@JoergS5 @o_lampe
That's ok if the center is unusable, it will just have a smaller donut shaped build volume.

Sorry, the big blue wheel on the right is just a filament spool and is unrelated to the kinematics.
X is rotational, Y is rotational, and Z is linear.
The arrow labeled 62mm is just a distance measurement from the min and max travel points.
It is similar to the dual disk printer but I think mine wont have crosstalk parameters if I am understanding crosstalk correctly for polar kinematics.

I'm designing a fun little printer with minimal components but I am struggling with the kinematics. I believe that this is equivalent to SCARA kinematics but the bed spins and acts as the proximal arm which means that the X direction changes as the bed spins. If I am incorrect about this then what kinematic style would this be?

I would consider the center of the bed to be 0,0.

The distal arm is 31mm long and its axis of rotation is exactly 31mm from the center of the bed in what I would call the +X direction. This means that there is theoretically no dead-zone in the center of the bed.

The bed (proximal arm) can spin continuously because there is no heated bed and in theory doesn't require homing.

The distal arm has 180 degrees of rotation. According to the documentation, a positive rotation is counter-clockwise when viewed from above and 0 degrees for the distal arm is defined as when the distal arm is in line with the proximal arm. This makes me think that the B parameter of the M669 command would be -180:0 since the arm swings 180 degrees in the negative (clockwise) direction from 0 degrees which is the maximum angle the distal arm can reach.

Would this be the correct M669 command?
M669 K4 P31 D31 A0:3600 B-180:-0 X0 Y0

Would the crosstalk parameter be 0?
Would the X and Y parameters be 0 since the nozzle can reach the center of the bed?

Even the example provided here seemed confused about the crosstalk parameter.
https://duet3d.dozuki.com/Wiki/ConfiguringRepRapFirmwareSCARAPrinter#Section_Movement_section_of_config_g_file

Edit: here is more of my config file so far.

; Drives
M569 P0 S0						; physical drive 0 goes backwards
M569 P1 S1						; physical drive 1 goes forwards
M569 P2 S1						; physical drive 2 goes forwards
M569 P3 S0						; physical drive 3 goes backwards
M584 X0 Y1 Z2 E3				; set drive mapping
M350 X16 Y16 Z16 E16 I1			; configure microstepping with interpolation
M92 X8.889 Y8.889 Z400 E415		; set (steps/mm) or (steps/degree) θ Axis: 200 steps/rotation * 16 microsteps/step / 360 degrees/rotation = 8.889 steps/degree
M566 X300 Y300 Z10 E400.00		; set maximum instantaneous speed changes (mm/min) or (degree/min)
M203 X12000 Y12000 Z450 E1200	; set maximum speeds (mm/min) or (degree/min)
M201 X400 Y400 Z30.00 E250.00	; set accelerations (mm/s^2) or (degree/s^2)
M906 X800 Y800 Z800 E800 I30	; set motor currents (mA) and motor idle factor in per cent
M84 S30							; Set idle timeout

; Axis Limits
M208 X-62 Y-62 Z0 S1	; set axis minima. Are these in mm or degrees?
M208 X62 Y62 Z110 S0	; set axis maxima. Are these in mm or degrees?

I wand to add a third hotend but i dont want to buy the whole duex expansion board just for one more thermistor and heater. Is there a way i can add those components more cheaply like with a solid state relay hooked up to the heater pins on the expansion headers (to reduce the current requirements through the pins)? The thermistor is low current enough that would it be possible to plug it directly into the expansion headers? Thanks.

@mrehorstdmd Thanks for the thorough reply. I did see your blog post before I posted this and that was definitely an inspiration to this idea. I was going for a maxwell kinematic coupling and you are using a Kelvin kinematic coupling.

I'm not sure which is best. Both allow for thermal expansion of the bed but the Kelvin style expands by the countersunk point. The maxwell one expands by a weird point in the middle. But once the bed had reached temp it shouldn't shift much. Just don't change the bed temp mid print.

The only thing I thought that could be improved in your design is that the area enclosed by the three points is smaller than it could be which could possibly cause the bed to tilt if you print something heavy outside that area.

I think I preffer the bottom arrangement more than the top one. The top one just makes it easier to adjust manually since you can change the pitch and roll but if I use assisted bed leveling it won't matter much.

If you think it's very stable then maybe I will just make it manual adjustment and only motorized it if I really need to fix it a lot.

@dc42 thanks for the suggestion about the assisted bed leveling. I didn't know about that. I guess you might be right about just making it a manual adjustment but I thought that as long as I can make it cheap enough then it would be nice to motorized it. The real question is how stable will it be. Every bed gets unlevel after enough time/use/abuse. I think I will make it manual first and see for myself how stable it can be if I use a lock nut or something.

I am working on a new 3D printer design and I wanted to have it level the bed for me but the only way I have seen this done is to use 3 independent z motors which I see an expensive solution to a small problem. Why add two more z motors and linear rails to go with them just to add about 10mm of travel to two points on the bed? I say two because with 3 point leveling one point can be at a fixed height (but can still tilt around that point) and the other two can be adjusted to match the fixed point and I am intending on using this system with a z probe on the head.

Just to clarify, I am thinking something like this

Why not instead create a cantilever bed with only 1 z motor but make the two adjustable points sitting on adjustable shims (with a v grove on top). The shims could be screw driven adjustable parallel like this one.

The shims could be controlled with a cheap motor such as a sg99 servo modded to be continuous (this does remove the position feedback though) or a 28BYJ-48 stepper. These are only about $2 USD each and personally, I think about $10 would be worth having the printer level itself.

The main issues I see with this idea are:

• I am not sure how reprap firmware handles independent z motors that are not controlled by a typical bipolar stepper motor. (28BYJ-48 is unipolar)
• How would it treat the motors since they have vastly different travel distances
• This Duet wiki post says "If you have two Z motors, the calibration process adjusts the bed tilt along a line between the two leadscrew positions, on the assumption that the tilt at right angles to this will not change." which would not be the case if one of the points is fixed height (but can still rotate).

Any thoughts? Would this require change to the firmware or could it be done with a macro? Could I use any other pins to power a continuous servo or a 28BYJ-48? Also note that I have not actually used a Duet before but it is the controller board I think I want to use in my design.

oh awesome. Thank you very much!

I guess the main and more general question is still unanswered but I assume you cannot store variables and do math within a macro?

im sorry if this has been asked before but I am playing around with the idea of making a printer that has 3 z motors that can automatically mechanically level the bed. This will likely involve probing three locations on the bed and storing the distance traveled at each position in a variable then rotating each motor by a certain value. calculating that value will at least require multiplication and will probably require all three . can this be done with a macro or will it require customizing the firmware or is there some other method like a c++ standalone function that could rewrite the macro since it is just a text file? thanks and sorry again if this has been asked before.