Listen To Your Lathe

[Mark Mandell]

Got to spend some time on the lathe last w-e and confronted a rather strange situation.

Have some dead-green walnut and decided to rough out a few HF's and bowls. Had one blank about 20" wide by 12" thick. The initial roughing went well enough, and I decided to do a closed form rather than a deep bowl. Got the profile done with a faceplate, removed it and chucked it in the vic 120 with a 5" tenon. Brought up the TS and re-trued the profile so all was running smoothly. All pretty usual.

I removed the ts and started to open the form with my bowl gouge. Moving into the form I started to get this low frequency vibration from the cut. Not the first time I've gotten a rubbing sound from wet wood, but as I kept making cuts the noise got stronger and all of a sudden my lathe (850lb Stubby 1000) started to dance like the piece was way out of balance. Huh? I stopped the lathe to see if the piece had slipped in the chuck, but all was well and the profile was dead true with no wobble. Turned the lathe back on and everything ran smooth. Take another couple of cuts and, first the sound, and then we're dancing again!

This appears to be a turner's example of induced harmonic resonance from the vibration in the cut setting my pile of steel to music. There's no doubt that if I hadn't stopped the lathe to check this out, my lathe could have easily oscillated out of control and I could have gotten hurt.

Moral of the story: When your lathe talks to you, you better listen.

 

[MichaelMouse]

Of course, you may have something unique going there, but I would do a PM on the lathe. I think something's working loose.

 

[Mark Mandell]

Of course, you may have something unique going there, but I would do a PM on the lathe. I think something's working loose.

Lathe and parts are rock solid. I can see two possible causes or (likely) a combination of the two.

1. The noted induced harmonics.

2. This was dead-wet, just-cut wood that was mounted in face grain orientation and was throwing a steady stream of walnut juice as I worked. It's possible that a resulting difference in free water content created a slight x/y imbalance in the piece even though the form was running true.

Reason I think it was a combination is that after I stopped the piece (to stop the dance) and then restarted at the same speed (about 1k) there was no movement. It took the vibration of the cut to start things going.

Putting this down as the effect of a little "Bowl Gouge Music", but it's still something to watch for if others are turning wet wood.

mm

 

[Ruth Niles]

Just curious, Mark, have you tried turning another piece of wood approx. the same and the lathe ran like a charm?

I know the Stubby has nothing in common with the General but when I experienced this same phenomenon, it was the rear bearing.

Ruth

 

[waltben]

I've noticed the same sort of thing (but never got my 750 moving about) on occasion. If I lower the speed a bit, it'll go away, or at least change. If I bring my tool rest closer to where I'm cutting, that will fix it too. I think you're right that it's caused by some sort of vibrational resonance caused by the tool and the characteristics of the wood fiber.

I've noticed a similar sort of thing doing spindle turning on my mini, and squaring the bottom and top of something with a parting tool. The tool will start to chatter a bit and vibrate the lathe. When I pull the piece off, there'll be a pattern on the flat part from it.

 

[Mark Mandell]

Just curious, Mark, have you tried turning another piece of wood approx. the same and the lathe ran like a charm?

I know the Stubby has nothing in common with the General but when I experienced this same phenomenon, it was the rear bearing.

Ruth

Bearings are tight and smooth, Ruth. A bit of spindle lash was the second thing I checked for (after examining the chuck).

Getting ready to mount a 24" piece of Spalted Sycamore. That should prove interesting . . .

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[KEW]

If it is indeed harmonic vibrations, you should be able to adjust the speed slightly and eliminate it entirely.
I cannot claim any true expertise, but I do not believe the harmonic frequency required to make your lathe "shudder" or dance would be in a range which you could induce. If you can hear it, it must be above 20Hz which would mean the lathe would feel like a vibrator when you touched it it might even drift around the shop, but I wouldn't call it dancing around (which would be much lower vibration).

 

[KEW]

Bearings are tight and smooth, Ruth. A bit of spindle lash was the second thing I checked for (after examining the chuck).

Getting ready to mount a 24" piece of Spalted Sycamore. That should prove interesting . . .

mm

Play with the speed control and see what happens!

 

[Mark Mandell]

Play with the speed control and see what happens!

