Thread: Do I need a keyway?
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?
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.
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 cornerJohn S -
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.
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?
The Following User Says Thank You to GEOFFREY For This Useful Post:
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.
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.
Last edited by Jonathan; 30-07-2013 at 05:48 PM.
By dsc in forum Stepper & Servo MotorsReplies: 9Last Post: 25-02-2014, 02:25 PM