Tolerances are not as tight as possible. Authoritative tolerance marking should simultaneously meet: functional requirements, manufacturing accessibility, and inspection feasibility, and try to avoid imposing unnecessary high-cost constraints on non-critical features.
Example: 20 ±0.05 mm. Suitable for general structural dimensions that are insensitive to deviation direction.
Example: 20 +0.02/0 or 20 0/-0.02. Suitable for dimensions that are sensitive to assembly direction and clearance/interference direction.
Example: 19.98 ~ 20.02 mm. Suitable for key features with high-precision fit or clear requirements for upper and lower limits.
When drawings do not mark dimensional tolerances one by one, general tolerance standards must be used to define the "default allowable deviation". Otherwise, the same drawing may get different understanding and results in different factories.
Write in the title block or technical requirements:"Tolerances not noted are in accordance with ISO 2768-m"(or national standard system), and separate stricter tolerances are marked on key dimensions.
| Linear size segment (mm) | fine f (reference) | medium m (reference) | Coarse c (reference) |
|---|---|---|---|
| 0.5 – 6 | ±0.05 | ±0.10 | ±0.20 |
| >6 – 30 | ±0.10 | ±0.20 | ±0.50 |
| >30 – 120 | ±0.15 | ±0.30 | ±0.80 |
| >120 – 400 | ±0.20 | ±0.50 | ±1.20 |
| >400 – 1000 | ±0.30 | ±0.80 | ±2.00 |
The IT level describes the "width of the tolerance zone", and the coordination code (such as H7/g6) describes the "position of the tolerance zone and assembly relationship". Correct use can significantly improve interchangeability and assembly consistency.
| Sample dimensions | IT6 (reference) | IT7 (reference) | IT8 (reference) | Engineering Tips |
|---|---|---|---|---|
| Ø20 mm | ≈ 0.013 mm | ≈ 0.021 mm | ≈ 0.033 mm | The larger the size, the larger the absolute tolerance value of the same grade is usually. |
| Ø50 mm | (Varies with size segment) | (Varies with size segment) | (Varies with size segment) | It is recommended to use standard tables or enterprise tolerance libraries for unified calculation. |
When part functionality depends on relative positional relationships (coaxiality, positioning, runout, perpendicularity, etc.), GD&T is often more efficient and manufacturable than infinitely tighter dimensional tolerances.
Roughness is not "the brighter, the better", but is strongly related to friction, sealing, fatigue and coating adhesion. It should be defined separately according to functional surface, appearance surface and post-processing plan.
| Roughness | Sight/touch | Typical process | Common applications |
|---|---|---|---|
| Ra 3.2 | Visible knife pattern/texture | Conventional CNC / Conventional molding surface | Structural surface, non-appearance surface |
| Ra 1.6 | More delicate | Finishing / Optimizing Tool Paths | General appearance surface, mating surface |
| Ra 0.8 | smoother | Finishing + light polishing | Seal pre-treatment, sliding surface |
| Ra 0.4 (and lower) | close to mirror | Polishing/mirror finish | Mirror appearance, key surface of transparent parts |
When assembling multiple parts, each dimensional tolerance accumulates along the assembly chain. If superposition analysis is not performed, it is easy for a situation of "single piece to be qualified but assembly to be poor" to occur.
Used in high-reliability, zero-failure tolerance scenarios (such as safety parts, critical seals).
It is used in scenarios where mass production and size distribution are controllable, emphasizing the assembly yield in a statistical sense.
The table below is for "Early Design Assessment and Communication". The actual achievable accuracy is affected by materials, structural rigidity, thermal deformation, clamping and inspection methods; it is recommended to conduct engineering evaluation before quoting for critical dimensions.
| Craftsmanship | ≤100 mm (reference) | 100–500 mm (reference) | ≥500 mm (reference) | Key influencing factors |
|---|---|---|---|---|
| CNC machining | ±0.10 mm | ±0.20 mm | ±0.30 mm or higher | Thin-wall deformation, clamping, temperature rise, tool path and inspection |
| Injection molding | ±0.10–0.30 mm | ±0.20–0.50 mm | Higher (needs evaluation) | Material shrinkage, wall thickness difference, warpage, mold temperature control and parameters |
| Sheet metal fabrication | ±0.10–0.20 mm | ±0.15–0.30 mm | Needs assessment | Bending springback, bending radius, hole margin, positioning datum and unfolding strategy |
| Vacuum casting | ±0.20–0.50 mm | ±0.30–0.80 mm | Not recommended for high accuracy | Material system, mold aging, batch stability and post-processing |
Every time the tolerance is tightened by one level, it not only increases the processing time, but also increases the frequency of clamping, temperature control, tool wear control and inspection, and brings yield risks. Focusing tight tolerances on key features is the best engineering and cost solution.
