When looking at the firmware calculations, here are those Geometry parameters that seem to matter, and wether thay are missing.
Spoiler: Chain Stretch and Top beam flex corrections seem very important and missing.
|Parameter||Description||Impact on XY coordinates||Present in firmware?|
|motor spacing||distance between the center of the gear box shafts||0.5:1 on horizontal, 2:1 on vertical||Yes|
|motors y offset||Vertical distance from the gear box shaft (motor) to the top of the workspace||nil on horizontal, constant 1:1 on vertical||Yes|
|rotation radius||‘extra’ distance to add to the actual chain length to reach the bit||1:1 proportional on horizontal, 2.5:1variation on vertical||Yes|
|chain sag||flexible chain/line will sag under it’s own weight in a catenary curve||lower corners get up to 4mm horizontal, 0.6mm vertical error||Not registering the real sled and chain weigth. But the parabola approximation is more efficient. although it needs calibration. The firmware CoordinatedMovement() migh need to be rewriten to improve accuracy by breaking long moves into small ones. ( sag correction warps space!)|
|chain tolerance||gap between chain links to allow them to move||creates asymetrical horizontal displacement errors. 0.13% error yields up to 4mm horizontal error in the oposite corner, and 4mm vertical error along the workspace top||Yes|
|chain stretch||elongation due to chain tension (rubber band effect)||1mm horizontal near the workspace top. 4 mm vertical near the workspace top center.||NO but is simple to implement. It would require a chain tension estimation. See the MaslowCNCModelOnOctave|
|sprocket “12 o’clock” error||Is calibrated using Ground Control||The error shows up to 1:1 on the horizontal oposite side, and creates a vertical error amplified on the workspace center top.||Yes by careful calibration|
|Chordal error||the amount of chain fed out for a given amount of rotation is not a constant as the effective radius of the sprocket changest||Neglectable||No|
|top beam flex (front/rear)||potential for the top beam to flex forward slightly, changing the effective motor spacing||TBD||No|
|top beam flex (up/down)||The weight of the sled moves from side to side, changing the downward force on the ends of the beam||0.5:1 Proportional to sled horizontal position, and parabola on vertical up to 1:1||NO would need a parameter and a simple calculation. See the MaslowCNCModelOnOctave|
|top beam tilt (compared to workpiece)||if the top beam is tilted compared to the workpiece, coordinates are wrong||TBD||No, but seemd small based on a thread dicussion where a pull request did not conclude|
|top beam tilt (compared to gravity)||if the top beam is tilted compared to gravity, tensions on the chains will not be what’s expected, which will throw off the position||TBD||No, but using a level to install the frame at a level position could be a critical step on frames like the 10 feet top beam for a 4x8 workspace.|
|triangulation kit flex||If the triangulation kit doesn’t keep the end of the chain the exact same distance from the bit, (due to flexing under tension, shifting of joints, or other reasons) this will affect the effective chain length||TBD could not yet be measured out of the other uncontrolled parameters||No|
|sled rotation||if the sled rotates to the point that the triangulation kit hits a limit, the line of the chain effectivly bends||TBD if occurs in normal operation||No|
|triangulation kit sag||the triangulation kit is heavier than the same effective length of chain||TBD if significant||No|
|encoder error||the encoder measures rotation of the sprocket using discrete steps. Movement less than a step size cannot be measured. However this error does not accumulate||Neglectable||No|
|backlash||If tension on the gearbox moves from being on the sled side to being on the back side, the chain position gets an offset equivalent to the gears play.||Neglectable if design prevents reverse of balance||No|
|weight of cords/vacuum hose||depending on where the sled is, there is a different amount of weight added to the sled by the cords/vacuum hosts||Would create chain stretch and Sag correction change but sag correction is usefull in lower corners only||No, but once stretch and sag will be tuned, we might want to see if this item becomes significant.|
|Loose router fit into router base||depending on how loose the clamp is to allow sliding with some types of Z axis mod||Would let the router bit deflect under lateral forces during routing||No. Carefull mechanical assembly is the way to go|
That is why I carefully waxed and adjusted the clamp latch to not hold the router body and yet not let the router body move sideways (affecting the bit position).