How to reduce the vibration of a bench lathe ( Emco Compact 8 )

[Ischgl99]

Preload is the amount of load placed on the bearings to reduce the amount of play in the spindle. This lets the spindle run true with minimum deflection under load. When preload is too tight, the bearings can overheat and seize. If the preload is too little, you get excessive deflection of the spindle, but you also reduce the natural frequency of the spindle system. Lets say for example with proper preload the natural frequency of the spindle is 3,000 rpm, if the preload is too loose, that could drop into your operating range.

The right amount of preload in these machines is a bit of art, you need to adjust and test. If it is difficult to turn the shaft by hand, preload is too high, you should be able to spin the chuck and get a few rotations before it stops. When you run the machine up to speed for a while, it should get a bit warm. If it is too hot to touch, too much preload. If you decide to try adjusting the preload, make a mark on the preload nut and spindle so you know where you started so that if you wanted to go back to that setting, it is easy to do.

 

[compact8]

Just tried to adjust the spindle bearing preload as advised. The strange thing is that no matter how hard I tighten the preload nut, I could not get the preload increased as evidenced by the fact that the spindle rotated just as freely as it was when turned by hand. I bought the machine new some 25 years ago and it has been well kept in a dry environment so the contact of related parts being frozen by rust is unlikely.

The Emco user manual does mention preload adjustment but it is very brief. Have done a bit of searching and found this Grizzly manual : http://cdn2.grizzly.com/manuals/g4003g_m.pdf which says that a "small sledge or heavy dead blow hammer" should be used to break the spindle free from the bearing, kind of scaring for me . With no sign of any spindle play and good finish of cut I am getting, I will not consider doing so for the time being.

This is the construction of the headstock of my lathe :



The finding so far is that vibration occurs only if heavy chucks are mounted on the spindle. If the chuck is lighter like my collect chuck, the level of vibration is much lower. To be more precise, the low-frequency components of the vibration, which shake everythings is cut down by a lot. If the problem cannot be solved, the only way to get around it is to use collect chuck when 850 rpm is required but that will be rather inconvenient.

 

[mikey]

The way the spindle bearings are adjusted at the factory is to do it cold, not after running the lathe. A chuck is mounted on the spindle and spun "hard" by hand. If it makes more than 1-1/2 revolutions before stopping then you need to increase preload until the chuck stops within 1 to 1-1/2 revolutions. This comes from one of the Emco engineers at the factory so I have to assume he knows how they do it.

Once preload is set, check static spindle run out. It should be zero to 0.0002" TIR. If it is not, then it is time to change bearings. If you do change them, look for at least ABEC 5/P5 bearings to restore accuracy.

 

[compact8]

The way the spindle bearings are adjusted at the factory is to do it cold, not after running the lathe. A chuck is mounted on the spindle and spun "hard" by hand. If it makes more than 1-1/2 revolutions before stopping then you need to increase preload until the chuck stops within 1 to 1-1/2 revolutions. This comes from one of the Emco engineers at the factory so I have to assume he knows how they do it.

Once preload is set, check static spindle run out. It should be zero to 0.0002" TIR. If it is not, then it is time to change bearings. If you do change them, look for at least ABEC 5/P5 bearings to restore accuracy.

I have taken a slow-motion video with my iphone and counted 6 turns when the 3-jaw chuck was spun hard by hand. The adjustment was done when the lathe was cold but still I could not change the level of preload.

 

[mikey]

The only way that tightening the preload nut will have no effect is if the rear bearing is locked up against something such that it cannot move. Might be time to disassemble the spindle and have a look.

 

[jwmay]

The finding so far is that vibration occurs only if heavy chucks are mounted on the spindle. If the chuck is lighter like my collect chuck, the level of vibration is much lower. To be more precise, the low-frequency components of the vibration, which shake everythings is cut down by a lot. If the problem cannot be solved, the only way to get around it is to use collect chuck when 850 rpm is required but that will be rather inconvenient.

What about buying a little smaller chuck? If you've proven it's that. Did this problem begin with the use of these larger chucks you mention? Did it suddenly appear, or has it been slowly getting worse, to the point that you are now addressing it? Springs wear out. I think you said it's not the spring. But they don't last forever. 25 years is a pretty good run.

 

[compact8]

What about buying a little smaller chuck? If you've proven it's that. Did this problem begin with the use of these larger chucks you mention? Did it suddenly appear, or has it been slowly getting worse, to the point that you are now addressing it? Springs wear out. I think you said it's not the spring. But they don't last forever. 25 years is a pretty good run.

As far as I can remember, it has been like that for a long time. Although the lathe was bought 25 years ago, it didn't get much use. Too many other hobbies .....

 

[jwmay]

Too many other hobbies .....

I know how that goes. Sometimes it seems the world conspires to keep me out of the shop. But you made a good pick 25 years ago eh? I get the impression these lathes have a good reputation. It may not be the culprit, but springs are usually fairly cheap. I also wondered, as already mentioned, about a gas charged cylinder to replace that spring. Any thoughts on that?
I watched your videos again a few times. Can you tell us a little more about the mechanism by which the tension is applied? Is it a cam attached to a hand lever? Does ot both rotate on an axis, and slide in/out? I can't guess why it would matter, but it's action seemed odd in the video. As if it were cammed, and then the pin seemed to lean, and then the entire thing seemed to shift away from the lens. Perhaps I'm seeing things.

 

[compact8]

Can you tell us a little more about the mechanism by which the tension is applied?

The mechanism is very simple. The tensioning wheel is attached to the end of a short lever ( #3 in the diagram below ) which is in turn attached to the spring. The handle is there just to lift up the tensioning wheel when the belt is being re-positioned. When things are set, the handle is not touching any part of the mechansim. The tensioning wheel just sits on the belt so it can go up and down depending on the tension of the belt and that of the spring.

 

[jwmay]

Such an interesting problem! Probably not to you... but this is unique to me. You've replaced that little round belt too? It does quite a dance at below the resonance. The sound of it though, reminds me of a time I tried to use a "not exactly correct" Gates timing belt in a pulley set. Needs must, as I'm sure you understand. I wouldn't think that's the issue, as you ordered the exactly correct part for your machine.