Tuesday, March 17, 2009


Wedgewood Update - Z Axis Begins

I've been away for much of the weekend, and crook yesterday so I've not had much time to work on RepRap. Next weekend is a 2 day chainsaw course, so I'm working on Wedgewood whenever I can. Here you can see the beginnings of the Z axis mounting scheme, using the PLA bearing holders I printed earlier to anchor the Z axis studding (one on the top that's hidden from view, and one on the bottom). I've removed the deposition bed so you can see the bearing holder.

The two nuts in the middle of the studding will be held captive by the X axis mountings. Given the weight of the head and X axis, it is unlikely that I'll need any anti-backlashing but it can be done if need be.

For the moment I'll use 2 tin can stepper motors to drive the Z axis, which neatly eliminates the need for a chain or driveshaft to link the two halves and gives me more torque where it is needed most. I've only got 2 of the Jaycar YM2751 steppers as the depot ran out of stock, so we'll see if they get stock before I can finish the Z axis.

The object of the exercise? To see if the Z axis on a Wedge design is going to need guide rails or not.

Vik :v)

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Hey, Looking good! However one thing that was getting to me was that where's the weight of the vertical axis going to go. In your design you've got the motors at the top with the bearings at the bottom. Surely you could have the vertical threads getting pushed through the bearings and breaking them?

In my design I put the motors at the bottom, keeps the weight off the A frame and by having the rods connected at the top with washers as the bearing surfaces it should be okay.
Well, you definitely want the weight of the rods and the Z carriage on a bearing, rather than on the motor. Motor bearings -- especially in tin can steppers -- are not really designed to shoulder pretty big weights. They don't have thrust bearings, usually.

Putting that weight on a couple of standard skate bearings is not exactly ideal, but those things are probably standard 6000-series size, or at least could be easily replaced by that, and those standard bearings exist as thrust bearings just like they exist as standard radial bearings.

At that, the weights are not so large that the axial loads would be too much for regular radial bearings, in the short term, I suspect.
In my experience tin can steppers have a few mm of end play. However, the magnets pull the shaft to one end or the other.

If you load the shaft in the opposite direction the rotor is displaced and the torque reduced slightly. So mounting them one way up is better than the other.
Looking at that z-axis arrangement I really, really hope that you get enough torque out of those tin can steppers to do what's needed. That part really worries me. I think your x and y will be just fine.
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