Is this correct GD&T?
Is this correct GD&T?
(OP)
here is a copy of the print I am talking about
h ttp://i168 .photobuck et.com/alb ums/u179/m opar4u/pos itionaltol erance.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?
h
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?





RE: Is this correct GD&T?
However others on this forum are far more experienced so can give a better answer.
Gotta go...
RE: Is this correct GD&T?
"Art without engineering is dreaming; Engineering without art is calculating."
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Is this correct GD&T?
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.
RE: Is this correct GD&T?
RE: Is this correct GD&T?
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 01-18-07)
RE: Is this correct GD&T?
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
RE: Is this correct GD&T?
RE: Is this correct GD&T?
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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RE: Is this correct GD&T?
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.
RE: Is this correct GD&T?
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
RE: Is this correct GD&T?
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.
www.qmsi.ca
RE: Is this correct GD&T?
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.
www.qmsi.ca"
So in this case the true position already includes symmetry, right?
drawoh
Point well taken
RE: Is this correct GD&T?
Dave D.
www.qmsi.ca
RE: Is this correct GD&T?
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
RE: Is this correct GD&T?
RE: Is this correct GD&T?