How the processes differ
CNC machining removes material from a solid billet using cutting tools to achieve the final shape. It is a subtractive process - flexible, capable of very tight tolerances, and applicable from single prototypes to batches of several thousand parts.
Powder metallurgy (PM) compresses metal powder into a die under high pressure, then sinters the compact in a furnace to bond the particles. It is a near-net-shape process - parts emerge from the die close to their final dimensions, with minimal material waste and no machining required for most features. The economics of PM are entirely driven by volume: the tooling cost is high, but the per-part cost is very low at scale.
When CNC machining is the right choice
- Low to medium volumes (1 to ~5,000 parts per year). Machining has no tooling cost - you pay per part and per hour of machine time.
- Complex external geometries, undercuts or features that a die cannot produce.
- Tight dimensional tolerances. CNC routinely achieves
±0.01-0.05 mm. PM sintered tolerances are IT8-IT10; sizing can improve this to IT6, but adds an operation. - Wide material range. CNC works with virtually any engineering metal - steel, aluminium, titanium, brass, copper, engineering plastics. PM is primarily limited to iron and steel, with some stainless, brass and copper PM grades available.
- Prototyping and early-stage development. A CNC part can be produced from a drawing in days. PM tooling takes weeks.
- Parts that require welding, threading into thin walls, or features that only work in wrought material.
When powder metallurgy is the right choice
- High volumes (typically 10,000+ parts per year). At sufficient volume, PM part cost is dramatically lower than machining - often by 50-80%.
- Complex internal geometry. Features like internal splines, blind keyways, and cam profiles that would require multiple machining setups can be formed directly in the die.
- Self-lubricating bearings. Oil-impregnated PM bearings (sintered bronze, sintered iron) are a well-established PM product with properties impossible to replicate by machining.
- Near-net-shape efficiency. PM produces almost no waste material. For expensive alloys or high-volume production, this is a significant cost driver.
- Consistent part-to-part quality. Because every part is pressed in the same die, dimensional variation between parts is very small.
Cost comparison across volumes
| Volume (pcs/year) | CNC machining | Powder metallurgy | Recommended process |
|---|---|---|---|
1-100 |
Low-medium | Not viable (tooling cost) | CNC machining |
100-1,000 |
Medium | High (tooling amortisation) | CNC machining |
1,000-5,000 |
High | Medium | Depends on geometry |
5,000-20,000 |
Very high | Low-medium | Powder metallurgy |
20,000+ |
Impractical | Very low | Powder metallurgy |
The crossover point depends heavily on part complexity, material and the specific geometry. For a simple steel bushing, PM may be viable from 2,000 parts/year. For a complex aluminium part, CNC may remain competitive at much higher volumes.
Tolerance and surface finish
This is where CNC machining has a clear advantage. Sintered PM parts come out of the furnace with IT8-IT10 tolerances on most dimensions. Sizing (a cold-pressing operation after sintering) can improve critical bore and OD dimensions to IT6, but this adds cost and is limited to specific features.
CNC machining routinely achieves IT6-IT7 across the whole part, with IT5 or tighter achievable on specific features through grinding or honing.
If your part requires tight tolerances across multiple features - bearing seats, precision bores, close-fitting shafts - CNC machining is almost always the better process, regardless of volume.
Material properties
PM parts have a density of approximately 85-95% of wrought material, depending on the alloy and sintering process. This means slightly lower tensile strength and fatigue resistance compared to the equivalent machined part. For most structural applications this is not a problem - PM parts are used in automotive transmissions, power tools and industrial machinery worldwide - but for fatigue-critical aerospace or motorsport applications, wrought machined material is generally preferred.
Can the processes be combined?
Yes - and this is often the optimal solution. A PM blank can be machined after sintering to achieve tight tolerances on critical features, while the PM process provides the near-net shape and keeps overall cost low. This hybrid approach is standard practice for automotive components such as connecting rods, sprockets and valve seat inserts.
Summary
- Choose CNC machining for low-to-medium volumes, complex geometry, tight tolerances, wide material choice, or prototyping.
- Choose powder metallurgy for high volumes, simple-to-moderate geometry, self-lubricating applications, or when minimising material waste is critical.
- The cost crossover point is typically 1,000-10,000 parts/year depending on part complexity.
- PM tolerances are IT8-IT10 sintered; sizing improves critical features to IT6.
- Consider a hybrid approach - PM net-shape with post-sinter machining - for high-volume parts with a few tight-tolerance features.