Do I need a keyway?

[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.