and a purchase order. The drawing is what defined the deliverable. Parts (the real objects shipped in crates on pallets to customers) did not get accepted or rejected based on a CAD model.

In either case, so what. It's a CAD model with no tolerances. How are you defining the tolerances?

 

[gearguru]

The term "Basic size" is used in ISO 286 (metric standard ISO system limits and fits), not "nominal". Symmetrical +/- tolerances are rather exception when using that standard.

 

[ajack1]

The Tick, I think you are a little behind the times, as BARM states in the automotive industry, or at least body panels and various other pressings the model is the master.

You will get a drawing but it only shows a few basic dimensions, usually holes and slots as well as the clamp areas for inspection and what areas have any specific tolerances other than the general as well as material spec.

They do not attempt in any way to give enough information to manufacture the part, it is purely to the model.

 

[vc66]

If you're using the model, with no tolerances, does that mean that the machinist cuts it perfectly, or the part is bad?

Unless you're putting tolerances on the model, it's no good.

Try physically making a perfect part, with no tolerances... see where it gets you.

V

 

[ctopher]

There is no perfect part, this is why tolerances exist.

Chris
SolidWorks/PDMWorks 08 2.0
AutoCAD 06/08
ctopher's home (updated 10-07-07)

 

[TheTick]

Apparently I've said something without actually writing it. An ability I didn't know I had. This thread, and my posts in it, are about use of tolerances on prints. I'm not debating the merits of building from CAD models. Apparently some have lost the ability to read paragraphs along with the ability to read prints.

 

[ajack1]

I think you will find that anyone who has not taken a vow of silence has the ability of saying something without actually writing it down.

The way parts are toleranced, at least in the automotive industry is exactly as I described it above. However the part is NOT dimensioned, the model is master.

 

[vc66]

I guess I still don't understand how one manufactures a part without tolerances...


V

 

[powerhound]

ajack1

Are the suppliers of the CAD model providing a file with the tolerances embedded into it? I know at least one CAD software can do it because I've seen it but I don't know which one it was. As far as I know the only thing you will be able to get from the CAD model is the nominal dimension without tolerances. Are you saying that the customer only provides drawings with the tolerances that they really care about and they don't really care about the other dimensions?
Whatever the case may be, Fundamental rule 1.4 (a) says that each dimension MUST have a tolerance except for those SPECIFICALLY IDENTIFIED as reference, MIN, MAX, or stock. From the threads, I don't see that the CAD dimensions are specifically identified as anything. Would one be correct in assuming that they are all reference? What if someone assumed all OD's were MAX and all ID's were MIN; or vise-versa? I'm no stranger to the idea of "well it's always worked before so what the problem?" but it only takes one multi-million dollar assumption to change the way everything goes. If a company has been allowing this sort of practice for years then suddenly hundreds or thousands of parts get made that will not work, someone has to pay for it. Who will it be; the company that provided the untoleranced data or the company that agreed to build parts based on untoleranced data? It's a dangerous game, in my opinion.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[ajack1]

There are tolerances, usually a general profile and surface tolerance and occasionally hole tolerances, which are normally unilateral but there are not dimensions, they come solely from the model.

As to if they are embedded in the model I do not know but they are readable as prints. We only have a basic version of Catia for reading in data, this is a requirement of most automotive manufacturers.

This might not comply with any standard and I cannot categorically state that all automotive manufacturers work this way but certainly the likes of Ford, GM, Audi, VW, Honda, Toyota and BMW all do and have done for a good few years.

 

[BARM]

Yes, as ajack1 as stated above this has been my experience too.

So what did I do when the math data and the GD&T did not match? I inform the customer and he signs off to it. We still use/need the tolerance from the GD&T but apply it to the size(s) of the CAD model.

In fact, to inspect the part you need the CAD model. The model is used to compare against the actual part produced. Think CMM.

Note, tolerances can come from the original drawings (GD&T) or signed notes and emails.

It this the best method? No but it's our reality in this environment of "Got to have it now!"

Note: There is software out there including SolidWorks in which you can include all your GD&T into a 3D model. The idea of this is to reduce the reliance of 2D drawings. ie: the drawing not up to date with the CAD model.

