Position of Flats to Cylinder

[[To...]]

Imagine a cylinder parallel to the X axis. This cylinder has 2 flats, top and bottom, sort of like 2 wrench flats. The cylinder is Datum A. The callout on the flats is Position of .002 with MMC to A with MMB. I'm guessing a Symmetry Plane is in order to create a feature of size but I need to take advantage of the MMB on A. Best Fit Bore pattern seems to only work in X and Y, but I'm guessing a secondary alignment can fix that.

Any ideas on how to best attack this?

Thanks, in advance.

[[Jo...]]

When you say you need to take advantage of the MMB modifier, do you mean that the part would fail otherwise or are you just trying to set up the program per the print?

The reason for the question is this: Because Calypso simply won't calculate datum feature shift under certain conditions, even when it's legal (i.e., Profile of a surface) I just inspect parts at RMB. If it passes RMB then it passes MMB. If it fails RMB, then I have more work to do.

[[An...]]

_

[[To...]]

Please sign in to view this quote.

Technically, the question came from our customer, so I don’t really know but I believe it's really only about setting up Calypso per print. I know some of you guys are great for coming up with “work arounds”, so I figured I’d post the question.

[[To...]]

Here is an example of my part.
.
.

[[An...]]

_

[[Aa...]]

Please sign in to view this quote.

I don't understand this. There's no such thing as "double bonus". MMB/MMC on a datum does not give bonus.

I get that you're trying to illustrate that when there is only one controlled feature, or the controlled features are all shifted the same direction, you can add them together with the same effect, but that's not quite accurate, though it appears to be in your 2D illustrations. In situations where the datum features and controlled features have different extents, for example, the befit of the MMB is not necessarily 1:1. (Imagine the cylinders in your illustration went twice as far into the page as the slots. And keep in mind that MMC and MMB cover rotation, as well as translation.)

In Tom's particular case, it appears that his features have the same "depth", so this workaround might be valid.

But I do want to strongly caution against grabbing it as a general principle.

[[To...]]

I am trying to visualize a hard gauge to check this. Please feel free to correct me.

So, if I interpret per ASME Y14.5-2009, Paragraph 4.11.6.1 Determining the Correct Maximum Material Boundary (MMB). Datum A is a external feature of size, the appropriate MMB is the smallest value that will contain the datum feature of size while respecting datum feature precedence.

Does this mean I make a cylindrical gauge at MMC of Datum A or does this gauge capture the Outer Tangential Element of Datum A? This would obviously allow wobble of A, though I question the need to have wobble on the primary datum. I've attached my conceptual gauge.

In Figure 7-56, it talks about changing the position tolerance to 0.000 and adjusting the feature tolerance, though I'm not sure if this applies to my application.

Then I attach 2 flat components perfectly centered to the cylinder at MMC plus the position tolerance?
.
.

[[Aa...]]

🙂 Spot on with the conceptual hard gauge.

You're right that 7-56 doesn't apply to your application. 0 (M) is a design option. You're not designing the toleranced part.

[[To...]]

So, the $64,000 question. How does one make Calypso measure this similar to the gauge?

[[Aa...]]

Think of it as a profile encompassing the cylinder and the flats. The outer boundary is exactly where you drew your conceptual hard gauge. There is no inner boundary.