My misconception about lathe vibration, and how to deal with it.....

Discussion in 'Newbie' started by odie, Dec 30, 2012.

  1. Matt Lewis

    Matt Lewis

    Joined:
    Dec 2, 2006
    Messages:
    122
    Location:
    Suwanee, Georgia
    Home Page:
    I am thinking that any vibration of the steel is very small and likely not of major concern to the turner. To me the bigger problem is control of the "feed irregularities" which I suppose falls under the tool presentation category. In other words, it is a bigger matter to maintain consistent orientation of the cut with regard to rigidity in relation to the fulcrum. Of course sharp tools are a necessity, but I think are more important to the turner with regard to "feed irregularities" than any vibration that might be enhanced in the steel. Try turning a piece of Aluminum on the wood lathe and see how hard it is to keep the tool fixed so that it maintains consistency of the cut. Although a sharp tool improves your chances as does keeping the tool rest very close to the piece, you will most likely have some chatter marks because of your inability to keep the tool fixed with regard to its cutting orientation rather than because of any small amount of vibration that is occurring in a naturally flexing, but very ridged piece of HSS. I suppose really what I am saying is that I think the vibration within the tool steel that does occur is at a microscopic level and really not as big an issue as the macro level movement of the tool orientation that is inevitable in free hand turning provided you have employed other measures such as limiting tool rest distance in relation to the tool's girth/thickness. I think that any pattern forming on the wood is more likely a function of wood flexing and feed irregularities than tool steel vibration. That is why I wondered about the use of the term vibration because to me vibration is an oscillation from equilibrium about some mean value which I don't believe is necessarily what is happening with the tool orientation itself.
     
    Last edited: Dec 31, 2012
  2. odie

    odie

    Joined:
    Dec 22, 2006
    Messages:
    4,432
    Location:
    Deep in the woods
    I tend to think along the lines as you do, Matt....

    If there is any flexing of the steel, it is microscopic, and won't be of concern to a traditional turner.

    Those were some powerful posts by Dale, but he's just kidding himself if he thinks our turning tools flex to any degree that will have any realistic effect on turning with hand held tools. For a rigidly mounted tool, such as with a metal lathe, and add a long overhang, such as on a wood lathe.......you might see such a thing.......but, for a lathe turner, your hands and arms will move long before the steel will.

    Somewhere back in time, I seem to remember a photo of a bent turning tool........in that case, there was one horrible catch! :eek:



    ooc
     
    Last edited: Dec 31, 2012
  3. Matt Lewis

    Matt Lewis

    Joined:
    Dec 2, 2006
    Messages:
    122
    Location:
    Suwanee, Georgia
    Home Page:
    Now, I will say that I turn Aluminum all the time for tool handle inserts on my wood lathe with great success, but I have had to tinker with the tool shape, feed and speed and presentation quite a bit to reduce the chatter and effort it takes to keep the tool as rigid as I can by hand. It isn't an easy task, but very doable. The chatter marks are typically easy to clean up with abrasives. I have also turned a limited amount of steel by hand as when making cup centers from steel Jacob's mandrels, turning down washers and turning down nuts. I wouldn't recommend doing it a lot, but it works for small jobs.
     
  4. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    Well again I'm not an engineer but let me show you what is happening. I don't know if it's the steel or not, but. I use the tools in the same orientation of cut, both sharpened on the same jig and both have handles identical in length. One makes screeching noises and the other doesn't. That's on the 3/8" detail vs 3/8" spindle gouge. The only difference is the size of the metal. See photos below. You'll notice there is almost no overhang on this tool so the vibration must be a sort of harmonic along the shaft. Could it be my hand moving and not the steel? Sure but I find it hard to believe that I can repeat the cut over and over with the same results. Why would one do it an the other not, unless it's the thickness of the steel.
    On the bowl gouge vs detail gouge. Again this is something I've repeated several times with the same results. The bowl gouge can chatter which shows up in the cut while the detail gouge doesn't. In this case there is about 3" of overhang. These 2 tools are both Thompsons made from the same steel with virtually identical handles and both sharpened using the Wolverine jig. These are 1/2". The bowl gouge has a bottom flute thickness of .227" and the detail gouge is .400" It could simply be the summer winter wood harness differences set up a vibration that is seen more by one tool than the other. I don't know but then again. I can repeat the results several times so in my mind that points toward the differences in the steel.
     

