In theory, a part designed at 20.00 mm should print at 20.00 mm.
In reality, that almost never happens.
Tolerances exist because manufacturing is not perfect, especially in additive manufacturing. Every functional part must account for variation — in materials, machines, orientation, temperature, and post-processing — or it risks failure the moment it’s assembled.
Functional design isn’t about hitting exact dimensions.
It’s about predictable fit, movement, and performance.
What Is a Tolerance?
A tolerance is the allowed variation from a nominal dimension.
Example:
- Nominal size: 20.00 mm
- Tolerance: ±0.20 mm
- Acceptable result: 19.80 mm – 20.20 mm
Without tolerances, parts may:
- Bind instead of slide
- Crack during assembly
- Require force to fit
- Wear prematurely
- Fail under load
Why Functional Parts Fail Without Tolerances
1. Assemblies That Don’t Assemble
Press-fits, snap-fits, and sliding parts are the most common failure points.
Two parts printed “perfectly” often interfere with each other once real-world variation stacks up.
This is known as tolerance stack-up — where multiple small deviations compound into a major issue.
2. Material Shrinkage & Expansion
Different materials behave differently:
- PLA shrinks very little
- PETG expands more
- ABS and nylon shrink significantly
- Carbon fibre filaments reduce shrink but add stiffness
Designing one tolerance for all materials almost guarantees failure.
3. Print Orientation Changes Dimensions
FDM prints are anisotropic:
- XY dimensions are usually accurate
- Z dimensions vary more
- Hole diameters print undersized
- Vertical walls behave differently to horizontal features
A hole designed at 6 mm may print at 5.6 mm — every time — unless compensated for.
4. Forcing Parts Masks Bad Design
If a part “needs a tap with a mallet,” that’s not success — it’s stress being locked into the part.
This leads to:
- Cracks around fasteners
- Long-term creep
- Sudden failure under load or heat
Good tolerances eliminate force during assembly.
Common Tolerance Mistakes
- Designing holes at nominal fastener size
- Assuming printer resolution = dimensional accuracy
- Ignoring post-processing effects (sanding, reaming, heat)
- Copying injection-moulded dimensions into FDM designs
- Designing snap-fits without flex allowance
Designing With Tolerances in Mind
Practical Guidelines (FDM Printing)
While every printer and material differs, these are reliable starting points:
- Clearance fits: +0.2 mm to +0.4 mm
- Sliding parts: +0.3 mm to +0.5 mm
- Press fits: material-specific and test-based
- Bolt holes: oversize and clamp with fasteners, not friction
- Moving parts: always allow for debris and wear
Testing a small tolerance coupon before committing to a full print saves time and material.
Tolerance Is a Design Tool, Not a Fix
Tolerances should be:
- Intentional
- Tested
- Documented
- Adjusted per material and use case
They are part of the design process, not something you “fix” after a print fails.
Final Thought
Functional parts don’t fail because printers are inaccurate —
they fail because designs assume perfection.
Designing with tolerances means your parts:
- Assemble smoothly
- Perform reliably
- Survive real-world use
- Scale from one-off to production
That’s the difference between a printed object and a functional part.