Sunday, August 30, 2009


Mendelssohn Z axis operational

I've got the Z axis going on the Mendelssohn design. In the picture here (RepRap is tilted so the bottom faces the camera) you can see the ball-chain drive connected up to the two Z threaded drive bars. The shiny pulley is a 608 bearing with some great big washers either side of it. Side note: If you put M4 nuts on an M4 bolt, the exterior diameter of the nut is 8mm and it fits perfectly as the axle for a 608 bearing.

The 4.5mm beaded chain gears are the same as used on the RepRap Child/Phoenix machine that built Mendelssohn. Printing a gear with a grub screw and captive nut was too hard, so I devised a clamp-on gear that fastens onto the NEMA17 shaft using an ordinary hose clamp. The tensioning pulley makes installation of the Z drive chain much easier on this prototype than the Darwin.

If you want to see it in operation, here's a video of it. I've stuck a mole grip on the X axis to add a bit of weight, and in case you can't see the ammeter it's running at 250mA. It ran like this for 20 mins, cycling up and down without overheating the motor or the EasyDriveV3 stepper driver. The OLPC is just powering the Arduino board - though some magic is needed to stop the OLPC powering down its USB ports at inconvenient moments. Once that's sorted I'll be able to power a RepRap using Ralith's minimalist RepRap command line utilities.

Vik :v)

PS I've since made the Y axis move back & forth using helicopter belt but need to print new bits to do it properly. People are asleep now, so I'll leave that for tomorrow.

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Looking good! How's the stiffness of the top of the triangle in the X direction? It seems like it could use some cross bracing there. I like the lack of a Z coupling, and the nicely unconstrained threaded rod drives.
Great progress Vik.

I made a PLA boss with a split and a captive nut with a bolt across the split to tighten it onto a stepper shaft. It worked but the grip was not that good. Looks like the pipe clamp idea will work a lot better as it squeezes from all directions.

How fast does it go?

Do you need the idler pulley, can't the belt just return diagonally?
Stiffness in the X direction seems just adequate at this point. When I bash stuff around up there we'll know for sure :) Also, I still have to fix the circuit board holder in place.

It doesn't go very fast - less than 1mm/sec at this point but I've not experimented with speed much.

I need the idler pulley to tension the ball-chain - it stretches for a little while at first as the burrs wear off. I suppose we could swivel or slide the whole motor to maintain tension, but this works for the moment.

Vik :v)
i like how you are progressing, it looks like it is coming along very well. i do look forward to seeing how much it dose stiffen up when you get that electronics mount on the frame.

when i do get going on making my reprap i think i will make this one.

what size of ready rod are you using.?
I'm using M8 threaded rod, brigt zinc plated, and 8mm polished steel bar. I want to test 10mm thick-walled aluminium tube at some point too.

Vik :v)
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