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birchy
29-07-2013, 07:45 PM
Probably a silly question...but do I need a keyway on my ballscrews?

For the X axis, I'm using direct drive 25mm Oldham Couplings which do not have a keyway. The stepper shaft is 8mm, the ballscrew shaft is 8mm, so I'm assuming that the 2 grub screws provided will be sufficient?

For the Y axis, I'm going to be using T5 x 16 wide pulleys and either a 1:1 or 2:1 ratio. The stepper shaft is 8mm, the ballscrew shaft is 10mm. I have no machinery to create a key as I only have one milling machine and it's the one I'm converting...

For the Z axis, I'm using T5 x 16 wide pulleys and a 3:1 ratio. The stepper shaft is 8mm, the ballscrew shaft is 10mm. Again, I have no way to key the shafts unless I take them to a machine shop.

Soooo...are there any other ways I can fit these? Will the grub screws hold? Should I "dimple" the shafts to seat the grub screws better? Or maybe I can roll pin right through at 90 degrees to the grub screws? Or should I just stop arsing about and pay someone else to key the ballscrews and pulleys?

kingcreaky
30-07-2013, 08:44 AM
In my opinion, id say a grub screw is fine... dimple the shafts and use loctite on the grub screw... perhaps even use two grubscrews (on on either side).

personally, I just use one, with pointed head grub and do it up man-tight.

Matt

John S
30-07-2013, 11:24 AM
Never use opposing grub screws, all it does is supply two pivot point for the shaft to rock on.
If you are using two then put them at 90 degrees so the screws force the shaft into a corner

Jonathan
30-07-2013, 01:50 PM
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 (http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors/Oldham-Couplings)

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

birchy
30-07-2013, 07:28 PM
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
30-07-2013, 07:33 PM
What do you more experienced fellas think?

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

GEOFFREY
30-07-2013, 07:37 PM
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
30-07-2013, 07:38 PM
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.

birchy
30-07-2013, 07:40 PM
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?

Jonathan
30-07-2013, 07:47 PM
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 (http://www.mycncuk.com/forums/linear-rotary-motion/4562-14mm-ballscrew-nut-vmc260.html#post30762), 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.

GEOFFREY
30-07-2013, 07:49 PM
3mm, no shaft flats or dimples, but the grub screws are tightly screwed in. G.

Jonathan
30-07-2013, 07:52 PM
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
30-07-2013, 08:00 PM
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.

kingcreaky
30-07-2013, 08:21 PM
what he said....:witless:

birchy
30-07-2013, 10:41 PM
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.