Last week we finished replacing the Z ballscrew in my Mazak PFH-4800 Horizontal. It had only been 3 years since it was last replaced albeit poorly.
The screw had developed about .001 backlash which is fatal on a horizontal. The screw started making noise six months after it's last replacement and I assumed it was just a noisy thrust bearing. I was wrong.
Here is what we found:
The end support bearings (away from the motor) had been spinning on the ballscrew because of lack of preload in the end support block. The bearing set was thinner than the housing by .001. We added a .002 shim to correct the preload problem.
The screw was not correctly captured on the shoulders of the screw. This required grinding the shim plate under the motor by .0094 to correctly capture the screw.
The new bearing set on the motor end was .0015 thinner than the set that came out resulting in no preload on the outer races. A .002 shim fixed that.
The ballscrew was .004 out of alignment vertically when assembled. Both ends allowed no vertical adjustment as the motor end was pinned in place and the end support allowed no adjustment. This was a Mazak manufacturing defect that surprised the hell out of us. This was the 3rd failed ballscrew since the machine was new and had never been caught before.
When the last ballscrew was replaced we had to change servo parameters from original to make it run smoothly. This should have been a dead giveaway but I trusted the technician.
In the end this cost me $8,500 in parts and another $3,500 in labor.
It worked out correctly this time because I personally did an analysis of the failure modes rather than trust the tech. I would have had yet another failure if I hadn't insisted on checking all variables myself.
Now it sounds the best it ever has and is set to all the factory tuning specs.
This is just a warning that you can't just drop in a new screw without checking all of the variables.
Needless to say I am a little pissed that the tech ignored all of these things 3 years ago.
Dan
The screw had developed about .001 backlash which is fatal on a horizontal. The screw started making noise six months after it's last replacement and I assumed it was just a noisy thrust bearing. I was wrong.
Here is what we found:
The end support bearings (away from the motor) had been spinning on the ballscrew because of lack of preload in the end support block. The bearing set was thinner than the housing by .001. We added a .002 shim to correct the preload problem.
The screw was not correctly captured on the shoulders of the screw. This required grinding the shim plate under the motor by .0094 to correctly capture the screw.
The new bearing set on the motor end was .0015 thinner than the set that came out resulting in no preload on the outer races. A .002 shim fixed that.
The ballscrew was .004 out of alignment vertically when assembled. Both ends allowed no vertical adjustment as the motor end was pinned in place and the end support allowed no adjustment. This was a Mazak manufacturing defect that surprised the hell out of us. This was the 3rd failed ballscrew since the machine was new and had never been caught before.
When the last ballscrew was replaced we had to change servo parameters from original to make it run smoothly. This should have been a dead giveaway but I trusted the technician.
In the end this cost me $8,500 in parts and another $3,500 in labor.
It worked out correctly this time because I personally did an analysis of the failure modes rather than trust the tech. I would have had yet another failure if I hadn't insisted on checking all variables myself.
Now it sounds the best it ever has and is set to all the factory tuning specs.
This is just a warning that you can't just drop in a new screw without checking all of the variables.
Needless to say I am a little pissed that the tech ignored all of these things 3 years ago.
Dan