Manufacturing guide
Manufacturing Tolerance Guide
How to communicate functional dimensions, datums, fits, geometric relationships, surface roughness, and inspection priorities without over-tolerancing.
Guide introduction
How to use this guide in a real RFQ.
A tolerance defines acceptable variation, but it also influences process selection, setup strategy, tooling, scrap risk, inspection effort, and cost. The most useful tolerances protect function while allowing reasonable variation elsewhere.
This guide explains how to structure drawing requirements for machined, molded, cast, and sheet metal parts. It does not publish universal capability numbers because achievable results depend on geometry, size, material, process, finish, quantity, and measurement method.
Chapter 01
Start from function and assembly
Tolerance decisions should follow how the part locates, moves, seals, aligns, or transfers load.
Identify critical characteristics
Critical characteristics often include fitted diameters, mounting patterns, sealing surfaces, alignment features, bearing seats, optical locations, connector interfaces, and assembly gaps. Mark them clearly and explain any special inspection or documentation requirement.
- Trace each critical dimension to a real function.
- Separate safety or validation requirements from ordinary fit dimensions.
- Avoid labeling every dimension as critical.
Use dimensions and fits consistently
A limit dimension, plus/minus tolerance, or fit designation should be applied consistently with the mating part and material condition. Stack-up analysis belongs at the assembly level, not only on each isolated component.
Control surface roughness where it matters
Roughness may affect sealing, wear, sliding, fatigue, optical contact, or appearance. Specifying it on every surface can add work without supporting function. Identify the relevant face and whether the requirement applies before or after finishing.
Chapter 02
Build a datum system that reflects the real part
Datums create the reference framework for manufacturing and inspection.
Choose stable functional references
Primary, secondary, and tertiary datum features should constrain the part in a repeatable way and resemble how it mounts or locates in the assembly. Small, flexible, rough, or inaccessible features often make poor primary references.
- Use broad stable locating surfaces where the function permits.
- Avoid conflicting datum schemes on related features.
- Confirm that the datum features remain usable after finishing.
Control relationships, not drawing appearance
Position, profile, perpendicularity, parallelism, and runout can express functional relationships more clearly than chains of coordinate dimensions. The selected control should match the feature and the inspection strategy.
Chapter 03
Match tolerance to process and measurement
The same numerical tolerance can represent very different risk on a short machined bore, a large molded wall, or a welded enclosure.
Consider process-specific variation
Machining is influenced by setups, cutter access, material removal, wall stability, and heat. Molding adds shrinkage, flow, cooling, and resin behavior. Sheet metal adds bend and weld variation. Finishing can add or remove material after the primary operation.
Select an inspection method with suitable access
Calipers, micrometers, height gauges, pins, thread gauges, optical systems, and CMMs solve different measurement tasks. A requirement may be difficult to verify if the feature is hidden, flexible, poorly referenced, or tighter than the method can resolve reliably.
Define reporting and sampling
State which characteristics need recorded values, the requested report format, sampling expectation, and any first-article or final-inspection requirement during quotation. Inspection scope should not be inferred after production.
Comparison table
Tolerance planning by manufacturing route
The table highlights dominant sources of variation; actual capability must be reviewed feature by feature.
| Route | Variation drivers | Drawing priority | Inspection focus |
|---|---|---|---|
| CNC machining | Setup relationships, tool reach, wall stability, heat | Datums, fits, feature relationships, finish condition | Critical bores, positions, runout, threads |
| Injection molding | Shrinkage, flow, cooling, ejection, resin | Assembly interfaces, warp-sensitive geometry, cosmetic zones | Fit, profile, repeatability, approved process condition |
| Sheet metal | Bend radius, springback, sequence, welding | Mounting planes, openings, formed and welded assembly | Angle, alignment, diagonals, hardware location |
| Die casting | Tooling, shrinkage, porosity, ejection, secondary cuts | Machined interfaces, sealing areas, draft-sensitive features | Post-machined dimensions and casting condition |
| Prototype processes | Build orientation, master quality, material simulation | Fit, function, appearance purpose | Project-specific comparison to design intent |
Practical design guidance
Actions to take before requesting a quotation.
TIP 01
Tighten only the functional features
Prioritize fits, alignment, sealing, motion, and critical interfaces instead of the entire drawing.
TIP 02
Use stable functional datums
Choose references that locate the real assembly and can be reproduced during manufacturing and inspection.
TIP 03
Account for post-processing
Clarify whether limits apply before or after anodizing, plating, coating, heat, welding, or assembly.
TIP 04
Make inspection part of the RFQ
Define critical characteristics, report expectations, sampling, and measurement constraints before quotation.
Guide FAQ
Questions about tolerances.
Why does Kewei not publish one tolerance for every process?
Capability changes with geometry, size, material, setup, wall condition, finish, quantity, and inspection method. A feature-by-feature review is more useful than a universal claim.
Can a general tolerance block replace all feature callouts?
It can control ordinary dimensions, but functional fits, datums, geometric relationships, threads, roughness, and critical inspection requirements still need explicit definition.
Should dimensions apply before or after surface finishing?
The drawing should state this for finish-sensitive features. Coating, plating, anodizing, blasting, polishing, and masking can affect the final condition.
What should be included in an inspection report request?
Identify the controlled revision, characteristics, limits, units, report format, sampling expectation, and any material or finish documentation required.