PM728-VT spindle bearings

[Bryanaverill]

My machine has very slightly noisy spindle bearings. I ran the motor with the belt disconnected to verify it wasn't the motor. It's the spindle making noise. Out of curiosity, I pried the seal out of the top bearing on noticed the balls were not circulating with the inner race, even when I simulated the belt tension, meaning the inner race is sliding across the balls. Since the noise is not a huge issue, I just put it back together.

Question. Has anyone installed tighter C2 clearance bearings?

 

[ddillman]

The balls should be rolling. If not it could be a problem in the long run.

 

[Ischgl99]

The balls should definitely be rotating, and the cage as well with them. If they are not, the bearing will not last much longer.

You need to be very careful with a C2 bearing, they have low clearance and it doesn’t take much heat to cause them to seize. I don’t have this mill, what clearance bearings are installed? In case you are not aware, unmarked clearance is CN (normal), C3 is greater than normal, and C2 is less than normal clearance. If you have C3 bearings in there, going to C2 is two steps less clearance.

 

[markba633csi]

Sounds like the bearing is defective- if you just bought this machine I would let PM know and perhaps take a video of the bearing in action
(or rather, no action) They should cover the cost of replacement
It begs the question, what is the lower bearing doing?

 

[Bryanaverill]

Yes indeed. It's difficult to get a video since the drive pulley and seal needed to be removed and I can only see the balls not rolling while turning the spindle by hand. All four bearings are now in question. I'm not opposed to putting quality bearings in if it's worthwhile. Rebuilding the head will be a pain though.

 

[GB21]

If you decide to replace them I am very interested in your experience. I have been looking at VFD's and plan on upgrading to more power/speed but the bearings will need replaced and I have not seen anyone do it yet on the 728.

 

[GingerMidget]

I too have noise in the spindle bearing, especially if I place much tension on the belt. My noise seems to come from the top bearing and is worsened with belt tension. My machine is new and has probably 2-3 hours total run time. I don't really know what to do about it. It bothers me and I am keeping an eye on it. Let me know what you find or do.

 

[van462]

It appears I have the same issue o my machine. I also had to switch from a 5/8" drill chuck to a 1/2" as the spindle return spring doesn't support the additional weight and it drifts down.

Come to think of it I only hear the noise when a chuck is installed. I haven't heard it with other accessory.

Edit- I think I found my issue. I removed the spindle cap, and compressed the spring cup to remove the retaining c clip. I found a metal chip in the grease. Likely why I mainly heard a "dry bearing sound" only when raising the quill. The weight of the 5/8" chuck also likely placed a light load on it.

 

[B2]

Hi @van462

from a 5/8" drill chuck to a 1/2" as the spindle return spring doesn't support the additional weight and it drifts down.

I have a PM940M and the return spring has just the opposite property. It pulls up so hard that if you release the handle it bangs up hard. So don't let your hand slip off of the handle. It looks to me like it might be possible to adjust the spring tension. Have you tried this? I was thinking about trying this, but was not so sure that there was not some trick to getting the spring wound back up if I release it.

Some folks say that they power tab by just letting the tap pull the quill down as the motor turns. Likewise they back it out by reversing the motor. Because of my strong spring this does not work. The tap may not start well or tends to tear out the first threads when backing out. So for power tapping I think you would just like to have the upward force to be low or virtually non-existent. Ideally one would have a little downward force at the start of tapping and then have zero force downward and only a slight force upward when backing out. Pretty hard to accomplish both without operator intervention, so maybe best is just a very SMALL upward force all the time.

Dave L.

 

[van462]

The 940 has an adjustable quill return spring much like a bridgeport. While holding the outer spring housing loosen the center bolt and back off the spring housing. I believe it has notches to help key it and not solely rely on the center bolt.
The 728 quill spring isn't adjustable. Its a simple spring that is compressed. I may be able to fit in a spacer to preload the spring a bit more to help with the quill return.