Heat-Set Insert Kits: What to Buy for Threaded Inserts in 3D Prints

A standard M3 heat-set insert from CNC Kitchen is 5.7 mm (0.224 in) long, 4.6 mm across the knurl, and goes into a 4.0 mm (0.157 in) hole, according to the dimension table the maker publishes with the product. Those three numbers decide most of what you need to buy: the inserts themselves, a tip that fits both the insert and your iron, and a temperature setting your iron can actually hold.

This guide is about the shopping side. How to model the boss, chamfer the hole and press the insert in square is covered in our heat-set threaded inserts guide. If you are still deciding whether you need inserts at all, read 3D printing threads and bolts first, because printed threads cover more jobs than people expect.

Three purchases that have to fit together

A working insert setup is three parts: a bag or kit of brass inserts, an installation tip sized for each insert diameter, and an iron that accepts that tip. Most frustration comes from buying them separately from different ecosystems. A tip set made for the TS100 shaft will not slide onto a Hakko T18 heater, and a kit of M3 inserts does nothing for you if the only tip you own is the conical one that came with the iron.

Buy in this order: first decide the thread sizes your parts use, then pick an insert family with a published dimension table, then match a tip set to the iron you already have. Buying a new iron comes last and is often unnecessary.

What a usable insert datasheet contains

Before you order a bag of inserts, look for a table with at least four values per size: length (L), outer knurl diameter, recommended hole diameter, and minimum wall thickness around the hole. CNC Kitchen lists all four, plus a centre point diameter for the smooth lead-in section. Unbranded bulk bags often list only the thread size and length, which leaves you to measure with calipers and guess the hole.

The hole diameter in the table is the one number you will type into CAD for every part. If a seller cannot give it to you, the cheap price stops being cheap once you have reprinted a bracket three times to dial in the fit.

Metric sizes from one maker's table

These values come from the CNC Kitchen dimension table as published on the product listing at retailer 3DJake (all values in millimeters). The minimum wall is the plastic you need on each side of the hole, so the smallest round boss is hole diameter plus two walls.

Read the last column before you buy M5. An 11.6 mm (0.457 in) boss is wider than many enclosure walls, so a part that looks like it needs M5 may fit an M4 insert and a slightly longer screw much better.

Short, standard and Voron-length inserts

CNC Kitchen sells M3 in a 5.7 mm standard length and a 3.0 mm short version, and M4 in 8.1 mm and 4.0 mm. The short versions exist for thin lids and panels: a 3.0 mm insert fits a 4 mm thick wall with plastic left underneath, while a 5.7 mm insert would punch through.

The M3 Voron variant (4.0 mm long, 5.0 mm knurl, 4.4 mm hole) is named for the Voron printer project and needs its own 4.4 mm hole. If you print Voron parts, buy that variant; if you design your own parts, the standard M3 with its 4.0 mm hole is the more common target in published models.

Imperial threads and the 1/4"-20 camera mount

US makers working with #4-40, #6-32 or 1/4"-20 hardware need inserts in those threads, and they are less common in hobby-oriented kits. The CNC Kitchen table includes a 1/4"-20 insert at 12.7 mm (0.5 in) long, 8.7 mm knurl diameter and 8.0 mm hole, with a 3.3 mm minimum wall. That works out to a boss of at least 14.6 mm (0.575 in), which fits a tripod plate or camera rig without much trouble.

For smaller imperial sizes, check that the tip set you buy covers them. The CNC Kitchen tip set for TS100/TS101 lists #2-56, #4-40, #6-32, #8-32, #10-24, 1/4" and 5/16" next to the metric sizes, so one set handles mixed hardware.

Why a conical soldering tip lets the insert down

A widespread claim says any soldering iron tip works for inserts. It does work, in the sense that the brass gets hot and sinks. Three things go wrong with a standard conical tip, though. The cone wedges into the insert's thread bore, so the insert sits on a point and tilts as soon as your hand moves. There is no flat face to show you whether the insert is square to the surface. And when you lift the iron, the cone can catch the insert's rim and pull it partway back out of the soft plastic.