Hope this helps.




---SolidWorks 2008 SP3.0---

 

[gearguru]

I also work for supplier to automotive companies.
We are creating the 3D solid models using the mean dimensions (median as drawoh calls it). Only some complex castings (as gearbox cases) are made using the "mathematical data" - the 3D model. The simplified drawing with requirements/tolerances/GD&T for the important features is part of the documentation. Any other parts have fully dimensioned 2D drawings including all tolerances, GD&T etc. The critical part drawings are usually also verified by the OEM's.
We supply the same OEM's as ajack1 mentioned, plus Chrysler.

 

[fcsuper]

That's what ASME Y14.41 is for. Complete tolerance information must still be included with the model though.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ewh]

As far as using primarily CAD data, as others have posted, it is done. A few critical dimension on the print along with notes which specify that the CAD data is the MASTER data model and a general profile tolerance note. When the drawing is signed off, it is the same as signing off on the model. Unfortunately, in my experience, those that have the authority to approve the drawing don't have the ability to even interogate the model, trusting that it was modeled and checked appropriately.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[mmolt]

I think we've gotten a bit off topic here. ...and what I'm reading is that some parts are so complex it doesn't make sense to attempt to create an all encompassing drawing to go with the model. For instance, one can imagine the complex curves associated with todays automobile exterior sheet metal parts...those are probably nearly impossible to dimension and have the dimensions mean anything to a human. So, in those cases some assumptions must be made about how good the not so critical features will end up, for other dimensions that are critical, some callouts on a drawing must be made to communicate the allowable deviation from nominal.

Regardless, in my line of work, the drawing IS without a doubt the binding contract with the supplier. The drawing is what is signed off on and agreed upon...otherwise a supplier could give you almost anything, and you would be responsible for the bill, regardless of whether the part meets the requirements of the design. The drawing is the DEFINITION of what is required.

Now, back to the unilateral question. I guess where outlawing unilateral tolerances doesn't make any since is in the case of fits and holes. In the case of drilling a hole, the standard tolerances are almost always unilateral.
For instance, according to a table in front of me listed as "drilled hole tolerance (under normal shop conditions)" a 1/4" drilled hole should have a +.0063 -.001 unilateral tolerance. So, if I were to do what the manufacturing engineer wants I would need to model the hole as .25265 and some change instead of exactly .250, so he could have his symmetric tolerance. To take that further, I don't think there is a standard drill that size, so once again his CNC setup guy would have to make some manual adjustments to his program right?

-Mike

 

[CheckerRon]

mmolt: What you are defining i.e. + .0063/-.001 is an unequal, bilateral tolerance, not a unilateral tolerance.
Unilateral is one of the tolerance values is zero.
(ASME Y14.5M-1994 ¶ 2.3.)
Bilaterals can be equal (±.010) or unequal +.005/-.001, with the primary value being the "target" dimension.
Back to your original arguement (and post)with the manufacturing engineer. Was he insistent on target dimensions with equal ± tolerances, or would he accept a target value (e.g. drill size) with an unequal tolerance value?

 

[mmolt]

Pardon me Ron, you're right, it is indeed bilateral, but nonetheless, the manufacturing engineer still has the same problem with it. He said he only wants us to use symmetric tolerances, so yes, he is insistant there is a target dim with an equal plus/minus tolerance.

Listen, I don't have a problem TRYING to always use symmetric tolerances, but I'm not going to agree with his demand to never use anything but symmetric tolerances. Like I mentioned, I feel it would mean throwing away some pretty good communication tools on our drawings.

-Mike

 

[ajack1]

That pretty much seems to sum it up Mike. It does seem strange to me that you can produce a car body part or an airplane wing in this way but a ¼” drill hole becomes a problem.

The shafts and holes standards go back to the days of pencils and paper I can see no good reason to change them, especially if all that matters is what is “drawn” on a 2D print.

I must say I have never seen a .250 drilled holes dimensioned as .25265 +/- .00365 that just seems to cause confusion for no good reason.

 

[dgallup]

Carrying a dimension to 5 decimal places means that you have to have inspection equipment capable of measuring to 6. Is your manufacturing engineer and quality department prepared to do that?