What Is the Standard Tolerance for Machined Parts? A Professional Guide to ISO 2768 and ISO 286 for Bronze Components

There is no single fixed “standard tolerance”—such as ±0.1 mm—that applies to every machined dimension. For bronze components such as bushings, slide blocks, and thrust washers, the practical standard is a two-tier system that combines ISO 2768 (general tolerances for unspecified dimensions) with ISO 286 (limits and fits for mating surfaces). This guide explains both standards, gives the tolerance tables engineers use, and shows how to apply them by component type.

1. Quick Answer: The “Two-System” Rule for Bronze Tolerances

Definition: The two-system rule assigns every dimension to either a general-tolerance standard or a fit-based standard, depending on whether the dimension affects function or assembly.

2. ISO 2768 — General Tolerances for Unspecified Dimensions

Definition: ISO 2768 (ISO 2768-1:1989 for linear/angular, ISO 2768-2:1989 for geometrical) defines default tolerances for dimensions that carry no individual ± callout, removing ambiguity between shops. When the title block states General Tolerance: ISO 2768-m, it acts as a safety net for every untoleranced dimension.

ISO 2768-1 Linear Dimensions — Class m (Medium)

Apply ISO 2768-m to: bushing lengths, non-functional outer footprints of slide blocks, chamfers, and mounting holes.

3. ISO 286 — Limits and Fits for Mating Surfaces

Definition: ISO 286 (ISO 286-1 and ISO 286-2) defines letter–number fit classes such as H7 and g6 that set the permitted size range for mating parts, so they assemble with the intended clearance, transition, or interference.

4. Tolerance Strategy by Component Type

A. Bronze Bushings (Sleeves)

• Inner Diameter (ID): Specify an ISO 286 fit, e.g. φ50 H7. Do not write only ±0.02, because that value alone does not state the fit intent (sliding vs. fixed).
• Outer Diameter (OD): Usually a press fit. In the basic-hole system the housing bore is machined to H7 and the bushing OD to p6 (or s6) to create the required interference.
• ID collapse after pressing: Pressing a bushing into a housing typically shrinks the ID by roughly 0.01–0.04 mm for a 1.5–2.5 mm wall. Finish-bore after installation when a tight ID is required, or compensate in the pre-fit sizing.

B. Bronze Slide Blocks

• Functional surfaces: Control geometry, not just size. Specify Parallelism and Flatness (GD&T) to prevent the block from binding or sticking during movement.
• Mounting holes: May follow ISO 2768-m.

C. Bronze Thrust Washers

• Thickness (t): Controls axial end play and is often held tighter than ISO 2768, commonly ±0.02 mm. Set this value against the assembly’s axial clearance budget rather than in isolation.
• Surface finish: Control Ra to reduce wear; the required value depends on load and speed.

5. High-Temperature & Self-Lubricating (Graphite-Impregnated) Bronze

This is where the generic H7/g6 guidance needs refinement for the service conditions your components face:

• Thermal expansion: At elevated temperature the shaft expands and the effective clearance shrinks. Verify that the H7/g6 clearance stays positive at the maximum operating temperature before finalizing the fit.
• Solid-lubricant (graphite) bushings: In dusty or high-temperature service, F7/g6 is often preferred over H7/g6; the larger bore tolerance reduces lubricant-film shear and debris jamming.
• OD interference by housing material: cast iron / steel → m6; thin-wall steel → r6; aluminum → p6 (or m6 with retaining compound) to limit hoop stress in the softer material.

6. Common Tolerance Mistakes to Avoid

1. Over-tolerancing: Applying ±0.05 mm to every feature raises cost with no functional gain. A common machining rule of thumb is that halving a tolerance roughly doubles the cost.
2. k6 on the bushing OD: A transition fit such as k6 can allow the bushing to spin under load; an interference fit (m6/r6 on the OD) is usually safer for the housing interface.
3. Ignoring ID collapse: Sizing the bore without accounting for press-in shrinkage produces a final ID that is too small.
4. Missing fit intent: Writing only ±0.02 without stating H7/g6 leaves the assembly behavior undefined.

7. FAQ — Standard Tolerance for Machined Parts

Q: What is the default tolerance for CNC machined parts?

Most CNC shops operate at ISO 2768-m (Medium) as a default. This typically means ±0.1 mm for small dimensions and up to ±0.5 mm for larger ones. Dimensions that affect fit should receive an explicit ISO 286 fit.

Q: Why use H7/g6 for bronze bushings?

H7/g6 is a widely used sliding (clearance) fit: the H7 hole is machined slightly larger than the g6 shaft, guaranteeing a positive gap. That gap allows for thermal expansion and the formation of the lubricating oil film between a rotating shaft and a bronze bushing.

Q: Can I just use ±0.05 mm for everything?

You can, but it is usually costly. Tightening tolerances where they are not needed increases machining time and tool wear, driving up part cost. Reserve tight values for the few features that drive fit and function.

8. Summary

By combining ISO 2768 for unspecified dimensions with ISO 286 fits for mating surfaces, you keep machined bronze parts functional, interchangeable, and cost-appropriate. State the fit intent explicitly (H7/g6, H7/p6), account for ID collapse after pressing, and refine the fit for high-temperature or self-lubricating service.