Is this correct GD&T? 2

[mopar4u]

here is a copy of the print I am talking about

http://i168.photobucket.com/albums/u179/mopar4u/positionaltolerance.jpg

I have issues with the true postion .002 and the paralellism .005 callout.

With the true position are you supposed to assume that the drafter wants you to find the symmetry plane of the two flats and compare that to the centerline? There are no basics as well.

With the paralellism, are you supposed to assume the drafter wants you to find the symmetry plane of the two flats can compare that to datum C?

Are these two callouts correct?

 

[KENAT]

Just taking a real quick look I'm thinking the parrallellism is unnecesary as the positional is tighter.

However others on this forum are far more experienced so can give a better answer.

Gotta go...

 

[MadMango]

Runout would seem more correct here.

"Art without engineering is dreaming; Engineering without art is calculating."

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[mopar4u]

How would you inspect runout on two flats? I agree, some kind of runout would be the correct callout but inspecting runout on flats???

I would also like to know if the callouts are correct. Unfortunately our drafting team has little to no knowledge in the GD&T area, our quality department has all the GD&T knowledge (which is what I'm in). Before I toot my horn I want to make sure I'm barking up the right tree. The callouts don't seem correct but i'm not 100% sure.

 

[mopar4u]

I'm thinking a symmetry callout would be appropriate?

 

[ctopher]

You are showing parallelism to itself.

Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 01-18-07)

 

[drawoh]

mopar4u,

I agree with ctopher on the parallelism. I would guess that the drafter is trying to control the parallelism of one surface to the other. If the tolerance is +/-.005", this could be out by .010". It is badly drawn.

I have no problems with the symmetry. This is described in ASME Y14.5M-1994 in Section 5-13. The positional tolerance provides a control.

JHG

 

[ringman]

This example has some other problems I believe. The angle and the flat on the bottom are inadequately defined. If he is needing the top surface parallel to C it needs to be handled with a separate callout. Curious to know that the datum feature A is.

 

[Heckler]

The TP callout is okay as long as clocking is not important. Then parallel to itself is wrong. Does that flat need to be a datum? But your total runout is wrong. It should be just to datum -A-. Also, as a side note, total runout would be an expensive shaft. I would just use runout.

One last thing to ponder when it comes to assigning datums to a part is it needs to clearly define design intent

Best Regards,

Heckler
Sr. Mechanical Engineer
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[mopar4u]

The total runout is to be checked using datum A and Datum B of equal importance, hence the A-B in the frame. Datum A and Datum B are our bearing journals.

Heckler, I agree, that parallelism callout is wrong. I guess the more that I look at it, the more symmetry seems to fit what the drafter is looking for. The symmetry callout could replace the true position and the so called parallelism. Like I said, our drafters have very little gd&t knowledge.

thanks for the help everyone, great site.

 

[drawoh]

mopar4u,

Be careful with symmetry. It is not the same thing as parallelism. A surface profile might work too.

I suggest a chat with the designer. You need to know exactly what it is they want. We could be very confused.

JHG

 

[dingy2]

How is this - NO!

Your parallelism of the C/L of the feature references one side of the same feature. Go figure that one. Take the parallelism out for sure.

Positional to datum B is OK. This is just showing symmetry and it is only for the width of the feature so that is OK.

I don't know how you established B but that is another question.

Dave D.

http://www.qmsi.ca

 

[mopar4u]

"How is this - NO!

Your parallelism of the C/L of the feature references one side of the same feature. Go figure that one. Take the parallelism out for sure.

Positional to datum B is OK. This is just showing symmetry and it is only for the width of the feature so that is OK.

I don't know how you established B but that is another question.
Dave D.

http://www.qmsi.ca"

So in this case the true position already includes symmetry, right?

drawoh
Point well taken

 

[dingy2]

Positional in this situation does reflect symmetry. You are correct there.


Dave D.

http://www.qmsi.ca

 

[drawoh]

mopar4u,

There are two possible issues with symmetry.

1. Your nominal geometry is symmetric.

2. You need symmetry. It is much more important than the other conditions you can control through GD&T.

I think the second condition is unlikely.

Let's try to figure out what this thing does that requires all those accurate tolerances.

Example:

Your shaft extends through a pair of accurately centred bearings and the flat section engages a slot. We want to minimize backlash in the mechanism by mechanical tolerances.

Your dimensional problem with the flats is to keep them within the MMC that will fit into the slot, but to keep them as close as possible to the slot's MMC to minimize play.

Use profile tolerances. You can show that your nominal geometry is at MMC.

The resulting conforming part will be close to symmetric, but you really do not care.

Example:

Wrenching flats.

Who locates wrenches accurately?

Specify limit dimensions with MMC being less than the MMC of the wrench. Apply a true position tolerance to the dimension to centre it. Unless this thing rotates at high speed, the true position can be sloppy.

Symmetry and parallelism are controlled, adequately.

You cannot apply dimensions and tolerances to something without understanding what it does.

JHG

 

[ringman]

This problem is not unlike some of the examples in the Y14.5 standard, inasmuch as they do not represent the total definition of the part nor its interface component. Without those, it is IMPOSSIBLE to provide a solution.

 

[tk369]

kenat is correct.