    Attached Files:

  5. Dale Miner

    Dale Miner

    Joined:
    May 13, 2007
    Messages:
    201
    The flex is small, but it is cumulative. Each revolution adds to the surface pattern, and the gouge flexes in more than one direction. As it flexes, the contact between the wood and steel changes, changing the cutting action. The pattern that you can see in the bottom of a deep bowl with a long tool overhang is occurring at a frequency above what our hands and arms can vibrate at. Flexing of the tool, the tool rest, the banjo, and the spindle all contribute to a greater extent than the flabby human elements.

    I just went out to the shop, clamped two inches of a 5/8" gouge in my vise, and with a surprisingly small amount of effort,was able to move the handle enough to visibly notice. Not scientific, but noteworthy. I would agree that the least ridgid component is the wood when it comes to vibrations with higher multiples of the rpm, but still think that the tooling and entire system can at times become a factor. The human component is no doubt the weakest part of the system when a one or two times rpm vibration is the problem.





    Sharp tools well presented, couldn't agree more.
     
  6. odie

    odie

    Joined:
    Dec 22, 2006
    Messages:
    4,432
    Location:
    Deep in the woods
    Dale........

    Although I hadn't considered this much, I'm inclined to think your hypothesis is true. The only thing that I'd question, is to what degree it applies to us as woodturners. In the case of a captive tool, it would seem it would be more applicable.......I don't have one, and don't do hollow forms, so I'm speaking strictly from a theoretical standpoint with that statement......but, it's certainly more rigid than hand held........

    The entire system, rest, banjo, lathe bedways, etc., would have some amount of residual effect, but the truth is that the tool is held by the human element on a fulcrum. Tip goes down, handle responds equally by swinging up......and that handle is in your hands.

    I've been giving some thought to your tuning fork example........and, it seems to be an apples and oranges sort of thing, as it applies to woodturning. It's a given that any steel will have a harmonic resonance, but again, this is a matter of degree, and how it applies to woodturning. It does, however, establish that harmonic resonance is fact, and to some degree it undoubtedly does apply to woodturning......the question is when, where, and how much?

    I don't know if you are applying the harmonic resonance to those spiraling ridges we sometimes get, but the answer to that (as you have confirmed) is sharp tools presented well.

    I have a theory of my own on how those spiraling ridges get started and get increasingly worse. (First and foremost.......this takes a tool that isn't as sharp as it should be.) My theory is that a dull tool will ride the grain's alternating hard and soft spots, on and in-between the annular rings. The tool will cut better where the wood is softer, and worse where the wood is harder. This ultimately causes the tool to rise and fall with the grain, and begins the process of leaving the spiraling ridges. Once the spiraling ridges have started, then the grain of the wood is no longer influencing the tool, but the ridges themselves are. The ridges get increasingly worse as the tool is now riding the ridges. It's only my theory, and I can't guarantee it's true, or not, but as I observe the action at my lathe, it does seem to apply.

    The answer is sharp tools presented well, and when I see/feel the spiraling ridges first start to happen......I immediately resharpen.

    ooc
     
    Last edited: Dec 31, 2012
  7. Dale Miner

    Dale Miner

    Joined:
    May 13, 2007
    Messages:
    201
    Odie,

    Your assesment at least parrallels the 'cumulative' part of my explanation. You have noticed that the ridges spiral, and are not in straight axial/radial lines. Wheter it is the wood that starts the pattern, or a flex somewhere in the system that does the initial patterning, the next revolution of the wood brings the gouge into contact with that now slightyl higher spot. Flex, bounce, arm movement, or whatever, the next high spot will be slightly later in the revolution and the spiral ridge is formed. The amplitude or size of the ridge will also be increased up to the point that the system can control the formation of the ridge.

    Let me bounce this one off you. Doing the bottom of a deep shape bowl with a fingernail ground gouge. Bevel contact is barely possible withoug hitting the side of the bowl. Straight tool rest, about 2.7" overhang. Overall length of tool is 30 inches. Soft maple. Very little early wood/late wood difference in the bottom as most of the cutting is not into the end grain. 750 rpm. Bowl held in a chuck with extra thickness in the tenon. A spiral pattern develops with 18 rigdes. The ridges are .007" valley to peak. 750 rpm x 18 ridges has a frequency of 13500 cpm, or 225 cycles per second. If nothing were flexing, then the end of the handle would have to be moving .070" with the passing of every ridge. At 225 times per second our hand would literally be numb in a short time.

