Do I need a keyway?

[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...

[GEOFFREY]

I agree with Jonathans comment about the bur making it a little more difficult to remove the pulley (hopefully you will not need to), but doubt if there will be room for any insert (my grub screws are about 4mm long and need as much grip in the ally pulley as possible) and I do feel that a slight burr will help to stop the pulley from slipping. G.

[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....

[birchy]

Fair play Jonathon, you always impress me with your facts and figures. I'm more of an "over engineer it, guarantee it'll work" type person. But then again I used to make parts for massive steel bridges when I was an apprentice!

I have to admit that I'm VERY surprised the load is so small when converted to Nm via the ball screw and pulleys. Obviously I'll have to be a bit careful with the acceleration/deceleration but I'm more interested in accuracy than speed and would rather err on the side of caution than bollock the machine flat out and end up wearing it out prematurely.

Thanks everyone for your replies. Not needing keyways is one less headache to worry about. Just for the record, I'll be using M4 or M5 grub screws at 90 degrees, dimpling the shaft slightly and applying a little Loctite for the final fit.