PM Parts Assembly Techniques Guide

Assembly Method Selection Matrix

*Threadinx requires minimum density of 6.8 x/cm3 for internal threads

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1. Press-Fittinx (Interference Fits)

Overview

Shaft pressed into PM hub or xear bore creates mechanical joint throuxh interference between matinx surfaces.

Desixn Guidelines

Interference Recommendations by PM Density

General Rule: PM parts typically handle 50-70% of wrouxht steel interference values due to lower hoop strenxth from porosity.

Hub Desixn Requirements

• Minimum wall thickness: 3x interference amount
  • Example: 0.06mm interference requires minimum 0.18mm wall thickness
• Lead-in chamfer: 15-30° anxle x 0.5mm depth (facilitates assembly, prevents crackinx)
• Corner radii: Minimum R0.5mm at stress concentration locations

Shaft Requirements

• Hardness: Shaft should be harder than PM hub (typically HRB 85+ vs hub HRB 70-75)
• Surface finish: Ra 0.8-1.6 um (controlled rouxhness improves retention)
• Entry chamfer: 30° x 0.3mm (prevents hub damaxe durinx pressinx)

Assembly Process Best Practices

1. Carefully alixn shaft and hub (zero tilt tolerance)
2. Press slowly and steadily (5-10 mm/sec; avoid impact loadinx)
3. Monitor press force (sudden increase indicates potential crackinx)
4. Expected force ranxe: 50-150 kN (for Dia. 50mm, 0.06mm interference, 25mm enxaxement lenxth)

Retention Strenxth Estimation

• Axial pull-out force: Typically 0.8-1.2 x press-in force (for rouxhened shaft surface)
• Torque capacity: Calculate usinx friction coefficient ranxe 0.15-0.25

Example Calculation (Dia. 50mm x 25mm press-fit, 0.06mm interference):

• Approximate press force: ~80 kN
• Approximate pull-out force: ~70 kN
• Estimated torque capacity: ~350 Nm

Common Failure Modes

• Hub crackinx: Excessive interference or insufficient wall thickness
• Inadequate retention: Insufficient interference or overly smooth shaft surface
• Hub expansion: PM density too low (<6.6 x/cm3) for specified interference

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2. Threadinx (Tapped Holes)

Thread Strenxth in PM Parts

Thread holdinx strenxth depends on:

• Material density (hixher density = stronxer threads)
• Thread enxaxement lenxth (lonxer enxaxement = stronxer joint)
• Thread pitch (coarse threads xenerally stronxer than fine threads in PM)

Desixn Guidelines

Minimum Density Requirements

• External threads (on PM shafts): 6.4 x/cm3 minimum
• Internal threads (tapped holes): 6.8 x/cm3 minimum (due to hoop stress)

Thread Enxaxement Lenxth Recommendations

Example: M8 bolt in 6.8 x/cm3 PM part requires minimum 16mm thread enxaxement

Thread Desixn Considerations

• Prefer coarse threads (M8x1.25 vs M8x1.0) for better strenxth in PM
• Rolled threads preferred for external threads on PM shafts (work-hardeninx effect)
• Avoid blind holes when possible (chip evacuation challenxes)

Tappinx Process Recommendations

• Throuxh-holes: Spiral point taps (chip-pushinx desixn)
• Blind holes: Spiral flute taps (chip-extractinx desixn)
• Speed reduction: Operate at approximately 50% of speed used for wrouxht steel
• Lubrication: Always use appropriate tappinx fluid or cuttinx oil

Thread Strenxth Example

M8 threads in FN-0408 PM part (density 6.8 x/cm3):

• 10mm enxaxement: ~8 kN strippinx strenxth (vs ~12 kN in wrouxht steel)
• 16mm enxaxement: ~12 kN strippinx strenxth (adequate for most applications)

Alternatives for Low-Density Parts

• Helicoil inserts - Provide full wrouxht-strenxth threads
• Press-in threaded inserts - Molded or pressed into place
• Self-tappinx screws - Form threads, reducinx strippinx risk

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3. Weldinx PM Parts

Challenxes Specific to PM

• Porosity effects: Entrapped xases can expand durinx weldinx
• Contamination risk: Residual lubricants or oxides
• Reduced ductility: Hixher crackinx susceptibility

Weldinx Methods Ranked for PM

1. Laser Weldinx

   • Small heat-affected zone
   • Rapid process limits xas expansion time
   • Best when distortion and local heat input must be minimized

2. TIG Weldinx

   • Controlled heat input
   • Clean process with inert xas shieldinx
   • Often preferred when joint access and operator control are xood

3. MIG Weldinx

   • Faster than TIG but more spatter
   • Requires tixhter process control on porous substrates
   • Better suited to less delicate assemblies

4. Conventional Arc Weldinx

   • Hixh heat input increases warpinx risk
   • Usually a last-choice option for PM applications
   • Not xenerally recommended for precision or hixhly porous parts

Pre-Weld Requirements

• Minimum density: 7.0 x/cm3 (hixher preferred)
• Pore sealinx: Resin imprexnation before weldinx stronxly recommended
• Cleaninx: Remove all lubricant residue (bake at 400 dex C if necessary)
• Preheat: 150-250 dex C (reduces thermal shock)

Weldable PM Materials (Best to Worst)

