Help! Why is my Lathe vibrating so much?

[Stefan Antwarg]

I have a jet 1442 with the iron legs plus I have added 250 lbs of sand. I am trying to turn the inside of an 11.5 inch bowl. It was fine when I had the speed set to the 2 lowest speeds. But, when I increased the speed to 850, it vibrates too much. It's sitting in the basement on a concrete floor and my wife even felt it upstairs. The bowl is attached to a chuck with a tenon, so I assume the tenon is centered. Any ideas.

Thanks,
Stefan

[Dale Thompson]

Stefan,
If the workpiece is balanced, your problem MAY be that the legs of your lathe are not contacting the floor uniformly. If that is the case then NO amount of weight will solve the vibration problem at high speed. Basement floors are usually NOT flat. They are pitched toward a drain. Check to see if you have one or more legs that are "light". If that's not the problem, you may have a bearing or chuck dilema.

I hope that this helps.

Dale T.

[Don Henthorn Smithville, TX]

Are you sure each leg is bearing an equal amount of the load? I would suspect that the lathe may be "reaching" to get one leg on the floor. Just a wild guess.

[Tom Hintz]

Stefan,
If you do not have the bowl itself rounded out, that 11.5" diameter is plenty to shake the teeth out of just about anything until it gets more in balance.
Also, check to make sure the chuck stays tight. I have had the piece get slightly loose during turning, not so much the chuck loosening, but rather all the forces of the piece throwing weight around during the rounding process compresses the wood slightly at the jaws. Tightening them ocasionally can help as well.
Certainly checking the legs is a good idea regardless. It's tough to be accurate if the lathe can move even a small amount.

[Stefan Antwarg]

The lathe does not move when I push up or down at any leg. Plus, I made sure it was level during assembly. So, I am pretty sure the legs are all contacting the floor equally. Tonight I will make sure the bowl is in the chuck securely and maybe do a few more passes over the outside to make sure it is round.

Stefan

[Richard Allen]

Hi Stefan

You very likely are doing everything right and that the lathe is very likely performing properly. 850 RPM for an 11 1/2" bowl blank is close to the max recommend speed (29 MPH where the rule of thumb I use for lathe speed is to keep it under 30 MPH Diameter times RPM less than 10000).

If anything was a little off that top speed is likely to be the issue.

I have not used this lathe. Most lathes in this class have adjustments that can be made to the feet. That adjustment is typically in the form of a bolt that is turned with a open end wrench. You can "feel" the pressure for each of the legs with the wrench. Adjust the headstock feet so the wrench pressure feels the same. This is a back and forth procedure as increased pressure on one leg will mean decreased pressure on the other leg. Do the same for the tailstock feet. Don't expect the headstock and tailstock to have the same pressure.

Most lathes have a harmonic speed where the lathe will vibrate. When a hunk of wood is added that harmonic can be intensified or changed to a slightly different speed. It is possible that the "850" speed is one of the harmonics for that lathe. Running the lathe 100 rpm faster or slower can eliminate the vibration.

An 11 1/2" hunk of wood can be a LOT of weight for a lathe like that. Depends on species, moisture content, lengths of the piece. When you bring the tailstock up to add support for the wood does the vibration reduce? If it does you might want to use the tailstock for a good portion of the roughing process to take some of the pressure off the headstock. Obviously when you are down to your last inch of wall thickness you want to have the tailstock out of the way. But for removal of a good portion of the mass of the wood the tailstock can be a big help. You can use your bowl gouge or a scraper or whatever to hog out the center with the tailstock in place.

The headstock on this lathe moves, swivels I think. Make sure everything is secure and tight.

Often wood will be round but unbalanced. Sapwood, inclusions, how the wood has been drying. There are many reasons why a hunk of round wood might not be balanced. Balance the hunk of wood on a coke bottle or something similar to see if the piece is naturally out of balance. If it is out of balance the turning process will very likely eliminate much of that balance "issue".

Good Luck

Good Luck

[Chris Padilla]

Richard,

Actually, harmonic is not quite the correct term to use. You are looking for the word resonance. All things have a natural frequency associated with them and if there is a force on the system that is close to this natural frequency, we say that the system resonates at that frequency.

In this case, the mass of the wood, the mass of the lathe and the rpm chosen to spin the mass must be very near the natural frequency of the lathe/wood as a system to cause these oscillations or vibrations.

As you said, raising or lower the rpms should help one past the resonant frequency found.

[Stefan Antwarg]

I understand that thing about harmonics - or resonance. When I increased the speed further to 1100, the vibration was much less.

Is it possible that using too small of a tenon can cause extra vibration? It was a bigger bowl than what I turned at my recent lesson with Bill G. But the tenon was smaller. A little over 2" in diameter and only about 1/2 of the max. depth.

Stefan

[Tom Hintz]

Stephan,
Probably the simplest thing to do is to chuck something else up to confirm the vibration goes away. If it does, tough it out and turn the piece you have in there. I have had to "turn through" vibration states on a few larger pieces but they did go away at some point as the piece became lighter.

[Jim Becker]

As you can see from many of the comments, there are a whole bunch of things that can individually or in combination cause vibration when you are turning an object. Even sanding can cause it if there is enough difference in density within a piece or there are hidden inclusions. Varying the speed is the best relieve if everything else is in order, although that is a little more difficult if you don't have the ability to change speeds in small incraments. 1100 RPM for an 11" bowl is a bit fast for safety based on the formula that Richard provided...be careful!

As to your tenon size...2" isn't too small depending on strength of the species, but I personally would prefer something larger for that size object. The tenon doesn't need to be long...in fact, a tenon should never bottom out in the chuck and best results are usually had when there is a small shoulder that the outside of the jaws rest on to insure that the workpiece is fully supported all the way around in addition to the portion that is inside the jaws.

[Bill Grumbine]

I understand that thing about harmonics - or resonance. When I increased the speed further to 1100, the vibration was much less.

Stefan

Hi Stefan

Now you are starting to think like the rest of us - since it is vibrating, let's see what happens when I go faster!

It does sound like you just hit the frequency with the piece. The tenon is only too small if it breaks off, but then it is too late to do anything about it. :eek: That is something you will develop a feel for as you gain experience. I usually determine how big I want the bottom of the bowl to be, and then make the tenon as big as I can within the constraints of the design of the piece. For example, on the maple burl bowl I just posted, the one with the wing, I made a huge tenon, even though it was a relatively small and light piece. I needed to tenon to make a ring bottom for this piece, and so I had to put the #3 jaws on the Stronghold. The tenon was close to 5" in diameter if I recall. Ordinarily I would have used a tenon in the 2" or smaller range for an artsy piece.

Good luck with it, and I will look forward to seeing how you end up with your bowl.

Bill