Designing Threaded Insert Pockets in CAD: Hole, Boss, and Test Coupons

Start From the Insert Drawing, Not the Screw Diameter

For an insert pocket, the screw’s nominal diameter alone does not define the hole diameter: the insert’s internal thread and knurled outer body are different dimensions. In its guide for heat- or ultrasonically-installed inserts, SPIROL says inserts are generally about twice the screw diameter. That is guidance from this paper, not a dimension to copy for every SKU: use the recommended hole for the selected insert model.

Before you open your CAD program, collect the inputs from the supplier drawing: straight or tapered style, knurl outer diameter, length, thread pitch, whether the insert has a head, and the recommended hole diameter. SPIROL states that it publishes recommended hole sizes per insert style in its design guide, and that the correct hole depends on the plastic, the chosen insert, and the installation method together.

If you have not chosen hardware yet, the options are covered on the heat-set threaded inserts page. Here we assume the part number is known and the datasheet is in front of you. Without that datasheet, every dimension is a guess you will need to confirm with coupons.

Depth, Flush Height, and Counterbore From the SPIROL White Paper

For inserts installed by heat or ultrasonics, SPIROL recommends a hole deeper than the insert and gives a minimum of insert length plus two thread pitches. The manufacturer explains that the assembly screw should not bottom out, which could cause jack-out. This depth is guidance in that paper for those methods; also check the exact screw and assembly against the supplier drawing.

SPIROL’s guide for these inserts recommends the installed top sit flush, with maximum protrusion stated as 0.13 mm. For headed inserts, it recommends a counterbore to achieve that seating: diameter 0.5 to 1.3 mm larger than the head and minimum depth equal to head thickness. Use the dimensions for the selected model.

The same SPIROL guide describes hole geometry for straight and tapered inserts: a straight hole has a maximum 1° included angle, and a tapered hole an 8° included angle; match insert style to hole. The manufacturer says an oversized hole reduces holding performance, while an undersized hole can induce stress, cracks, or flash. These recommendations are from its heat/ultrasonic insert guide.

Model a Real Boss Around the Pocket

In its heat/ultrasonic insert hole guide, SPIROL’s diagram labels the optimum boss diameter as two to three times the insert diameter. Its text says the relative multiplier decreases as insert diameter grows. It also says the wall should prevent bulging during installation and withstand the recommended screw torque. The outside boss diameter and the radial wall thickness around the hole are different dimensions: do not apply this multiplier as radial wall thickness. This is supplier guidance, not a holding result measured here.

A separate SPIROL guide for FDM assemblies recommends using outer wall loops to create a solid feature: at least 1.5 times the knurl diameter; for an M6 insert, it gives six loops with a 0.4 mm nozzle. It describes 2 times the knurl diameter as maximum performance potential. Its wall-loop curves are captioned PLA, 15% infill, and SPIROL INS 29 M6 short; the infill curve uses PLA, two loops, and the same insert. Keep those results and figures within their stated conditions, without a universal performance ranking.

For your CAD model, you can add a boss around the hole and inspect in slicer preview how the wall loops form that area; this is a proposed check, not a result observed here. For the mating part, SPIROL’s heat/ultrasonic guide says its clearance hole must be larger than the screw’s outside diameter but smaller than the insert pilot or face to prevent jack-out. Use the dimensions for the selected hardware.

Heat, Glue, or Press-In: Three Different Pockets

For thermal installation in a thermoplastic, Formlabs describes an example using a soldering-iron tip: the insert heats and softens nearby material, then it is pressed into the part. The guide says to follow the instructions for the exact hardware and allow the part to cool before driving a screw. It does not set one temperature for every insert or thermoplastic.

For an SLA thermoset resin part, Formlabs also describes using a heat-set insert as a glue-in insert: size the glue-in pocket from the insert’s widest diameter, keeping that bore distinct from the boss’s outside diameter, use CA or epoxy in that example, and allow the glue to cure before driving a screw. SPIROL’s separate 3D-printed assembly paper recommends cold installation for thermosets and compares press-in, self-tapping, and expansion styles with different trade-offs. Choose by the material and the exact hardware instructions.

Formlabs describes screw-to-expand inserts as installed with a simple press fit; adding a screw pushes the knurl into the material, and the guide cautions that these inserts may loosen at elevated temperatures. Formlabs also says a captured-nut pocket should be accessible after printing. As a CAD check, confirm access required by the chosen installation method and tool rather than replacing supplier instructions.

Coupons: Measure the Gap Between Model and Print

SPIROL says 3D printing tolerances often exceed the hole tolerance recommended in its catalog and lists typical values by process; those figures do not predict a particular machine’s result. The manufacturer names post-print drilling as one option that reduces variation, with a risk of compromising reinforcement in FDM outer walls. Its alternative is iterative development of the hole size.

One check you can run is a plate with several hole diameters near the supplier recommendation and the boss intended for the final part. Print it with the material, profile, and installation method under consideration, then compare the fit of the exact insert. SPIROL mentions iterative development; this article reports no coupon printed or measured by its author.

Compare seating and look for flash or boss bulging; record a result only for that insert, material, profile, and geometry. If you change a parameter in a way that could affect fit, do not assume the earlier result still applies. The dimensional accuracy guide covers checks on the printed part.

Build the Pocket Parametrically

In FreeCAD or another parametric modeler, create named variables for the selected insert drawing’s dimensions: hole diameter, depth, boss, and counterbore if needed. If several pockets use the same insert, linking those features to shared variables makes consistent edits easier; this CAD organization does not validate the assembly.

In the CAD review, check access for the chosen method and tool, along with the screw and mating part against the insert drawing. For SPIROL’s heat/ultrasonic guidance, specifically check that the screw does not bottom out in the pocket and that the mating-part clearance hole follows the diameter relationship in its guide.

Formlabs also describes printing threads directly and recommends larger sizes—M6 or above—in its guide, along with profiles designed for plastic and fillets to reduce stress concentrations. This is an option distinct from inserts, not a comparison establishing a universal choice based on the number of disassembly cycles. Decide from the screw model, material, access, and the selected process instructions.

Sources and limits

• 3D Printing Threads and Adding Threaded Inserts to 3D Printed Parts (With Video) | Formlabs
• How to Design the Proper Hole for Heat / Ultrasonic Inserts | SPIROL
• How to Select a Threaded Insert for Your 3D Printed Assembly | SPIROL

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.