Heat-Set Inserts: Hole Size, Boss Design, and PCB Mounting

Flux can design PCB mounts for heat-set inserts and check the board and enclosure together. Add your board geometry and insert drawing to the project. Ask Flux to place the supports, check fastener and tool clearance, and analyze the mounting stack. Its ECAD and MCAD workflow keeps electronics and mechanical parts in one project.

Start with a request like this:

Design a PETG enclosure for this industrial controller board. Use M3 heat-set inserts and align the supports with the four PCB mounting holes. Use the attached insert drawing for the bores. Check board clearance, fastener access, and the mounting tolerance stack.

A heat-set insert hole is sized from the insert manufacturer's drawing, not from the screw thread. M3 describes the internal thread. It does not specify the hole in the plastic. For the CNC Kitchen M3 × 5.7 insert used here, the drawing specifies a 4.0 mm installation hole and a 4.6 mm outside diameter. The printed hole, surrounding boss, and screw length must work together.

This guide works through a serviceable mount for a 1.6 mm PCB in a printed PETG housing for an industrial controller. Use the worked dimensions below to specify the insert holes, a 10 mm boss, an M3 screw stack, and a coupon test plan in Flux.

What heat-set inserts do

Heat-set inserts give thermoplastic parts reusable metal threads. A heated brass insert melts the plastic around its external knurls. When the plastic cools, it locks around those features. The screw engages the brass, so repeated service does not cut new threads into the printed part.

They suit removable PCBs, inspection covers, and mounting brackets. Use them with thermoplastics such as PETG, ABS, or nylon. Cured thermoset resin does not soften in the same way; select a fastening method made for that material.

For a board mount, the complete load path is screw head → washer → PCB → insert face → insert knurls → plastic boss → housing. Design that path, not just a cylindrical hole.

What hole size does an M3 heat-set insert need?

Start with an exact insert family and length. The CNC Kitchen dimensions and design guide gives these values for its standard M3 × 5.7 insert:

The same guide lists a 4.4 mm hole for its M3 × 5 × 4 VORON insert. Both inserts accept M3 screws, but their host holes differ. Do not apply a generic “M3 hole” value across suppliers or insert styles.

Keep two values in your drawing notes: the required physical hole and the CAD diameter that produces it on your print process. A CAD circle of 4.0 mm can print smaller than 4.0 mm. Use a coupon to establish the compensation for your printer, material, orientation, and slicer settings.

Worked example: a boss for an industrial controller PCB

The example board is 80 × 50 × 1.6 mm. Four mounting-hole centers sit 5 mm from the board edges: (5, 5), (75, 5), (5, 45), and (75, 45) mm from its lower-left corner. These are design inputs for the example, not measurements from a built product.

1. Set the boss diameter

Choose a 10.0 mm outside boss diameter. Around the insert's 4.6 mm maximum outside diameter, the remaining radial plastic is:

Radial plastic = (10.0 − 4.6) / 2 = 2.7 mm

This exceeds the guide's 1.6 mm minimum wall even when measured from the full insert envelope. It also falls within SPIROL's general boss guidance of two to three times insert diameter: 9.2 to 13.8 mm for a 4.6 mm insert. The selected insert's drawing remains the primary reference.

Add a root fillet and ribs where the boss meets the housing floor. Keep the support face flat. Check the sliced print so that perimeter paths form continuous material around the bore and connect to the floor. An infill percentage alone does not describe that structure.

2. Leave space below the insert

Specify a straight blind hole 6.7 mm deep from the boss support face. That is 1.0 mm deeper than the 5.7 mm insert. It follows CNC Kitchen's extra-depth guidance. It also matches SPIROL's insert-length-plus-two-thread-pitches rule for a standard M3 × 0.5 thread:

Minimum hole depth = 5.7 + 2 × 0.5 = 6.7 mm

Make the support face 8.0 mm above the inside housing floor. The blind bore then leaves 1.3 mm between its bottom and that floor. Add the housing floor thickness below this; do not confuse it with the remaining material inside the boss.

Install the insert flush with the support face. An insert that sits high can rock the board. An insert that sits low can load the plastic before the PCB contacts the metal face. Keep this plain insert's hole mouth straight, as the supplier recommends.

3. Design the PCB interface

Use 3.4 mm finished, non-plated clearance holes for the M3 screws in this example. Keep copper and components clear of the screw head, washer, boss, and insertion tool envelope on the relevant layers and board faces. A non-plated hole alone does not create that keepout.

For the nominal 10 mm boss, reserve a 5 mm radius beneath the PCB from each hole center. Add your component placement margin outside that footprint. Reserve the selected washer's actual diameter on top. Keep intentional chassis-ground connections separate from ordinary mechanical mounting holes.

SPIROL specifies that the mating clearance hole must be larger than the screw and smaller than the insert's pilot or face diameter. Check the selected insert face against the 3.4 mm board hole. Keep the board supported at each fastener so tightening does not bend it over an unsupported span.

