Z axis gantry vs Z axis printbed

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I'm curious on this design concept where the z- axis carries the gantry instead of the pritnbed

Gantry is lighter but houses moving items, print bed is heavier and require gearing and a method to hold the bed during power off.

has anyone tried both and can comment on it?

Haven't tried a printer that moves the hotend in X Y and Z but I see some possible pros and cons to this configuration. First it would be easy to enclose and also the bed assembly and mounting is a lot simpler. The main draw back I see is that acceleration forces are now bearing on the Z axis rails which could produce Z wobble. Wire management is a little more complicated this way too. I can't really think of any huge advantage or disadvantage to this approach.

What is the easier to make and keep square ?
See also this: [forums.reprap.org]

In doubt, go Delta !

Edited 2 time(s). Last edit at 03/09/2018 11:06AM by MKSA.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

I made a printer with 300 mm square bed using belts to lift it in the Z axis (700 mm) and it's working well (click the Ultra MegaMax Dominator link in my sig, below). I would not lift the XY stage unless the printer is really big making it impractical to lift a much more massive bed. I have tested the Z axis in my printer to lift a 4 kg print mass (and the bed assembly weighs about 3.5 kg) at 20 mm/sec. It uses a 30:1 worm gear reducer to prevent the bed from dropping when power to the Z motor is off. It works perfectly.



The Rino worm gear reducer is a relatively expensive part to use- it and the keyed 8mm output shaft cost about $140. I have located some worm gears from a Chinese supplier and await delivery to test a printable gearbox I have designed. It will reduce the cost of using a worm gear reduction for a similar Z axis to about $40.

One of the other issues with moving the XY stage up and down is that the cables have to be lengthened and controlled. The bed has only heater and thermistor leads to deal with.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I view moving bed vs moving gantry to be very similar, as I in practice I feel like the bed and gantry probably have similar moving mass. There are, however, a few drawbacks to a moving gantry. The first is of course that side forces now apply through all three axes instead of only two; the second is that your bowden tube must become longer to accommodate Z travel.

With a moving bed you can essentially extend the Z axis to any height no cost, but if you are moving a bowden extruder a huge distance there will be complications. You could mount the bowden extruder on the Z axis I guess.

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6911...
the second is that your bowden tube must become longer to accommodate Z travel.

Just mount the extruder to the gantry.

The biggest challenge with an XYZ moving gantry is keeping it rigid. A flat bed on Z is easy to keep rigid and can be kept relatively light as a result. With an XYZ frame you have a square that will act a bit like a picture frame when you take the glass out. You can reinforce it, but in practice its a lot easier to have a bed that moves in Z.

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the_digital_dentist
I made a printer with 300 mm square bed using belts to lift it in the Z axis (700 mm) and it's working well (click the Ultra MegaMax Dominator link in my sig, below). I would not lift the XY stage unless the printer is really big making it impractical to lift a much more massive bed. I have tested the Z axis in my printer to lift a 4 kg print mass (and the bed assembly weighs about 3.5 kg) at 20 mm/sec. It uses a 30:1 worm gear reducer to prevent the bed from dropping when power to the Z motor is off. It works perfectly.

Z%2Baxis%2Brev3.jpg

The Rino worm gear reducer is a relatively expensive part to use- it and the keyed 8mm output shaft cost about $140. I have located some worm gears from a Chinese supplier and await delivery to test a printable gearbox I have designed. It will reduce the cost of using a worm gear reduction for a similar Z axis to about $40.

One of the other issues with moving the XY stage up and down is that the cables have to be lengthened and controlled. The bed has only heater and thermistor leads to deal with.

Ya good point on the cabling. I do think going belt on the z-axis is going to be a trend, Voron V2 is starting to use belt in the Z axis .

Curious, why didn't you try a 2 stage gear reduction using pulley and closed loop belts? Maybe 2 (4:1) to give 16:1 reduction. GT2 pulleys of various sizes are very common and I think GT2 backlash is going to be better than gearboxes.

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Origamib
The biggest challenge with an XYZ moving gantry is keeping it rigid. A flat bed on Z is easy to keep rigid and can be kept relatively light as a result. With an XYZ frame you have a square that will act a bit like a picture frame when you take the glass out. You can reinforce it, but in practice its a lot easier to have a bed that moves in Z.

Yup your right, I'm guessing that is the reason why Voron v2 went with 4 belt driven Z axis gantry at 4 corners.

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MKSA
What is the easier to make and keep square ?
See also this: [forums.reprap.org]

In doubt, go Delta !

I have a delta as my 2nd printer.
Ikessel
The prints are really good that I'm now trying to figure out how to make my current corexy as good as it, reusing most of the hardware.

Edited 2 time(s). Last edit at 03/12/2018 10:59PM by Vigilant.

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Vigilant
Curious, why didn't you try a 2 stage gear reduction using pulley and closed loop belts? Maybe 2 (4:1) to give 16:1 reduction. GT2 pulleys of various sizes are very common and I think GT2 backlash is going to be better than gearboxes.

The worm gear reducer doesn't have any backlash because the weight of the bed assembly plus print always keeps the gear teeth engaged on the same side, whether the direction of motion is up or down. In that respect the worm gear acts like a lead screw and the disc gear like a nut.

I specifically chose a worm gear drive to ensure that the bed won't drop when Z motor power is cut. A bunch of belts and pulleys could get the same level of drive reduction but wouldn't necessarily guarantee that the bed won't drop. The specific worm gear assembly I used isn't guaranteed to be irreversible, but it would take a much larger load that my printer will present to back drive the motor.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
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The Rino worm gear reducer is a relatively expensive part to use- it and the keyed 8mm output shaft cost about $140. I have located some worm gears from a Chinese supplier and await delivery to test a printable gearbox I have designed. It will reduce the cost of using a worm gear reduction for a similar Z axis to about $40.

As an alternative, I wonder whether using a stepper motor with a planetary gearbox might do the job, for example this one [www.omc-stepperonline.com] ? It would depend on the weight of the bed being sufficient to take out the backlash, but not too much to overcome the detent torque of the motor (which unfortunately they don't specify). The maximum permissible torque on the output shaft is specified as 3Nm so if the belts are driven by 20 tooth GT2 pulleys then in theory it could support a 48kg bed without exceeding the torque rating.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

It might work, but it provides only one driven output (which may be OK in some printer designs), and that reduction ratio is going to give weird full-step resolution with any pulley you can find. That 20 tooth pulley and 2 mm pitch belt will give 95 full steps per mm which translates to full step layer thickness intervals of 0.0105263... mm. That's worse that what I have in UMMD (waiting on new pulleys to arrive to fix that). It does hit nice, round layers at 0.2 mm intervals (multiples of 19 full steps). There are probably planetary gearbox/pulley combos that allow a little more freedom in selecting print layer thickness.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]