Dowels, pegs or magnets: how to join 3D printed parts
Every seam in a split model needs something that lines the parts up and something that holds them. Here is how the usual methods compare, with clearances and magnet pocket sizes that work on FDM printers.
Six ways to join printed parts
| Method | Alignment | Strength | Removable | Best for |
|---|---|---|---|---|
| Separate printed pins | Very good | Good once glued | Yes, if dry-fitted | Most glued assemblies |
| Wooden dowels or metal rod | Very good | Very good | Rarely | Large statues, heavy props |
| Integrated pegs | Good | Medium: pegs snap between layers | Yes | Small and medium parts, nothing to lose |
| Magnets | Weak on their own | Low in shear | Yes | Parts that come apart for transport or access |
| Dovetails and keys | Good in one direction | Good | By sliding | Flat panels, signs, terrain tiles |
| Glue only (butt joint) | Poor | Depends on the glued area | No | Walls too thin for a connector |
Printed lying down, a separate pin is stronger than an upright peg, which is held only by a few layers.
Which method for which project
- Permanent display piece: printed pins or wooden dowels, plus glue. The pins align, the glue holds.
- Large or heavy model: wooden dowels or a threaded rod through several parts, set in epoxy.
- Must come apart: magnets, ideally with a pin or peg so the parts cannot slide, for example a helmet faceplate or a model that travels by car.
- Thin shells (under about 6 mm): there is no room for a hole. Glue the butt joint and reinforce it from the inside with a printed strip or fibreglass cloth (see gluing and finishing seams).
Clearance: always print a test first
On FDM printers a clearance of 0.1 to 0.3 mm per side is typical: about 0.15 mm in PLA, 0.2 mm in PETG, ABS and ASA, less in resin. It also depends on flow calibration and hole direction: vertical holes stay round, horizontal ones sag at the top.
- Print a tolerance test in your material and settings, for example with the free tolerance test generator.
- For glued joints, pick the gap where the pin slides in with light pressure, since glue needs room. For dry joints, pick the snug one.
- Make each pin about 0.5 mm shorter than the two holes combined, so it never holds the seam open.
- Trim the elephant foot on cut faces that sat on the bed.
In the SplitSTL editor you set the clearance once for every hole, and the export includes a fit-test coupon: block 2 uses your clearance, block 1 is 0.05 mm tighter, block 3 is 0.05 mm looser.
Magnet sizes and pocket dimensions
Neodymium disc magnets are sold as diameter × thickness in millimetres. Make the pocket 0.1 to 0.2 mm wider than the magnet and about 0.2 mm deeper, so it sits just below the surface without holding the seam open.
| Magnet (Ø × thickness) | Pocket Ø | Pocket depth | Min. wall around | Typical use |
|---|---|---|---|---|
| 5 × 2 mm | 5.1–5.2 mm | 2.2 mm | 1.5 mm | Miniatures, small lids |
| 6 × 2 mm | 6.1–6.2 mm | 2.2 mm | 1.8 mm | Figure bases, small parts |
| 6 × 3 mm | 6.1–6.2 mm | 3.2 mm | 1.8 mm | Small parts, a bit more pull |
| 8 × 3 mm | 8.1–8.2 mm | 3.2 mm | 2.4 mm | Helmet and armour parts |
| 10 × 3 mm | 10.1–10.2 mm | 3.2 mm | 3 mm | Medium props, panels |
| 12 × 3 mm | 12.1–12.2 mm | 3.2 mm | 3.6 mm | Large removable sections |
Leave at least 1 mm of plastic under the pocket so it does not show through. Two small magnets spaced apart resist twisting better than one big one, and pull drops fast with distance: even thick paint between magnets weakens the joint.
Gluing magnets with the right polarity
A magnet glued the wrong way round repels its partner, and you usually notice once the glue has set. Mark them first:
- Stack all the magnets into one column; they line up by themselves.
- Mark the top face of each one with a permanent marker before taking it off the stack. Every marked face is now the same pole.
- Glue the magnets marked side out in one part and marked side in on the mating part.
- Check each pair with a loose magnet before the glue sets.
CA (super glue) sets in seconds and suits snug pockets, but its bond to the nickel plating is brittle: a magnet can pop out after a drop. Two-part epoxy fills gaps, takes shocks better and leaves time to turn a flipped magnet round, so use it for worn or handled items. Never push magnets in with a soldering iron: common neodymium grades weaken above about 80 °C.
Frequently asked questions
What clearance should I use for printed pins?
Start with about 0.15 mm per side in PLA and 0.2 mm in PETG, ABS or ASA, then print a tolerance test. Glued joints need a slightly looser fit than dry joints.
Can magnets hold parts together on their own?
They pull parts together but resist sliding poorly. That is often enough for display pieces; for handled parts, add a pin, a peg or more magnets spaced apart.
Super glue or epoxy for magnets?
CA for tight pockets on static pieces; epoxy for worn or handled items, because it fills gaps and survives shocks better.
Does SplitSTL add connectors automatically?
Yes. Choose separate pins with matching holes, integrated pegs or magnet pockets, set diameter and clearance, and every cut gets them, with each part number engraved on its cut faces.