8.4 Zero Positional and Orientation Tolerancing at MMC and at LMC

8.4 Zero Positional and Orientation Tolerancing at MMC and at LMC

Quick Answer: Para. 2.8.3 defines the effect of zero tolerance at MMC: the tolerance becomes totally dependent on the size of the unrelated actual mating envelope of the considered feature. No tolerance of position or orientation is allowed if the feature is produced at its MMC limit of size — at MMC it must be located at true position, or be perfect in orientation, as applicable. Para. 2.8.5 gives the mirror image for zero tolerance at LMC, where the tolerance depends on the actual minimum material envelope. The design payoff is that zero tolerancing converts unusable size tolerance into usable position tolerance without changing the virtual condition, which is why it is the standard treatment for clearance holes whose only requirement is assembly.

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The Problem Zero Tolerancing Solves

Consider a clearance hole for an M8 fastener, sized ⌀8.40 – ⌀8.70 with ⌖ | ⌀0.4 Ⓜ | A | B | C.

• MMC = ⌀8.40, so the virtual condition is $8.40 - 0.40 = \varnothing 8.00$.
• A hole produced at ⌀8.50 earns 0.10 bonus and may be off position by ⌀0.50.
• But now imagine a hole produced at ⌀8.35 — undersize. It is rejected for size, even though its actual mating boundary might sit perfectly at ⌀8.00 and the fastener would assemble without complaint.

The size limit is doing work the function never asked for. Zero tolerancing removes that artificial rejection.

The Zero-at-MMC Conversion

Where a tolerance of position or orientation is applied on a zero tolerance at MMC basis, the tolerance is totally dependent on the size of the unrelated actual mating envelope of the considered feature. No tolerance of position or orientation is allowed if the feature is produced at its MMC limit of size; and in this case, it must be located at true position or be perfect in orientation, as applicable. (Para. 2.8.3)

The conversion is mechanical. Move the geometric tolerance into the size tolerance so the virtual condition is unchanged:

                 ZERO TOLERANCE AT MMC — TOTAL TOLERANCE vs. SIZE

   total position tol
        ⌀0.70 ┤                                              ●  (hole at 8.70)
              │                                        ●
        ⌀0.40 ┤                            ●  (8.40)
              │                      ●
        ⌀0.20 ┤              ●  (8.20)
              │        ●
        ⌀0.00 ┤  ●  (8.00 = MMC: must be at TRUE POSITION)
              └──┬─────┬──────┬────────┬────────┬────────┬──►  actual mating size
                8.00  8.20   8.40     8.55     8.70

Every point on that line has the same inner boundary of ⌀8.00. The gage never changes; only the accept/reject decision at the small end of the size range does.

The Trade the Exam Tests

Zero tolerancing is not free. It transfers the entire burden onto the size dimension:

• At exactly MMC the hole must be perfectly located — an impossible manufacturing target, so in practice the shop must run the process away from MMC.
• The size tolerance appears enormous (⌀8.00 – ⌀8.70), which can alarm a reviewer who does not read the feature control frame.
• It is appropriate only where assembly clearance is the sole requirement. If minimum edge distance, wall thickness, or a bearing fit also matters, the enlarged size range is dangerous — the ⌀8.70 end is still governed by the LMC limit, so the designer must set that limit for function, not for convenience.

Zero Tolerance at LMC (Para. 2.8.5)

Where a tolerance of position or orientation is applied on a zero tolerance at LMC basis, the tolerance is totally dependent on the size of the actual minimum material envelope of the considered feature. No tolerance of position or orientation is allowed if the feature is produced at its LMC limit of size; and in this case, it must be located at true position or be perfect in orientation, as applicable.

Same mechanism, opposite direction. Zero at LMC protects minimum wall thickness or minimum stock: at LMC the feature is at its worst case for material, so it must be perfectly located; as the feature moves toward MMC, material is regained and tolerance is released.

Zero Orientation Tolerance at MMC (Para. 6.4.4)

The orientation section carries its own application of the same idea. Para. 6.4.4, Application of Zero Tolerance at MMC, applies zero perpendicularity, parallelism, or angularity at MMC to a feature of size — most commonly a perpendicularity refinement on a hole already located by position. The interpretation matches para. 2.8.3: at MMC the feature must be perfect in orientation, and tolerance accrues only as the feature departs from MMC.

Trap: candidates read ⌖ | ⌀0 Ⓜ | A | B | C and conclude the drawing demands perfection. It does not. It demands perfection only at MMC, a size the process is expected never to produce. The real requirement is the virtual condition boundary, which is exactly the MMC size itself.