    From experience, the above is not an impossibility, especially with a less than fresh edge. I do know that the end of the gouge does not move .070" 225 times per second. It may move that much once or twice per revolution. In the bottom of the bowl with the extra thickness and smaller diameter area, intuitively there is little chance for the wood to be flexing away from the tool to form the ridges. If the handle of the gouge is not moving .070" at each ridge, and the wood is not moving away from the tool .007" at each ridge, then somewhere in the system is a flex that allows the spiral ridges to form. The flex could be in the tool, could be in the tool rest, could be in the spindle assembly, could be in the compressibility of the wood, or in the lathe bed itself. Actually, there is some flex in all the elelments. In the above example, there will be one or two elements that have a harmonic frequency close to the 225 cpm figure. A sharp tool that cuts with only enough bevel pressure to cut the tops of the spirals but not follow the spirals into the valleys flexes the system the least, and will go a long way to cutting down on sandpaper costs. Curved tool rests for short overhangs and gouges ground specifically for deep bowl bottoms; better yet, providing the extra length of the tool rest doesn't become an issue (and it easily can).

    I guess it still comes back to sharp tools well presented are best.
     
  8. John Jordan

    John Jordan AAW Advisor

    Joined:
    Jun 24, 2008
    Messages:
    283
    Location:
    Cane Ridge (Nashville), TN
    Home Page:
    Listen to Dale, he is 100% correct. Tool chatter is not uncommon-that is the steel flexing/rebounding. It may or may not be accompanied by wood chatter. Wood chatter is usually when the wood is thin. The incidence of both can be lessened by the "sharp tool, well presented". Sometimes, though there needs to be more mass in the tool as it is farther off the tool rest. That's why we use larger shafts on hollowing tools as we go deeper, or larger bowl gouges on deeper bowls. And FWIW, there is no difference in the chatter/flex of tool steel vs. plain/mild steel.

    I see a lot of less than skilled turners in my classes that get tool chatter/vibrations, and we talk about ways to identify what exactly is happening. Sometimes the chatter is caused by less than a solid connection between the wood/tenon/chuck/faceplate/spindle shoulder. One needs to recognize which of these different elements is the problem.

    I know a number of very talented and skilled turners who exploit the tendency of a tool to chatter when extended to cut intentionally cut spirals in the work. And the spirals are indeed caused by the tool flexing, not the growth rings. Spirals are also often the result of simply pushing too hard, and each cut follows what came before, and it gets worse.

    And a second FWIW, out of balance pieces, and the shaking and bouncing around of the lathe is not "vibration".

    John
    ____________
     
  9. odie

    odie

    Joined:
    Dec 22, 2006
    Messages:
    4,432
    Location:
    Deep in the woods
    Dale......

    Was in the shop for the past few hours, and just came in to have something to eat.......

    I appreciate your taking the time to give a detailed explanation. I'm sure I'm not the only one who does! When you consider the numbers, it does make a little more sense. 225 times per second is much higher in frequency than I'm feeling through my hands, and that's the point you made that made me reconsider my thinking...........thanks.

    Now, all of this is great for discussion, but I seldom get those spiraling ridges anywhere near as often as I did years ago.......and, when I do, I know what to do about it......whether or not I had a proper understanding of the physics!

    John J......."vibration" not the best word for out of balance condition.......oscillation would have been the better term, unless there is yet another term that is better.......

    Supper done......I'm going back out to the shop! :D

    ooc
     
  10. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    4,317
    Location:
    Lakeland, Florida
    Home Page:
    Unless I missed it nobody mentioned the tool rest vibration.
    Working on the end of the tool rest more vibration than working over the center post.
    When I hollow I try to work over the top of the post as much as possible.

    Deep bowl often gets us working on the edge of the tool rest with a lot of gouge overhanging the rest.
    Sharp tool, light cuts, short bevel help.

    I find the gouge bevel rub increases vibration shortening the bevel by grinding the heel away helps.


    Al
     
    Last edited: Jan 3, 2013
  11. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    Al I was discussing this with Vic from Bestwoodtools one day. He makes his tool rests out of a special steel designed for NASA that is a low vibration steel.
    I think that's one of those things that would be hard to tell where the problem is coming from, us, the tools, toolrest, overhang etc. So it's good to try all of it because I'm sure it's a cumulative effect.
    I mean who would think that you would get vibration from that huge 12" Powermatic cast iron tool rest. But as you said you can tell a difference when your turning from the end vs the middle. It does depend on the cut your making however. I have a very hard time telling the difference on spindle. Probably because I'm using very sharp tools with no overhang.
     
  12. MichaelMouse

    MichaelMouse

    Joined:
    May 16, 2005
    Messages:
    3,540
    Lets see, the tool, the rest - how about the bed? Take your thumb, put it on the rim of your thin wood, and press. See how both it and your flesh compresses? Now try it on that 1/2 cylinder of alloy steel.

