Extended stance for vibration

[john lucas]

We keep discussing weight when it comes to vibration but my friend John Roberts was turning a lot of off center hollow vessels. He bolted 8 foot long pieces of 1/2" thick by 4" angle iron to each leg section of his Powermatic 3520 and all the walking and vibration went away. It is a pretty big trip hazard but he had a big open shop, I don't
I haven't tried this yet(I added 300lbs of gravel to mine) but I'm considering extending the stance a little while at the same time adding removable wheels. Anyone tried this. I was thinking about just 3 or 4 inches on my side and maybe 6" to a foot on the backside. I could maybe add a cabinet to the back to both give more storage, have a track to slide the dust collector and lamps and also reduce the chances of tripping over the angle iron.
Good idea or bad? Any suggestions

 

[MichaelMouse]

I would think less of a splay would be enough. I advocate diameter plus ~1/3 on the spread. Of course you might want to go more if the centerline is way up there. Figure the max velocity/mass difference is exerted at around 10:00, extend a line from there to center and draw a perpendicular tangent to the max swing. That should be enough.

Six feet either side isn't enough if the setup isn't rigid, of course. Much less if it is.

 

[Bill Boehme]

That's a lot of steel that he added and I think that it was the additional mass more than anything else that was responsible for the decrease in lathe vibration. Your sandbags are a lot cheaper and are better at absorbing energy than the steel. Basically, the added steel is just lowering the frequency of some of what engineers refer to as "dominant poles" (in essence, structural resonance frequencies).

Something to consider is that a better "lathe friendly" solution is to balance the load as much as possible at the beginning and then slow the lathe down while turning the rough piece of wood into something round and inherently better balanced. While it is good to ad mass to a lathe -- it is only good up to a point. If used as a substitute to take the place of balancing a very heavy chunk of wood, the dynamic loads on the spindle bearings will essentially be the only compliant component in the system that absorbs the energy from the unbalanced wood. The amount of energy into the bearings is proportional to the square of the angular velocity -- in other words, if you were to double the speed of the spindle, the energy would be increased fourfold. Most lathes use ordinary general purpose ball bearings that are intended primarily for torque loads along with light radial loads, and even lighter axial loads so it is not doing them a favor by subjecting them to conditions with heavy unbalanced chunks of wood, especially at high speeds. I think that the bearings on the Powermatic may be double row angle bearings, but it is still good to be kind to your bearings.

 

[Joe Greiner]

What he said, especially "be kind to your bearings."

And, deliberately off-center to produce vibration cries out for a slow lathe speed, with cutting speed provided by another machine, such as a router, Dremel, or Lancelot/Arbortech. I have an auxiliary drive for my lathe, in a rotisserie motor mounted outboard of the headstock to turn CW (LH thread), then cut freehand with Lancelot (avoid climbing cuts, which could confuse the drive). A power planer is an alternate for some shapes. The router or Dremel works better hard-mounted, e.g. in a cross-slide vise to mimic the tool support in a metal lathe; could also follow a pattern.

The rotisserie drive also facilitates coated finishes, to reduce runs. Mask the bed and everything else, of course.

 

[Fog Tanner]

Most of my turning these days is offcenter. My lathe weights 1100 pounds, but I can still get it vibrating. Part of that is due to one of the offcenter chucks I use placing an amount of steel off the centerline. And part of the vibration is due to the speed. With a lot of the cuts being 70% air, I like to keep the speed up for a smother cut.

In order to get the speed I want/need, I went to a balancing system. Since I am using a Vicmarc, I bought their balancing system. It mounts on the outboard side where the handwheel goes. I start by balancing the piece with the lathe off, then turn it on and check the vibration. If I am not happy with it, I fine tune it until I get it as smooth as it will get. Since going to the balancing system, I have saved several hours on some of my pieces, what used to take five hours now takes two.

I also use this when roughing out large bowl blanks. For 20" bowls, that I might normally have to start at 200 rpm, I can start at around 500-600 - which is where I would normally run at after it was roughed down. The only down side to this is as I progress towards the finished shape, it starts to vibate and I have to reset the weights to zero to balance the piece.

One of my offcenter chucks has adjustable counter weights built into it. They are good up to a point, due to their size. I know that Kelton makes something like this except much larger and would work for 24" bowls. This might be another alternative for balancing. With a sliding headstock lathe, you will always have a slight amount of movement between the headstock and the bed, so balancing the rotating mass would probably provide more benifit than adding cross beams to the legs. I tried using my Vicmarc Escoulen chuck on my sliding headstock lathe and found that it was OK up to the point. The headstock was going to have a certain amount of movement.

There is also one other point to consider when "tieing down" the lathe. If the rotating mass is unbalanced, the stress will have to go someplace. With a ridig supported lathe this means being absorbed into the lathe, which could be flexing of the cast iron and over a period of time could lead to stress problems. Without a balancing system, I would think you be better off with a cabinet or weight box that had a couple of hundred pounds of lead shot in it.
The lead shot would do a lot better job than gravel in absorbing vibration.
Fog

 

[john lucas]

I definitely agree on balancing the piece. I do that when I am purposely turning off center work. I just thought that since I needed to make it easier to move the lathe (I have too many tools and not enough space) while I was at it I would see if there was a way to make the lathe more stable for the day to day roughing of bowls. If I could speed up the lathe just a few hundred rpm it would speed up the start of the process. I don't have much time to turn since my job is eating up more of my time so I have to make the most of it.