Process Capability on Basic Dimensions

S

setowouh

#1
Just recently, I was working on a PPAP for a wheel bearing assembly for my automotive customer. There’s a PCD call out and a between bolt distance specification on the bearing assembly drawing. The PCD and bolt to bolt specification are shown as BASIC dimensions. My customer wants me to do a 125 pc capability study on the Wheel PCD and bolt to bolt distance. How would you perfom a capability study on a basic dimension?
 
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bobdoering

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#2
How would you perform a capability study on a basic dimension?
There is no true position or similar GD&T location tolerance?

You can develop a distribution of distances between parts, but you can not calculate a capability index without a specification.

Attaching a sanitized print (only the dimensions in question, no customer info) would be handy.
 

Stijloor

Staff member
Super Moderator
#3
Just recently, I was working on a PPAP for a wheel bearing assembly for my automotive customer. There’s a PCD call out and a between bolt distance specification on the bearing assembly drawing. The PCD and bolt to bolt specification are shown as BASIC dimensions. My customer wants me to do a 125 pc capability study on the Wheel PCD and bolt to bolt distance. How would you perfom a capability study on a basic dimension?
You can't. A basic dimension is a theroretically exact dimension that is used to specify True Positions, True Profiles and Datum Targets.

Look at the positional tolerance call out (located in the feature control frame) and see how much the feature is permitted to vary from its true position. If you have a sketch/drawing for us that could help.

Capability studies on features that are controlled by tolerance of position are extremely challenging and not everyone agrees on how to perform them especially when modifying symbols such as M and L are used.

There are a few threads here at The Cove Forums about capability studies on tolerance of position.

I am sure that some of my Fellow Members here will chime in.

Stijloor.
 
S

setowouh

#4
There is no true position or similar GD&T location tolerance?

You can develop a distribution of distances between parts, but you can not calculate a capability index without a specification.

Attaching a sanitized print (only the dimensions in question, no customer info) would be handy.
Here's the spec.

Bolt Hole True Position: 0.2mm
No of Bolts on the HUB: 5
Distance from Center of bolt to Center of Next Bolt: 67.18 (Basic Dimension)
Pitch Circle Diameter (P.C.D.): 114.3 mm (Basic Dimension)
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
why don't you ask your Customer?
how would they perform a capability study on a basic dimension without tolerances? what are they really looking for?

sometimes we make these things too complicated...
 

Stijloor

Staff member
Super Moderator
#6
Just recently, I was working on a PPAP for a wheel bearing assembly for my automotive customer. There’s a PCD call out and a between bolt distance specification on the bearing assembly drawing. The PCD and bolt to bolt specification are shown as BASIC dimensions. My customer wants me to do a 125 pc capability study on the Wheel PCD and bolt to bolt distance. How would you perfom a capability study on a basic dimension?
Threads on "Tolerance of Position Capability Analysis."

Stijloor.
 

Paul F. Jackson

Quite Involved in Discussions
#7
I would do "process control" analysis of the bolt circle on the X and Y coordinates from one common "processing orientation" for the sample.

It is very important that the control is analyzed on pattern in a common process orientation if there are multiple machines segregate them as well.

You would have to convert your basic (if they are detailed like you say).

(R)_ (A)_ (X)_ (Y)
(57.15)_ (0)_ (0)_ (57.15)
(57.15)_ (72)_ (17.66032)_ (54.35288)
(57.15)_ (144)_ (-46.2353)_ (33.59193)
(57.15)_ (216)_ (-46.2353)_ (-33.59193)
(57.15)_ (288)_ (17.66032)_ (-54.32588)

Then if the data appears to be "in control"... you should balance the rotational devation among the five bolt holes, re-compute your rotated X and y coordinates, then go ahead and figure your diameter of position deviation for each, and finally estimate the conformance capability.

Your position deviation distribution is likely going to be skewed toward zero position deviation so you will have to use a skewed distribution model to better predict the probability of a defect.

BTW... It may be more appropriate to use the BCD acronym "bolt circle diameter" instead of P.C.D.

I doubt that there are variable tolerance modifiers on the wheel studs since the "center hub pilot diameter" and the beveled lug nuts both contend to constrain the wheel location regardless of hole clearances... but if there are you will either have to ignore the MMC tolerance or do considerably more reading on this forum to figure out how to accomplish the analysis.

Either way the attached presentation should help you to understand the rotational balancing.



Paul
 

Attachments

Last edited:
D

David DeLong

#8
Here's the spec.

Bolt Hole True Position: 0.2mm
No of Bolts on the HUB: 5
Distance from Center of bolt to Center of Next Bolt: 67.18 (Basic Dimension)
Pitch Circle Diameter (P.C.D.): 114.3 mm (Basic Dimension)
I just bet that the positional tolerance also has a diametrical tolerance zone since bolts are round. I just can't see positional tolerances on round features of size without the diametrical tolerance zone.

I might even agree with Paul if there is no diametrical tolerance zone but if there is, well, it gets complicated.

I would also suggest that you talk to the customer. I have found that sometimes they don't know what they want or, if they do, how to achieve it.
 

Paul F. Jackson

Quite Involved in Discussions
#9
Dave,

I would assume that the 0.2mm tolerance zone is diameter zone as well.
If it isn't... it most likely should be functionally because the studs are round yada, yada, yada.

I might even agree with Paul if there is no diametrical tolerance zone but if there is, well, it gets complicated.
I would say just the opposite!

If there was not a diameter symbol then it would be complicated or if there were MMC symbols then it get more complicated.

It is not that complicated if there is a diameter as it seems we both assume.

Paul
 
R

rogercheng

#10
I would do "process control" analysis of the bolt circle on the X and Y coordinates from one common "processing orientation" for the sample.

It is very important that the control is analyzed on pattern in a common process orientation if there are multiple machines segregate them as well.

You would have to convert your basic (if they are detailed like you say).

(R)_ (A)_ (X)_ (Y)
(57.15)_ (0)_ (0)_ (57.15)
(57.15)_ (72)_ (17.66032)_ (54.35288)
(57.15)_ (144)_ (-46.2353)_ (33.59193)
(57.15)_ (216)_ (-46.2353)_ (-33.59193)
(57.15)_ (288)_ (17.66032)_ (-54.32588)

Then if the data appears to be "in control"... you should balance the rotational devation among the five bolt holes, re-compute your rotated X and y coordinates, then go ahead and figure your diameter of position deviation for each, and finally estimate the conformance capability.

Your position deviation distribution is likely going to be skewed toward zero position deviation so you will have to use a skewed distribution model to better predict the probability of a defect.

BTW... It may be more appropriate to use the BCD acronym "bolt circle diameter" instead of P.C.D.

I doubt that there are variable tolerance modifiers on the wheel studs since the "center hub pilot diameter" and the beveled lug nuts both contend to constrain the wheel location regardless of hole clearances... but if there are you will either have to ignore the MMC tolerance or do considerably more reading on this forum to figure out how to accomplish the analysis.

Either way the attached presentation should help you to understand the rotational balancing.



Paul
i cannot download this file. pls. let me know how to do
 
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