Tolerance Zone of R is ambiguous 3

[tgwow]

ASME Y14.5m, 1994 Section 2.15.1 Radius Tolerance states:

"A radius symbol R creates a zone defined by two arcs (the minimum and maximum radii). The part surface must lie within this zone...."

Section 2.15.2 Controlled Radius Tolerance states:

"A controlled radius symbol CR creates a tolerance zone defined by two arcs ( the minimum and maximum radii ) that are tangent to the adjacent surfaces..."

How do you connect the two arcs? Because no matter how you do it, there's a zone that is zero wide. Wouldn't you need to connect the two arcs with straight edges? And if you did, wouldn't that be the tolerance zone of CR?

What does the tolerance zone look like for R2+-2?

 

[ctopher]

Same definition, but:
R; Flats and reversals allowed
CR; Flats and reversals not allowed

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

 

[powerhound]

The R2 +/-2 is a silly callout because you don't have to have a radius at all and the part will be to print. It also allows all kinds of crazy geometry on that corner and still you would have a good part. I wouldn't preoccupy myself with this issue because the dimension, as specified, doesn't really make sense. If you give your customer a goofy looking corner that is still to print they are just going to reject it and you'll make the part again to keep him happy. Slap a decent looking radius on that corner and send him on his way. If you want to push the envelope, leave a sharp corner and see what he says...haha.

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[tgwow]

ctopher,

How could the tol zones be the same when CR requires tangency and R does not? The tol zones in the figures in ASME for R and CR appear the same but are they really the same? Try drawing it for the example of R2.4 +/- .3 from ASME. Because the arcs aren't require to be tangent, the zone can move anywhere between the tangency point of the minimum radius and the sharp and you still satisfy the definition of R. (See attached pdf)


powerhound,

This may seem silly but it is fundamental in understanding what R means. So, is it possible to draw the tol zone of R2 +/-2 using the ASME definition?

 

[fcsuper]

R does have less restrictions than CR. R used to mean some of what CR now means, but CR was created (I believe) to address the issue of tangency and fairness of curve. So, yes, the start of a R can have a variety of possibilities, where CR has only one. A minimum, R can be used to break a sharp edge. A minimum, CR can be used to establish an actual radius form from the two surfaces.

Matt Lorono
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Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[powerhound]

tgwow,
Regarding your drawing; the arcs are required to be tangent per 1.8.5 and 1.8.6 of the standard but not tangent to each other, they must be tangent to the corner that they are supposed to fillet. You would draw the minimum arc tangent to the edges of the corner and then the maximum arc tangent to the edges of the corner. The bottom view of your drawing is correct.

Regarding the "silliness" comment; what I meant was that if a sharp corner is acceptable then why even put a radius there? It's just one more thing that QC will have to inspect. The tolerance zone for a 2 +/-2 will look like a triangle with one of the legs as a 4 radius.

Powerhound, GDTP T-0419
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Inventor 2008
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[tgwow]

powerhound,

Section 1.8.5 and 1.8.6 requires that the actual part geometry be tangent to the sides but says nothing about the tolerance zone.

Therefore, why would you draw the min and max arcs tangent for R? Additionally, the definition of the R tolerance zone does not state inclusion of the tangent sides as boundaries.

By definition, wouldn't the tol zone for R2 +/-2 be just the max arc (since the min arc of zero just disappears) located somewhere in the corner (not required to be tangent)?

 

[KENAT]

If I had it on the drawing I'd have thought "R4 MAX" would be a more logical callout.

Depending on the application the 'silliness' point about why even break the edge may have merit.

KENAT, probably the least qualified checker you'll ever meet...

 

[tgwow]

Kenat,

We're kind of going on a tangent but for parts which are molded I've seen requests from suppliers who would want the CAD model modeled at nominal which would mean in order to "show" the dimension from the model on the drawing for "R4 MAX" and keep it parametric, "R2 +/-2" would be prefered over "R4 MAX".

The alternative is to model at R2 then fudge a dimension "R4 MAX" on the drawing.

 

[KENAT]

Yeah a bit off tangent, more related to thread1103-212575 but is an example of where wanting to go straight from the model makes it difficult to use certain standard drawing/tolerance conventions.

I'm always tempted to tell those suppliers that if the drawing is the master then the model is for reference only and maybe they'll actually need to do some work and go in and modify things.