    Anyone remember the third law?
     
  13. Richard Jones

    Richard Jones

    Joined:
    Jan 18, 2006
    Messages:
    87
    Location:
    VA
    3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Laws.

    or........

    III. For every action there is an equal and opposite reaction.

    Newton was a great ballistician. Also proof that an underhand grip with index finger wrapped offers more control than an overhand grip, for certain things. Gives you that push-me-pull-you not achieved with an overhand grip. Reserved for detailed spindle work only, as you don't want to use up all that mojo at the same pace....... Hey, I could use BOTH grips. Underhand when needed, overhand when needed. What a concept.

    Science, you gotta love it.
     
    Last edited: Jan 4, 2013
  14. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    Well I was photographing a test that our engineering department was doing on setting up some testing apparatus for testing the structure of bridge parts. This was a grant that came in after the bridge collapse a few years back.
    They have 20 foot columns buried in the ground that are various thicknesses but none less than 20". Some are steel, some are concrete. They have vibration sensors placed up and down the columns in various locations. They strike the column with a hammer and record vibrations. The sensors are sensitive enough to record you hitting the column with your hand. Now this is a 20" concrete pillar, about that same size on the steel ones. So I'm pretty sure my lathe will vibrate if I hit it with my fist. So I rest my case.
     
  15. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    4,317
    Location:
    Lakeland, Florida
    Home Page:
    John,

    the earth vibrates too. :)

    So our lathes are vibrating all the time. We just don't notice it.

    Al
     
  16. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    And the earth is rotating over 1000 miles an hours. I think I need a faceshield
     
  17. Matt Lewis

    Matt Lewis

    Joined:
    Dec 2, 2006
    Messages:
    122
    Location:
    Suwanee, Georgia
    Home Page:
    And to think Odie had a rather interesting thread going on this topic. Too bad.
     
  18. robo hippy

    robo hippy

    Joined:
    Aug 14, 2007
    Messages:
    1,872
    Location:
    Eugene, OR
    Hee, Hee, Hee, this got funny.

    Vibration comes from all sorts of things.

    Ever make a musical/rythmic instrument out of your old school 1 foot rulers and a table of desk top????? Well, hang it out a little bit, and just a thunk, no vibration. Hang it out a bit farther, and you get some twanging. You can move a bit out and back and get different degrees of twanging. Hang it out some more, and you lift the table end off, and get no vibration. There is a range that you can use with little or no vibration. Too far and you get chatter. Farther, and the tool can get yanked out of your hand no matter how big you are.

    The wood will vibrate with tool pressure. You can spin a balanced piece of wood pretty fast and it will not vibrate. Most of the tool generated vibrations on the wood are caused by too heavy handed bevel rubbing. Quoting some one, "the bevel should rub the wood, but the wood should not know it". We can use steady rests, either mechanical, or just you hand to dampen the rub. Negative rake scrapers will exert almost no pressure with a gentle touch. When the bowl is getting hollowed out, because there is less solid wood on the other side of the cut, the wood can flex/deform/vibrate. Ever notice how little vibration there is when turning the outside of the bowl?

    When you are cutting, as you cut, you get different 'resistance' to the cut as you go through side grain and end grain, also as you cut with the grain and against the grain, some times called up hill and down hill. This is generally minimal, and not really noticeable unless you start really fighting the cut. The more you fight it, the worse it gets, and this is when you get those nice swirly patterns. "Hold the tool as you would a bird. Too tight and you kill it. Too loose and it flies away".

    Grain orientation can also allow for some distortion of a bowl at higher speeds as it spins. Minimal at slower speeds and thicker bowl walls, and more at higher speeds and thinner walls.

    robo hippy
     
  19. MichaelMouse

    MichaelMouse

    Joined:
    May 16, 2005
    Messages:
    3,540
    A sonarman can hear a shrimp pass gas at half a mile. Now tell me what the deflection was on those members, and the mechanical advantage at the point of impact, and we'll talk.
     
  20. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    MM You can thump on an I beam with your knuckle and it will ring. Of course it rings louder if you hit it with a hammer. Is this not vibration or am I calling it the wrong thing.
    We were working in the attic of our building running new wiring for some lights we installed and I hit my head on a huge I beam that is one of the roof supports. It actually rang and my boss heard it. He made the comment I hurt the beam more than my head. What is it that makes that noise? I may be confused but I think it's vibration. If that's the case then why can't you have vibration at the end of a tool rest with the winter wood hitting your tool at hundreds of times a second.
     

Share This Page