Size Dimensions to Avoid

[@ThePowerhound]

Nice video. The only thing I have to say about it is that a size dimension only exists when it’s defining a regular feature of size. It’s not about whether or not it has a +/- tolerance on it. The dimension that controls the step is simply a dimension, not a size dimension.

[@patientestant]

Informative. Most of the time lathe operations are perpendicular and the tolerance is wide enough that the ambiguous call out is not an issue. If you are talking about tighter tolerances and on the edge of a tolerance, then this can be relevant, but if you have a relatively wide tolerance then you are right parts can look horrible and still pass, but that often is not an issue, because it is common to know how to avoid having such a large discrepancy. So for simple parts, I am not sure you really need to have datum’s and profile call outs. Once people learn this though, it will help clarify exactly when the parts pass or fail.

[@crashfactory]

GREAT video. thank you!

[@jenshavla4673]

Very informative, thank you.

[@Hobz79]

I wouldn't read too much into the drawing, it is just an imaginary example and we don't know anything about the other parts it mates with. The designer would have done a tolerance analysis to make sure the parts still fit. The ID could definitely have been datum B, with the bolt pattern or keyway datum C, but without knowing the design it's impossible to say which way is correct

[@keydi98]

To have less références maybe, B embed an axis and à plane we can use to position anything else.

[@MichaelMachado2]

Any reason to not just replace any and all size dimensions with profile callouts?

[@keydi98]

That would be ambiguous, all the features are by definition separates it is difficult to join them unless you put an unique tolérance zone by using the UF modificator.... however that would cost too much... for nothing.....

[@Gdandtbasics]

Thats a great question, and we cover that in our training courses. But the major reason is that profile will control Size, Location, Orientation, and Form all to the same value. Whereas if we control the Location and Orientation with the Position symbol, we can have different size/form tolerances via the size dimension and rule #1. This gives more flexibility to tolerance appropriately to the functional intent of the feature.

[@radon3292]

Why does a profile have B datum for secondary? It makes no difference

[@Gdandtbasics]

You're absolutely correct, none of the features that utilize this profile callout need B as a datum feature, however its still considered good practice to include it just incase that UOS gets applied to a feature in future revisions that DOES utilize it. Basically it doesnt hurt to include it and make all the feature control frames have the same DRF.

[@TAH1712]

There's no point to any 'B' reference. I really do feel sorry for all the machinists that think 'I'm being making great parts that never get rejected - but now I'm not sure I fully understand the drawings anymore !' Buying parts, the specifier has the advantage - shop owners now have to decode the drawing to get back to even the simple features. It is obviously a big competitive advantage to fully appreciate the drawing and identify the dead easy vs the extremely difficult part features. Looking at the drawing it makes no sense to me that the boss isn't datum B with the slot being positioned relative to the bore ID. pattern of holes positioned to the boss and then the slot positioned Looking at the drawing it makes no sense to me that the boss isn't datum B and no sense that the boss ID and OD have differing positional tolerances yet both have the same tolerance of size, how exactly does that 'functionally' work on real parts. The slot position is relative to the bore ID and to make the holes as datum B , the last feature to be produced as a 'functional' datum I still don't understand. Why has the OD flange the same position tolerance with a +/- 0.015 tol as both the boss ID and OD with size tols of +/- 0.001 ? Why has 21 people liked this without anyone questioning anything going on with this part drawing?

[@keydi98]

I had a hard time understanding all your points, the drawing for me is not that bad.... I just wished the ID to be at least a référence..... but Either way when the function requiered are fullfilled it is okay.... By the way the remark concerning the ambiguous linear dimension is true... in Iso gps that type of dimension was prohibed since à lot of time.

[@Gdandtbasics]

One thing to not forget as a designer, datums should be selected due to their functional importance, which is why the bolt pattern was selected as datum B. It sets the location of this part in the final assembly after datum feature A. Then all other features should be located with respect to this functional location. Thus the key way is clocked with respect to the pattern of holes. The size dimensions should be selected once again from the functional design of the part where a good tolerance analysis will lead you to the appropriate amount of tolerance for all features of size as well as surfaces. And lastly, at the end of the day, this drawing was created to specifically discuss the stepped size dimension rather than all the other aspects of the example.

[@TAH1712]

@@Gdandtbasics Also, 'At the end of the day' it would really help if simple drawings were complete without any conjecture - as your are fully responsible for your output, I read your last sentence with incredulity - You can not on the one hand, be precise with logic to stipulate unambiguous definition, yet sidestep other illogical tolerances by saying 'this drawing was created to specifically discuss the stepped size dimension rather than all the other aspects of the example.' Sorry - but that's my sincere opinion.