The trade off between the possible extra cost of this compared to the impact of worrying about the kind of issues modelling strictly nominal (as in mid range with unilateral tolerances) cam cause need to be weighed.

KENAT, probably the least qualified checker you'll ever meet...

 

[dgallup]

I mentioned this in the unilateral tolerancing thread but it seemed to go over everyones head. With Pro/E & I assume other CAD programs you can have an R4 max. dimension (fully parametric model driving dimension) which is set to regenerate the geometry at the middle value (in this case 2) so the drawing calls out R4 max. and the model measures R2. The drafters, design engineers, manufacturing engineers, CAM jockies & mold makers are all happy. This is not a fudged in dimension.

The same technique can be used for asymmetric tolerances like shaft & clearance holes where the tolerances are both positive or negative. The dimension can read 1.000+.005/+.001 and the model will measure 1.003. You can also set the dimensions to regenerate at the maximum or minimum value to do tolerance stackups. The state of the dimension (max, nominal, middle or min) is indicated by color.

 

[tgwow]

Thanks dgallup, that's a nice tip! I tried it and it worked. Will need to train everyone so they know what's going on though.

 

[powerhound]

Fig 2-18 shows an obvious separation of the tangencies of the 2 arcs that make up the tolerance zone. Although the points of tangency are not specified, an extension of principle based on 1.8.5 and 1.8.6 is totally appropriate in this case. There are lots of implications that can be made that are not specifically stated in the standard and this is one of them. The tangency rule applies not only to actual features but to tolerance zones as well.

There is really nothing more I can add to clarify this any further. I hope this is good enough.

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Inventor 2008
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[tgwow]

powerhound,

Thank you for your responses.

Extending the principle based on 1.8.5 and 1.8.6 does seem appropriate and reasonable as an implication. However, I still think my original premise is still true due to the deficient definition in the standard. The tolerance zone for R is ambiguous as defined in the standard. Therefore, the only practical use of R that I can see is to indicate a break edge.

 

[fcsuper]

tgwow,

Yeah, I can see other uses for R as well, but one of the bigger uses is to break edges.

I've noticed that a lot of people do not understand radius symbols. I had one boss years ago who questioned me every time I used SR (spherical radius) per the standard. The conversation used to go like this:

Boss:"You have a typo here on your drawing."
Me:"That's not a typo. It is a spherical radius."
Boss:"What's a spherical radius?"
Me:"I'm dimensioning this spherical radius with the spherical radius symbol."
Boss:"I've never seen that before" ::looks at me like I'm making things up::
Me:"It's the standard since 1994."

We had this conversation twice for separate revisions the same drawing. Then one day, we was changing the drawing for some other reason and then removed the S from SR on the SR callout.

I didn't throw it in his face. I just simply added the S back on the next revision. He stopped bugging me about it after that. However, this likely drew out so long because we didn't have a copy of ASME Y14.5M in house at the time. (as this for hijacking a thread? )

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

 

[KENAT]

dgallup, I recall you bringing it up but I don’t think all CAD systems can do it quite like that. I don't think ours can although I haven't tried to do it for quite a while. However does seem a good idea.

As to “What does the tolerance zone look like for R2+-2?” based on 2-18 isn’t it something like my attachment, or am I missing something?



KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

YES!!!! This is exactly what I was trying to describe when I said "The tolerance zone for a 2 +/-2 will look like a triangle with one of the legs as a 4 radius." . The contour of the corner can fall anywhere within this zone and still be good if specified as R. A specification of CR will fix this. Whatever the radius is, it has to be smooth with no flat or reversals.

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Inventor 2008
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[tgwow]

Kenat,

The problem with your sketch is that the definition doesn't say use MIN R, tangent lines, and MAX. It just says to use MIN R and MAX R. Additionally, the MAX R arc doesn't have to be tangent to the sides so you can move the MAX R arc towards the corner and tilt it.

Even if we make the assumption that you can use the tangent sides, the MAX R arc doesn't have to be tangent to the sides so you can move the MAX R arc all over the place as long as it intersects the sides. Therefore, the tolerance zone for R is not well defined at all.

 

[KENAT]

tgwow, I'd say 1.8.5 is in effect, no doubt about it and as such my sketch which matches fig 2-18 is correct.

KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

KENAT,
Agreed. Arcs are to be tangent. That is clearly stated in the standard. See Fig. 1-29 which is referenced to in 1.8.5.

Powerhound, GDTP T-0419
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Inventor 2008
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