When parts have to fit together, accuracy matters far more than appearance. MJF holds tolerances well for a powder process, but it is important to understand what it can and cannot reliably do so that your assemblies work first time rather than after several attempts. A little knowledge here saves both money and frustration when the parts finally arrive.
We tune builds for accuracy as part of our MJF & SLS printing service, and we will always tell you honestly when a particular feature needs a different approach. Setting expectations correctly up front is far better than discovering a fit problem once the parts have arrived on your bench.
What accuracy to expect
As a rough guide, MJF typically holds around plus or minus 0.3mm on smaller dimensions, with a small percentage tolerance accumulating over larger lengths. That is more than good enough for the vast majority of housings, brackets and general assemblies, where a snug rather than a perfect fit is all that is needed for the part to do its job.
Several factors feed into the final dimensions, including the part size, the geometry, the wall thickness and how the part is positioned within the build. Larger flat parts can be a little more prone to slight movement than small compact ones, which is worth keeping in mind for big panels and long spans. It is also worth remembering that the very first measurement off the machine can settle slightly over the following day as the part finishes cooling and relaxing, so for critical work we let parts stabilise before checking them against your drawing.
Designing for a good fit
Rather than printing to nominal dimensions and simply hoping, it is far better to design in deliberate clearance wherever parts mate. Holes can close slightly during fusing, so plan any critical bores oversize or drill them to size afterwards for a guaranteed result that does not depend on luck.
For threads, you can print them directly for light duty use, or design in clearance and add threaded inserts or tapped holes where the joint must be strong and used repeatedly. Choosing the right approach for each fastening point makes the whole assembly more reliable over its life.
Measuring and quality checks
For parts where fit really matters, it pays to check the first article carefully before committing to a full batch. Simple measurements with callipers confirm the key dimensions, and a quick trial assembly catches any tight spots before they multiply across a run.
If you have drawings with critical dimensions marked, share them and we can verify those features against your tolerances. For very tight work, 3D scanning and photogrammetry can compare a finished part back to the original model in fine detail. Keeping a record of those measurements across batches also builds a useful history, so any slow drift can be spotted early rather than after a problem reaches a customer.
When tolerances are critical
If a particular feature demands very tight precision beyond what the process holds naturally, there are several reliable ways to achieve it without abandoning MJF for the whole part. The trick is to print most of the part and finish only the features that truly need it, which keeps the overall cost sensible.
- Drill or ream critical holes after printing.
- Add metal inserts for accurate, durable threads.
- Machine key faces where flatness is essential.
- Combine MJF with CNC machining for hybrid parts.
Talk to us about tight features
Mark up the dimensions that genuinely matter and send them along with your model when you get a quote. We will tell you exactly what MJF can hold on its own and where a quick finishing step will guarantee the fit, so there are no surprises when the parts land on your bench.