1. Tolerance Grades
What is a tolerance grade? According to national standards, tolerance grades are understood as follows: Definition—tolerance grades determine the level of dimensional precision. National standards define 20 grades (IT01, IT0, and IT1 through IT18);
the higher the number, the looser the tolerance, the lower the machining precision, and the lower the manufacturing cost. Application Range—IT01 to IT4 represent ultra-high precision (gauge blocks, standard measuring tools); IT5 to IT7 represent precision fit grades (equipment spindles, precision hole-and-shaft assemblies); general machinery manufacturing typically uses IT6 to IT10, while precision machining employs IT5 or higher. Essence—tolerance is a composite concept involving both the basic dimension and the tolerance grade, not merely an isolated number: a ±0.005 mm tolerance corresponds to IT6 for a 50 mm shaft but IT3 or even IT2 for a 150 mm shaft, resulting in vastly different production costs. Correct Notation—drawings and inquiries must use standardized terminology (e.g., 50 mm IT6) so manufacturers can provide an accurate quote; tolerance grades are the language of precision—specifying the wrong grade leads to incorrect quotes and defective finished products.
2.
Functional Grading
Determine tolerances based on function.
In accordance with industry design principles, tolerances are established based on the core concept of “tolerating the function, not the part”: tight tolerances are applied only to features that directly affect assembly, sealing, motion, or safety, while tolerances for other features are relaxed. Feature classification includes: interference fits (extremely tight, with tolerances of ±0.005 to 0.015 mm and interference measured in microns—excessive looseness causes parts to slip out, while excessive tightness leads to stress cracking during assembly); precision fits (e.g., bearing housings and locating pin holes, ±0.01 mm); general functional surfaces (±0.05 to 0.1 mm); and non-critical contours (untoleranced dimensions). A common mistake is applying tight tolerances to every dimension, which drives up costs without improving functionality; tight tolerances should be reserved for mating surfaces, bearing bores, and functional interfaces. Categorizing by function—where different features on the same part have different tolerances—is the essence of tolerance design.
III.
Fit Classification
Classification based on fit.
Industry standards define fits as follows: Fit classifications—clearance fit (constant clearance; hole tolerance zone lies above the shaft tolerance zone; used for bearings and sliding guideways), transition fit (may result in clearance or interference; used for locating pins and shaft couplings), and interference fit (constant interference; requires press-fitting; used for gears and pulleys under high-load rotation). Determination steps—first, define the assembly relationship (interference, transition, or clearance) → consult manuals for recommended IT grades → verify against the capabilities of available machining equipment → tighten the tolerance by one grade for critical mating surfaces and loosen by one grade for non-critical surfaces → specify the IT grade and fundamental deviation letter for both the hole and shaft on the assembly drawing (e.g., 30H7/f7) → use CMM for the first-off inspection and limit gauges for batch production. Examples—H7/g6 with 0.025 mm clearance (bearings); H7/p6 with 0.01 mm interference. Hole and shaft notation—uppercase letters for holes, lowercase letters for shafts—the nature of the fit determines the position of the tolerance zone;
a combination like H7/f7 represents a standard solution in tolerance design.
IV. Unspecified Tolerances
Unspecified tolerances and their notation.
According to the national standard GB/T 1804, untoleranced dimensions are handled as follows: Definition—dimensions without specified tolerances are subject to “free tolerances” (general tolerances), meaning they are not without tolerance entirely; Standard—linear dimensions without specified tolerances follow the GB/T 1804-2000 “m” (medium) class: 0.5–3 mm (±0.1), 3–6 mm (±0.1), 6–30 mm (±0.2), 30–120 mm (±0.3), and 120–400 mm (±0.5); Directional Convention—negative for outer diameters, positive for inner bores; Classes—f (fine), m (medium), c (coarse), and v (very coarse) (ISO 2768 uses the same four-class system); Notation Method—for mass production, specify the tolerance class symbol (aligned with specialized gauging tools); for small-batch or single-piece production, specify the deviation values (upper deviation at the top right of the basic dimension, lower deviation at the bottom right; deviation figures are one font size smaller; for absolute values, indicate the sign followed by the number); Title Block—general tolerances are listed in the title block as default limits to keep the drawing clean—untoleranced dimensions serve as the design’s default fallback;
specifying “m” class establishes a legally binding range for all unmarked dimensions.
V. Cost and Capability
Manufacturing capabilities and costs.
Based on industry processing capabilities, tolerances and costs generally correspond as follows: Standard CNC machining typically achieves ±0.01 mm for precision features (such as bearing bores, shaft diameters, and mating surfaces); tolerances tighter than ±0.005 mm require specialized processes like grinding, honing, or lapping. The ±0.02 mm level marks the threshold for precision machining (requiring fine milling with shallow cuts and temperature control, 100% inspection with micrometers, and a cost multiplier of 1.3). The ±0.01 mm level requires fine milling combined with grinding or other special processes, 100% inspection via CMM (Coordinate Measuring Machine), and a cost multiplier of 1.8 or higher. Regarding defaults: many machine shops default to ±0.05 mm unless a tighter tolerance is specified. Regarding selection: choose the lowest acceptable grade that meets functional requirements; precision can be increased slightly as a buffer if process conditions allow and cost increases are minimal. Regarding verification: consult the machine shop about equipment capabilities before finalizing design tolerances (not every shop can achieve ±0.01 mm). Costs escalate significantly with each step toward tighter tolerances; define precision based on functional requirements rather than specifying high precision merely for the sake of appearance.
Conclusion
How should tolerances be determined for custom automation components?
The conclusion is to follow a four-step process: classify functions, grade fits, verify equipment capabilities, and establish a default tolerance for unspecified dimensions.
Grades—National Standards (GB) range from IT01 to IT18 (20 levels total): general machinery uses IT6–IT10, precision fits use IT5–IT7, and ultra-high precision uses IT01–IT4. Functionality—Apply tight tolerances only to features affecting assembly, sealing, or operational safety (interference fits: ±0.005 to 0.015 mm; precision fits: ±0.01 mm; general functional surfaces: ±0.05 to 0.1 mm), while relaxing tolerances for other features. Fits—Determine the type (clearance, transition, or interference) → consult manuals to select the IT grade → verify equipment capability → tighten the tolerance by one grade for critical surfaces → specify the fit (e.g., 30H7/f7) → validate via first-article inspection (CMM) and batch gauging. Unspecified tolerances—Apply GB/T1804-m class (e.g., ±0.2 mm for the 6–30 mm range); apply negative deviations for external diameters and positive deviations for internal bores; tolerance classes range from f (precision) and m (medium) to c (coarse) and v (very coarse). Costs—A tolerance of ±0.02 mm increases cost by a factor of 1.3; ±0.01 mm increases it by 1.8 or more;
tolerances tighter than ±0.005 mm require grinding or lapping.
Regarding industry practice, remember one rule when setting tolerances: be lenient with non-critical features but strict with fits—classify specifications based on function to achieve the optimal balance between cost and precision.
