Datum feature in line
Datum feature in line
(OP)
Please see attached. When the datum feature symbol is in line with a toleranced feature, does the bonus apply to positional when mmc is tagged
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RE: Datum feature in line
When the circle M is in the datum portion of the feature control frame, it's not called bonus tolerance, it's called datum shift. Your question mentions bonus but the drawing shows the M in the datum portion.
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum feature in line
RE: Datum feature in line
It is permissible to say that the effective positional error between the two actual center planes will appear to have a .017 variation, but as Powerhound stated the position tolerance zone itself has not grown. Rather, the tolerance zone is still .012 but it's now allowed to shift around (because the item it's trying to position to has a slop of .005).
So again, your logic is on track, but in GD&T terminology we just have to be careful about how we say it.
It should also be noted that the logic presented here only works in simple cases where there's a single datum feature and it's acting in the same direction as the toleranced feature. If the notch were perpendicular to datum A, then the extra "datum shift" of .005 that we're talking about is not linearly additive.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Datum feature in line
To clear up one of your statements:
"the "A" datum is the center of the 1.500 regardless of its size."
No, since you added the MMC modifier, datum A is the center of a 1.505 wide boundary since the MMC of datum feature A is 1.505. Had you not added the modifier then your statement would be true.
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datum feature in line
I think that we have had this discussion before though.
Dave D.
www.qmsi.ca
RE: Datum feature in line
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Datum feature in line
1. What is datum A?
As it was already mentioned, presence of (M) modifier after datum letter A in positional feature control frame causes that datum A is the center plane of 1.505 wide boundary.
2. What is size of positional tolerance zone and how is the zone located/oriented to datum A?
The tolerance zone is two parallel planes spaced 0.12 apart and perfectly centered on datum center plane. The size of the tolerance zone does not change (it is always 0.12), the tolerance zone cannot move or shift relative to datum A.
3. So what is actually allowed to shift?
It is the datum feature A (and in consequence the feature being positionally controlled) that can shift. If the size of datum feature is smaller than 1.505 (but greater than or equal to 1.495), and some initial results of positional callout check show that .500+/-.005 feature is out of its allowable position, the part can be adjusted (shifted) inside 1.505 boundary in order to make .500+/-.005 feature better fitting inside the positional tolerance zone. Example: if the size of datum feature A is 1.498, the part can be shifted up to .007 (1.505-1.498) to bring the toleranced feature back inside its positional tolerance zone.
As for logical application of this callout, imagine that OP's part must assemble with a slot of 1.505-1.515 width, and the .500+/-.005 groove must be centered to the slot and not to the width of OP's part. Wouldn't the callout be a way to go?