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DATUM / MODIFIER GUIDE

PERFECT FORM AT MMC — RULE #1

For an individual regular feature of size, ASME Rule #1 normally requires the actual surface to remain within a perfect-form boundary at its maximum material size. Local sizes must also remain within the stated limits. No separate envelope modifier is added.

Datum required
No — envelope is not datum-related
MMC / LMC applicable
No — a default size/form requirement
Control type
Default size and form relationship
What it controls
A perfect-form boundary at maximum material size
Typical features
Mating shafts, bores and width features

INTERACTIVE APPLICATION

Rule #1: drawing and interpretation

2D shaft side view, vertical diameter dimension Ø20 ±0.1 with extension lines from the upper and lower cylinder outlinesØ20 ±0.1
DRAWING CALLOUT

Read the annotation

⌀20 ±0.1 gives shaft limits 19.9–20.1. The complete shaft must fit inside a perfect ⌀20.1 cylinder. A bore has the opposite material relationship: its smallest size establishes the maximum-material boundary.

Key points

  • For this external shaft, the largest permitted size defines an ideal cylinder that must enclose the complete feature. Local size limits still apply. The envelope is a size/form requirement, not an axis-location tolerance.
01 / PURPOSE

Why use this annotation?

Prevent a bowed or irregular feature from meeting local size checks while failing to fit its ideal mating boundary.

02 / APPLICATION

Where is it applied?

Shafts, bores and width-type mating features where maximum-material fit matters.

03 / DRAWING

How to apply and read the annotation

2D shaft side view, vertical diameter dimension Ø20 ±0.1 with extension lines from the upper and lower cylinder outlinesØ20 ±0.1
Original schematic. Example values use millimetres; a production drawing needs its complete requirements.

Steps to add it to a drawing

  1. Define the eligible feature size and limits.
  2. Apply the default perfect-form boundary at MMC unless a stated exception or override applies.
  3. Identify the complete extent of the feature.
  4. Check local size and the maximum-material perfect-form boundary.

How to read it

⌀20 ±0.1 gives shaft limits 19.9–20.1. The complete shaft must fit inside a perfect ⌀20.1 cylinder. A bore has the opposite material relationship: its smallest size establishes the maximum-material boundary.

Common mistake

Rule #1 limits form, not feature location or orientation relative to other features. Add the appropriate datum-related geometric controls when those relationships matter.

ASME Y14.5-2018 (R2024)

Rule #1 normally requires perfect form at MMC for an individual regular feature of size. See paragraphs 5.8.1–5.8.3 for applicability, exceptions and the independency override.

ASME Y14.5 edition reference. Examples are learning schematics; use the complete requirements on the drawing.

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