Abusive use of Continious feature?

[supergee]

Hello all,

I've tried seaching the standard and the threads but can't find a clear answer.

A student gave me a drawing of a sheet metal part that is bended to look like a "U". Two coaxial holes are made on each side of the "U". The student decided to make the holes Continuous Features in order not to calculate the Frtz value... I know...students are sometimes lazy...

Is this abusive? I mean, logically, in that particular case, the holes would be laser cut (or any other process) on the sheet metal BEFORE it would be bent into shape. Using <CF> would imply the holes being drilled together after the bending. (I know the standard does not dictate the manufacturing process)

 

[Burunduk]

What tolerance was the CF symbol applied to? Size? Position?

 

[3DDave]

A tool is a tool is a tool.

How does CF avoid the upper segment/sole segment for positioning the holes? What calculation is being avoided?

 

[Burunduk]

I would understand how CF could replace specifying the FRTZF (for alignment) but not the PLTZF.

 

[supergee]

Burunduk, I wrote Platz but I was thinking FRTZF. you are right.

By applying the <CF> to the size of the feature, the holes are concidered a single feature: it is the combined geometry of both hole that need to respect rule#1. the relative position of the holes with regard to each other doesn't need to be specified. all that is left is the position of the hole pattern on the part.

As for the calculation avoided, I wanted to students to share the total tolerances of an assembly between all parts while thinking of the manufacturing limits. Sheet metal bending being less precice than turning a part on a lathe, they would have to figure how to distribute the tolerances amoung the parts. Instead, they bypassed the bending issue by drilling after bending. the added work station adds more cost than necessary to the process.

Again, I beleive the final answer would be okay, I was just wondering if specifying <CF> on features that are not really continuous feature is abusive. For instance, on the crankshaft of a gas engine, would it be ok to say that two journals (I beleive it is the English term) are continuous features even though they clearly are not. since it is not possible to machine them together.

lots of English word I am not sure how to spell... sorry for the typos.

 

[Burunduk]

Supergee,
I think it is more about the functional requirement, less about the manufacturing process. If a single pin has to fit through the two holes in the U shaped piece of sheet metal, CF is not a bad choice.
Think of it this way - CF on the size limits for the two coaxial holes is equivalent to a datumless zero@MMC position tolerance. That could be a second segment of a composite or a multiple single segment position control. All these would result in the same limits on the geometry and describe a functional requirement that is possibly appropriate to the application.

 

[supergee]

Burunduk,

That is more or less the answer I was expecting from the students: a datum less tolerance on the last segment with a value (zero ideally but I am a leneant teacher) at MMC.

If i understand your opinion correctly, <CF> could be used regardless of the fact that the features are not a real extension of one another?

 

[Burunduk]

Supergee, if you expected the datumless zero@MMC position, the CF on the size of the holes provided an equivalent answer, so it can't be bad.

I say that the idea of CF is to specify that two or more features are treated as a single feature for some specific tolerance evaluation. It doesn't mean that the two features are actually perfectly or near-perfectly aligned, although they should be aligned to some level to conform to the requirement. If the features are produced in a single machining operation, that certainly helps, but not a mandatory condition. Hope this makes sense.

 

[supergee]

Burunduk,

Thanks for your time. Your arguments make sense. I feel uncomfortable with it but, since the standard doesn't specify it, I guess it is not wrong.

Have a good day

 

[CheckerHater]

Little late for discussion, but your student is not lazy enough.

It still legal to point at 2 holes and say "this is 1 hole":

https://files.engineering.com/getfi...-330b-4e52-8f4b-6f14128ade2e&file=Capture.PNG

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