Post-processing options when the raw print is not accurate enough
When to use reaming, drilling, machining, sanding or calibration fixtures to reach tolerances that FDM cannot hold as-printed, and what each option costs.
Most FDM parts do not need any secondary operation to hit their functional tolerance. A small number of features on a given part, usually a bore, a shaft seat or a flat mating face, sometimes do. Knowing which operation to apply, and being clear that it is a targeted step on one feature rather than a blanket rework of the whole part, keeps the cost of tight tolerances proportional to what actually needs them.
This guide walks through the post-processing options we use most often to bring a specific dimension closer to nominal, when each one makes sense, and what it costs relative to designing around the raw print tolerance instead.
Reaming for precise round bores
Reaming is our default recommendation whenever a bore needs to hold a bearing, a shaft or a dowel pin with a tolerance tighter than roughly plus or minus 0.1 mm. We print the bore undersize by a defined allowance, typically 0.3 to 0.5 mm on diameter, then ream it to final size. This gives a round, accurately sized bore regardless of the minor out-of-roundness typical of a raw printed hole, at a cost of a few minutes of manual work per part.
Drilling and light machining
For holes that were not modelled at all, or where a standard drill size is more practical than modelling and printing the exact geometry, drilling after printing is often faster and more accurate than trying to print the hole directly, especially on small diameters below about 3 mm where printed holes tend to close in slightly or come out undersize. Light face milling on a flat mating surface is useful when flatness matters more than the raw printed surface can guarantee, for example a sealing face or a surface that must sit flush against a machined part supplied by the customer.
| Operation | Typical achievable tolerance | Best suited for |
|---|---|---|
| Reaming | +/- 0.02 to 0.05 mm | Round bores for bearings, shafts, pins |
| Drilling | +/- 0.05 to 0.1 mm | Small holes, standard fastener clearance |
| Face milling | +/- 0.05 mm flatness | Sealing or mating flat surfaces |
| Sanding / filing | No fixed value, manual fit | One-off parts, small batches |
| Calibration fixture | Depends on fixture, +/- 0.1 mm typical | Correcting minor warp on flat or planar features |
Calibration fixtures for warp and flatness correction
On larger flat parts in materials prone to warping, such as ABS or ASA, a simple fixture that clamps or lightly reheats the part into a flat reference plane after printing can correct minor deviation without machining. This works best on thin, mostly flat geometry and is not a substitute for the orientation and cooling strategy discussed in our warping guide, it is a targeted correction for the residual deviation that remains after those measures.
Manual fitting for one-off parts and small batches
For a single prototype or a very small batch, light sanding or filing of a bore or a slot is an acceptable way to correct a fit that came out tighter than intended, and it is often faster than reprinting. This is not something we plan into a repeatable production process, since manual fitting introduces variation of its own and does not scale past a handful of parts, but it remains a reasonable pragmatic fix during prototyping.
- Problem
- Bore printed to nominal 12 mm came out roughly 11.8 mm and slightly out of round, bearing would not seat.
- Change
- Bore reprinted at 11.6 mm nominal with a defined reaming allowance, then reamed to 12 mm H7 after printing.
- Result
- Bearing press-fits consistently across the batch, added cost was about three minutes of manual reaming per part.
Deciding whether post-processing is worth it
Before adding a secondary operation, we check whether the tolerance can instead be met by adjusting the clearance design, as covered in our fits guide, or by reorienting the part so the critical dimension sits in the more accurate XY plane rather than in Z. Post-processing is worth it when the feature genuinely cannot be loosened, for example a standard bearing bore diameter that is fixed by the bearing manufacturer. It is not worth it when a small design change would achieve the same result without adding a manual step to every part in a production run.
- Check if the clearance design alone can absorb the required tolerance
- Check if reorientation moves the critical dimension into the more accurate XY plane
- If neither is sufficient, select the lightest post-processing operation that reaches the required tolerance
- Define the printed allowance needed for that operation and update the model accordingly
- Confirm the result on a first article before releasing the process for a full run
Frequently asked questions
- When should I plan for reaming instead of trying to print a bore to final size?
- Whenever a bore needs a tolerance tighter than roughly plus or minus 0.1 mm or must be genuinely round, such as a bearing seat. Print it undersize by 0.3 to 0.5 mm on diameter and ream to final size.
- Does post-processing add significant cost to a printed part?
- For a single targeted operation like reaming one bore, the added cost is usually a few minutes of manual work per part, which is small compared to redesigning around an unrealistic raw print tolerance.
- Can warping be corrected after printing without machining?
- On thin, mostly flat parts, a calibration fixture that clamps or lightly reheats the part into a flat plane can reduce minor warp. This works alongside, not instead of, proper orientation and cooling strategy during printing.
- Should I rely on manual sanding to fix a tight fit in a production run?
- No. Manual sanding is acceptable for a single prototype or very small batch, but it introduces part-to-part variation and does not scale. For a production run, fix the clearance design or add a defined post-processing step instead.
- How do I know if post-processing is actually needed or if a design change would be enough?
- Check first whether adjusting the clearance or reorienting the part to place the critical dimension in XY solves the problem. Post-processing is worth it mainly when the tolerance is fixed by an external component, such as a standard bearing.
Have your part reviewed before production
Send us your CAD file together with the application, load and operating conditions. We review geometry, orientation, material and tolerances and come back with concrete change proposals and a quote.
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