Delta Calibration

FirePick Delta Calibration

FirePick Delta (FPD) is a high precision machine. It is also an extensible, affordable open source machine. To reduce end-user cost, FPD can be built using a wide variety of manufacturing techniques that result in parts whose dimensions can vary significantly from nominal design dimensions. You may also want to customize your FPD and change one or more dimensions. It is therefore critical that you calibrate your individual FPD for highest accuracy to account for these manufacturing and assembly variances.

The kinematic model for FirePick Delta involves at least 21 variables. This is considerably more complex than a Cartesian machine and requires corresponding care and attention for calibration. When you calibrate your FPD, you are matching FireStep's internal kinematic model to your actual machine so that it can control your FPD precisely. Any one of these variables can throw off your machine's accuracy.

NOTE: FPD has rotational symmetry about the central X0Y0 axis, with arms positioned at even angles of 120-degrees from each other. It is tempting to assume that similar parts for the three axes are identical. However, ignoring inter-axis differences will definitely decrease the accuracy of your FPD. 3D-printed parts tend to vary in practice even when printed on the same printer from a common design.

Definitions

See Glossary

Other Error Sources

Actual machines always differ from their ideal design. Most variances are introduced during manufacture as well as assembly. For example, 3D printed parts tend to vary more than injection molded parts and certainly more than CNC machined parts. And assembly variances are introduced by the very looseness that facilitates assembly. There are, however, other sources of error you need to be aware of during calibration.

Digitization Error

Digitization error is caused by using a digital specification for an analog coordinate. Stepper motors can only have discrete positions measured in microstep pulses. Given any stepper position, we can compute a precise machine position; however, we cannot make the machine move to arbitrary positions. FPD digitization error is typically on the order of ~0.005 mm for a 16-microstep FPD using 200-steps/revolution steppers.

You cannot eliminate digitization error, but you should expect it as you analyze calibration results. In particular, if a number seems off by 0.005, perhaps it's just digitization error and therefore ignorable.

NOTE: This is actually one of the benefits of 400 steps/revolution motors. They tend to run slightly smoother and quieter than their 200 steps/revolution cousins because of reduced digitization error.

Calibration

Calibrate FireStep as follows:

Step 1. [Calibrate steppers](Stepper Calibration)

Step 2. [Configure MTO-FPD](Configure MTO-FPD)

Step 3. [Calibrate latch backoff](Latch Backoff)

Step 4. [Calibrate Z probe](Z Probe)

Step 5. [Calibrate home/bed angle](Z Bowl Error)