Do I need a keyway?

[Jonathan]

Since all your stepper shafts are 8mm, that implies the motors are no more than 3 or 4Nm. Grubscrews will hold, but make sure you threadlock them as otherwise they can vibrate loose.

A nicer way is to machine the couplings/pulleys such that they clamp on to the shaft, like these for instance:

Oldham Couplings - Arc Euro Trade

But be careful not to make the coupling weak in doing so.

[birchy]

I'm using Oldham Couplings on the X axis but they're the grub screw variety. The motors are 3Nm. My main concern is the Z axis...I've just weighed the head and the AC motor and it comes in at a whopping 37Kg. It will probably be closer to 42Kg by the time I fit the drive, handles and other gubbins. There's no counter weight or gas strut, so will these motors be man enough at 3:1 ratio? Is the 1605 ballscrew a bit small, particularly the 10mm pulley shaft?

I'll probably get the Y and Z shafts/pulleys keywayed but I'm now a bit concerned that I've undersized the Z axis. What do you more experienced fellas think?

[kingcreaky]

What do you more experienced fellas think?

in other words, john, jonathon or Jazz. rest of us need not apply

[GEOFFREY]

in other words, john, jonathon or Jazz. rest of us need not apply

Not one of the above, but I have been running my machine for about 10 years now and the pulleys are each fitted with 2 grub screws at 90 degrees. G.

[birchy]

Not one of the above, but I have been running my machine for about 10 years now and the pulleys are each fitted with 2 grub screws at 90 degrees. G.

That's handy information! What size grub screws?

[GEOFFREY]

3mm, no shaft flats or dimples, but the grub screws are tightly screwed in. G.

[Jonathan]

3mm, no shaft flats or dimples, but the grub screws are tightly screwed in. G.

Which reminds me, if you've got no flats or dimples on your shaft, then put a small piece of brass bar under the grubscrew, such that the brass presses onto the shaft and not the screw. That way the shaft wont be marked, since brass is much softer than the steel, so if you need to remove the pulleys there wont be an annoying burr to stop them sliding off easily...

[birchy]

I'll be thankful for ALL opinions. I meant more experienced than ME...noting that this is my first build. I'm not completely new to engineering, but this my virginal flight on anything CNC.

[Jonathan]

in other words, john, jonathon or Jazz. rest of us need not apply

You forgot Irving - I'm about to answer this just using information he posted long before I joined the forum.

What do you more experienced fellas think?

I think it's not a matter of experience, it's just applying a simple equation. From experience I can say that there's nothing to worry about, but lets be more precise:

You're concerned that the ballscrew is lifting a 'large' mass and that as a result may be too high, so calculate the torque. Using the standard formula I posted here, the torque required to lift 42kg with a 5mm pitch ballscrew is 0.36Nm, but if you've got a 3:1 ratio the motor only needs to provide a third of that, so 0.12Nm. Either way the motor will output that torque up to a satisfactory speed. There will be a bit more torque due to acelleration, but still not enough to be a problem.

If the torque on the ballscrew shaft is 0.36Nm (as above), then at 5mm radius the force will be 73N, so so long as the grubscrews impose enough frictional force to oppose that, it wont slip... again, the force isn't very big so you'll be fine with two grubscrews.

Edit:

That's handy information! What size grub screws?

I'd use M4 or M5 if they will fit...you'll certainly fit M5 in the bigger of the two pulleys.

[kingcreaky]

what he said....