Threads and Snap Fits

Four ways to put a screw in a print

Method	Good for	Watch out for
Printed threads	Large threads, lids, jars, parts that assemble without hardware	Small sizes print poorly; plastic threads are less wear-resistant than metal ones
Tapped hole	Quick, cheap threads in a plain printed hole	Needs a tap designed for plastic; the hole must be the right size
Thread-forming (self-tapping) screw	Fast assembly with no extra step	Can crack brittle materials
Metal insert or nut	Strength, wear resistance, frequent assembly	An extra installation step and the right boss size

Formlabs' guide compares these options and says to choose by material, by how many times the part will be assembled and taken apart, by the strength needed, and by how many extra steps your workflow can take.

Printed threads

Make sure the thread is real geometry

Many CAD programs show threads as a picture on a plain cylinder. In Fusion, the Thread tool makes either a cosmetic thread, which only applies the appearance of threads, or a modeled thread, which is cut into the geometry. Fusion's help notes that you may want to model a thread if you are 3D printing the part, and that milling usually does not need it. A cosmetic thread exports as a smooth hole or post, so tick Modeled before you export. Other CAD programs have their own thread or helix features; whichever you use, confirm the thread is visible in the slicer preview.

Design rules for printed threads

Formlabs' guidance for printed threads Rule of thumb:

• Stick to larger sizes: M6 or 1/4-20 and up.
• Reduce stress concentrations with fillets.
• Use thread profiles designed for plastics.
• For smaller threads, customize the profile. Formlabs' example is a semicircular profile on both screw and nut with a 0.1 mm offset Starting point, which it found gave better engagement and wear.

That offset comes from Formlabs' SLA and SLS printers. Your printer, process and orientation may need a different clearance. Print a short test of the thread before the full part Measure it.

On FDM, a thread printed with its axis vertical forms each turn from stacked layers, and the stair-stepping of the flanks shows. Prusa notes that FDM prints are stronger in one direction than another, like wood, so think about which way the load pulls on the thread; see Strength and Print Orientation.

Tapped holes and thread-forming screws

Formlabs calls tapping with a tap designed for plastic a quick, economical way to add threads, with no extra design steps. Model the hole at the tap drill size for your thread, then tap it.

Thread-forming (self-tapping) screws go straight into a plain hole; follow the screw maker's guidance for the boss (the post the screw goes into). Formlabs recommends them for ductile, high-elongation materials, and warns that brittle, low-elongation materials may crack, so take care and wear eye protection when driving screws into brittle prints.

If printed holes come out off size, OrcaSlicer's X-Y hole compensation expands or shrinks holes in XY, and its Polyholes option converts round holes into polygons that print more accurately. Check a printed test hole with the actual screw before changing the model.

Metal inserts and nuts

Metal threads are strong and wear-resistant. Formlabs describes heat-set threaded inserts, which are pressed into thermoplastics with a heated tip, and notes they can be glued into thermoset (resin) parts instead. It found that heat-set inserts in both cases offered better security and strength than screw-to-expand inserts. For resin parts, Formlabs' method is to design the boss to match the insert's widest diameter, hold the insert in place with a bead of cyanoacrylate (CA) glue or epoxy, and let the glue fully cure before installing a screw. Use the glue's label or SDS for skin protection and ventilation. For heated installation in FDM parts, hole sizes, tools and safe use are in Heat-Set Threaded Inserts.

Snap fits

A snap fit has a protruding hook on one part that catches on an undercut on the other. Formlabs describes four common types: cantilever (the most common, an arm with a hook at its free end), U-shaped (a cantilever bent back on itself), torsion and annular (a ring that snaps over a lip).

Design guidance

Formlabs says there are no exact correct dimensions, because the right size depends on material and overall size, and lists these guidelines:

• A longer hook reduces stress at its base.
• A lower hook height reduces stress and the force needed to assemble and take apart.
• Taper the hook (a trapezoidal, elongated hook) rather than making it straight and rectangular.
• Use curves at the edges of the hook.
• There is no perfect clearance; test and iterate.

Orientation decides whether it breaks

A snap-fit arm bends every time it is used. On FDM, Formlabs advises orienting the parts for strength in the XY plane, not Z, and reports that a cantilever loaded across the layer lines has its elongation at break reduced by about 50 percent and tensile strength by about 20 to 30 percent Rule of thumb. Lay the arm flat on the bed so it bends within the layers. Formlabs describes its SLA parts as isotropic, so on resin the orientation matters less for strength, though brittle resins are still a poor choice for a hook that flexes often; pick a tough or flexible resin from the resin maker's range.

Clearances for screws and snaps

Prusa reminds designers that parts that fit together need a clearance and that there is no single universal value; it depends on size, orientation, geometry, calibration, settings and material. Start from Fits and Tolerances, and measure your printer with a dimensional accuracy check. For hinges and joints that print already assembled, see Print-in-Place Hinges and Joints.