Definition and the Calibration Model
It’s important to make sure you use the correct language when describing your issues and the definitions of accuracy and precision are often confused.
Accuracy is how close a measured value is to a desired value. You want to cut a 12-inch square and you get an 11-inch square, you accuracy is poor.
Precision is how close multiple measured values are. You want to cut a 12-inch square and do so in 5 passes around the plywood and each cut perfectly overlaps the previous, your precision is excellent.
I found this image that I think really clears it up (taken from https://keydifferences.com/difference-between-accuracy-and-precision.html) :
Maslow has so far been shown to be very precise, regardless of how far off the calibration is. Precision is determined by the hardware and software of the machine (motors, encoders, controller, firmware) and the current generation does an excellent job… even an amazing job for what it is.
Accuracy, however, is controlled by two things, the model of the system built into the firmware and the measured values of the parameters used by the model. This article discusses only the triangular kinematics model based upon usage of the ring kit or one of the linkage kits. We’ve seemed to have moved on from the original quadrilateral system (though a few still use it).
Currently, the model considers the following primary factors that are subject to calibration:
- Distance between motors
- Rotational radius (distance from end of chain to the router bit)
- Chain sag
- Vertical height of motors above worksurface
- Chain stretch (in development)
- Chains pitch tolerance (in development)
- Sled weight (in development)
The following diagram presents sled positioning and chain properties:
The idea is that to move the router to a specific location, the firmware calculates how much chain to feed out from each side and the math is based upon a triangular relationship between the motors’ gearbox sprockets and the router bit (hence, triangular kinematics)