Capabilities for bigger machine



  • I’m impressed by the Duet board. My dilemma is that I want to build a very large printer using what I know from my othet cnc router and plasma build.
    The size is going to be - 6x6x4 ft cube around 1800x1800x1200mm.
    I don’t need large X motor as it only need to move the print head, Y motors might need to be tandem to run the gantry, Z needs 4 motors to lift the entire frame. What is the most powerful stepper I can run from the board? Can I pin out from the existing XYZ to external drivers ( ie Gecko Drive )?
    Also,
    It will need a large heated bed, can I run 4 heat beds from the board?
    The 3D printer realm is so fantastic, I want to learn it all.



  • The first thing I would decide is the XY motion mechanics and system. There are various methods of achieving XY motion:

    • H-Bridge - this is the normal gantry with a motor for X and a motor for Y. Either X or Y is doubled, with the other one riding on top of it. H-bridge can also be flipped to be X and Z (like in the Prusa i3 style design). This is a very common way to build a 3d printer.
    • CoreXY - this is an advanced build style where both motors control both directions. The main advantage of this is if done correctly it is extremely stable - for all movements, both motors are used and it pulls the head on 2 different positions in the same direction. Motors are stationary mounted, meaning the head have less inertia, giving better speeds etc.
    • Cross - this is the same method used in the Ultimakers. Each of the X and Y axis have 2 stationary rods at both the Min and Max (these can be doubled for extra rigidity; typically the one is above the other). Then on each of these rods is a small gantry block, connecting to the belt (belts are also on both sides, though only driven by a single motor). Also between the gantry blocks, is another rod. These extra rods create a cross, and having your head (usually a bowden style head, to be very light) there, gives you precise control and movements.

    These are the methods I have came across before, but there are others as well.

    Ideally one would prefer to move as little mass around as possible - smaller mass, less inertia - greater speeds and acceleration possible (being able to print faster saves a lot of time, though it will take a long time to build any object of that size - in my limited experience we are talking about weeks for a single print, unless you use something like the Moarstruder or E3D Volcano that is built for extremely large amounts of filament).

    On the H-bridge some experts recommend running a single axis with a single motor, connected with a belt on at least one side. This means you spend time squaring it, and the 2 sides will remain in sync. With different stepper motors you have the downside that they can go out of sync every time the motors are disengaged (requiring a homing position). Sometimes we don't have a choice, and need to run 2 motors for the axis (typically in CNC applications, where having the extra power of another motor is crucial) - just know they go out of sync and plan for a way to sync them again (some people run them to a side until both skip steps).

    For the Z axis, I have seen a 1200x1200x1200 printer from an expert that uses a single Z motor with belts running to all the lead screws - again it is leveled and synced once and remains in sync. That machine only used 3 lead screws, but it worked.

    The most powerful stepper you can run from the board: The stepper motors are able to provide up to 2.4A/phase of current. Typically Hybrid stepper motors are more powerful than others. Running the board in 24V Vin, allows motors run faster than 12V (due to the Back EMF - use the Back EMF calculator at https://reprapfirmware.org/).

    On my latest build (a CNC machine), I will be running 3A rated stepper motors at the 2.4A maximum allowed in the firmware (actual limit is 2.5A). These motors are Nema 23 with holding torque of 2.2Nm, for a 3D printer this would simply be overkill. Running at this high currents, require active airflow over the board to protect the drivers (they will shutdown if they run too hot, but best to keep them cool). Ideally one would run a stepper motor at 80-90% of rated current to ensure they remain cooler - with adequate cooling of the stepper motors self, one can run them at higher than rated current, but typically we don't.

    There are 2 ways to increase the torque of stepper motors: increase the current (this is where the current limit plays its role); increase the number of coils in each set (this is why some motors get very long and these are fine).

    As I understand, only the other stepper driver signals (expended on the expansion boards Duex2 and Duex5) can be run on external motors. Typically the builtin drivers (and expansion boards) should be sufficient for a 3D printer (capable of running up to 10 steppers).

    Heatbeds: The Duet boards only have a single heatbed output able to supply up to about 18A at Vin (newer versions of the board may have this reduced to 17A). Requiring more current than this, will blow the mosfet controlling the heatbed; however the best alternative solution is to either run this output to more Mosfets (for DC heatbeds) or Relays (for AC heatbeds) which control all of them at the same time. Both the Mosfets and Relays come in pre-made modules that you can integrate quite easily.


  • administrators

    @jacotheron said in Capabilities for bigger machine:

    The Duet boards only have a single heatbed output able to supply up to about 18A at Vin (newer versions of the board may have this reduced to 17A). Requiring more current than this, will blow the mosfet controlling the heatbed;

    Actually it's not the mosfet that's the limiting factor, it's the PCB traces. Even using 2oz copper and duplicating the traces on both sides, it's difficult to carry large currents on a PCB.



  • I'm running a TInyG on a big CNC router. The TinyG is a "one board" solution for desktop CNC, and therefore has drivers on board that have similar limitations, one or two amps.

    I'm using Gecko G201X at 70V to drive (large, I don't remember) NEMA motors.

    The point of this is... anything that will take Step/Pulse can be driven from almost any board. Yes, with the Duet, it is best to "skip" the onboard, and use expansion... Still quite a few ports available, and Duet "mapping" commands make it VERY easy to use any physical port for a given logical axis.



  • @jacotheron said in Capabilities for bigger machine:

    Heatbeds: The Duet boards only have a single heatbed output able to supply up to about 18A at Vin (newer versions of the board may have this reduced to 17A). Requiring more current than this, will blow the mosfet controlling the heatbed; however the best alternative solution is to either run this output to more Mosfets (for DC heatbeds) or Relays (for AC heatbeds) which control all of them at the same time. Both the Mosfets and Relays come in pre-made modules that you can integrate quite easily.

    Or "SSR", Solid State Relay. That's what I'm doing on my 600mm (24") delta, with about a 1800 watt heater. Duet output that would go to the bed heater (i.e. 12 or 24V) goes to the input (control) side of the SSR. 110V/120V AC is switched on the other side.

    Works great.



  • If using external drivers you can hook up multiple drives to a single step/dir output. I run a machine with 5 axis motors but use only 3 external drive outputs on the duet (2 motors on Y, 2 on Z, 1 on X) for a 1000mm^2 machine. Could hook up four drivers for the Z axis using this same technique.

    Might have to shift the logic level of the output signal depending on the external driver. I just use this one: https://www.filastruder.com/collections/electronics/products/duet-expansion-board

    As others have mentioned it's easy to hook up a SSR for mains power to the existing heater outputs with no change. Could even connect multiple SSRS together without an issue if you have multiple heated areas. Just have to be cognizant that you can't PWM most SSRS very fast when running AC through them. Probably want to run the bed heater in bang-bang mode (which it is by default) or cap the frequency to the ac mains (50/60hz).


 

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