?? Best way to design the Z-axis movement ??

I want to build a large printer (1200 mm x 1200 mm bed). At this stage, I'm favouring a flying gantry design where the print bed is stationary and the X and Z axes are fitted to a gantry which moves on a track system attached to the side of the print bed assembly.

The question I am trying to solve is: Should the Z axis assembly be moved by a system which has steppers on each side of the gantry like a Prusa, with the X-axis acting as the connection between the gantry sides as in this picture:

Attachment 620

or have the Z axis on a plate moved along the X-axis and only having one stepper on the Z-axis to move it up and down as in this picture:
Attachment 621

I would add support brackets to the Prusa system for strength and rigidity. The only problem to overcome with the second design is to decide how much movement could be build into the Z-axis before it began to lose accuracy due to wobble.

What do you think?

Old Man Emu

I can see in the latter design how you would suppress z-axis wobble in one dimension (the x) by having the z axis move vertically on either a plate or two shafts. The further apart the shafts, the less wobble I can see a problem with z-axis wobble in the y dimension, though. Eliminating it would require a reasonably stiff x-axis and minimal play in whatever the hot end is moving on in z.

I have seen a rather clever design that used a single z-axis stepper where the stepper was mounted at the top connected to two lead screws using a chain.

I'm beginning to think that the best of both worlds would be to merge the two by having Prusa set up for the Z and X axes fixed to the gantry of the second design.

Old Man Emu

Wouldn't that introduce more wobble than the traditional moving print bed, as you're moving and decelerating more mass?

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Originally Posted by 3dkarma

Wouldn't that introduce more wobble than the traditional moving print bed, as you're moving and decelerating more mass?

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I think that he electronics can cover that by setting the acceleration rate.

Don't forget that I'm looking at a 1200mm bed length. If I use a movable bed, then the machine will have a footprint that is over 2400mm (say 2500mm to allow for exterior parts of the frame). The moving gantry would have a footprint of, say, 1500mm for the same 1200mm bed size. We don't want to need an aircraft hangar to put this in. The movable gantry design would fit onto a dining table sized support.

Old Man Emu

You're right - I missed that bit about the size of the print bed. It does seem like tacking the prusa z/x onto your y carriage seems the best bet. Would stability be improved by having the two x rods on the same horizontal plane, instead of the same vertical plane? Are you thinking of using extrusion instead of rods?

If I were building something that large, I wouldn't move the bed at all. Fixed bed, move the heads in X/Y/Z. Then you're not shifting the mass of the build around.

The configuration in the second picture looks like it would have more precise positioning (less flexing).

With the gantry design, the print bed and object are stationary, so there is movement of the mass there. There is no real need for the extruder to be any larger than the extruder of a desktop printer (using Wades Gears or similar), and that moves side to side, relative to the gantry. Provided that the gantry/X-axis assembly was rigid, there should be little wobble.

In this set up, for any particular layer, neither the Y axis (print bed) nor Z axis (in the +Z direction on each layer) moves as the layer is being extruded. Only the movements are the Gantry moving along the Y-plane and the X-axis assembly moving side to side. This printer is not likely to be used for printing small, high feature things like figurines. It is to be used where it is necessary to print long or wide things such as a bumper bar for a car.

Old Man Emu

Quote:

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Originally Posted by 3dkarma

. Would stability be improved by having the two x rods on the same horizontal plane, instead of the same vertical plane? Are you thinking of using extrusion instead of rods?

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I have a design for a gantry machine that uses two rods for the X-axis track. With linear bearings that will make that assembly very rigid.

Since this machine is going to be semi-industrial, I am going to use bright bar instead of the nice shiny chromed bar normally used on Prusas. Have you ever priced that shiny bar? I'd need to sell my first-born son to buy the nearly 6 metres I'd need for the X and Y runners. A bit of lubrication will let the bearings slide.

OME