Adafruit's product page for the Virtjoule insert tip describes exactly this: the dedicated tip has a vertical wall so the iron "pulls right out after pressing," and because its shoulder is square "it's easier to know if the insert is square in the piece" (Adafruit product 4239). A conical tip can also reach the threads themselves, where molten plastic then has a path into the bore.

How to tell which tip fits your iron

Tip sets are built for a heater standard, and the heater decides compatibility. Check which one your iron uses before ordering:

1. TS100/TS101 style shaft: CNC Kitchen's "Installation Tips SET compatible with TS100 & TS101" fits this group, and the maker lists Pinecil and Miniware irons as examples.
2. Pinecil: PINE64 sells its own "Pinecil Threaded Insert Tips Set and Adapter" with M2, M2.5, M3, M4, M5, M6 and M8 tips plus an adapter.
3. Hakko T18 or 900M style: CNC Kitchen's "Installation Tips SET compatible with 900M & T18" covers these irons, with narrow tips for M2 to M3 in tight spaces and standard tips for M4 to M8.
4. Weller: Third-party listings disagree about which Weller models their tips fit. Measure your iron's heater barrel before ordering, or pick an iron from the groups above.

Pinecil, TS-style irons and the Hakko FX-888DX

If you have no iron yet, two routes make sense for inserts. The PINE64 Pinecil V2 sets between 100 and 400 °C (212 to 752 °F) and runs on USB-C at 12 to 20 V and 3 A or a 12 to 24 V barrel jack, per PINE64's product page. PINE64 recommends a 65 W USB-C PD charger at 20 V; with a weaker charger the iron gets less power to work with.

The Hakko FX-888DX station uses T18 tips and, according to its user manual, sets between 50 and 480 °C (120 to 899 °F). A bench station suits you if the iron lives next to the printer and also does electronics work. The Pinecil suits a small desk or a toolbox that moves between workspaces.

Iron temperature: the maker's rule, worked through

CNC Kitchen's guidance on its insert listings is to install at 10 to 20 °C above the temperature you printed the part with. That ties the setting to your filament instead of to a fixed number:

All of these sit well inside the Pinecil's 400 °C ceiling and the FX-888DX range, so the iron you choose will not limit you on common filaments.

Where the displaced plastic goes

The knurl is wider than the hole, so the plastic in that ring has to move somewhere. For a standard M3 insert the upper bound is a ring 4.6 mm outside, 4.0 mm inside and 5.7 mm deep: 0.785 × (4.6² − 4.0²) × 5.7 ≈ 23 mm³. For M5 the same calculation gives 0.785 × (7.1² − 6.4²) × 9.5 ≈ 70 mm³, about three times as much. The knurl grooves take up part of that, so real displacement is lower.

That volume rises as a lip around the hole or drops to the bottom of the hole. A blind hole modeled exactly as deep as the insert leaves the plastic nowhere to go but up into the threads. Model the hole deeper than L, which is why the datasheet length matters as much as the hole diameter.

When you should not buy inserts

Some problems look like an insert shortage and are something else:

• The screw goes in once and stays: A self-tapping screw or a printed thread holds fine for parts you assemble one time.
• Every hole comes out the wrong size: If a 4.0 mm hole prints at 3.7 mm, the insert will crack the boss. Fix dimensional accuracy first, then buy hardware.
• The load pulls along the layer lines: Inserts do not fix weak layer adhesion around them. Print orientation or a captive nut pocket may be the real solution.
• Resin prints: CNC Kitchen's own listing says to glue inserts into resin prints instead of melting them, so an iron tip set does nothing there.

Decision table: what to buy for which bench

Ad: Linked product names in this table are affiliate links. If you buy through them, we earn a commission at no extra cost to you.

Building a heat-set insert kit that matches your parts

Start with the thread size you use most and buy that insert in a 100-piece bag with a published hole diameter. Add the matching tip set for the iron you already own; only buy an iron if you have none or if yours uses a heater no insert tip set supports. Set the iron 10 to 20 °C above your print temperature, model the hole from the datasheet and deeper than the insert, and calibrate your hole sizes before blaming the hardware. For the installation steps themselves, the insert installation guide picks up where this list ends, and the home workshop setup guide covers where the iron and its stand belong.