How to split an STL that's too big for your print bed

Draw the seam where you want the join, add alignment pins, cut, export, print. Five minutes, free, in the browser.

Most models that don't fit a printer are only a little too big in one direction. Scaling the model down loses the size you wanted, and the usual fix — a flat plane cut wherever the software happens to put it — leaves a straight line across the model's face that shows after painting. The better answer is the one a sculptor would give: put the join where a join already belongs, at a fold, a belt, an edge, and pin the two pieces so they go back together straight.

1. Load the model and check the size

Open the STL or 3MF in Model Splitter — drop the file onto the viewport, or use Open. The Size fields show X, Y and Z in millimetres. Compare them with your printer's build volume (a Bambu A1 is 256 mm each way; an Ender 3 is 220 × 220 × 250; a Prusa MK4 is 250 × 210 × 220).

If the model is over in one direction only, one cut is enough, and you get to choose where it goes. If it is over in two or three, jump to much bigger than the bed below and let the auto planner do the first pass.

A model with holes or flipped faces will refuse to cut. Inspect & repair tells you, and Repair — safe only fixes most of it without changing the shape.

2. Draw the seam where you want the join

Choose Band in Cut Tool and draw a loop around the model where the join should be — around a limb, a waist, the base of a neck. Then drag the orange line into the place it will hide: the edge of an armour plate, the fold of a sleeve, under a belt. Click to pin a point in place; right-click a pin to release it. Smoothing eases out a dragged stretch so the seam doesn't kink.

Two other ways to draw the same seam:

• Line — one stroke across the model, and the loop is inferred through the back. Quickest, but turn the model and check where it went.
• Rope — draw what you can see, turn the model, carry on from the loose end, and close the rope at its start. Use this when the back matters.

Whichever you use, turn the model right round before you cut. The seam you didn't look at is the one that ends up across a face.

3. Add alignment pins

In Joints, leave the type on built-in pins and set Count — two is enough for a limb, three or four for a torso. A pin is part of one piece and fits a socket in the other. They are for alignment, not strength: the glue holds the pieces, the pins stop them sliding while it sets and make a curved seam go back together in exactly one position.

Pin thickness and length are sized from the cut face, and Clearance is the gap around the pin. The defaults print and fit on an FDM machine. If you find your pins need forcing, come back and raise the clearance a little; if they rattle, lower it.

Prefer something you can take apart? Magnet sockets make recesses for magnets instead — choose a preset or type in the magnet's measured size.

4. Cut, and look at both pieces

Press Cut. Rotate the result and look at the cut faces and the pins — the app shows both pieces, and the pin should sit well inside the face with room around it. If the seam needs to move, Back to editing returns the uncut model with the loop still there to adjust.

When a cut is refused, the message says why: usually the cut face is too narrow for the pins you asked for. Fewer or thinner pins, or a seam through a fatter part of the model, fixes it.

5. Export and print

Export gives you the pieces as separate STLs (a zip when there are many) or as one 3MF with every piece in it, which most slicers open as a plate. Save the project too if you might want to move the seam later.

In the slicer, put each piece's cut face down on the bed — it is flat, it needs no support, and it hides the first-layer texture inside the join. Print the socket piece with a little extra care on the first layers, since the socket is what the fit depends on. Then dry-fit, glue, and clamp along the seam with tape or a rubber band while it sets.

Much bigger than the bed?

When a model is over in two or three directions, choose Auto multi-plane, pick your printer from the list or enter its build volume, and Model Splitter proposes the fewest planes that make every piece fit. Keep clear reserves a margin at each edge of the bed. Every plane can be moved afterwards, and you can still draw a curved seam on any single piece if a plane lands somewhere ugly.

For a life-size print this is the difference between a grid of boxes and a set of pieces that follow the model — on a 1.8 m figure the planner needed 58 pieces where a plain grid needed 112.

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