Fasteners, inserts & assembly fit

A screw joins a stack of parts

A clearance hole lets a screw pass through. A threaded hole engages its thread. A heat-set insert provides a metal thread within suitable thermoplastic. These are different features even when they look like similar circles from above. The five-hole clearance coupon teaches passing fit; it is not automatically the correct bore for any insert.

Read a fastener as a specification: thread diameter and pitch, length, head shape, material or grade, and any required washer or locking method. M3 describes a nominal metric thread size, not one universal screw. Two M3 screws can have incompatible lengths or heads. Organize parts by their complete identity, not by a bag labeled small screws.

Length must fit both sides of the joint

Stack thickness is the total material and washer thickness beneath the head before the threaded destination begins. For an ordinary non-countersunk screw, a 10 mm under-head length through a 6 mm stack leaves approximately 4 mm projecting. This arithmetic does not prove enough thread engagement or enough clearance at the bottom of a blind hole.

A screw that bottoms out can feel tight without clamping the intended surfaces. On a motor, excess protrusion may reach something that must remain clear. Use the actuator's actual drawing and mounting instructions. Countersunk screws use a different length convention, and a compressible washer or spacer changes the stack. Measure the real arrangement instead of guessing from the exterior.

Tightening torque is the turning force applied while fastening the screw. If the required motor-mount torque is unknown, leave final tightening pending and obtain a specification that applies to the actuator, fastener and receiving materials. A generic screw chart does not by itself account for the motor housing or a printed mounting part.

Inserts need a designed installation

Heat-set inserts work by softening nearby thermoplastic so it settles around the insert's outer features. Their specified bore, depth, surrounding material and installation process matter. Two inserts with the same internal thread can need different holes. Obtain the exact supplier drawing before buying a matching insert or modifying a printed part.

For a practice installation, use an appropriate scrap specimen and a temperature-controlled tool with a suitable tip and stand. Follow the chosen insert supplier's procedure. Let the joint cool before testing it. Plastic can creep under sustained compression, so tightening harder is not a universal cure for loosening. Compression limiters or other design changes require engineering review, not improvisation.