Sure, that changed everything when I tried it. Mind you I slowed things DOWN; wasn't about go faster . . . . no guts, no glory

When I did slow it down and restart the cut, I (naturally) got a completely different sound. Had no effect on the lathe. Leads me to reason that there were a number of factors at work, but the bottom line was/is that I quickly responded to a situation before it got out of control.

First time in 4 years that I actually used my lathe's mushroom kill switch.

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[robo hippy]

It could be related to what happens with a slightly unbalalced piece of wood. You turn the speed up and it vibrates slightly. Turn it up some more and the vibration goes away. Also, I have found that when I am turning larger pieces, some times I get vibration when I apply the tool, but not when it is spinning free. I always attributed this to the attachment to the lathe (recess for me, tenon for others) as being a bit minimal for this size of piece. 5 inch tenon on a 20 inch diameter piece is a bit minimal, 6 to 7 would be more standard (1/3 to 1/2 the diameter of what you are turning), but is there a chuck that size?
robo hippy

 

[Bill Pottorf]

I always attributed this to the attachment to the lathe (recess for me, tenon for others) as being a bit minimal for this size of piece. 5 inch tenon on a 20 inch diameter piece is a bit minimal, 6 to 7 would be more standard (1/3 to 1/2 the diameter of what you are turning), but is there a chuck that size?

Definitely, the 5" jaws on the VM120 are much too small for the diameter being turned here. Vicmarc makes the multi jaws which I always use for large bowl blanks to be cored or turned. You can then get up to the 1/3 of the diameter which should be used for this size of blank.

Bill

 

[MichaelMouse]

Lathe and parts are rock solid. I can see two possible causes or (likely) a combination of the two.
2. This was dead-wet, just-cut wood that was mounted in face grain orientation and was throwing a steady stream of walnut juice as I worked. It's possible that a resulting difference in free water content created a slight x/y imbalance in the piece even though the form was running true.

Reason I think it was a combination is that after I stopped the piece (to stop the dance) and then restarted at the same speed (about 1k) there was no movement. It took the vibration of the cut to start things going.

I've taken wood which was laying wet down, dry up and cut it at 90, which will certainly do what you say if you're turning too fast. Paradox in that applying the tool normally slows the rotation, especially if you're hogging. Slower rotation, as others mention, is going to make less of the imbalance. Square of velocity and all.

Good to hear that slowing, which is the first reaction I have to any out-of balance situation, has done it for you. Don't be in a hurry, and just let that wood come to the tool and curl away. Easier on your elbow, too.

 

[Mark Mandell]

. . . applying the tool normally slows the rotation, especially if you're hogging.

Michael, taking a full chip with a 1/2 bowl gouge will not slow down a 3 hp Stubby. I've done so, but it took the big McNaughton set or a 1" Glazer to do it.

Good to hear that slowing, which is the first reaction I have to any out-of balance situation, has done it for you.

Once I have a workpiece round and true, I often turn at 1k. Brings off the lightest "wood wool" shavings and gives silky smooth cuts. In this case I slowed the lathe only to halt the dance, but I stopped cutting completely. When the rocking stopped, I brought the speed back up slowly looking for the oscillation to reappear; didn't happen until I restarted the gouge cut. If it were caused just by the unbalanced wood, I'd have expected it to oscillate without any cutting. Didn't happen. That's why I find the situation so strange and worth posting here.

Since it was a hollowform, I racked the bowl gouge and hooked up my hollowers. Had a little vibration, of course, but no trouble hollowing the piece at about 900 rpm with no lathe dancing.

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[MichaelMouse]

Michael, taking a full chip with a 1/2 bowl gouge will not slow down a 3 hp Stubby. I've done so, but it took the big McNaughton set or a 1" Glazer to do it.

Not sure I understand. On the one hand you say that your machine does not react under load, and yet your problem is that it is reacting under load. Most unusual. Find the spot where there's room to flex and you find your problem.

 

[Mark Mandell]

Not sure I understand. On the one hand you say that your machine does not react under load, and yet your problem is that it is reacting under load. Most unusual. Find the spot where there's room to flex and you find your problem.

