[anemachinebc]

We had a ball screw snap off at the coupler between the servo motor and the screw itself. Has anyone had this happen to them before? This machine wasnt crashed, it just broke while it was running a program.

I just love HAAS machines.







Thanks
Adam

If HAAS made a plane would you fly in it?

[Geof]

Do you have pictures of the broken ends? My guess is that this would be a fatigue fracture and I am interested to see what the initiation point was for the crack.

[samhyto]

Wow, like geof siad, would love to see pictures of the crack? Sounds like it was just a defective ball screw?

But.....there could be some other underlying issues.

Which axis, since the Z is always doing alot of changes of inertia, and there is alot of mass in the head?

Is it a SS machine, these have faster pitch screws?

Has the operator noticed higher than usual loads on the effected axis recently?

How old is the machine as well?

Sam

[asuratman]

It happened to me also, it's broken inside the coupling. I made beveled cut on ballscrew and bar. Re-welded and machined. It works OK. But I do not know how strong. Maybe you just try, better than you throw it. If it broken in the screw, just cut and machine. The, use it for another purpose. Thanks.

[macrobina]

Do you have pictures of the broken ends? My guess is that this would be a fatigue fracture and I am interested to see what the initiation point was for the crack.

Dear Geof, We had same problem. Ball screw of our CNC broken when running program. Our CNC has a coupling but it is correct. I attached pictures of broken ball screw. Please answer me why.

[handlewanker]

Hi, I doubt it is a torque thing. more likely a flexing that will crack even a mild steel shaft over time.

It doesn't take much axial misalignment to make the shaft flex......by that I mean axial out of line not eccentric misalignment.

Eccentric misalignment is easy to detect as you can see it with the naked eye, but axial misalignment is a lot harder to detect, and it will break a shaft.

This becomes more of a problem if you have a coupling that is grub screwed to the screw and the stepper spindle, and so making a solid coupling where the slightest misalignment is a problem.

Mounting the stepper axially to the face of the machine makes it difficult to align squarely, unless you have two faces that you can gauge for squareness to each other before bolting up.

This is a vital exercise and if you neglect it you get ........a broken shaft.

You need to have a solid drive that cannot move radially under pressure from the screw when under load or you'll lose steps and have backlash etc.

Rubber flex couplings achieve this for various drives to cure misalignment, but they do compress radially and give you some axial movement that would simulate backlash and a few steps lost when under pressure, but when the load is off the couplings are once again aligned.

The finishing cut is therefore not affected by cutter pressure causing the screw to go out of sync with the stepper or servo spindle.

I think a coupling that has something like a hard rubber bush in the centre with bonded metal ends for grub screws to mount on the screw and motor spindles would align both while still being stiff enough to drive.

For best results you need to align the two spindle ends as closely as possible.

BTW.....the solid couplings commonly used with 2 screws to attach the motor to the screw are quick fixes and cannot be considered as ideal coupling devices......they require 100% perfect alignment which in most CNC machines is hard to achieve.