SL-30 Big Bore Issues

[fdane]

Recently we had a Haas tech come to our shop to check out our SL-30 Big Bore after a crash. The wedge was out .003 and the turret was out .002". He realigned them. However, big problems occured when he was working on the machine.

At one point he asked us to remove the 15 inch chuck and gave some drill sargent-like orders to our guys during the process. Basically, he insisted that we cycle the closer pedal over and over and over again while the chuck was suspended from a hoist. The idea was to make it easier to remove the big draw tube nut. After the chuck was removed, the closer began to leak like crazy. We pulled the actuator/closer off and tried to take it apart, but couldn't because the drawtube was rusted to it. We wound up having to buy a new closer/actutaor. A $1500.00 alignment turned into a $9200.00 bill.

Is there anyone who could shed some light onto the possible cause of this failure? Did the side load from the chuck fail it? The closer was 2003 vitage and had lots of rust on the draw tube side of it and was heavily pitted. Could rust have gotten into the seals and blown the away? Is it standard practice to support the 15 inch chuck with a lift and cycle the closer while unscrewing the drawtube nut? Was the Haas tech blowing smoke?

Thanks a lot for you input.

[Geof]

Here is my conclusion and analysis based on my experience with rusty hydraulic equipment.

Releasing the chuck and operating the actuator is what caused the damage. But that was not the reason for the damage.

When a chuck and actuator are asembled the amount of travel in the chuck mechanism is less than the maximum possible travel of the actuator; it has to be so that the actuator does not bottom out. This means that the nicely finished surface on the actuator piston rod (piston tube I guess it is) does not travel full distance through the seals. During use, with water based coolant getting everywhere, you can get a lot of rust inside the spindle and down around the actuator. On the piston tube this rust can occur all the way up to the limit of travel in the seals.

Then you removed the chuck mounting bolts and operated the actuator many times. Now the piston tube moves further in each direction because the free hanging chuck no longer limits its travel. So the nasty, corroded, pitted, yucky section of the piston tube goes in and out of the seal and destroys it.

To some extent the rust was not the reason for the damage, the root cause of the damage was lack of preventative maintenance. But who takes off the chuck and actuator on a regular basis to clean it and coat everything with grease to prevent rust? I think you have learnt an expensive lesson, a bit more expensive than mine.

[fdane]

Geof

Your reply makes sense. Basically, if the atuator were new or not such a rust bucket, it would not have failed by means of internal abrasion.

I have another problem and would like to run it by you. I will post it within the hour.

Thanks a lot. Great insight!

Forrest

[fdane]

We made the original service call because one of our guys crashed our SL30-BB.

The crash occured when the set-up man put a 5 inch long 1.875 diameter spade drill in the wrong turret pocket and ran it into a chuck jaw. The drill did not break or even permanently bend but the bottom of the bolt-on tool holder shattered and the drill bent out of the path of the chuck after the 1st jaw hit it.

Besides the turret and wedge misalignment, mentioned earlier, a couple of other problems have been observed. First, the z-axis whines, when the rapid is set to anything above 25%. It was always silent before the crash. Secondly, about .015" backlash in the X-axis appeared.

The Haas tech said the z-axis noise was normal (thanks for the $150/hr diagnosis, Haas). He had me turn the ball screw by handle and said, "see, it feels fine".

The X-axis backlash he traced to a loose nut at the bearing support casting end of the ballscrew. The only problem with this is: I think he could be wrong. I believe that thrust backlash is controlled by the bearing sleeve at the servomotor end of the ballscrew. I have been reading my Haas Service Manual and it looks like the bearing sleeve contains angular contact bearings
that handle the thrust force and thrust backlash duties. When he tightened the locknut at the bearing support casting, I think he masked the problem. Worse, I don't think the bearings in the bearing support casting are intened to handle thrust loads.

Any insights would be highly appreciated.

Thanks al Lot!

.

[Geof]

Sorry to say, I think you have toasted either the ballscrews, the thrust bearings or both. When you gave a stationary or slow moving ballscrew or thrust bearing a hefty wallop it is possible to brinnell the races; the balls actually make permanent depressions in the race due to the massive overload. Now when they run fast, as the balls bump over these depressions the bearings sing.

Also I think you are correct about the thrust bearings being at the servo end and I think the other bearing is supposed to float to allow for expansion of the ballscrew when it gets warm. Tightening things to make the far end immovable may not be a good idea.

I would suggest getting in there, releasing whatever the technician tightend and try to find out where the backlash is coming from; in the bearings or in the nut on the ballscrew. If it is the bearings maybe they can be retightened to get the backlash down to an acceptable level.

If you can tighten things up to get acceptable backlash I would suggest for the moment the least expensive approach is to keep on using the machine. Ignore the noise from the ball screws if the machine can still reliably put out work within tolerance.

And start putting money aside to replace both ballscrew assemblies.

If you cannot get acceptable parts out of the machine it will be a case of spending the money now.

[fdane]

Hi Geof,

The ballscrews may be fried, but the X-Axis backlash did not come from the ballscrew. When the tech tightened the floating end, it could not have affected the internal preload on the ballnut. It could only affect the motor end thrust load. I think it is now pulling the thrust bearing in reverse. The Haas manual says to be sure to tighten the locknut on the motor end thrust bearing first (15 ft-lbf) and the bearing housing end last (4 IN-lbf). Anyhow, I think the thrust bearing is shot, but I would love to hear from a Haas tech to be sure. I don't have a lot of confidence in the dealer.

Is there a Haas tech on the board, who might know if this indicates a bad thrust bearing?

Thanks a lot,

Forrest

[haastec]

Yes, it is quite possible that the thrust bearing is shot. As it degrades the noise will increase. At this point, I would try to tighten both lock nuts and see how well it makes parts. If the noise continues to increase in volume then replace the thrust bearing. At $300+ for the thrust bearing, it is worth running a while to help confirm.

Just to clarify, the support bearing lock nut needs to be tight also, otherwise the ballscrew will move back and forth against the nut with the direction of travel; major backlash.

It has been a while for me with a lathe, but next time you need to take the chuck off, power the machine off while the chuck is in mid stroke. This should take all of the hydraulic pressure off allowing you to more easily unscrew the drawtube. Same effect as cycling open/close really fast several times.

Also, be prepared as Geof said that the ball screws themselves may be the issue now or sometime in the future. Another possible source of the noise.

Good luck!

[fdane]

We ran 30 large parts on the machine and experienced something rather weird. The length on three of the parts varied tremendously from the rest of the parts(.005 to .05") . We can't figure out what could be causing this. The machine was crashed as the turret approached in the Z direction, so the z-axis ballscrew, thrust bearing, turret and linear trucks come to mind. However, the depth of a counterbore on the part to the face location were perfectly aligned (two different tools). It's as if the whole part walked out of the spindle in three cases.

AT firts we held the part with bored steel jaws and stopped it against the face of the 15" chuck. One of our guys thought the chip guard might have flexed out during the chuck opening sequence, because he found flattened chips btween the drawnut and the tubular end of the chipguard. We changed the set-up to machined aluminum jaws with the part stopped against a step in the jaws. After changing the set-up we ran 8 more sparts without a hitch, but have our confidence shaken nevertheless.

Any ideas about this situation..?

f

[Geof]

Listen to the guy who thinks the chip guard might have flexed. I have had similar things happen with chips getting packed inside.

[fdane]

That is facinating. I'm glad someone else has, at the very least, had the same suspicions. On the big 15" chuck there is a nice space for chips to accumulate in front of the nut. When you open the chuck the chips mash up against the chip guard. We need to look into this a little more. Time for a dial indicator..!