LMC Bonus

[[Jo...]]

Most of the time a print will have an MMC modifier for bonus tolerance added to position tolerance. As I understand it, if it were a hole, Calypso calculates the diameter based on functional, or max inscribed size, then your bonus is derived from that.

I have a couple of prints that use LMC for a hole which will be machined to final size. The theory would be, any "EXTRA" material over the LMC condition would allow clean up. While that sounds logical, there is no additional consideration of form of that diameter. So How does Calypso calculate that diameter ?

[[Jo...]]

For a little clarification.. When I check the evaluation settings on that diameter it is using LSQ for position and diameter. I believe it should be using Outer Tangential. That is the true limit of the size we are trying to control. This could influence position calculation too, as form will shift fitted diameter.

[[Cl...]]

Are you not using separate evaluations? One for position, one for size?

I always use (as taught by Zeiss) LSQ evaluation for TP, then in my LMC drop-down evaluate accordingly for (ID/OD).

[[Jo...]]

Please sign in to view this quote.

When you select a modifier, Calypso prompts you for a new feature(characteristic) creation to be evaluated for the bonus. When MMC is selected, it will generate another characteristic using Max inscribed evaluation. That I would consider to be correct, however the LMC selection is using LSQ. I can, and probably should change to Outer Tang. LSQ or Max Inscribed would have portions of the feature that would not clean up during machining process.

LSQ for position makes no sense to me. You are trying to simulate a hard gage using functional size, and position.

[[Ri...]]

By default - unless you change your Defaults - it will use LSQ. If you want it to use the functional size (Outer Tangential) that is up to you.

Max Inscribed/Min Circumscribed take form into the equation because they are using the three most extreme points to construct the circle.

Regarding LMC/MMC - that is just nomenclature to let you know where the "bonus tolerance" applies to. In this case, as the part approaches the lower end of the tolerance (or the least material condition) the more bonus tolerance it gets.

[[Br...]]

You should be using Inner Tangential for LMC. The reason we always use Outer Tangential for everything is because by default, all size tolerances (per Y14.5) have an envelope of perfect form at MMC. When we use Outer Tangential, this best reflects what the standard defines as the Unrelated Actual Mating Envelope or UAME. This is necessary for evaluating a position tolerance at MMC or RFS.

With LMC, things get a bit trickier. First, the size tolerance no longer has an envelope of perfect form at MMC; It has an envelope of perfect form at LMC. So a better functional check here is going to be using Inner tangential. Additionally, Y14.5 para 2.8.4 states: "Where a geometric tolerance is applied on an LMC basis, the allowed tolerance is dependent on the unrelated actual minimum material envelope of the considered feature."

So for a position tolerance at LMC, we do not use the UAME, but rather the Unrelated Actual Minimum Material Envelope. To do this will require the Inner Tangential Evaluation.

[[An...]]

See attached.

Contribution_02_03_2020_2a.pdf