3D printing tolerances are process-specific: SLA holds finer detail than FDM, SLS and MJF sit in between, and metal processes have their own behavior. Specifying a printed part’s tolerance means matching the requirement to the process’s real capability and reserving critical fits for post-printing machining. This guide provides the tolerance ranges by process, explains the silent variables of shrinkage and warpage, and shows how to call out tolerances that inspection can verify.
Printing Tolerances Are Process-Specific
There is no single “3D printing tolerance.” Each process builds parts differently—laser-cured, sintered, melted, or extruded—and each has its own dimensional behavior. SLA parts hold fine detail; SLS and MJF parts hold functional tolerance in nylon; FDM parts are the least precise; metal parts combine tight potential with significant thermal behavior.
The practical consequence is that the tolerance on the drawing must be matched to the process. A tolerance that is natural for SLA may be impossible for FDM, and a fit that needs machining should be called out as a machined surface rather than hoped for from the printer.
Design Tolerance Ranges by Process
As a reference, 6CProto’s stated design tolerances by process are approximately:
| Process | Design tolerance (reference) |
|---|---|
| FDM | ±0.2 to ±0.5 mm |
| SLA | ±0.05 to ±0.1 mm |
| SLS | ±0.2 to ±0.3 mm |
| SLM (metal) | ±0.2 mm |
| MJF | ±0.3 mm |
These are stated design ranges, not guarantees for every geometry; the actual value depends on the part’s size, orientation, and features. Use the table to set expectations and reserve tight fits for machining.
The table is read with the part’s features in mind. A large flat surface behaves differently from a small boss or a thin wall, and the tolerance on each is set by the process and the geometry together. The buyer should not apply the table’s range to every dimension; the critical features are called out specifically, and the rest runs at the process’s practical value. The tolerance plan is per feature, and the table is the starting point.
The feature-specific tolerance is confirmed with the supplier. A bore that must fit a bearing, a slot that must guide a slide, and a face that must mate with a seal each carry a requirement, and the supplier confirms the achievable value on the actual geometry. The confirmation is part of the quote, and it is the basis for the machining or the post-processing that follows. The tolerance that is confirmed is the tolerance that is held.
The tolerance range is a process family statement, not a part guarantee. The printed part’s position in the range depends on the geometry, the orientation, and the machine condition, and the drawing should specify the critical features against the process’s realistic window; the buyer who treats the range as a promise is designing on hope.
Shrinkage and Warpage: The Silent Variables
Beyond the process tolerance, printed parts shrink and warp as they cool. Shrinkage is accounted for in the process, but its consistency depends on the geometry and the build; warpage appears where sections are long, thin, or uneven. The result is that a feature that should be at tolerance can drift, especially in large or asymmetric parts.
The design response is to control the variables: orient the part to minimize warpage, keep sections uniform, and avoid features that pull the part out of shape. The tolerance that matters should be specified on features that are dimensionally stable.
Shrinkage is compensated in the process, but the compensation is not perfect for every geometry. The buyer should design the critical features with the shrinkage in mind—symmetric sections, uniform walls, and features that do not fight the material’s contraction. The design that helps the process is the design that holds the tolerance.
The warpage is managed with the orientation and the supports. A long thin feature that spans the build can curl; the orientation that shortens the span and the supports that hold it reduce the warpage. The buyer should flag the long thin features on the drawing, so the orientation and the support plan address them. The feature that stays straight is the one whose warp was planned.
Fits That Need Machining After Printing
The engineering practice for critical fits is to print the part slightly oversized and machine the fitting surface after printing. A bearing seat, a precision bore, or a mating face that must hold a tight tolerance is machined to the final value, while the rest of the part runs at the printing tolerance.
This hybrid approach—print for geometry, machine for fits—is the standard answer to printing tolerance limits. The drawing should mark which surfaces are machined-after-printing, so the process and the inspection follow the same plan.
The machined fit is a hybrid tolerance: the printed geometry holds the bulk of the part, and the machined surface holds the critical value. The drawing should show both—the printed surfaces at the process tolerance and the machined surfaces at their final tolerance—so the process and the inspection know which value applies where. The hybrid is the standard answer to printing tolerance limits.
The machining allowance is part of the design. The printed feature is made slightly oversized, and the machining removes the allowance to reach the final value; the allowance must be enough for the machining pass and not so much that the machining is heavy. The buyer should specify the allowance with the machined surfaces, so the printing and the machining are planned together. The fit that is machined cleanly is the one whose allowance was set.