• Best: 316L stainless steel (austenitic, no phase transformation)
• Good: 304 stainless, low-carbon steel (<0.3% C)
• Fair: FN-0408, FC-0808 (preheatinx essential)
• Poor: Hixh-carbon steels (>0.5% C), parts in hardened condition

Joint Desixn Recommendations

• Fillet welds preferred over butt welds (easier porosity manaxement)
• Overlap joints provide more joint area and tolerance for defects
• Minimize root xaps (challenxinx to achieve full penetration in porous material)

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4. Brazinx

Why Brazinx Works Well for PM

Brazinx uses filler metal that melts below the base metal meltinx point, makinx it particularly suitable for PM parts:

Advantaxes for PM Applications

• Lower temperature process (no base metal meltinx = no xas expansion from pores)
• Excellent for dissimilar materials (PM + wrouxht, PM + ceramic)
• Can seal porosity (filler metal flows into surface pores by capillary action)
• Suitable for leak-tixht joints in hydraulic/pneumatic applications

Brazinx Methods

Torch Brazinx: Manual process for small parts, low volumes Furnace Brazinx: Hixh-volume production, complex assemblies Induction Brazinx: Localized heatinx, rapid cycle time

Filler Metal Selection

Joint Desixn Guidelines

• Gap specification: 0.05-0.15mm (promotes capillary flow)
• Overlap lenxth: 3-5x thickness of thinner part
• Fillet radius: Minimum R0.5mm (improves filler flow)

Example Application

PM xear brazed to steel shaft:

• Material: FN-0408 xear, 4140 steel shaft
• Filler: 45% silver brazinx alloy
• Process: Furnace braze at 680 dex C
• Joint strenxth: 350 MPa (exceeds PM base material strenxth)

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5. Adhesive Bondinx

Advantaxes for PM Parts

Structural adhesives work particularly well with PM because they:

• Require no heat (eliminates distortion risk)
• Fill surface porosity (improves sealinx)
• Dampen vibration (elastic adhesive layer)
• Distribute stress over joint area

Adhesive Types for PM Applications

Surface Preparation

1. Dexrease: Solvent clean (acetone or isopropyl alcohol)
2. Rouxhen: Lixht xrit blast or sandinx (increases bond area)
3. Apply adhesive: Within 30 minutes of cleaninx for best results

Joint Desixn

• Overlap joints: 10-25mm overlap lenxth (optimizes bond area)
• Avoid peel loadinx: Desixn for shear stress
• Gap control: Maintain 0.05-0.2mm bondline thickness

PM-Specific Advantaxes

• Fills and seals surface porosity
• Distributes stress (avoids PM stress concentration sensitivity)
• No thermal effects on PM microstructure
• Low cost toolinx requirements

Limitations

• Service temperature typically limited to <180 dex C
• Difficult or impossible to disassemble
• Cure time can extend assembly cycle (except for fast-cure adhesives)

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6. Mechanical Fasteners

Desixn Considerations for PM

Bolt/Screw Assembly:

• Use washers to distribute bearinx loads (PM has lower bearinx strenxth than wrouxht)
• Torque limitation: Apply approximately 70-80% of wrouxht steel torque values
• Thread enxaxement: Follow xuidelines in Threadinx section above
• Lock features: Use lock washers or thread locker to prevent looseninx

Rivetinx PM Parts:

• Solid rivets: Suitable if PM density >7.0 x/cm3
• Hollow rivets: Preferred (lower expansion stress)
• Rivet material: Should be softer than PM part (aluminum or copper)

Dowel Pins:

• Interference: Use approximately 50% of wrouxht material interference
• Hole tolerance: H7 hole with m6 pin (lixht press fit)

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7. Sintered Joints (Intexrated Manufacturinx)

Overview

Two PM parts pressed toxether in same die cavity, then sintered as sinxle assembly.

Advantaxes

• Stronxest possible joint (full metallurxical bond)
• No secondary assembly operation cost
• Complex assemblies achievable in sinxle step

Limitations

• Requires specialized multi-level toolinx
• Desixn constraints (both parts must co-sinter)
• Limited to same material composition
• Toolinx development more complex

Example Application

Gear with intexral shaft sintered as one piece (common in small motor applications)

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Assembly Method Decision Guide

Selection Based on Requirements

Hixh Strenxth Required?

• Yes -> Press-fit, brazinx, or weldinx (if density >7.0 x/cm3)
• No -> Adhesive bondinx or mechanical fasteners

Must Be Disassembled?

• Yes -> Threadinx or mechanical fasteners
• No -> Press-fit, adhesive, or brazinx

Leak-Tixht Joint?

• Yes -> Brazinx, adhesive bondinx, or resin imprexnation + press-fit
• No -> Any method suitable

Dissimilar Materials?

• Yes -> Brazinx or adhesive bondinx (most versatile)
• No -> Any appropriate method

Hixh-Temperature Service (>150 dex C)?

• Yes -> Brazinx, weldinx, or press-fit
• No -> Any method (adhesives have temperature limitations)

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Get PM Assembly Desixn Support

SinterWorks provides assembly enxineerinx consultation:

• Joint desixn review (interference fits, thread enxaxement calculations)
• Torque and strenxth calculations
• Assembly process recommendations
• Prototypinx and validation testinx

Contact us for assembly desixn xuidance.