Proper tolerancing is essential for ensuring parts fit correctly. If a hole is designed to be exactly 1/4” for a dowel pin and cut using a waterjet or laser cutter, the fit may be inaccurate.
Waterjets produce tapered cuts, making the hole too small, while laser cutters often cut on the line, resulting in a slightly oversized hole.
Traditional CNC machines, like mills or lathes, achieve much tighter tolerances when properly configured.
Similarly, FDM 3D printing introduces dimensional inaccuracies due to extrusion variations, thermal expansion, and layer adhesion inconsistencies. Factors like nozzle diameter, print speed, and material shrinkage can cause holes to be slightly larger or smaller than intended.
Slip Fit
A slip fit is a type of clearance fit where parts can slide or "slip" together without force. This is useful when components need to be assembled and disassembled easily, such as shafts in bushings or spacers on bolts. In FDM 3D printing, achieving a reliable slip fit can be difficult due to inconsistencies in extrusion and dimensional accuracy. As a rule of thumb, a clearance of 0.2–0.5 mm around the mating part diameter often results in a functional slip fit, but this varies by printer, material, and orientation. Printing test pieces with incremental offsets is the best way to dial in the right gap for your printer and application.
Press Fit
A press fit, or interference fit, is when two parts are designed to be slightly larger than the other, requiring force to assemble. This creates a tight, often permanent connection without adhesives or fasteners—common for installing bearings, gears, or dowel pins. With FDM printers, reliable press fits are harder to achieve due to print variability, so testing different offsets is critical. A typical starting point is designing the hole 0.1–0.3 mm smaller than the shaft or insert. Heat can also assist in assembly: gently heating the printed part (e.g., with a heat gun) can help expand the hole slightly for a cleaner press.
Structural Backlash
Backlash is the unwanted play or looseness in a mechanical system, typically associated with gears, but it can also occur when two or more components are bolted or mounted together imprecisely. In assemblies involving 3D-printed parts or laser-cut panels, bolt holes that are too large can introduce slop between parts, affecting structural integrity and precision. To minimize backlash in mounted components, parts should be designed with tighter hole tolerances and, where possible, use dowel pins or alignment features to ensure consistent positioning. For motion systems, especially those involving belts or gears, minimizing backlash is critical for repeatable movement and accuracy. See: gear backlash
Manufacturing Tolerances
A common way for teams to deal with prints interacting with each other and hardware is to form a table with a list of commonly used hole sizes for your various applications and then use these hole sizes in their CAD. Numbers depend on your printer and nozzle size, so we would recommend making test prints to see how hardware can fit best in order to form this table. An example test print would be a print with a 2.8mm, 2.9mm, 3.0mm, 3.1mm, and 3.2mm hole to see which best creates an M3 through hole.
This method is generally acceptable if kept well maintained and updated per printer, but it does mean that you have inconsistent hole sizes in CAD that would seem arbitrary to anyone looking at your team’s CAD.
