Tolerance Stackup Basics

A worked example

Say you stack three spacers in a slot. Each spacer is meant to be 10 mm, but is made to ±0.1 mm. The slot is meant to be 30.3 mm, also ±0.1 mm. Will the spacers always fit?

• Three spacers, each up to 0.1 mm over: 30.3 mm total.
• The slot at its smallest: 30.2 mm.
• Worst case: 30.3 mm of spacers into a 30.2 mm slot — it jams.

Each part was within spec. The stack is what failed. That is the whole idea: tolerances accumulate, and you have to check the total, not just the individual parts.

The worst-case method

The simplest and safest stackup method is worst-case: add up every tolerance that affects your gap, assuming they all go the wrong way at once.

Worst-case is conservative — it assumes every part is at its limit simultaneously, which is rare. For mass production engineers use statistical (RSS) methods that give a looser, more realistic number. For one-off and small-batch parts, worst-case is the right call: it guarantees the fit.

Where stackup bites a 3D-printed or machined part

• Bolt patterns across two parts. Two plates each with their own hole-position tolerance must still line up. This is exactly why loose-fit clearance holes exist — the extra clearance absorbs the stackup.
• Press-fits and snap-fits. The interference is small (tenths of a millimetre), so a little stackup flips a press-fit from “tight” to “loose” or “cracks the boss.” See press-fit and snap-fit design.
• Lids and enclosures. A box plus a lip plus a wall thickness plus the lid — four tolerances in a row before the lid even touches. Plan clearance into the design.
• 3D printing adds its own swing. FDM parts carry roughly ±0.2–0.5 mm on top of your design tolerance — covered in 3D printed part tolerances. Treat the printer’s accuracy as another term in the stack.

Five ways to beat stackup

• Fewer parts in the loop. Each dimension you remove removes a tolerance. One part that does the job of three has no internal stack.
• Add clearance on purpose. A gap that is meant to be there is far cheaper than a tight fit that sometimes fails.
• Tighten only the dimensions that matter. A tighter tolerance costs money; spend it on the one or two dimensions in the critical loop, not everywhere.
• Use slots or adjustable features. A slot instead of a hole lets a fastener absorb position error at assembly.
• Make one part the reference. Locate everything off a single datum face so errors don’t chain part-to-part.

Design parts that fit the first time

Describe the part and its mating fit to PartWork.ai — “add 0.3 mm clearance around the lid” — and you get editable geometry you can check in the 3D viewer before committing. Because the part is parametric, when a fit is off you change one number instead of rebuilding. See creating parts and modifying parts.