The only items with any "flex" are the gouge and the tool rest, but even there it's minimal and should not effect a lathe of this size. My lathe "appeared" to be reacting to the vibration of the cut in progress; the longer the cut, the move severe the oscillation became. The only real variable that I can see was the density of the wet wood and, possibly, a change in that density by water release through the centrifugal force of the spin.

So, was it the Gouge? The Cut? The Speed? The Water? The opening of the form which increased the water loss? The sugar-drag on the tool's bevel cutting through the sap wood? The ghost of a Revolutionary War solder who was killed where the tree grew? Or a dozen or so other factors that I'm not aware of that might produce the effect?

I don't understand either, and since this is so unusual (to my limited experience), I posted here to see if anyone else has run across the same thing.

Thanks anyway.

MM

 

[Richard Baker]

As mentioned, the problem would seem to be the excitation of a natural harmonic vibrational frequency of the "system", initiated by beginning the cut. Once excited, the harmonic can increase in amplitude with very little driving force. Wasn't it the Tacoma Straits (or Narrows) bridge that did that? The "system" of the workpiece, chuck, spindle & bearings, headstock, lathe bed, banjo and toolrest, and the tool (and anything else attached) forms a very complex mechanical structure with numerous natural frequencies, nearly all of which are well damped nearly of the time.

You just got lucky and hit the natural frequency jackpot. A little slower might be better. A 20" piece at a grand would have me hiding in the furthest corner.

 

[waltben]

My first post was:

I've noticed the same sort of thing (but never got my 750 moving about) on occasion. If I lower the speed a bit, it'll go away, or at least change. If I bring my tool rest closer to where I'm cutting, that will fix it too. I think you're right that it's caused by some sort of vibrational resonance caused by the tool and the characteristics of the wood fiber.

I believe you experienced the same thing only more pronounced. I don't think it's got anything to do with the water content, since I've had this happen with dry wood more than wet. The ghost bit may be part of it, since a lot of my wood comes from around the Manassas battlefield area and other parts of Virginia.

 

[Charlie Knighton]

ghostbusters

The ghost bit may be part of it, since a lot of my wood comes from around the Manassas battlefield area and other parts of Virginia.

do you get more for a piece if it has a ghost in it, and even more if you can id the ghost????????? :cool2:

 

[MichaelMouse]

Eino and Toivo were out cruising the woods for birdseye logs one day when they came to a stream. They took off their shoes and began to cool their feet in the water, until Eino noticed, with horror, that a sow bear was on the opposite side of the stream, and her two cubs behind them. Panic set in as he asked Toivo over and over again "what shall we do?"

Toivo began pulling on his boots, which prompted Eino to say "you can't outrun that sow, even with your shoes on."

Toivo said "I don't have to outrun her, I only have to outrun YOU!"

Which is the proper philosophy when turning, as well, You only have to apply enough force or present enough resistance to free up a shaving. If you do more, you risk slowing the lathe when engaged, bouncing the tool over high spots, and closing open spaces between your chuck and your piece, only to let them open again when not under stress. If it happens again, go back to a sweep and peel motion which includes only the high spots. Don't throw shavings, let them drop as they slide off the tool. That way you stress the system the least, regardless the horsepower, torque or weight involved.

To create a resonance, you first have to have a regular oscillation. Something has to be loose or flex. If you're extended too far over the rest - always a bad move, you could set up a bounce in the tool. If you're digging too hard you can set up a flex and recoil in the hold on your chuck. If the piece is already thin, or you're hacking rather than shaving, you can experience flex in the wood itself. I assume you're experienced enough to know all this, therefore I suggest something loose in the lathe/wood interface, or a loose pulley grubscrew, a loose bearing, as others have suggested, or a sort of off-the wall possibility that your rpm sensor may be too high, attempting to maintain speed by application of additional power whenever the piece slows by a minute amount.

If the piece rotates under no load without setting up an oscillation, I don't think it's balance. The Delta M, difference in mass in the wood does not change. In an unlikely scenario, it is lighter at the precise point at which you tool encounters greater resistance cutting upgrain, but it's a real coincidence if so. Reduce your speed to reduce the effect of the Delta M and/or lighten your cut to take a shaving rather than a chunk, and it should pass.

