True Position with only 1 Datum - Referenced datum is the plane the hole is on

D

David DeLong

#11
Here is what I would do in a bit of detail using a CMM.

There must be 3 holes marked "A" on the drawing although I only saw one.

Pick 1 hole and use it for an origin. Obtain its center.

Go to the second hole and measure its size. Many CMMs will perform this function automatically but if your CMM doesn't, measure it since you will have to calculate the tolerance. From the actual hole size, subtract its virtual condition size which will be - - - (MMC size (4.70) - the tolerance in the feature control frame (0.1) = 4.69. From the actual size subtract 4.69 and you will have a diametrical tolerance zone of more than 0.1 mm. That is the actual tolerance depending upon its size. If one calculated a diametrical tolerance zone of 0.11, as an example, it has a radial tolerance of 0.055 mm.

From the first hole or origin go to the theoretical center using the basic dimensions shown. The actual center should be within the calculated radial tolerance.

Go the the third hole and perform the same operation as the second. One must calculated its actual tolerance.

Now you have confirmed whether or not the 3 holes are within their tolerances except it is better using a checking fixture since we can confirm the position simultaneously.

Please note - I don't know who ever considered this a SC but it certainly is not applicable in this situation.

GD&T is tough at times but I have been training in it for over 20 years.
 
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J

JasonFdz

#12
Dave....... You are the man, Ive been trying to figure this out for days. Is this a really common call out? I can't ever remember seeing it before.
 
Q

qualitytrec

#13
Brings back memories.
Looks like a seatbelt component.
The -A- is the only reference datum called out in the TP because the feature you are measuring is the -B-. Auto sometimes does this, especially on this type of component.
There are three holes that make the -B- datum, I see two in the scan. I would use these (all three) in a best fit setting then check tolerance and position based on the best fit then set the third datum.
If this is the first production run I would use each of the three holes as an origin seperately and check the others just to make it easier to tell which if any need to be corrected.
This will be a fun part to program if you have a good way to hold it. Multiple datums was always fun. I miss programming CMM-but love what I am doing now.

Mark
 
T

True Position

#14
The easy way to do this in Calypso is to take your primary datum plane and the origin, rotating along the origin to bring one of the -B- datum (A) holes to the correct distance from the axis per the drawing then checking the true position of each -B- hole to the plane / origin / rotated alignment.

Who ever drew that really make it far too confusing just by poorly labeling everything. If the holes were labeled s, t, u, v etc instead of the same letters as the datums it would be much more clear. Labeling holes as B as part of datum F is also just adding to the confusion.
 
D

David DeLong

#15
Dave....... You are the man, Ive been trying to figure this out for days. Is this a really common call out? I can't ever remember seeing it before.
It is very common to have only 1 hole as datum B and it should be reflected in the feature control frame with a perpendicularity requirement referencing datum A.

In your case, it is a pattern and it is unusual. Trying to set it up on a CMM is really difficult since you end up creating 1 of the 3 holes as an origin but, in reality, all 3 holes should be checked simultaneously with a checking fixture.

This pattern (datum B) should also be used for anti-rotation so there is no requirement for a tertiary datum.

Hope this info helps.
 
T

True Position

#16
It is very common to have only 1 hole as datum B and it should be reflected in the feature control frame with a perpendicularity requirement referencing datum A.

In your case, it is a pattern and it is unusual. Trying to set it up on a CMM is really difficult since you end up creating 1 of the 3 holes as an origin but, in reality, all 3 holes should be checked simultaneously with a checking fixture.

This pattern (datum B) should also be used for anti-rotation so there is no requirement for a tertiary datum.

Hope this info helps.
From how I was looking at it, datum A(the center gear) is the origin and the pattern(datum B, made up of holes labeled A) is used to set the part rotation. If you pick one of the holes as a set dimension from one of the axis you can then check each of the 3 holes individually to the center feature.
 
D

David DeLong

#17
From how I was looking at it, datum A(the center gear) is the origin and the pattern(datum B, made up of holes labeled A) is used to set the part rotation. If you pick one of the holes as a set dimension from one of the axis you can then check each of the 3 holes individually to the center feature.
Datum A is a plane and also the primary datum. The part during assembly should be mounted on this surface.

Datum B or secondary datum is a pattern of 3 holes labeled "A". It appears that the part must mount on datum A and then 3 fasteners would protrude through the holes simultaneously. The 3 holes must be in position to each other and also perpendicular to datum A. With MMC shown in the feature control frame, the best checking method would be a checking fixture with 3 pins of virtual condition set at true position (theoretical centers). It will be tough to measure on a CMM.

In theory, this situation is called a "feature relating tolerance zone framework" sometimes know as the "FRTZF" which just means the features within the pattern are a concern and not the pattern location.

All dimensions should now come from this 3 hole pattern. It will control both the X and Y locations of all features and not used just for orientation.

If the secondary datum was a single hole, then all dimensions still would come from this datum but we would need another hole or some other feature for orientation or a tertiary datum. In this situation, we have a pattern of 3 holes which also locks the orientation. A tertiary datum is not required.
 
T

True Position

#18
Datum A is a plane and also the primary datum. The part during assembly should be mounted on this surface.

Datum B or secondary datum is a pattern of 3 holes labeled "A". It appears that the part must mount on datum A and then 3 fasteners would protrude through the holes simultaneously. The 3 holes must be in position to each other and also perpendicular to datum A. With MMC shown in the feature control frame, the best checking method would be a checking fixture with 3 pins of virtual condition set at true position (theoretical centers). It will be tough to measure on a CMM.

In theory, this situation is called a "feature relating tolerance zone framework" sometimes know as the "FRTZF" which just means the features within the pattern are a concern and not the pattern location.

All dimensions should now come from this 3 hole pattern. It will control both the X and Y locations of all features and not used just for orientation.

If the secondary datum was a single hole, then all dimensions still would come from this datum but we would need another hole or some other feature for orientation or a tertiary datum. In this situation, we have a pattern of 3 holes which also locks the orientation. A tertiary datum is not required.
I'm sorry, my other monitor's resolution was so low I never saw the 2nd drawing earlier. I agree and that's how I would probably measure it. It's annoying but not impossible to measure if you just set two holes to each other and check the positions of all 3. It would let the pattern float around anywhere but at least would keep them to each other.
 
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