DFM Guide · Assembly Features

Snap fits.
Press fits.
Self-locating.

Assembly features — snap fits, press fits, threaded inserts, locating pins, dowel pins. Right feature for the job reduces assembly time and improves quality.

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01 · Key principles

Key principles.

Press fits

Cylindrical interference

Shaft slightly larger than hole. Permanent assembly. Standard ISO 286 H7/p6 typical. Holding force from friction.

Snap fits

Plastic flexibility

Elastic deformation engages mating feature, then springs back. Plastic parts only. Permanent or removable per design.

Locating pins

Position datum

Dowel pins position mating parts precisely. H7 reamed holes, ±0.005 mm achievable.

Threaded inserts

Plastic threading

Heat-set or ultrasonic inserts provide metal threads in plastic. Stronger than self-tapping.

Magnetic features

Fast assembly

Permanent magnets for fast positioning, alignment. Common for tooling, displays.

Bayonet mounts

Quarter-turn

Twist-and-lock mounts. Fast assembly/disassembly. Lens mounts, quick-change tooling.

FAQ

Press fit vs snap fit?

Press fit: any material, requires force, generally permanent. Snap fit: plastic only, no special tools, can be removable. Choose by material and disassembly need.

How tight to specify press fit?

Light press: H7/n6 (0.005-0.015mm interference per 10mm). Standard: H7/p6 (0.015-0.035mm). Heavy: H7/u6 (0.035-0.08mm). Choose per holding force needed.

Snap fit failure modes?

Plastic creep over time loosens snap. Heat softens plastic, releases snap. Repeated cycles fatigue. Design for finite cycle life if removable.

Threaded inserts vs molded threads?

Inserts handle higher torque, repeated assembly. Molded threads cheaper but limited cycles. Heat-set inserts standard for quality plastic assemblies.

Adhesive bonding alternative?

Adhesives provide gap-filling bond, work with any clearance. Slower (cure time) but tolerant of dimensional variation. Reversible with heat.

Designing for disassembly?

For service-friendly products, design assembly for non-destructive disassembly. Threaded fasteners or removable snap fits. Avoid permanent press fits where service needed.