what are the disadvantages of a one-plate gantry?

[WayOutWest]

Hi, I've located a shop in the US that will laser-cut small runs of steel at a reasonable price, and I'd like to start customizing my Liteplacer. Big thanks to Juha for being willing to make the DXF's available. Aside: they can only cut 1.0mm steel, not 1.5mm -- but for the low-force plates in the gantry (not like the X/Y motor plates, for example) this shouldn't be a problem, right?

One of the more major changes I want to try is moving to a HiWin linear bearing for the Z-axis, and perhaps a double-head setup (as many previous designs display you can get a second head "for free" if you don't need both to be "down" at the same time).

The current Liteplacer design uses a pair of makerslides for the X-axis, and a pair of steel plates (one on each side of the makerslides) for the gantry. One reason for this is that it provides a secure, well-centered place to mount the very powerful and long-stroke threaded-rod stepper for the Z-axis.

Assuming that it was not necessary to mount the Z-stepper this way, is there a big reason not to eliminate the back steel plate and one of the X-axis makerslides? This would result in the gantry riding on a single makerslide using four V-wheels the same way the (current) Z-axis makerslide rides. I figure if it works for the Z-axis it should work for the X-axis too, no? This would also let me lay a much larger dragchain along the X-axis; right now I'm constrained by it having to fit in the small hole between the makerslides and the bottom face of the Z-stepper. I also wouldn't have nearly so much weight hanging off the front gantry, pulling it away from the makerslides.

My plan is to then mount a HiWin rail on the remaining front plate and use a much weaker, much shorter-stroke drive for the Z-axis, maybe even a simple hobby servo or one of those ubiquitous DVD-drive-head steppers. I'm already limited to about 30mm of Z-travel due to a mistake I made in one of my other customizations (long story), so I know I can live with a much shorter Z-travel.

[Mark Harris]

I dont know that a DVD motor would have enough torque to lift the head?

[JuKu]

The stability of the gantry would suffer, I think. Looking the system sideways, there is rather little in the V wheels preventing the gantry rocking clockwise and anticlockwise.

[WayOutWest]

The stability of the gantry would suffer, I think. Looking the system sideways, there is rather little in the V wheels preventing the gantry rocking clockwise and anticlockwise.

Thanks, sounds reasonable, but why isn't this a problem for the Z-axis?

[JuKu]

Because it is supported by the stepper motor shaft. Besides, the weight is smaller.

[WayOutWest]

Please forgive me for being so dumb about mechanical issues... I am learning here...

I completely understand what you mean about the back gantry plate preventing rotation around the X-axis.

The analogous problem for the Z-makerslide, which has no similar secondary gantry+vwheels, would be rotation about the Z-axis (not the threaded shaft -- the makerslide). I'm still confused about why this doesn't happen.

Because it is supported by the stepper motor shaft.

But that support is coupled through only the three rubber posts, which do not prevent rotation about the Z-axis...

http://www.liteplacer.com/the-machine/a ... motor-nut/

... have I misunderstood how all of this works?

Besides, the weight is smaller.

Ah, okay, I think I understand this... in the rotation-around-the-X-axis case you have to worry about the static load of the Z-makerslide's weight, which is always pulling on the gantry. In the rotation-around-the-Z-axis-case there is no similar load; in fact you really shouldn't ever encounter significant forces like that (this is a pick and place, not a CNC). This makes more sense.

I'm currently most tempted to move to a mounting scheme where the Z-stepper is mounted almost directly above the head's center of gravity, like the image below from the OpenPNP list and can be shifted in the positive-Y direction to counterbalance the weight of the pickup head. So I'm starting to think that I can get away with only one makerslide as long as the head's center of gravity is directly above the V-wheels. I'm probably wrong, but mistakes are how you learn...

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