Screw Bosses

Screw bosses are the go-to way to put screws into 3D printed parts without splitting the plastic, stripping the threads, or crushing the surface under the screw head. Design the boss around your fastening method (self-tapping, heat-set insert, or through-bolt), then make sure the boss has enough “meat” (perimeters and outer diameter), a filleted base to spread stress, and ribs/pads where loads would otherwise pry the boss off the part.

TL;DR

Pick the screw method first (self-tapper vs heat-set insert vs through-bolt), then design the boss with a generous outer diameter, lots of perimeters, and a base fillet; add ribs or a pad if the screw load could bend or crush the boss.

Screw Bosses (Cross-Section)Topic-specific diagram for the concept, checks, and tradeoffs in this lesson.Boss ODPilot holePerimetersBase fillet
Cross-section labels for the features that prevent splitting, stripping, and base cracking.

What a Screw Boss Does

A screw boss is a raised cylindrical feature that carries a screw hole. It provides enough plastic for threads (or an insert) to grip, helps the hole stay round, and spreads clamp and pull-out loads into the surrounding part through thicker walls, pads, ribs, and smooth transitions instead of sharp corners.

Choose the Fastening Strategy First

Model the boss around how the screw will be used, because the hole size, required wall thickness, and torque tolerance change a lot: Self-tapping screw: the screw forms/cuts threads in plastic, creating outward “hoop” stress that can split thin bosses. Heat-set insert: the insert carries threads; the boss must resist insertion heat and still have enough wall thickness around the insert. Through-bolt + nut: the boss may become a clearance hole plus a washer seat; strength depends more on crushing resistance and load spreading than thread grip.

Boss Geometry Rules of Thumb (What Matters and Why)

Perimeters over infill
A boss is mostly walls. Add perimeters so you get a solid ring around the hole; infill doesn’t stop splitting as effectively as continuous walls.
Outer diameter (OD)
Make OD clearly larger than the pilot/insert hole so there’s plastic to resist hoop stress and pull-out; thin rings crack or split during tightening.
Base fillet
Fillet the boss-to-floor/wall junction to reduce stress concentration; sharp corners are where bosses commonly crack off.
Height and bracing
Keep the boss only as tall as needed for thread engagement; tall skinny columns bend and snap—use ribs/gussets or a surrounding wall if height is unavoidable.
Top lead-in
Chamfer the hole entrance to help the screw start straight and prevent chipping/peeling at the rim.
Under-head load spread
Use a counterbore/washer seat or a thicker pad so clamp force doesn’t crater the surface or collapse the boss top.

Reinforcements That Prevent Real Failures

  • Ribs/gussets from boss to nearby wall to resist bending and tear-out
  • A thicker pad under the boss to prevent crushing and to increase stiffness at the base
  • Counterbore or washer seat to spread load under the screw head
  • Move the boss away from edges/corners, or add local thickness, to avoid splitting into a nearby thin wall

If Your Boss Fails

Boss splits radially while tightening

Likely cause: Boss OD too small, too few perimeters, sharp base corner, or layer-to-layer weakness under hoop stress

Fix: Increase boss OD and wall/perimeter count; add a larger base fillet; reduce torque or switch to an insert

Threads strip out easily

Likely cause: Pilot hole too large, not enough engaged wall material, or weak material for repeated assembly

Fix: Reduce pilot diameter slightly; increase perimeters; increase engagement length; consider a heat-set insert for repeat use

Boss snaps off at the base

Likely cause: Bending/prying load with no ribs, thin floor, or sharp boss-to-floor junction

Fix: Add ribs/gussets and a larger fillet; thicken the floor/pad; reduce boss height or move the load path closer to a wall

Screw won’t start straight / cross-threads

Likely cause: No chamfer, rough/undersized printed hole from over-extrusion, or hole printed out-of-round

Fix: Add a chamfer; calibrate flow; drill/ream the pilot hole; slow down hole perimeters for better accuracy

Quick Validation Print (Do This Before Committing a Whole Part)

  1. Print a small coupon that includes the boss plus the same surrounding wall thickness and floor thickness as your real part.
  2. Assemble with the exact screw/insert and driver you’ll use; tighten to your expected torque (then push to failure on a sacrificial sample if you can).
  3. Look for whitening, hairline cracks at the base fillet, boss ovalization, surface crushing under the head, or layers opening up.
  4. Change one variable per iteration: pilot diameter, wall/perimeter count, boss OD, fillet size, adding a pad, or adding ribs.