Calculate screw length instead of guessing

Choose an M3 × 6 socket-head screw, with length measured under the head, and a 0.5 mm washer. The PCB is 1.6 mm thick. With the insert flush, nominal penetration into the insert is:

Penetration = screw length − PCB thickness − washer thickness

Penetration = 6.0 − 1.6 − 0.5 = 3.9 mm

That leaves 1.8 mm to the end of the 5.7 mm insert envelope. Confirm the supplier's usable thread depth and the screw's tip geometry when specifying effective thread engagement. The blind-hole floor is not a screw stop: a bottomed screw can push the insert out while the joint still feels tight.

For a tolerance check, assume the purchased screw is 6.00 ± 0.15 mm under the head, the board is 1.60 ± 0.10 mm, and the washer is 0.50 ± 0.05 mm. These are example acceptance limits; use the actual purchase specifications for your assembly.

Minimum penetration = 5.85 − 1.70 − 0.55 = 3.60 mm
Maximum penetration = 6.15 − 1.50 − 0.45 = 4.20 mm

The maximum remains 1.5 mm short of the nominal insert end. Include insert-length tolerance, installed depth, and usable thread depth in the release check. Set the tightening torque from tests of the actual insert, printed boss, PCB, and washer stack. A steel M3 torque table does not establish a plastic boss's torque limit.

Compare printed coupons before printing the housing

Print a small coupon with four labeled bosses. Use the same 10 mm boss diameter, 8 mm height, 6.7 mm bore depth, material, orientation, and slicer settings as the housing. Change only the CAD hole diameter.

This is a proposed comparison, not a set of measured results. Measure each printed bore with suitable gauges before installation. Record actual diameter, pilot seating, insertion behavior, final flushness, cracking, and screw torque response. Repeat the candidates to check process variation.

In CNC Kitchen's own PLA tests, holes printed about 0.25 mm undersize and a 4.2 mm CAD hole gave a useful balance of installation and retention. That is a reason to measure compensation, not a new universal 4.2 mm rule for PETG.

Choose the compensated CAD diameter that produces the specified physical bore and repeatable installation. Reject a hole that lets the insert fall in cold. Reject a process that cracks the boss or pushes a plastic plug into the screw path. Then test pull-out, rotation, and repeated service against your product's loads and torque requirements. Record the accepted process with the drawing.

Install heat-set inserts without damaging the mount

1. Install the inserts before fitting the PCB. Support the housing on a stable surface.
2. Use a temperature-controlled iron and an insert tip that contacts the brass without entering the thread.
3. Follow the supplier's temperature guidance for the plastic. CNC Kitchen suggests starting 10–20 °C above the printing temperature.
4. Align the insert with the bore. Let heat soften the plastic instead of forcing the insert into a cold hole.
5. Finish flush with a flat alignment tool, then hold position while the plastic solidifies.
6. Let the joint cool. Check the thread and seating face, then fit the PCB and tighten to the validated assembly torque.

Keep insertion-tool access in the mechanical model. A boss beside a tall wall can accept the final screw but still block the heated tip during assembly.

Carry the mounting stack into Flux

Flux is an AI hardware engineer. It designs the board and mechanical parts in one project. Its documented mechanical standards workflow covers preferred screws, heat-set inserts, engagement requirements, and PCB standoffs. Its fit and clearance workflow covers board supports, tool access, and tolerance stack-ups.

Add the board geometry and insert drawing to the project. Give Flux the selected dimensions, material, and mounting coordinates:

Design four PETG PCB mounts for this 80 × 50 × 1.6 mm board. The hole centers are (5, 5), (75, 5), (5, 45), and (75, 45) mm from the board's lower-left corner. Use CNC Kitchen M3 × 5.7 heat-set inserts. Make the bosses 10 mm outside diameter and 8 mm high above the inside floor. Use 6.7 mm deep bores with a 4.0 mm target physical diameter. Keep CAD print compensation as a separate parameter. Use 3.4 mm non-plated PCB holes, M3 × 6 screws, and 0.5 mm washers. Keep components and copper clear of the mounting hardware.

Then ask for the assembly check:

Check the board supports, screw penetration, and driver and insert-tip access. Show a cross section through one mount. Calculate minimum and maximum penetration from the screw, PCB, washer, and installed-insert dimensions. Create four coupon bosses with CAD bores of 4.0, 4.1, 4.2, and 4.3 mm for print comparison.

Open the assembly in the 3D view. Press M to measure the board height and surrounding clearances. After the coupon test, ask Flux to apply the accepted CAD bore compensation to the housing. Save the supplier, material, print process, and accepted mounting rules in Knowledge for the next design.

See Flux for mechanical design to build the board mounts and housing in the same project as the electronics. Keep the physical test record beside the design so the next revision starts with a defined mounting process.

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FAQs

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