Friday, 22 June 2012

The only way is up

I have always had my machines home away from the bed, with the Z limit switch at the top. Reprap software has always done the opposite, homing towards the bed. As I wanted Mendel90 to work with standard Reprap hardware and software I designed a bottom limit switch with a fine screw adjustment. Playing with it like that for a few days reinforced my opinion that homing upwards is much better.

I have clips in the corners of my bed, and being limited to using the Prusa PCB heater and the smallest bulldog clips I can find means the corners are no go areas. That means there is no homing sequence that guarantees not to crash the head. G28 homes X and Y to a corner and then homes Z, so that is a guaranteed crash. I had to home X first and then move to the middle before homing Y, then move to the middle and home Z. But if Z was already lower than the clips and Y was at one extreme then homing X would crash. So then I raised Z a few mm before homing but that means if Z was at the top it could ram the bar clamp.

After a few days I had crashed the head several times, ripped the tape on my bed and the final straw was homing Z with some crap on the nozzle. That stopped the nozzle reaching the bed so it failed to trigger the endstop. That caused it to pull the nuts out of the X ends and then spin endlessly so the levelling was lost.

I really hate levelling the bed when there are four points (it is easy with three, but without redesigning the PCB, I am stuck with four). So I went back to having a fixed limit switch at the top. It has the following advantages: -

  • Z homes to the top first, so it is always a safe operation no matter what the starting position is and what is on the bed or if there is any plastic stuck to the nozzle.
  • It prevents the Z axis ramming into the top, which is potentially damaging because it can exert quite a lot of force. Driving Z past the bottom is fairly safe because the nut traps are open at the bottom. That means the maximum force it can apply to the nozzle is the weight of the X axis.
  • Z = 0 is defined to be when the nozzle touches the bed (when hot) but it never has to actually go to that position. It is defined by putting the distance from the end stop in the firmware configuration so adjusting it is a matter of tweaking a number rather than a screw adjustment. I set it roughly by nudging to 8mm while rolling an 8mm bar under the nozzle. Then I extrude an outline, cool it and measure its height. I can then correct the number to get the first layer exactly the right height. There is no trial and error, just a simple adjustment procedure.
  • The end stop is much simpler as it no longer needs to be adjustable.
I had to hack Marlin to make it work. If the homing position is towards the top it does Z first instead of last.

I also like my X Y origin to be in the middle of the bed, not one corner. That means the G code doesn't need  to contain the origin offset, everything is sliced to be at 0,0. The comments in configuration.h implied that you could define the origin in the middle but then the soft limits didn't work. I changed the way the axes are defined to be more flexible. Instead of defining just the HOME_POS and the MAX_LENGTH I define the MIN_POS, MAX_POS and HOME_POS. For X and Y they are -100, 100 and - 102 as Mendel90 has exactly 200 mm of travel plus 2mm clearance each end and 2mm to the end stop. So the axis only ever hits the endstop during homing. After that the soft limits prevent it hitting any of the ends. Z cannot be driven lower than 0, so it can just touch the bed worst case.

The code is available on Github. I will remove the bottom endstop from the Mendel90 configuration as I don't see any need for it. Changing a number in the firmware is so much easier than tweaking a screw.

Since reverting to this method I have not had any head crashes or homing mishaps, it just works. I also have ooze free unattended starts with the following G code.

M83 ; use relative distances for extrusion
G1X5Y99F9000 ; Go to the middle of the front
G1Z0.05 ; close to the bed
M104 S200 ; set extruder temp
M190 S55 ; set bed temp & wait
M109 S200 ; wait for extruder temp
G1E5F50 ; extrude a blob
G1X40F4000 ;wipe 40mm along the edge of the bed
G1Z0.3 ;lift Z