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Lathe Wobble at 750 RPM

McKay Daines

McKay Daines

Joined
Nov 15, 2023
Messages
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Location
Salt Lake City, UT
Hello all,

I've been turning for about two years now and love it. I upgraded about a year ago to a Powermatic 3520B, and love that as well. I've had a persistent problem that I'm looking to troubleshoot with some more 'seasoned' turners ;) I've noticed that my lathe develops a wobble at around 700-750 RPM. If I have a slightly off-balance piece, it will do fine up to that range, I'll get a noticeable wobble at that speed, but it will go away if I go past it. You can barely notice the wobble without anything mounted, but it is much more pronounced with something mounted. I've replaced the spindle and bearings, gotten a new chuck, and spent about 4 hours trying to balance the feet (I'm on a concrete pad that is slightly angled). Is there anything I am overlooking, or do you have ideas on what I can do to diagnose? The belts seem fine, almost zero noise no matter the speed, and reverse works fine as well.

Thanks for your assistance,

McKay
 
It is normal to find a speed for less vibration at higher RPM than the worst vibration hit when increasing speed.

Your issue may be something else. I’d check the pulleyes and belt. Vertical alignment, free from dirt, pulleys locked in place tightly with the set screws.

You already did the leveling.
I do a fine tune level on my ONEWAY. After getting close with a level, I put a slightly out of balance piece on andi get it up to a noticeable vibration. Then with the lathe running adjust each foot up or down stopping at the least vibration.

If the sloped floor prevents level - metal shims (1/4” steel plates) under the low feet.
 
hockenbery,

I'll try the pulleys, thank you! I also did the same trick with the feet adjustment; couldn't get it perfect, but got it the best it was able. Maybe putting that metal shim like you suggested will help that as well.
 
I am not familiar with anyone doing a lathe leveling video, but there may be some. In order to get it 'perfect' I figured out what works for me. One is to make sure lathe is level cross the ways on both ends, and level front to back. I then mount an unbalanced piece on the lathe, and back off one of the lathe leveling feet till it is just floating off the floor. Turn on the lathe at medium speeds, enough for it to wobble, but not enough for it to start walking around the shop. Adjust till the one floating leg engages enough to stop the wobble. I may do this a couple of times. I then adjust that leg down maybe 1/16 or so of a turn. I do this because when I 'lock' that leg/foot in position by tightening the nut, it seems to lift that foot off the floor just a hair. I do mark on the concrete floor where the feet are positioned. The lathe still wants to walk around a bit when an unbalanced piece is mounted on it. You may have to repeat this a couple of times to get it perfect. If your lathe is on a garage floor, they are sloped, and you may need to anchor the feet to the floor, and the lathe will ALWAYS want to walk down hill.

robo hippy
 
My 3520c shakes at 950-1000 with an unbalanced piece. I have adjusted the feet several times. Eventually I get a piece that’s too off balance and the lathe suddenly starts to walk to a new location, throwing the leveling off again. I have decided to leave it as is and just not use that speed. At least this way I get warning before it starts walking.

I suppose more weight would help it stay put after leveling.
 
Mike Novak said:
My 3520c shakes at 950-1000 with an unbalanced piece. I have adjusted the feet several times. Eventually I get a piece that’s too off balance and the lathe suddenly starts to walk to a new location, throwing the leveling off again. I have decided to leave it as is and just not use that speed. At least this way I get warning before it starts walking.

I suppose more weight would help it stay put after leveling.
Click to expand...
Mike, mine does the same thing at the same speed, even with smaller out of round pieces. I’m sure there is some funky mass/moment/harmonic thing to explain it because as you know, the 3520c isn’t a light lathe. like you, I just don’t work in that range with unbalanced items.
 
It could be any rotating part of the drive system. Try removing the drive belt or at least moving it out of the way. (I do not know the internals of the lathe) and running it. if the vibration persists then remove the drive pulley from the motor and try again. It's doubtful it's the drive shaft as that is an item machined on a lathe and should be dynamically balanced. The pulleys may be machined castings and out of balance.
 
stu senator said:
It could be any rotating part of the drive system. Try removing the drive belt or at least moving it out of the way. (I do not know the internals of the lathe) and running it. if the vibration persists then remove the drive pulley from the motor and try again. It's doubtful it's the drive shaft as that is an item machined on a lathe and should be dynamically balanced. The pulleys may be machined castings and out of balance.
Click to expand...
It doesn’t happen without an out of balance blank. Smooth as butter otherwise. At least in my case, it isn’t the lathe being out of balance.
 
Then it's a harmonic in the lathe structure. If it is constant no matter where the tail stock is bolted tight then it's most likely not the cast iron ways.
It could be the legs. Let your fingers do the walking over all the safe to touch parts and see if its that. They may be able to be stiffened.
Also the old standby, add weight to the base.
If it doesn't really bother you live with it. Good luck either way.
 
robo hippy said:
I am not familiar with anyone doing a lathe leveling video, but there may be some. In order to get it 'perfect' I figured out what works for me. One is to make sure lathe is level cross the ways on both ends, and level front to back. I then mount an unbalanced piece on the lathe, and back off one of the lathe leveling feet till it is just floating off the floor. Turn on the lathe at medium speeds, enough for it to wobble, but not enough for it to start walking around the shop. Adjust till the one floating leg engages enough to stop the wobble. I may do this a couple of times. I then adjust that leg down maybe 1/16 or so of a turn. I do this because when I 'lock' that leg/foot in position by tightening the nut, it seems to lift that foot off the floor just a hair. I do mark on the concrete floor where the feet are positioned. The lathe still wants to walk around a bit when an unbalanced piece is mounted on it. You may have to repeat this a couple of times to get it perfect. If your lathe is on a garage floor, they are sloped, and you may need to anchor the feet to the floor, and the lathe will ALWAYS want to walk down hill.

robo hippy
Click to expand...
Is it safe to bolt a lathe to concrete floor? Like everything else, there are opinions both ways. I’ve thought of doing it but have been deterred by the stories of lathe bed cracking. Any experience with this?
 
I had a Sears bandsaw when I started back in the 80's and it had a horrible vibration when running. I basically disassembled it, squared things up, got the wheels running in the same plane and stiffened the flimsy base. It ran better and cut good, but the vibration was still there. Finally, I put a bag of lead shot on top of the housing and the vibration completely went away. It cut very smooth with a good blade. Two friends have now owned it since I payed with it years ago.
 
If a lathe is bolted to the floor with pads under the feet the bolts should also be isolated from the legs with the same type of pads and that includes the circumference of the bolt hole. Otherwise you are not really isolating the lathe from the floor due to up and side forces generated by the vibration.
 
Gerald, I have some spare rubber laying around the shop - might try that and see if it helps! I'll also take a second look at the belt and other moving parts, check the bolts on the legs.. and if that doesn't fix it, probably just deal with it :D Not worth re-pouring the concrete pad!
 
I am pretty sure I have heard of using hockey pucks under the feet of lathes. Hard enough to take a pounding, but soft enough to absorb some shock.

robo hippy
 
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