The following suggestions come from common manufacturing failure and rework scenarios and are suitable for design review of multi-process parts such as CNC, injection molding, sheet metal, and replica molding.
Here we sort out the tolerance issues that engineers, procurement and project teams most frequently confirm before drawing quotations, covering CNC machining tolerances, unindicated tolerances, ISO 2768, GB/T 1804, IT grade, hole-axis fit, GD&T geometric tolerance, surface roughness, inspection reports, cost and delivery time impact.
Conventional CNC machining structure dimensions can usually be ±0.10 mm as a reference for quotation communication. The details also depend on the material, size, clamping method, wall thickness, deep cavity and detection method. Critical holes, locating surfaces, sealing surfaces or assembly dimensions can be evaluated individually for tighter tolerances, but this will increase processing time, inspection costs and delivery risk. It is recommended to adopt a grading strategy of "tightening critical dimensions and applying general tolerances to non-critical dimensions".
If the drawings do not clearly indicate the unindicated tolerances, the supplier may perform according to the company's default rules, ISO 2768, GB/T 1804 or experience standards, which may easily lead to inconsistent understanding of quotation and acceptance. It is recommended to specify in the title bar or technical requirements that "non-note linear dimensions are in accordance with ISO 2768-m" or the corresponding national standard level; key fitting dimensions, datum surfaces, sealing surfaces and functional hole locations still need to be marked with tolerances separately.
ISO 2768-m and GB/T 1804-m are generally suitable for general structural parts, enclosures, brackets, non-precision assembly surfaces and most unindicated dimensions. They are used to define the default allowable deviations and reduce the workload of item-by-item marking on drawings, but are not suitable for replacing key features such as shaft hole fits, positioning pin holes, sealing grooves, and bearing locations. If the customer is not sure of the grade, he or she can first provide the usage and assembly relationship, and the engineer will determine whether local tightening is needed.
Yes. The smaller the IT value, the narrower the tolerance zone and the higher the dimensional accuracy, but the requirements for processing, clamping, temperature control and inspection are also higher. IT6 and IT7 are often used for more precise hole-shaft fit and positioning features, while IT8 and IT9 are more commonly used for general assembly structures. It is recommended to use high precision IT grades only on features that affect interchangeability, movement clearance, sealing or positioning and to avoid over-tightening of the entire drawing.
H7/g6 and H7/h6 belong to the ISO 286 hole shaft matching system. The first letters and numbers define the position and grade of the tolerance zone of the hole, and the following letters and numbers define the position and grade of the tolerance zone of the shaft. The two together determine the clearance fit, transition fit or interference fit. The actual tolerance value will vary with the diameter size segment. It is recommended to provide 2D drawings or clarify the matching purpose before quoting and production.
GD&T is recommended when part functionality depends on relative positional relationships, such as position, coaxiality, flatness, perpendicularity, parallelism, circular runout, and contour. GD&T can express assembly and inspection requirements more accurately than simply tightening linear dimensions, and is especially suitable for positioning hole groups, bearing locations, sealing surfaces, robot joints, automotive functional parts and precision structural parts. When marking, the reference A/B/C and inspection caliber need to be clearly specified at the same time.
meeting. The smaller the Ra value, the smoother the surface, which usually requires finer tool paths, slower feeds, secondary finishing or polishing. Ra 3.2 is commonly used for general structural surfaces, Ra 1.6 can be used for appearance surfaces or partial mating surfaces, and lower roughness is suitable for sealing, sliding or areas with high appearance requirements. It is recommended that only the critical surface be marked with a lower Ra, and the remaining areas be treated as conventional processing surfaces, which is beneficial to controlling costs and delivery times.
Worst Case is more conservative and suitable for safety parts, high-reliability structures, small quantities of assembly, or projects that do not allow failure; RSS is more suitable for scenarios with stable processes, mass production, and an emphasis on statistical yield. When making an assembly tolerance chain, customers should first clarify the functional clearance, reference path and closed-loop dimensions, and then choose a calculation method. In the absence of a tolerance chain analysis, it is recommended that engineers evaluate whether critical dimensions are too tight or too loose before quoting.
Excessively tight tolerances usually mean more clamping corrections, slower machining cycles, tighter tool control, higher inspection frequency and higher risk of scrapping, such as the need for CMM three-dimensional coordinate inspection, first article report or full inspection record. Setting too tight tolerances on non-critical dimensions will directly push up the quotation and lengthen the delivery time. It is more economical to concentrate tight tolerances where they really affect fit, movement, sealing and function.
It is recommended to provide the following information as much as possible so that engineers can more quickly determine tolerance accessibility, detection methods, costs and delivery times:
The more complete the information, the closer tolerance assessment, quotation and delivery time will be to real production results.
After uploading the file, add material, quantity, tolerance, surface treatment and delivery requirements, XProdLab will recommend next steps around manufacturability, cost and delivery path.
