Wobblycogs CNC Mk1

[Jonathan]

I contacted the supplier I linked to in my previous post to find their prices for the RM2010 ballscrews. He replied promptly with the following:


For:
RM 2010 -L1800mm*2pcs with 2pcs SFU2010 ballnut with end machining
Best price for you is USD360.00,include the freight by Fedex/
For:
RM 2010 -L1500mm*2pcs with 2pcs SFU2010 ballnut with end machining
Best price for you is USD325.00.


Clearly that's $121 (£76) more than the RM2510 from Chai, but with 2010 you could still use the 3Nm steppers and drivers and get better acceleration than with the 2510 ballscrews. The 5mm difference in diameter may not seem like much, but remember the torque required to accelerate a cylinder is proportional to the radius to the power 4, so 20mm needs only around 41% of the torque that 2510 requires.
The reason none of us mentioned RM2010 earlier was because Chai doesn't stock them and they are very expensive from England, but these prices change that even though they are a bit more expensive.

Since I'm using 50mm box for the frame I can't directly mount the screw bearing block onto the frame. I'm hoping to get around that by mounting the bearing block and motor on a plate (green) which then bolts onto the frame. The picture below shows the business end of the screw but there's a similar plate at the other end as well. Good idea / bad idea?

Good plan as it makes it easier to align everything since you can have adjustment via slotted holes in the plate.

I'm now thinking aluminium would be better as it would be easier to work (I want to provide some vertical adjustment holes for example). Will 5mm aluminium be strong enough?

No I don't think so, I'd say 10mm based on what looks about right!

The motor housing is drawn in 20mm aluminium which looks far thicker than I need but I have very little experience to go by. Would 16mm or even 12mm be strong enough? I'm going to drill and tap the edges to join the pieces together so it can't be too narrow :-). Ideally I'd like to use the same thickness plate for parts on the gantry and Z-axis so that I can just buy one large piece of plate which, fingers crossed, will be cheaper.

20mm is excessive, but as you say it's sometimes best to keep things the same thickness to make it more efficient when you buy the material. Have you put a pocket in the mount for the motor to rest in? It will make it easier to align the pulleys since you'll have more of the shaft extending through the aluminium, e.g:



6082 aluminium is readily available and cuts easily, so that's fine.