 

[Mark Mandell]

The ghost bit may be part of it, since a lot of my wood comes from around the Manassas battlefield area and other parts of Virginia.

Story Wood!!!!

I made some furniture out of cherry milled from a large tree on a farm near Carlisle, PA. In several boards, corresponding to growth from about 1900, I found some 12 slugs at 30 cal. or higher.

From thence developed the story of "wild braves" escaping from the Carlisle Indian School, getting drunk, and terrorizing the farm until the "pony soldiers" arrived, shot up the place, and dragged the escapees, war paint and all, back to the institution.

Now, while the Carlisle "school" was actually a cultural torture chamber to abuse and subjugate the "savages" into becoming "civilized", and some children did, in fact escape, there was no such "war party." However, the story makes a wonderful tale to fit into the lexicon of the stereotyped jingoistic misinformation that comprizes some of the history of our country. Since I incorporated as many of the bullit holes into the pieces as I could, it presents an unparalleled opportunity to mess with minds at parties when somebody asks about them.

 

[Charlie Harley]

Harmonics are strange animals. College tried for several semesters to learn me about vibrations, and the biggest thing I recall was the Tacoma Narrows Bridge falling in the water.

Harmonics can crop up at any time for any reason. I turn wood with the premise that NOTHING is perfectly balanced and that there will always be a rotating imbalance of some sort on the face plate. Even if a piece is dimensionally balanced, density variations will cause a mass imbalance.

Setting a gouge to the piece further complications manners. Anyone who's done squarallel wood work knows that it generally takes more force for a tool to cut cross grain than with the grain. Therefore, if the piece is spinning at 1000 rpm, it is being "hit" 2000 times per minute with an extra force as the gouge goes cross grain. The equation further complicates if a piece has a soft area.

The bottom line is that any given harmonic only exists at one rotational frequency. Speeding up for slowing down (with slowing being HIGHLY preferrable to turners) will take the piece out of its harmonic range. That's not to say that any given piece won't have another harmonic frequency, but slowing a piece will generally eliminate the problem.

 

[robo hippy]

Mark,
I am surprised that it takes a 1 in Glaser or the big McNaughton to slow your big lathe down. The Glaser, or for me, a 1 1/2 wide scraper can do it (perhaps that will change when I trade the PM for a Robust). The McNaughton has a 3/8 inch wide cutter, and if you add the spear point in, they you may have 1/2 inch of cutting surface in the wood. I have never understood how so small of a cutter can have such stopping power.
robo hippy

 

[Mark Mandell]

Have to say, Charlie, that this sounds most plausible. I wouldn't know Delta M from any other letter in the Greek alphabet , but Ive seen and read enough about vibrations, resonances (including the Tacoma Bridge story and now the Milwaukee collapse), and standing wave generation to suspect that this was what was happening. Your point on variations in the wood density was a good one as this walnut was a piece of wood taken from just above a "rams horn" in the log, and was comprised of unbalanced amounts of heart wood and some figure. It's most likely that there were density differences in the piece that, even if cut true and round, could react to the vibrations noted.

I have some other pieces from the same log so I may just try to (carefully!!) replicate the occurrance.

mm

 

[Mark Mandell]

Mark,
I am surprised that it takes a 1 in Glaser or the big McNaughton to slow your big lathe down. The Glaser, or for me, a 1 1/2 wide scraper can do it (perhaps that will change when I trade the PM for a Robust). The McNaughton has a 3/8 inch wide cutter, and if you add the spear point in, they you may have 1/2 inch of cutting surface in the wood. I have never understood how so small of a cutter can have such stopping power.
robo hippy

It gets multiplied by the distance from the center; the further out, the less force it takes. My machine runs on a 3/4" poli-v belt. I stalled the spindle with the McNaughton coring at a 20" diameter, but I didn't phase the motor. Remember too that the coring tool is a scraping rather than a slicing cut and even though you should clear the kerf often, that buildup of shavings in the cut creates a lot of friction between the long knife and the cut walls. That all adds up the a lot of force being put on the drive belt(s). I also don't have my belt tensions as high as others may do because it acts as a safety factor.

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