Agreeing on Critical Tolerances and Inspection
Connect functional dimensions with measurement methods and acceptance records.
TOLERANCE & VALUE
Precision where it matters. Budget well spent.
Tighter tolerances can be requested. They usually increase machining and inspection costs. Specify them for functional features; use agreed general tolerances for the rest.
01 / SEE THE DIFFERENCE
A 10 mm dimension. How much room is left?
Nominal size：10 mm (≈ 0.3937 in)
Choose a tolerance to compare the permitted size range.
PERMITTED SIZE RANGE
≈ 0.3933–0.3941 in
Total tolerance width 0.020 mm (≈ 0.0008 in)
For this 10 mm (≈ 0.3937 in) example, ±0.002 mm (≈ ±0.0001 in) gives a band 35× narrower than the general ±0.07 mm (≈ ±0.0028 in). This is a size comparison, not a price ratio.
02 / START WITH GENERAL TOLERANCES
A practical starting point for general dimensions.
An example for dimensions without an individual tolerance, when adopted on the drawing.
| Nominal size (mm) | Tolerance (mm) |
|---|---|
| 1–4 mm≈ 0.0394–0.1575 in | ±0.05 mm≈ ±0.0020 in |
| 4–16 mm≈ 0.1575–0.6299 in | ±0.07 mm≈ ±0.0028 in |
| 16–63 mm≈ 0.6299–2.4803 in | ±0.1 mm≈ ±0.0039 in |
| 63–250 mm≈ 2.4803–9.8425 in | ±0.2 mm≈ ±0.0079 in |
This is a reference table, not a universal standard. Agree on its scope and shared range boundaries on the drawing. Individually specified tolerances take precedence.
1.0000 in = 25.4 mm exactly. Inch values are rounded to four decimal places. Acceptance follows the original units specified on the drawing.
Why does tighter cost more?
More careful setups, tool-wear and temperature control, additional measurement, and sometimes extra finishing operations. The actual cost depends on the part and quantity.
Tighten for function
Fits, locating diameters and sealing interfaces: define the tolerance, datum and inspection method together.
Use general where it works
Clearance lengths and non-mating profiles can use agreed general tolerances when function allows.
Agree before quoting
Mark critical dimensions on your drawing. We review the manufacturing and inspection costs with you; drawing changes require your approval.
Pay for the precision your part needs.
Swipe the tables below to see the complete metric and imperial dimensions.
A Tolerance Is Only Part of an Acceptance Plan
A dimension states an allowable result. An inspection plan explains which characteristic is checked, how it is measured, on which parts and in what condition. Aligning both before production helps avoid a machined part being accepted by one method and rejected by another.
This is an engineering planning guide, not a measured capability study. It does not assign a blanket tolerance to every machine, material or geometry.
See the Difference: A Smaller Tolerance Means a Narrower Window
A tolerance defines how far a dimension may depart from its nominal value. ±0.01 mm (≈±0.0004 in) permits a deviation of 0.01 mm (≈0.0004 in) in either direction, giving a 0.02 mm (≈0.0008 in) total acceptance window. At ±0.002 mm (≈±0.0001 in), that window narrows to 0.004 mm (≈0.0002 in). Distinguish the permitted deviation in each direction from the full width of the acceptance window.
For specified critical dimensions in Swiss turning, special tolerance requirements in the range of ±0.01 to ±0.002 mm (≈±0.0004 to ±0.0001 in) can be discussed. Feasibility and a suitable acceptance method depend on the drawing, material, diameter and length-to-diameter ratio, geometry, surface treatment, quantity and measurement method. This range concerns individual dimensions under review; it is not a blanket commitment for an entire part or every feature.
For a nominal 10 mm (≈0.3937 in) dimension, compare three requirements:
| Tolerance requirement (mm / in) | Accepted dimension range (mm / in) | Total window width (mm / in) |
|---|---|---|
| General tolerance reference ±0.07 mm ≈±0.0028 in |
9.93–10.07 mm ≈0.3909–0.3965 in |
0.14 mm ≈0.0055 in |
| Special tolerance ±0.01 mm ≈±0.0004 in |
9.990–10.010 mm ≈0.3933–0.3941 in |
0.020 mm ≈0.0008 in |
| Special tolerance ±0.002 mm ≈±0.0001 in |
9.998–10.002 mm ≈0.3936–0.3938 in |
0.004 mm ≈0.0002 in |
Inch references are calculated from the original metric values using the exact conversion factor 1.0000 in = 25.4 mm, then rounded to four decimal places using the fifth decimal digit. These conversions are references; acceptance follows the drawing's specified original units.