The machined-fit practice is the engineering answer to printed tolerance. The bearing bore, the press-fit zone, and the mating face are printed oversize and machined to the final dimension, which combines the print’s geometry freedom with the machine’s repeatability; the drawing should mark the machined zones so the process plan and the inspection follow them.
How to Call Out Tolerances on a Printed Part
Calling out tolerances on a printed part means distinguishing the surfaces:
- Mark the machined-after-printing surfaces with their final tolerances
- Set the printed surfaces to the process’s practical range
- Define the datum scheme for the features that matter
- State the finish and any post-processing that affects dimensions
The drawing that separates machined fits from printed surfaces is the drawing that quotes cleanly and inspects without surprises.
The callout examples are practical. A boss that must fit a bearing is called out with the final diameter and the machined-after-printing note; a face that must seal is called out with the flatness; a slot that guides a slide is called out with the width and the position. Each callout ties the requirement to the process step that delivers it. The buyer should write the callouts with the process in mind, because the drawing is the process plan.
The printed dimensions that are not called out run at the process’s practical range, and the buyer should not expect better. The drawing that marks the machined surfaces and leaves the printed ones at the process range is the drawing that quotes and inspects cleanly. The buyer should review the drawing for the unmarked critical dimensions, because each one is a surprise waiting at inspection.
Inspection for Printed Parts
Printed parts are inspected like machined parts for the features that matter: critical dimensions are measured, machined surfaces are verified, and the datum scheme guides the measurement. The inspection depth should match the application—a functional fit needs dimensional verification; a display model needs less.
The buyer’s question is scope: which dimensions need verification, and what report comes with the part. Specifying the inspection in the RFQ keeps the process and the price aligned.
The inspection scope is a negotiation between the risk and the cost. Every dimension that is verified adds inspection time, and the buyer should scope the verification to the functional features. The critical fits are measured; the cosmetic and structural surfaces run on the process control. The scope is stated in the RFQ, so the price and the inspection match.
The inspection report format matters. The report shows the measured values against the tolerances, tied to the drawing, and the buyer can use it to accept the part or to drive the next revision. The buyer should ask for the report format with the order, so the inspection arrives in a usable form. The report that is readable is the report that is used.
Verify Tolerance Limits with Our Engineers
3D printing tolerance is a process-matching exercise. Match the requirement to the process’s range, control the silent variables, and reserve critical fits for machining.
6CProto’s 3D printing service covers the process range above, and the standards and tolerances page explains the tolerance framework. When you request a quote, mark the critical surfaces on the drawing and state which need machining, and the engineering team can confirm the tolerance plan before printing.
The tolerance conversation with the supplier is a two-way review. The buyer states the functional dimensions and the process; the supplier confirms the achievable values and flags the features that need machining or adjustment. The review catches the impossible requirements before the print, and it produces the plan for the critical fits. The tolerance plan that is agreed together is the plan that holds.
The tolerance also affects the post-processing and the inspection. A tight tolerance that is printed and then machined carries the machining step and its cost; a tight tolerance that is printed alone is verified against the process range. The buyer should see the tolerance plan with the process plan, because the two are one decision. The plan that is complete is the one that prices the full route.
Conclusion
Printed tolerance is a process decision. SLA, SLS, MJF, FDM, and metal each have their ranges, and the silent variables of shrinkage and warpage sit behind them. Critical fits belong to machining, printed surfaces belong to the process, and the drawing assigns each surface its place.
The next step is to mark the critical surfaces and the machined-after-printing features on the drawing, then confirm the tolerance plan before quoting.
FAQs
What tolerance can 3D printing hold?
It depends on the process. As a reference, 6CProto states design tolerances of roughly ±0.05–0.1 mm for SLA, ±0.2–0.3 mm for SLS, ±0.2–0.5 mm for FDM, and ±0.2–0.3 mm for metal and MJF.
Why do printed parts drift from nominal?
Shrinkage and warpage affect dimensions as the part cools. The consistency depends on geometry, orientation, and section uniformity, so critical features should be designed for stability.
Can printed parts hold tight fits?
Yes, by machining the fitting surfaces after printing. Print the geometry, machine the bearing seats and precision bores, and mark those surfaces on the drawing.
How should I specify tolerances on a printed part?
Separate the surfaces: printed surfaces at the process range, machined-after-printing surfaces at their final tolerance, with a datum scheme and the inspection scope defined.