True positon of a group of holes
True positon of a group of holes
(OP)
I have a customer drawing that was created by a group in Detroit. They did the GD&T. But of course the customer SQA person is in Mexico. They have a different interpretation of how a gage should be built for this - I am caught in the middle although I agree with the Detroit Engineer. Could I be wrong?
Here is a section of the part drawing
http://files.engineering.com/getfile.aspx?folder=4...
The control frame for true positon has two lines in the callout. The engineer and I interpret this control frame to mean that if we use gages to measure this (as opposed to CMM) we need 2 gages.
The first gage would have a pin at the mmc size of datum A 12.065 sticking out of a plate (DATUM B) Then around that pin would be 6 other pins spaced 60 degrees apart at a bolt circle of 40 mm. These 6 pins all have a size of 5.1-.05 (hole size tolerance) - .2 (true positon tolerance)= 5.85 mm. The part must fit over this gage into all holes simultaneously.
The second gage would be similar except there is no datum A pin and the 6 pins have a size of 5.1-.05 (size tolerance) -.05 (true positon tolerance)= 5.0 mm.
The Mexican SQA says it is only a single gage similar to the first one I mentioned except the size of the 6 pins is 5.10 - .05 (size tolerance) - .2 (first position tolerance) - .05 (2nd position tolerance) = 4.8 mm diameter pins.
Which is correct ?
On another note: Does the 6x marking automatically tell you to combine all 6 holes into the measurement simultaneously? one other person suggested that the first line of the control frame means that you build a gage with the datum A pin at 12.065 and only a single pin at 5.85. Then you test each of the 6 holes individually. My assumption is that the 6x implies that the holes are to be treated as one group. Is this true or just an assumption?
Here is a section of the part drawing
http://files.engineering.com/getfile.aspx?folder=4...
The control frame for true positon has two lines in the callout. The engineer and I interpret this control frame to mean that if we use gages to measure this (as opposed to CMM) we need 2 gages.
The first gage would have a pin at the mmc size of datum A 12.065 sticking out of a plate (DATUM B) Then around that pin would be 6 other pins spaced 60 degrees apart at a bolt circle of 40 mm. These 6 pins all have a size of 5.1-.05 (hole size tolerance) - .2 (true positon tolerance)= 5.85 mm. The part must fit over this gage into all holes simultaneously.
The second gage would be similar except there is no datum A pin and the 6 pins have a size of 5.1-.05 (size tolerance) -.05 (true positon tolerance)= 5.0 mm.
The Mexican SQA says it is only a single gage similar to the first one I mentioned except the size of the 6 pins is 5.10 - .05 (size tolerance) - .2 (first position tolerance) - .05 (2nd position tolerance) = 4.8 mm diameter pins.
Which is correct ?
On another note: Does the 6x marking automatically tell you to combine all 6 holes into the measurement simultaneously? one other person suggested that the first line of the control frame means that you build a gage with the datum A pin at 12.065 and only a single pin at 5.85. Then you test each of the 6 holes individually. My assumption is that the 6x implies that the holes are to be treated as one group. Is this true or just an assumption?





RE: True positon of a group of holes
The rule of simultaneous requirements applies regardless of the 6X notation. Even if every hole had its own control frame, all holes must still conform simultaneously unless it is specifically noted otherwise.
RE: True positon of a group of holes
"I believe it is frowned upon to solely control perpendicularity of a hole relative to a planar datum feature by referencing that single datum."
If you are referring to the lower box, the intent is not to control perpendicularity. The intent is to control the relationship of all 6 holes to eachother and datum B without tying them to datum A
"The rule of simultaneous requirements applies regardless of the 6X notation. Even if every hole had its own control frame, all holes must still conform simultaneously unless it is specifically noted otherwise."
Is this stated in Y14.5 somewhere?
RE: True positon of a group of holes
Section 4.19 of Y14.5-2009 says:
"A simultaneous requirement applies to position and profile tolerances that are located by basic dimensions, related to common datum features referenced in the same order or precedence at the same boundary conditions."
As a counterexample to the notion that the "X" is the indicator for a simultaneous requirement...What if I had 6X holes at one size, and 6X holes at another size, all needing simultaneous control? A more pointed example is in Fig. 4-39 of the 2009 standard, where it's a bunch of holes and a profile under simultaneous control.
RE: True positon of a group of holes
The numbers from Detroit are correct, however I doubt that the datum features precedence in the upper positional callout is trully functional for this geometry. I would think B should be primary and A secondary.
You are correct that the lower feature control frame controls not only perpendicularity to datum plane B, but also spacing within the pattern of 6 holes.
As for where it is stated that: "The rule of simultaneous requirements applies regardless of the 6X notation. Even if every hole had its own control frame, all holes must still conform simultaneously unless it is specifically noted otherwise." It is explained in para. 4.19 of Y14.5-2009 (page 76).
