The Purpose of Tolerances
"What are tolerances for?" — asked suddenly, even experienced engineers sometimes hesitate. The simple answer:
Interchangeability means parts can be swapped between assemblies. Function means the resulting fit provides the intended clearance, interference, or freedom of movement — within specified limits. Tolerances exist to achieve both simultaneously.
The English word tolerance originally meant something like "allowance for imperfection." Because perfectly identical replication is impossible, we allow a defined amount of deviation — that allowance is the tolerance.
A Brief History — The Factory Manager Who Nearly Got Killed
The concept of interchangeability is historically recent. In the 18th century, when rifle and lock parts were hand-filed by craftsmen, it was normal practice to fit each component individually to its mating part. One part was made to fit this specific counterpart — and no other.
The result could be highly functional, but the parts were not interchangeable. Swap a component from one assembly into another and the fit might be too loose, or too tight to assemble at all.
How radical the idea was
When interchangeability was first introduced into manufacturing, it was met with such fierce resistance from craftsmen that — according to historical accounts — the factory manager who attempted to impose it came close to being killed by the workforce. The idea that a part could be made to a specification rather than fitted to a specific counterpart was, at the time, incomprehensible. Custom fitting was simply how manufacturing worked.
Machine tool technology has since made interchangeability an unquestioned baseline. But even today, perfect replication remains impossible — which is precisely why tolerances are still needed.
JIS 2016 Revision — "Dimension" Becomes "Size"
The 2016 JIS revision reorganized terminology substantially. The headline change: many uses of the word "dimension" were replaced by "size."
"Dimension" was not eliminated — it still covers quantities like step heights, radii, and straight-line distances. "Size" is now the specific term for distance-like quantities associated with particular geometrical forms (circles, parallel planes, etc.).
The most visible change: the reference value in a tolerance specification.
In a callout like 50 +0/−0.5, the number "50" has been called
many things across companies and eras:
| Term | Status | Notes |
|---|---|---|
| Basic Dimension | Common in US CAD software | Translation of "Basic Dimension" — also used for boxed dimensions in GD&T (theoretically exact values) |
| Reference Dimension | Non-standard | Internal convention in some companies |
| Nominal Dimension | JIS 1998 (previous standard) | The formally correct term under the old standard |
| Nominal Size (読み: Nomi) | Non-standard | Informal reading of "Nominal Dimension" |
| Nominal Size (呼びサイズ) | JIS 2016 — current standard | From "Nominal" = "in name only." The officially correct current term. |
The 2016 revision also split what was previously called the "tolerance zone" into two distinct concepts:
| New term | Meaning |
|---|---|
| Size tolerance interval | The interval from the lower limit of size to the upper limit of size (a range concept) |
| Tolerance zone | Continues to be used in its original sense within geometric tolerancing |
Tolerance Quiz
Most people instinctively choose ④. But ±0.05 is the deviation (upper deviation = +0.05, lower deviation = −0.05). The tolerance is the difference between those two deviations: (+0.05) − (−0.05) = 0.1.
A common floor mistake
"What's the tolerance on this feature?" — "Plus or minus point-oh-five." Technically, ±0.05 is the deviation; the tolerance is 0.1. In everyday shop conversation this distinction is often blurred, but in formal documentation and geometric tolerancing, it matters.
Deviation, Tolerance, and Size Difference — Illustrated
Fig. 01 — Nominal size, deviation, tolerance, and size difference (shaft example)
For 30 ±0.05: upper deviation = +0.05, lower deviation = −0.05, tolerance = 0.10. The size difference (deviation of the actual measured value from nominal) changes with every measurement.
The English view
In English, both "deviation" (a limit distance) and "size difference" (a measured offset) are called deviation. Upper Limit Deviation, Lower Limit Deviation, and the deviation of an actual measured value are all "deviation" in ISO/ASME terminology. This shared word reflects a genuine conceptual connection that can get lost in translation.
Recent Changes in Drawing Practice
| Item | Previous practice | Current (JIS 2013–2016 onward) |
|---|---|---|
| Diameter symbol (φ) | Required on all features whose cross-section exceeds a semicircle | May be omitted when the feature is clearly circular in the view. Required on leader-line callouts. |
| How to read φ | "Pi" (パイ) — still common among veterans | "Phi" (ファイ) — the JIS-specified pronunciation |
| Tolerance notation | Upper/lower values stacked in half-height text | Same height, stacked two lines; single-line slash notation permitted to save space |
| Arrow terminator | Open 30° arrowhead (outline only) | Filled 30° arrowhead — now the global trend; better visibility |
3D CAD and tolerance notation
As 3D models increasingly define geometry, one-sided tolerance notation (e.g. +0.05/0) has become less common. Symmetric ±0.05 notation and geometric tolerance callouts are now the mainstream — aligning with model-based definition workflows.
MMC and LMC
MMC and LMC appear frequently in geometric tolerancing discussions. The word "material" (as in Maximum Material) comes directly from the English — it means physical material, as in the metal of the part.
Fig. 02 — MMC and LMC: shaft and hole comparison
MMC = tightest fit. LMC = loosest fit. The shaft is obvious; for the hole, remember that a larger hole means less material, so Least Material Condition = largest hole.
Why the hole seems "backwards"
A larger hole means more material has been removed — so a bigger hole = less remaining material = LMC. This is the counter-intuitive part. The cleanest way to remember it: MMC = tight fit, LMC = loose fit. Work from the fit, not from the size.
Summary — Warm-Up Complete
Interchangeability + function. Perfect replication is impossible, so tolerances define the acceptable range of imperfection.
±0.05 is deviation. Tolerance is the difference between upper and lower deviation: 0.10. Commonly confused — even on the shop floor.
"Nominal Dimension" → "Nominal Size" is the key terminology update. The JIS B 0401 old/new comparison table is the reference for mapping old terms to new ones.
Remember by fit tightness: MMC = tightest, LMC = loosest. For holes, larger = less material, so LMC = largest hole.
This article is the warm-up before geometric tolerancing. With the terminology and conceptual foundations in place, each geometric tolerance symbol becomes significantly easier to understand. Next: a closer look at size itself.