Resizing an STL: why scaling to 110 percent breaks every hole, and what to do instead

What the slicer's scale box actually does

Type 110 into the scale field and the slicer multiplies every vertex by 1.1. Every one. The outer wall, the inner wall, the screw holes, the clip that snaps on a 0.2 mm interference, the text on the lid. Nothing is exempt because the mesh does not know which parts are features and which are just material.

That is fine when the whole object is the feature. It is a disaster when the object has to fit something else.

Three things that break

Holes. A 3 mm screw hole scaled to 110 percent is 3.3 mm. Scaled to 90 percent it is 2.7 mm and the screw no longer goes in. Holes are the first casualty because their tolerance is a few tenths of a millimetre and scaling moves them by whole tenths.

Wall thickness. A 1.2 mm wall at 80 percent is 0.96 mm, which most slicers print as a single fragile line or skip entirely. At 130 percent a snap clip designed to flex at 1 mm becomes 1.3 mm and snaps instead of clicking.

Spacing. Scale a Pi case to fit a slightly larger board and the mounting holes move apart proportionally, which is not how the board's holes moved. The hole pattern on a board is fixed; the case outline is what you wanted to change.

The two cases where scaling is correct

• Proportional objects with no fit. Vases, figurines, decorative items, cookie cutters that should just be bigger.
• Compensating printer shrinkage. Scaling 100.3 percent to correct a material that shrinks is exactly a uniform correction, so uniform scaling is the right tool.

Non-uniform scaling (X only, say) extends the middle ground: a box stretched 10 percent in X keeps its wall thickness in Y and Z, but the holes still become ovals and the corner radii become ellipses.

What you actually wanted

You wanted one dimension to change and the others to stay. That is a parametric edit, and a mesh cannot do it because the dimensions are not stored anywhere. A parametric model stores them as variables: length = 92; wall = 2; hole_d = 2.7;. Change length and the walls, holes and clips are untouched because they are computed from their own numbers.

SpeakCAD models are parametric programs, whether you generated the part from a sentence or uploaded an STL and had it rebuilt. So the resize is a sentence:

The last clause is usually unnecessary (the AI keeps them by default) but it costs nothing and makes the intent explicit. The render shows the previous version as a ghost, so you can see that only the length moved.

Resizing a download in practice

1. Upload the STL or 3MF and say what it is: "a two-part enclosure for a 65 x 30 mm board".
2. Say what has to change and what has to stay: "the board is now 72 x 30. Keep 2 mm walls, 2.5 mm holes 3 mm from the corners, and the USB cutout centred on the short side."
3. Check the dimensions readout on the render, tap a face to confirm a number if you like, and download.

The rebuild is not a scaled copy; it is a fresh model with your numbers, so the holes are round, the walls are the thickness you asked for and the clips are designed for the new size.

When you only need it to fit the bed

If a model is a few millimetres too big for the bed and it has no fits, scale it 97 percent in the slicer and move on. If it has fits, ask for the same model at the bed size with unchanged holes, or ask for it split into two parts with an alignment pin, which is what people do for large flat parts.