Powder Metallurgy Tolerances After Sizing: What to Expect and How to Design for It

# Powder Metallurgy Tolerances After Sizing: What to Expect and How to Design for It

Powder metallurgy has a reputation as a near-net-shape process, but "near-net" does not mean "as-machined." If you are quoting a PM part for the first time-or reviewing a design that is coming from a machined-part background-understanding what tolerances are actually achievable after sizing is one of the most important things you can do before finalizing your drawings.

This post covers:

• The difference between as-sintered and sized tolerances
• Typical tolerance ranges for common features
• What geometry does and does not respond well to sizing
• Where you still need secondary machining
• Practical guidance for drawing callouts

What Sizing Is

Sizing is a secondary pressing operation performed after sintering. The sintered part is loaded back into a precision die and re-pressed at a controlled load. The goal is not to compact the part further-it is to cold-work the surface and correct dimensional variation introduced during sintering.

Sintering shrinks parts. The shrinkage is predictable at the process level (typically 0.5-2% linear, depending on material and atmosphere), but it is not perfectly uniform. Part geometry, support orientation in the furnace, and thermal gradients all create small but measurable variations. Sizing corrects these variations by forcing the part to conform precisely to the die geometry.

Coining is similar to sizing but applies higher force and is used to improve density in specific areas or to achieve very tight dimensional tolerances, surface finish, or flatness on specific faces.

As-Sintered vs. Sized: The Tolerance Gap

Here is a practical comparison for a typical iron-based PM structural part:

These are representative ranges for common structural alloys. Actual achievable tolerances depend on part geometry, material, density, sizing force, and tooling condition. All values should be treated as illustrative and verified with your supplier for the final design.

What Responds Well to Sizing

Not every dimension improves equally with sizing. The features that respond best are those that are directly in contact with the sizing die.

Bores (ID features) respond very well. A mandrel pressed through the bore during sizing can often produce tolerances in the IT6-IT7 range for many alloys and bore sizes. Bearing bore tolerances in the H6 or H7 range can be feasible in PM with sizing for well-behaved geometries, which is why PM is commonly used for self-lubricating bearings without secondary machining.

Outer diameters also respond well when the OD is fully enclosed in the die during sizing. Gears and sprockets sized in a closely fitting die can achieve pitch-circle tolerances that require no secondary grinding.

Axial height improves with sizing but is more sensitive to part geometry. Parts with wide flats and simple geometry hold axial tolerances more predictably than complex parts with multiple steps or thin walls.

Face flatness improves significantly after coining on a flat platen. This is useful for sealing surfaces, mating flanges, and parts that will be used as bearing thrust faces.

What Does Not Respond Well to Sizing

Transverse features-holes drilled perpendicular to the press axis, slots, cross-holes-are not affected by sizing because they are not contact surfaces in the sizing die. If you have a cross-hole or side-port that requires tight location tolerance, expect to machine it after sintering and sizing.

Very tall parts (high aspect ratio: axial height much greater than transverse diameter) can be difficult to size uniformly because the load does not distribute evenly along the height. Die springback and load variation can leave the middle of a tall part at a different dimension than the ends.

Parts with large projected area variation between levels (very thin sections adjacent to very thick sections) can show differential springback after sizing. These geometries should be reviewed with the process engineer before committing to sizing as the primary tolerance control.

Features in material that is not directly contacted by the die do not improve. If a feature is recessed into a pocket or behind a shoulder that contacts the die first, that feature may not be adequately sized.

Bore Tolerances in Practice

For buyers and engineers specifying bore tolerances, here is a useful frame of reference:

These values are illustrative. A short bore (L/D < 1) is easier to size uniformly than a deep bore (L/D > 2). If a bore will locate a bearing, shaft, or press-fit component, confirm the target tolerance class and whether it is achievable by sizing alone.

Gear Tolerances After Sizing

Gears and sprockets are one of PM's strongest applications precisely because sizing improves them at low cost. A typical iron-based PM gear after sintering and sizing can achieve:

• AGMA quality level 6-8 (application-dependent)
• Pitch diameter tolerance consistent with running gears at moderate loads and speeds
• Face width tolerance suitable for standard gear mesh

For high-precision gearing (AGMA 9+) or high-speed applications, grinding or finish machining may still be required. But for most industrial, automotive auxiliary, and appliance gears, sized PM is accurate enough without grinding.

When Machining Is Still Required

Sizing is not a substitute for machining in all cases. Expect to machine:

• Tight-tolerance threaded holes (sizing cannot produce threads; tapping is required)
• Cross-holes, side ports, and lateral slots
• Features with tolerances tighter than sizing can reliably achieve for the geometry
• Surfaces with cosmetic or contact-critical surface finish requirements beyond what sizing provides (Ra < 0.4 um, for example)
• Reference features that must be concentric or perpendicular to a sized bore but are not directly in the sizing die

A well-designed PM part minimizes the number of machined features. If your drawing has more than two or three machined features, it is worth asking whether those features are truly required or whether they are carryovers from a machined-part design that could be redesigned out.

Practical Advice for Drawing Callouts

When specifying a PM part:

1. Distinguish features by process. Identify which dimensions will be controlled by the press die (sintered), the sizing die (sized), or secondary machining (machined). Apply tolerances appropriate to each.
2. Avoid machined tolerances on pressed features. If you apply a +/-0.01 mm tolerance to a face height that will only be controlled by sintering and sizing, the supplier will need to machine it-adding cost you may not have anticipated.
3. Use tolerance classes, not just numbers. Calling out "bore: H7" communicates more clearly than a raw number, because the PM supplier can select the sizing tooling to match the class.
4. Discuss before finalizing. The cheapest way to achieve tight tolerances on a PM part is to design the geometry to be friendly to sizing. A 10-minute conversation with your PM supplier before drawing release can prevent expensive changes later.

Summary

• Sizing improves PM tolerances significantly, especially for bores, ODs, and face flatness
• Typical bore tolerances after sizing are IT6-IT7 for small-to-medium diameters
• Axial height tolerances improve with sizing but are more geometry-dependent
• Features not contacted by the sizing die do not improve; cross-holes and lateral features typically require machining
• Not all PM geometries are equally suited to sizing-verify with your supplier before committing to a tolerance requirement

If you are specifying a PM part and have questions about what tolerances are achievable for your geometry, contact our engineering team. We can review your design and identify whether sizing alone is sufficient or whether secondary machining needs to be factored into the cost.