With the '6X' notation the holes are treated as pattern (according to the patter definition given in para. 1.3.42), thus 6 pins must be used simultaneously, not one pin individually for each hole.
RE: True positon of a group of holes
Am I mistaken about the single position symbol in a common first cell of each segment in a composite positional tolerance?
I believe the datums are referenced in a way that breaks the rules of composite tolerancing, too. What am I missing here?
RE: True positon of a group of holes
Thank you for enlightening me
I am glad my perception was not that far off of what it really meant. Now to convince a Mexican - He doesn't want to pay for 2 gages - he thinks it is possible to make this in one gage.
RE: True positon of a group of holes
RE: True positon of a group of holes
It is technically doable with only one gage. The gage simply has to have a removable pin simulating datum feature A. For more details on gages and fixture design you may want to take a look to ASME Y14.43.
RE: True positon of a group of holes
My apologies to to you, Spurs, for my screwball assumption.
RE: True positon of a group of holes
RE: True positon of a group of holes
I agree with pmarc that the datum feature references for the upper positional tolerance are probably backwards functionally. However, unless the orientation error between datum features A and B is fairly large, I don't think it will make much of a difference for the position tolerance. For instance, 20 mm * (1 - cos(1°)) = 0.003 mm or so.
If you're willing to make some assumptions about the orientation errors of the holes, and verify as appropriate, then I think you could accomplish a lot with a single gage as follows:
Flat surface to simulate datum B.
6X pins at (5.10 - 0.05 - 0.05) = 5.00 diameter.
1X pin at (12.075 - 0.010 - 0.200) = 11.865 diameter.
I haven't fully thought it through, but I think this gage would be exactly correct if the holes all have perfect perpendicularity, and would generally become more conservative (reject more good parts, not accept bad ones) as orientation errors increase. Pin diameters could perhaps be adjusted slightly to account for any errors inherent in this approach.
A drawing change could of course make this gage completely correct, and may represent the functional requirements just as well.
- pylfrm
RE: True positon of a group of holes
Other than that the precedence is probably wrong. It says to fix the orientation of the part with a fixed diameter LMC pin that corresponds to the MMC center hole and then let some datum simulator plane perpendicular to the A simulator touch B at some point. With +/-.020 there is a 1/4 degree orientation when the hole is at the maximum size. The Axes of the referencing holes are projected from that plane, parallel to the A simulator/perpendicular to the B simulator, which means those hole true positions can wander a lot from where they are expected to be.
The Mexican guy is completely wrong.
Whoever is in Detroit should probably put down the pencil or check the drawings more carefully before they go out.
RE: True positon of a group of holes
1X pin at (12.075 - 0.010 - 0.200 + 0.050) = 11.915 diameter
- pylfrm
RE: True positon of a group of holes
RE: True positon of a group of holes
Gage 1: Check PLTZF: size: central pin 12.065. and size of six pins 4.85 each
Gage 2: check FRTZF (if composite used) size of six pins 5.00 (no central pin needed)
Gage 1 and gage 2 could be combined (as pmarc suggested) in one set of gage assembly with replaceable pins depending which callout is to be verified. Central pin shall be movable along with the six pins to verify the pattern of the six holes position.
RE: True positon of a group of holes
My suggestion would be correct if upper tolerance were removed from the 6X holes, and the the center hole were toleranced with position diameter 0.15 (M) to datum feature B, right? What I'm getting at is that these are actually very similar tolerance schemes, especially if the holes are accurately perpendicular to datum feature B. Both schemes allow 0.2 relative displacement between the 6X holes and the center hole.
No gage is perfect (due to gage tolerances), so I think abandoning theoretical perfection as well can be justified in certain circumstances.
Have I missed something here?
- pylfrm
RE: True positon of a group of holes
You could design one gage to check both tolerances. The concept is to imagine a single FCF that would covers the requirements of both the FCF's on the drawing. This could be done with POS|dia .05|A(M)|B|.
The gage would have a 12.065 pin for datum feature A, and six 5.00 pins for the hole pattern. If the part fits on this gage, then it conforms to both FCF's.
This approach is obviously very conservative and could reject a lot of conforming parts. But that is the price of simplicity - the requirements become tighter. With GD&T there is usually a trade-off between tolerance and complexity - to allow more tolerance, the specification (or inspection method) must be more complex.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: True positon of a group of holes
RE: True positon of a group of holes
That is the designer’s job and the OP’s (in general) are not aware how the part functions in the assembly. There are exception to this too.
RE: True positon of a group of holes
Thanks for the insight. I am new to the forum and really appreciate the "etiquette" response. With this post, it just seemed to me like there we a lot of responses that were based on what existing with the design "might be" - like reversing the datum sequence in one of the first posts.