Threading and Tapping Powder Metallurgy Parts: What Works and What to Avoid

Threads are required in almost every fastened assembly, and PM parts are no exception. But the standard approach to threads in PM - tapping after sintering - has specific requirements and limitations that differ from tapping wrought steel. The porous structure of PM changes tool behavior, thread strength, and the minimum design requirements for a reliable threaded connection.

This guide covers the practical rules for threading PM parts successfully.

Why Threads Cannot Be Pressed into PM Parts

Engineers new to PM sometimes ask whether threads can be pressed into the part - forming the thread profile in the die, eliminating the secondary tapping operation.

The answer is almost always no, for a practical reason: ejection.

When a PM compact is formed, the punches and core rods must be withdrawn vertically (in the press direction) to release the part. A threaded feature - either external or internal - has a helical profile that cannot be released axially. Removing the part from the die would require rotating it out along the helix (like unscrewing), which is mechanically complex and slow enough to make pressed threads impractical for production.

Some PM presses can produce helical gear teeth (with rotating tooling), but precision threads require a more consistent and controllable helix than this approach provides.

The practical rule: All production threads in PM parts are added by secondary machining (tapping for internal threads, thread rolling or cutting for external threads).

Secondary Tapping: What PM Changes

Tapping PM is similar to tapping wrought steel, with some important differences:

Tool Behavior in Porous Material

A standard tap cuts metal by plastic deformation and chip formation. In PM, the porous matrix means:

• The cutting edge encounters alternating dense metal particles and pore spaces
• Chips are interrupted and may not clear well - increasing the risk of tap binding and breakage
• Pore spaces reduce the effective cross-section at each thread flank, so the thread is slightly weaker per unit area than in solid metal

Tap Selection for PM

Spiral-flute (gun) taps are preferred over hand taps for PM. The spiral flute directs chips back out of the hole rather than into it, reducing chip re-cutting and binding risk.

Plug chamfer geometry (rather than bottoming or taper) is standard for through-holes and blind holes where chamfer length fits.

Uncoated or TiN-coated taps are commonly used. TiN coating reduces friction and extends life in abrasive PM material (stainless and high-hardness grades wear taps faster).

Tap speed: PM taps typically run at 50 - 0% of the speed used for equivalent wrought steel to allow better chip evacuation and reduce thermal load. Actual speeds should be verified for the specific alloy and tap geometry.

Lubrication During Tapping

Use cutting fluid designed for the PM alloy:

• Iron-based PM: sulfurized cutting oil or general-purpose tapping fluid
• Stainless PM: chlorinated or sulfurized tapping compound; stainless work-hardens and requires more aggressive cutting fluid
• Oil-impregnated PM: the oil in the pores provides some lubrication, but supplemental cutting fluid is still recommended for consistent tool life

Hole Preparation

Drill the pilot hole to the correct size for the thread type and PM material. PM porous material is slightly more compressible than solid steel - the effective drilled hole diameter after spring-back may be slightly smaller than the nominal drill diameter. Check that the tapped thread size is achievable with the standard drill for the tap.

For blind tapped holes in PM, ensure the hole depth is sufficient for the required thread engagement plus the chamfer length of the tap. Bottoming the tap against the end of a blind hole increases breakage risk.

Thread Strength in PM: Estimating Engagement Length

Because PM has lower tensile strength than equivalent wrought steel and the thread flanks have reduced cross-section due to porosity, the thread pullout strength per unit engagement length is lower than for solid metal.

A practical starting rule: thread engagement length in PM should be 1.5 - .5x the thread diameter (versus 1.0 - .5x for solid medium-carbon steel). Higher-density PM and heat-treated grades can approach the lower end of this range; standard-density as-sintered grades should use the higher end.

Example: an M6 threaded hole (6 mm diameter) in FC-0208-70 at standard density should have a minimum engagement of approximately 9 - 5 mm for a reliable joint. If the PM wall section is insufficient for this engagement, consider a thread insert or redesign the joint.

Minimum Wall Section for Tapping

Tapping too close to an edge or into a thin wall creates structural weakness in the thread and risk of the wall fracturing during tapping. Minimum wall thickness from thread edge to part surface:

• For standard iron-based PM: minimum wall ~1.5x thread pitch, typically <=1.5 mm for M4鈥揗8 threads
• For stainless PM: slightly more generous (stainless is more brittle during tapping); <=2 mm for M4鈥揗8

Blind holes close to a thin wall are higher risk than through-holes. Flag these in design review before tooling is released.

External Threads on PM Parts

External threads (on PM part studs, shafts, or bosses) can be produced by:

Thread rolling after sintering: Cold-rolling threads into a PM OD. Rolling is better than cutting for PM because it does not cut through the porous surface - instead it plastically deforms and densifies the thread flank. Thread-rolled PM threads have better strength and fatigue resistance than cut threads on the same material.

Thread cutting (turning) after sintering: Standard single-point threading on a CNC lathe. Generates chips; chip clearance and tool geometry must account for PM's interrupted cutting behavior.

Thread rolling is preferred where it can be applied - it improves thread strength without weakening the PM surface.

Thread Inserts for Improved Strength

When a PM part requires a tapped hole that must carry significant load - or when thread stripping during assembly is a concern - a threaded insert can be used to transfer the load to a stronger material:

Helical wire inserts (Helicoil type): A stainless steel coil is installed into a tapped hole (the hole is tapped to the insert's outer thread size). The insert provides a hardened, full-density thread surface inside the PM part. Load is distributed over the insert helix, not just the PM thread flanks.

Solid threaded inserts (press-fit): A threaded steel or brass insert is pressed into a PM bore. The PM bore must be accurately sized for the press-fit. This is a common approach for adding robust threads to PM parts in die-cast or assembly housings.

Sinter-in inserts: For some designs, a solid metal insert (knurled or hex head) is placed in the green compact before sintering. The sintering process bonds the PM material around the insert. This requires careful insert material selection (the insert must survive sintering temperature) and may require adjustment of sintering conditions to avoid distortion.

Thread Specification on PM Drawings

When specifying tapped threads on PM part drawings:

1. Specify as a secondary machined feature. Mark the thread with "TAP" or "MACHINE AFTER SINTER" to avoid ambiguity about whether it is a pressed feature.
2. Specify minimum engagement length. If the assembly joint is load-bearing, specify the minimum thread engagement on the assembly drawing.
3. Specify class of fit. Standard metric threads to ISO 965 (6H for internal threads) or UN threads to ASME B1.13M are appropriate. For PM, class 6H (medium clearance) is standard - tighter classes (5H, 4H) are more difficult to achieve consistently in porous material.
4. Note thread inserts if required. If the design requires a helical insert, specify insert type, thread size before and after insert, and installation method.

Summary

• Pressed threads in PM are not practical - all production threads are secondary operations
• Tap selection (spiral-flute, appropriate cutting fluid) matters more than in solid steel
• Thread engagement length in PM should be 1.5 - .5x thread diameter for reliable joints
• Thread rolling is preferred for external threads (better strength than cut threads)
• Thread inserts improve reliability for critical or high-load threaded connections

Contact us to discuss threading requirements for your PM part design. We can advise on engagement length, tap selection, and whether a thread insert is warranted for your joint.