Delta 4th axis length and limits?



  • Intro

    I working on a 'flying' extruder based on the newish addition of extra axis on delta's. I am wondering what happens if the usable travel length of the 4th axis is shorter or longer than what would be required?

    Setup

    For example, my delta has a homed height of 634mm with a 4th axis located at (0, 0). My extruder should be a be able to swivel in a directions for a maximum of 55° from the bed normal. Combined with a 446mm diameter bed, the virtual diagonal rod would have to be ~272mm.
    If we make the assumption the print head only moves up and down at (0, 0) you would need a usable travel length of 634mm , located from Z=272 to Z=906. Adding X/Y moment in the mix, you would need to lower the Z=272 to Z=156 for the hotend to be able to reach the bed edges. If my old cos and tan's are correct, a 4th axis travel of 790mm is required (from Z=156 to Z=906).

    Question

    How does the firmware deal with shorter or longer upper limits? What happens if the 4th axis endstop is located higher than the what the machine can reach based on all the rod legths etc. Same question for when the endstop if located lower, so the 3 main tower endstops cannot be reached. Bonus question, do you need a endstop at the bottom of the 4th axis as well?


  • administrators

    RepRapFirmware won't handle that configuration properly. However, you should find that the travel needed for the 4th axis is less than the travel needed for the main towers. I expect your main rods are about 540mm long and the carriages go up to about 1080mm. So between the extruder outlet at the top of its traveland the top of the machine, you should have around 170mm, in which to fit the extruder and the 4th axis mechanism. If you need more space, then the 4th axis mechanism will need to protrude from the top of the machine.

    You don't need an endstop at the bottom of the 4th axis.



  • @dc42 My diagonal rods are 500mm long with carriages mounted on 1000mm mgn12 rails, so you were fairly close 🙂 . Can you elaborate on what won't work in that setup (and do you mean my IP setup, or a setup with sorter limits?)


  • administrators

    What I mean is that you need to set the homing switch height for the 4th axis such that when all 4 switches are triggered, the extruder is at the correct distance from the effector, or at least close enough not to be a problem (you can fine-tune the 4th homing switch offset in the M666 command).



  • @dc42 said in Delta 4th axis length and limits?:

    What I mean is that you need to set the homing switch height for the 4th axis such that when all 4 switches are triggered, the extruder is at the correct distance from the effector, or at least close enough not to be a problem (you can fine-tune the 4th homing switch offset in the M666 command).

    I see, that is an important piece of info. Does RRF assume the homed height is the same for all axis, ie. once the machine is homed it is assumed the 4th axis can move 634mm down in my machine, or how does that work?


  • administrators

    Once the axes have been homed, RRF assumes that all axes can be moved by the amounts needed to reach the defined build area.



  • Thanks for the info @dc42, I think I can continue the design process. I see what I come up with in a few weeks/months and post here.



  • Some minor tweaking here and there is expected but normally the design for my 4th axis extruder should be completed. All the parts are printed and will probably installed tomorrow 🙂
    On the arm a Pi with a camera is mounted near the carriage. The extruder (Bontech BMG) will be installed in a gimbal style mount so it can swivel in all axis, to minimize the required bowden length.

    0_1564007053258_2019-07-25 00.11.13.jpg



  • Following with great interest.



  • @danal said in Delta 4th axis length and limits?:

    Following with great interest.

    I can already tell you that the first version I designed of the 4th axis worked quite well. The arm was printed to heavy and the carriage I used wasn't ideal but it did it's job.