Perpendicularity of a flange surface to a bore ID datum
Perpendicularity of a flange surface to a bore ID datum
(OP)
I have a very large flange part that has a 0.05mm perpendicularity callout to Datum -A- which is an ID bore diameter that is also very large and only about 21mm in depth. We setup a CMM program to inspect the part and depending on how many inspection points we put on this surface we can get whatever reading we want (Mitutoyo CMM is not equipped with a scanning head). So have a few questions here:
What is the proper number of inspection points and programming method (have PCDMIS software) for checking perpendicularity on a point style head CMM type machine for this type of check?
How do you set up the axis of Datum -A- if it is so short in land to ensure you have the true axis of Datum -A- established to take a good reading of the surface (again what is proper number of inspection points etc.)?
What would be the proper inspection method using a surface plate and indicator to do this open plate layout check and calculate the perpendicularity instead of using a CMM?
Thanks,
DS
What is the proper number of inspection points and programming method (have PCDMIS software) for checking perpendicularity on a point style head CMM type machine for this type of check?
How do you set up the axis of Datum -A- if it is so short in land to ensure you have the true axis of Datum -A- established to take a good reading of the surface (again what is proper number of inspection points etc.)?
What would be the proper inspection method using a surface plate and indicator to do this open plate layout check and calculate the perpendicularity instead of using a CMM?
Thanks,
DS





RE: Perpendicularity of a flange surface to a bore ID datum
What I've described is called total runout, but it's identical to perpendicularity for the flat surface, if I'm picturing your requirement correctly.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Perpendicularity of a flange surface to a bore ID datum
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RE: Perpendicularity of a flange surface to a bore ID datum
I am pretty sure different companies have different policies and take different approaches. In the attachment below you can find an excerpt from "CMM Measurement Strategies" guide by National Physical Laboratory. They, for example, refer to BS7172:1989. They even have special formulas for calculating how many measurements planes and how many point in a particular measurement plane should be considered when probing a cylinder.
http://files.engineering.com/getfile.aspx?folder=7...
If we are talking about IDs, from theory standpoint the datum axis should be the axis of a maximum cylinder than can be inscribed within the probed contour. If the ID is primary datum feature, this maximum inscribed cylinder is unrelated to any datum. If the ID is secondary or tertiary datum feature, the maximum inscribed cylinder must be oriented and/or located relative to higher precedence datums.
This is great question. In 99% of cases I have experienced, such short features (short comparing to the size of part) do not really function as primary datum features. In other words, they do not orient part sufficiently to be named primary datum feature in any geometric callout. Moreover these features do not produce repeatable datums.
Without knowing all the details of the part (its mating relationships in the assembly) it is hard to give good answer, but perhaps it is the face of the flange that should serve as primary datum feature, and the ID should be controlled relative to datum plane derived from that face (through perpendicularity callout). Then the inspection should go quite smoothly.
As for optional inspection methods, what J-P (Belanger) suggested will work provided that the part can be repeatably fixed in the chuck or mandrel. Due to short length of the ID this may be difficult to satisfy, that is why usually in such cases, when runout is verified, a planar feature (perpendicular to the datum axis) is defined as primary datum feature, and the cylinder becomes secondary datum feature. This stabilizes the part and enables repeatable measurements.
RE: Perpendicularity of a flange surface to a bore ID datum
Quote: “If the ID is primary datum feature, this maximum inscribed cylinder is unrelated to any datum.”
Is the maximum inscribed cylinder above the same with unrelated actual mating envelope?
IS there any difference between them?
And one more question: IF perpendicularity is called at MMC then how the bonus is calculated? (something like the difference between the actual mating envelope or the maximum inscribed cylinder –IF they are the same-- and the MMC size, my best guess)
Thank you
RE: Perpendicularity of a flange surface to a bore ID datum
1. There is no difference between the maximum inscribed cylinder and the unrelated actual mating envelope of internal feature of size.
2. Yes, you are correct. When perpendicularity is called out at MMC, bonus tolerance is the difference between size of UAME and the MMC size of a FOS (if we think about internal FOS).
RE: Perpendicularity of a flange surface to a bore ID datum
RE: Perpendicularity of a flange surface to a bore ID datum
By default you cannot subtract form error (profile, flatness) from orientation error (perpendicularity). There is a way to override this - by using tangent plane modifier (T in a circle) after tolerance value in the perpendicularity feature control frame - but as far as I see this is not the kind of callout you are dealing with.
If you have access to Y14.5 standard, check out fig. 6-18 in 2009 version, or fig. 6-43 in 1994 version.
RE: Perpendicularity of a flange surface to a bore ID datum
RE: Perpendicularity of a flange surface to a bore ID datum
There is one more thing I did not show in that attachment:
Note: It is beneficial for the number of points to alternate between odd and even on the circles, viz., in Figure 23 above seven points on the first circle, eight on the second, seven on the third, etc. This choice will enable any lobing effect on the circular cross section to be detected.
Below you can find 2 more pages from the very same document where probing strategy for planes is described:
http://files.engineering.com/getfile.aspx?folder=0...
RE: Perpendicularity of a flange surface to a bore ID datum
Also my part is a circular flange not a square plane (OD is 406mm and ID is 311.61mm so the width of the flange is 47.195mm) The flange has 16 10.5mmdiameter holes on a 387.35mm bolt hole pattern). So the square plane number of points sheet won't work for me. Is there some other number of points sheets that tells you on a flange surface the number of points you can send me?
Thanks,
DS
RE: Perpendicularity of a flange surface to a bore ID datum
Also, this document does not offer anything when it comes to circular surfaces, but does it really have to? The message is following - to achieve more or less uniform distribution of points, divide inspected surface into smaller fields (more or less similar in size and shape) and place at least one point in each field. I think this is doable also in your case, but of course the fields won't be square or rectangular (see another attachment).
http://files.engineering.com/getfile.aspx?folder=5...
RE: Perpendicularity of a flange surface to a bore ID datum
RE: Perpendicularity of a flange surface to a bore ID datum
Nevertheless, a very short bore like that is a terrible primary datum. I am assuming you are not the design authority and cannot change the drawing, so..
The old school way to use that bore as a primary datum is to machine a tight-fitting plug to push into it, and spin that plug on a stem or face machined into it in the same setup.
... while bitching continuously about the designer's skill set, of course.
I am not aware of any property or technique associated with a CMM that can make consistent measurements easier or faster, given such an unfortunate dimensioning scheme.
Mike Halloran
Pembroke Pines, FL, USA
RE: Perpendicularity of a flange surface to a bore ID datum
21* (.05/406) is the amount of axis variation that would entirely consume the perpendicularity tolerance. You'd want to lose no more than 5% -> about .0001 to repeatability error or less. Confirm this with the drawing in hand.
If it is hard to determine if the part passes, then that's unfortunate for the part function in the next assembly as it will likely not act properly there any better than in inspection.
RE: Perpendicularity of a flange surface to a bore ID datum