Dimensional Accuracy (3D Printing) – 7 Tips & Tricks

Achieving dimensional accuracy with 3D prints can be a pain. Use these easy tips and tricks in order to gain a headstart on achieving the highest tolerances possible!

What Is Dimensional Accuracy?

 

Dimensional accuracy, as its name implies, is the accuracy of dimensions. In this article, we are going to specifically focus on the dimensional accuracy of 3D printed parts. How good is it? And how can you improve it?

But first, why is dimensional accuracy important?

In fact, for many models it is unimportant. If you just need something for fun or decoration, you usually won’t be pulling out the calipers to ensure a tolerance of 1 Angstrom. However, if you need precise parts to be compatible with other hardware, for example, high dimensional accuracy is a must.

1. Check Your Printer's Accuracy

Before you start tightening belts and tweaking firmware, find out how accurately your printer is printing. To this end, a number of helpful test prints exist out there, with calibration cubes being especially useful.

Using a specific example, Thingiverse user iDig3Dprinting’s calibration cube measures 20 mm in the X, Y, and Z directions. If you print it, you should measure the actual dimensions of your print. The difference between the actual dimensions and 20 mm will equal the dimensional accuracy of every axis on your printer. (For precise measurements, use a caliper!)

For desktop FDM machines:

Greater than +/- 0.5 mm is BAD

Less than +/- 0.5 mm is AVERAGE

Less than +/- 0.2 mm is GOOD

Less than +/- 0.1 mm is FANTASTIC

Keep in mind that, most times, positive dimensional inaccuracies are better than negative ones. You can sand prints down to size, but it’s pretty hard to add material after printing!

This is a great place to start! Now, you know how much of a problem your printer’s tolerance is.

2. Check Working Units

Before all else, make sure that you are working with the right units. If your 3D model is using one unit, and your slicer uses another, you will obviously notice some discouragingly large or small prints.

So make sure you are only working in one unit of measurement, and move onto the next easy fixes.

3. Tension Belts

Next, check your printer’s belts. Both loose and overly-tight belts can cause dimensional inaccuracies.

Almost every printer uses a different belt tensioning process. Regardless of your printer type, tighten your belts so that the drive gears cannot slip and the belts give a guitar-like resonation when plucked.

If you are having trouble tightening belts, do the best you can and print a belt tensioner to finish the job. The springs of clothes pins also work quite well!

4. Check Linear Movement Components

Correct working units? Check! Belts tight? Check!

If you are still noticing dimensional inaccuracy, make sure that the linear rods, rails, and screws are all straight! If they are bent, accurate movement is impossible. Replace bent parts and run another test print.

If everything is perfectly straight, try slightly oiling the linear rods and screws. A little lubrication can go a long way!

5. Reduce Print Temperature and Lower Flow Rate

Now, if you have good dimensional accuracy but want to improve it even more, keep a lookout for blobs or stray extrusions. These excess bits of material on a print can affect its dimensional accuracy.

Reduce print temperature in 5-degree increments or flow rate in 5% increments until these excess extrusions disappear. Be careful, though! If you decrease the nozzle temperature or flow rate too much, under-extrusion and clogging are inevitable.

6. Compensate While Designing

Even though all these fixes can improve your printer’s dimensional accuracy, you cannot expect 100% accuracy from your prints. There will always be at least a tiny inaccuracy that lingers.

If you are designing your own parts to print, take the inaccuracy into account for the part to be printed perfectly on your machine.

For example, if your printer can only print down to +/- 0.1 mm, try making screw holes 0.1 mm bigger. Compensating for the dimensional inaccuracy of your printer during the design process will make your experience much smoother.

You can also scale other designer’s models to fit your machines tolerances. However, make sure (before you scale) that you won’t be messing up any important features of the model.

7. Adjust Steps Per Unit in the Firmware

This is a “last resort solution”. If you are experiencing significant (greater than +/- 1 mm difference) dimensional inaccuracy after tensioning belts and checking your linear rods, rails, and screws, chances are that your printer’s firmware has the wrong steps per mm set for your axis motors.

Do not resort to this solution early in the game. Most of the time, especially with bought 3D printers, the steps per mm is set perfectly. If you are experiencing dimensional inaccuracy and adjust this already-tuned setting, you will only be skirting around the actual problem. As a result, you might notice even worse tolerances.

To properly adjust steps per unit in your printer’s firmware, do not go from the dimensions of your calibration cube. Instead, measure (as precisely as you can) how far your axis are moving when told to jog a certain distance.

Assign the variable e to the expected distance, o to observed distance, and s to current steps per mm

Measure e from your cube and find s in your firmware

Calculate: (e/o) * s

Change the current steps per mm in your slicer with this new value

Repeat this for each axis

Note: You may need to assign different steps per unit for each motor even though they are the same kind of motor. Adjust each axis’s steps per unit based on the dimensions you measured from that axis.