Straightness tolerance effect on material boundary size.

[JacobCheverie]

Datum C - Cylinder
Datum D - Cylinder

The Position callout that I am interested in is the Position of the left side hole to [C(M)|D(M)] shown at the bottom left of the image.

Datum D has a straightness callout referenced at MMC.
Datum C has a Position callout at MMC to [D(M)]


The boundary size at MMB of datum feature C for DRF [C(M)|D(M)] is just a cylinder at the MMC size of C.
The boundary size at MMB of datum feature D for DRF [C(M)|D(M)] is unclear to me.

Per Y14.5-2009 4.11.6.1 I must calculate the MMB as the size at MMC plus any applicable geometric tolerance to the higher precedence datum so as to not violate datum precedence. In this case, there is no control on D with respect to C. All I have is the straightness tolerance on D, but that callout does not reference C as primary (i.e. it is not necessarily applicable while constrained to C.)

My question is: Do I reference datum D at a custom boundary size which is (Size at MMC + Straightness Tolerance)? Or do I reference it as is seen on the drawing?

I am thinking of measuring to [C(M)|D(M)[Ø.XXX]] where .XXX is (Size at MMC + Straightness).



STRAIGHTNESS.jpg

[louisd]

What is D? the shaft? Your image clip doesn't show it.
MMB is respective to the 'virtual condition' of the cylinder. The virtual condition of the D datum is the maximum material boundary as a perfect cylinder around the entire length and largest diameter of datum D cylinder. This would include any form error (bend/warp/barreling/bowing/hourglassing, etc) that the straightness would partially quantify.

You are obscuring so much here on the clip and post, that i can't help with a more practical example.

Edit 13May2020
- since you entirely edited your initial post-
The Position callout that you are interested in (Position of the left side hole to [C(M)|D(M)] shown at the bottom left of the image).
--Depicted as Datum E on the snip, correct?

I see your concern.
So Datum C is controlling total runout of E to C.
The only thing that Datum D is doing, is creating confusion in this case. Datum C is controlling itself to D effectively.
Since Datum C's FCF is controlling itself to D... Datum E being dependent upon C is all it needs. Adding D as a secondary datum is noise.

One can interpret this multiple ways, there's no clearly communicated method of understanding design intent.
1. Designer doesn't know what he/she is doing, it is an error in design. Datum C is all that's necessary for control of that cylindrical feature. Print needs corrections.
2. Datum C is controlling Total runout, PLUS Datum D is controlling perpendicularity of E axis to D axis (somehow while not conflicting with datum C controls).
3. Datum C is controlling Total runout, PLUS Datum D is controlling perpendicularity of E axis to D axis, AND location in Y axis to Datum D (somehow while not conflicting with datum C controls).

[JacobCheverie]

louisd Datum feature D is the shank, I accidentally covered it up in the image. I will update the image tomorrow. The question is whether the straightness tolerance effects the MMB size even while constrained to C.

[louisd]

read above

[Schlag]

Just remember for simplicity, If you REMOVE the MMB from consideration in the final TOL, you cannot pass bad parts but could possibly reject good parts.

[JacobCheverie]

louisd I don't know where you are getting Total Runout from... I edited the post since yesterday to make things a little more clear because you mentioned it was obscured. The axis of each cylinder lies in a different principal direction. One is along X, one along Y, one along Z. Yes, the Position of interest is applied to datum feature E.

Position of C with respect to D(M) controls Perpendicularity and how far C is off the axis of D.
Position of E with respect to C(M)|D(M) controls centerline distance and how far E is off the axis of D.

Both controls together are ensuring that each end is centered on the shank and the centerline distance is satisfied. My question is just how to compute the MMB of datum feature D at this point when referenced as secondary to C(M).

[JacobCheverie]

louisd I think that you are interpreting the geometry incorrectly. The reference to D is not noise. Without the reference to D, you would have Position of a cylinder to a perpendicular cylinder and effectively only be able to measure Position along one axis. This is a diametral callout. Datum feature E must also lie on D, hence the callout to D as secondary.

[louisd]

If you ahve three cylindrical features that are all perpendicular, your drawing is not following a 3-view standard, and the snip above does not depict this. My apologies for misunderstanding the print. What i see is C and E being coaxial and one being cut to diameter x on one side and the other being cut on the opposing end of the cylindrical axis they share to a different diameter.

[JacobCheverie]

louisd I understand, sorry for the confusion. I didn't know about a 3-view standard, I'm only about 2 years in to all of this. In any event, thanks for letting me know.