A narrower window leaves less room for process variation and measurement conditions, usually requiring more process control and inspection effort. This comparison describes dimensional requirements, not price multipliers or guaranteed process capability.
Use General Tolerances for Noncritical Dimensions
General tolerances provide a shared acceptance basis for dimensions without an individual tolerance, provided the drawing or order explicitly defines the applicable rules. The following is the general tolerance reference table used for this discussion:
| Dimension range (mm / in) | Permitted deviation (mm / in) |
|---|---|
| 1–4 mm ≈0.0394–0.1575 in |
±0.05 mm ≈±0.0020 in |
| 4–16 mm ≈0.1575–0.6299 in |
±0.07 mm ≈±0.0028 in |
| 16–63 mm ≈0.6299–2.4803 in |
±0.1 mm ≈±0.0039 in |
| 63–250 mm ≈2.4803–9.8425 in |
±0.2 mm ≈±0.0079 in |
These values come from the supplied reference table and are not a declaration of an ISO or other international standard. The ranges retain the original notation. The drawing agreement must resolve which range includes boundary values such as 4, 16 and 63 mm (≈0.1575, 0.6299 and 2.4803 in respectively), as well as dimensions outside the table and the features it covers. An individually toleranced dimension follows its stated requirement and the drawing's applicable rules. Dimensional tolerances do not replace position, runout, form or surface-roughness requirements.
Tighten Where Function Requires It and Spend Where It Matters
Tolerances can be requested to suit the application, but tighter requirements usually increase cost. Additional effort may include slower machining cycles, closer tool and workholding control, compensation and temperature management, more frequent measurement and greater scrap risk. The actual effect must be assessed alongside material, geometry, quantity and process.
Start with the function of each dimension. Tighten critical features such as mating diameters, locating holes and sealing interfaces to the level the function requires. For general lengths or clearance dimensions that do not affect fit, location or sealing, evaluate the agreed general tolerances. Any relaxation of an existing requirement still needs design approval; the manufacturer cannot make that decision alone.
When requesting a quote, mark critical features, identify mating parts and explain the functional purpose. State the general tolerances for the remaining dimensions. This directs manufacturing and inspection effort toward the features that determine performance, so the budget is spent where it matters.
Identify Functional Characteristics First
Separate assembly fits, datum relationships, sealing surfaces, running surfaces and appearance requirements from general dimensions. Retain the drawing's governing convention and resolve unclear datum references with the designer. A diameter measurement alone does not establish position, runout or every form requirement.
Number the reportable characteristics on a controlled drawing when a ballooned report is required. Agree which need actual measured values and which can use a specified pass/fail check. Never relax a requirement through an inspection shortcut without buyer approval.
Match Method to Characteristic
Dimensional hand tools, gauges, vision measurement, CMM, roughness and form instruments serve different measurement tasks. Mitutoyo's equipment-selection documentation distinguishes these categories. Choose a method for the characteristic and accessible geometry, not merely because a factory lists an impressive instrument name.
For a small visible profile, optical measurement may be worth evaluating; a hidden bore needs another access strategy. For 3D datum relationships, confirm alignment and probing. Thread gauging must use the correct standard and class. Surface texture needs its specified parameter and evaluation conditions, not just a photograph of a polished surface.
Record the Conditions That Affect Agreement
Confirm instrument suitability and calibration status, measurement location, part restraint, datum simulation and the material/treatment state. Identify whether inspection takes place before coating, after coating or after final grinding. For flexible or slender parts, agree how the part is held during measurement.
Define the report format before ordering. A useful report connects part number, revision, lot, characteristic number, requirement, actual result, method and disposition. These are suggested planning fields, not a mandatory report standard.
Separate First Article, Sampling and Full Inspection
First-article reporting establishes the agreed sample evidence. In-process checks monitor the agreed production characteristics. Outgoing sampling covers the agreed shipped-lot checks. None automatically means that every dimension on every part has been inspected.
If full-dimensional first article, 100% inspection, a specific sampling plan, PPAP or a capability study is required, declare the exact scope at inquiry. The selected partner, cost and available records must be confirmed before acceptance; no certification or study is implied by offering machined parts.
Changes and Nonconformance
Revisit the plan when a revision, material, treatment, process or inspection method changes. If a result is outside the agreed requirement, identify the lot, contain affected parts and agree disposition. A concession belongs to the buyer's approval process, not an unrecorded supplier decision.
See quality requirements and delivery for ElixirFab's coordination scope and prepare an RFQ before sharing your drawing.
Technical Reference
Mitutoyo: Measurement equipment selection explains general measurement categories. It is not evidence that ElixirFab or a partner owns those instruments.