Thread: From smiles to heartbreak

What sort of coupling do (did) you have between the motor shaft and the ballscrew?
I would have expected it to give way before the ballscrew.

I'm using alloy flexible couplers, I would have thought two small grub screws would give before breaking a 8mm shaft if the load was to much.

Regards,

kiwiken

Hi Ken,'
Has it sheared at the shoulder of where the 8mm joins the 6mm? What I am getting at is it the 6mm bit that has parted. If it has then it is logical becasue this is the weakest part of the dirve chain. The motor shaft is 1/4 inch making it 0.25mm larger than 6mm turned section.
As there is no binding of the screw the only thing I can offer is it broke because it is not up to the sudden acceleration and braking. 1200 acceleration is quite good and maybe 6mm shaft is just too small to hanfle these forces.
The standard for a 16mm screw is 12mm for the bearing and 10mm for the drive shaft. The ballscrews are quite soft in the centre of the shaft as it is only the outer thread that is case hardened.
I hope somebody else can think of a solution for you.

Looking at your photographs, the blue end plate seems to have the mounting holes drilled way out of square. Is this so, or is it just the angle of the photograph?


Edit:
Put a protractor on the end the shaft in the middle and make sure that all holes just touch the protractor when you spin the protractor.

Do not try to measure to the centres of the holes as you can easily make errors if you do it that way

Re edit: Smack head. COMPASS, not protractor.

Hi Rod,
The shaft is breaking where it steps down from 12mm to 8mm.
I've got 50mm to play with at the end of the screw so I'm going to get the end machined down then a mandrel to fit over the shaft.
The mandrel will be machined to the spec's of the broken piece and grub screwed to the shaft.
No sharp corner at the steps in diameter. Hopefully this will be stronger.

Bob
The hole are square, it's just the angle of the photo. Everything was drawn up in inventor and lazer cut out.

Regards,

kiwiken

sorry i must have miss read it , how long are the 2 ballscrews you have for sale ?



?????" Nut" carriage not on the same line as motor ???? did it snap when the axis was close to 0 ????

cheers ken

Hi Ken,
No ballscrews for sale.
I was working a the center of the table, which is about 625 from where it broken. Everything is on center.
The screws are 1483mm overall length and I have a 1250mm x 505mm cutting area.

regards,

Kiwiken

Hi Ken,
As you describe it I have concerns that it is something else causing it. Eliminating binding is the first step - I know you have checked but disconnect the good ballscrew and see what the other one feels like. The 387 motors have reasonable power so it might be driving through the bad bit. Check by turning the screw by hand. You are looking for something way out of whack to shear 8mm shaft. Just check your ballnut as sometimes a ball bearing can get in front of the return track and that locks the screw immediately. It will do this randomly so give it a good test. While you are at it check your bearing supports in case there is a crook ball bearing in those.

Perhaps you have an electronics problem. If one motor sets off in the opposite direction it might cause the problem. It would be fairly obvious as the gantry would crank out of allignment.

I am digging deep here as you seem to have covered it well already and offering up something to think about.
.

??? ball screw whipping , ?? (sagging)
dont know how you could measure that , you cant use a dial indicator because the ball screw isnt round

At the place the shaft broke, did the shaft have a small radius at all?

Well I'm back up and running. I made up a laser aligner to check the alignment of the screw and it's pretty close.
Had a mandrel made up to repair the broken screw. I was able to have the bearing retainer machined out to take a larger bearing, so I had the shaft machined to 10mm instead of 8mm.
Ran the router for about an hour cutting a Mayan calendar with no problems.
Now I have to work out what to cut next.

Kiwiken.

All smile, no heartbreak, goodonya
Cheers
Wolffie