Guides

Sketch it first: planning a 3D print on paper

The prints that work first time are usually the ones worked out on paper. Ten minutes with a pencil settles the sizes that matter, the fit and how the part sits on the bed, before you open CAD.

Measure what it has to fit

Most printed parts attach to something, hold something or replace something. Measure that first, with calipers if you have them: the hole spacing, the shaft diameter, the thickness of the panel it clips onto. Write those numbers down before you draw. They’re the sizes the part can’t get wrong. Everything else is your choice.

Sketch it in 3D, roughly

On isometric paper, block the part in as a box and carve the features out of it. Don’t worry about fillets or exact wall thickness yet. Get the shape, then put the must-fit sizes from the step above onto the sketch. If a feature is hard to read in 3D, add a flat front or top view beside it.

Plan the fit

Printed parts come out slightly different from the model, and holes tend to print a little small. Wherever two parts meet, decide the gap on paper: a sliding fit needs clearance, a press fit needs little or none. A few tenths of a millimeter is a common starting point on a desktop FDM printer, but it depends on the printer, material and settings. Write the clearance on the sketch, and print a small test of just the fit before the whole part.

Choose how it sits on the bed

Sketch the part the way it will print, mark the face that goes down, and check it for:

• Overhangs. Faces leaning out more than about 45° from vertical usually need support. Could the part turn over, or take a chamfer, so they don’t?
• Layer direction. Parts are weakest between layers. A clip or hook that has to flex should bend along the layers, not pull them apart.
• Holes. Holes that run up the print come out round. Holes lying on their side can droop at the top.
• The first layer. A large, flat face on the bed helps the print stay put.

Size the walls to the nozzle

Thin walls print cleanest as a whole number of lines. With a 0.4 mm nozzle, walls of 0.8, 1.2 or 1.6 mm are safe choices. Put the wall thickness on the sketch with the other key sizes.

Then model it

With the sizes, fit and orientation settled, the CAD session is mostly typing numbers off the sketch. Keep it beside you and tick off each dimension as it goes into the model. When the first print comes off the bed, mark what changed on the same page.