New bowl turner - Lathe RPM and Bowls

Discussion in 'Newbie' started by Ron Robbins, Apr 23, 2012.

  1. Ron Robbins

    Ron Robbins

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    I have been turning pens for a while now and have only done about 4-5 bowls. It seems when I do a bowl I get a lot of grain tear and chips. I think I am shear scraping pretty well with my bowl gouge but it still does not seem to clean it up enough.

    I recently read an older article in Shop Notes about bowl gouge usage and the lathe speeds they mentioned in the article seemed fast (600 for roughing and 1200 for hollowing). I thought if I could go at a faster speed then it might make for a nicer finish.

    Is there a rule of thumb here? Does a faster RPM get a nicer finish? (After roughing of course)

    Is there a reference somewhere by chance?

    Thanks for any help.
     
  2. john lucas

    john lucas

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    If your scraping your always going to get more tearout than a good cut. Shear scraping helps but on some woods is not enough.
    Most people have trouble with tearout due to tools not being sharp enough and pushing the gouge into the cut. This is where the speed thing comes in. When the wood is moving faster past the cutting edge you have less tendency to force the cut so it naturally cuts cleaner.
    Now hears the rub. Faster speeds mean more danger. You can get the same results buy simply relaxing and letting the tool do the cutting at the speed that it needs to. You will be moving the tool through the wood slower but your cuts will be cleaner.
    Experience will tell you when you can get away with faster speeds. I turn far faster than I used to but have 25 years under my belt. for beginners roughing should probably be about 250 to 500 rpm depending of course on the size of the bowl. Finish cuts can go from 1000 to 1500 rpm.
     
  3. hockenbery

    hockenbery AAW Advisor Staff Member

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    Ron,

    Never use a speed that you are uncomfortable with.

    You have to match feed rate of the tool to the speed of the wood past the tool
    This is most evident when hollowing and you have to slow the cut as you come to center.
    Sharp tools and proper technique will do more for a smooth surface than differences in speed.
    Light finish cuts reduce tearout.

    Bad things happen faster and more often at higher speeds.
    Faster speed can help on natural edge bowls with a interupted cut.

    I turn a bit faster speed than most people and lot slower than some.
    But I have practiced enough to feel comfortable at higher speeds

    Al
     
    Last edited: Apr 23, 2012
  4. MichaelMouse

    MichaelMouse

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    There's a law of Physics. Newton's second, that says force increases exponentially with velocity. Means if you spin fast, catch hard, the bowl flies farther and hits (maybe you) harder. You don't need inches per second to get a clean cut. Or even a clean scrape. What you need is a good tool, proper presentation, and enough energy to remove a shaving. I strive for continuous shavings, because, just as when planing flat stock, every time I have to start the cut again I leave an artifact.

    It seems I'm able to transport the tool across the piece at a rate that produces a continuous shaving when working between 500 and 700 rpm. That means the same spot comes by roughly from 8 to 11 times per second. That's the number that counts, not the inches/second. After all, the outer portion of the bowl is no smoother than halfway is it?

    So don't put unnecessary energy into the process, concentrate on carving the rotating wood. The problems you mention are control problems.
     
  5. odie

    odie

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    Yes, faster rpm does produce a better finish.....within certain stipulations. The principle can be mentally visualized when thinking of slicing a piece of paper with a razor blade. That razor blade will not cut cleanly if drawn across the paper very slowly, but as the speed of the cut is increased, there is a point where the cut is cleanly slicing the paper. The cut will get better with an increase of speed, but there is a point where increasing the speed will not improve the cut.

    This same thing applies to cutting wood with a sharp lathe tool. By increasing lathe rpm, there is a point where the cut being produced is as clean as is possible, and increasing the lathe speed beyond that point will not improve the quality of the cut. There is not a hard rule of what rpm will produce the best cut, because there are variables that will effect what that speed will be.

    > How sharp the tool is, and how well it is presented to the wood.

    > How far away the tool is from the radial centerline of the lathe.

    > The resistance of the particular piece of wood to cutting. (species, MC or moisture content, grain pattern, etc)

    The rpm will vary, but a general rule for getting a concept are these published recommendations from Dale Nish:

    (Diameter, roughing rpm, finishing rpm)

    5" 1200 to 1800
    6" 1000 to 1500
    7" 850 to 1250
    8" 750 to 1125
    9" 650 to 1000
    10" 600 to 900
    12" 500 to 720

    I generally go a little faster at the larger diameters, but this is a good starting point for you to get going with.......

    ooc
     
    Last edited: Apr 23, 2012
  6. Jake Gevorgian

    Jake Gevorgian

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    When I was young, I'd turn a bowl at a fast speed.

    As I got older, I started turning them slow...while enjoying every little cut of the wood. The fun part----for me---is to hear the wood as it cuts (my lathe remains relatively silent.)
     
  7. Ian Thorn

    Ian Thorn

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    I have seen on a demo the speed is min =6000 devided by the dia
    max =9000 " by the dia
    for an 8inch bowl min 6000 div by 8=750 max 9000 div by 8=1125 but as the others have said no matter what speed you turn at the tools must be sharp i carry a diamond hone all the time ,happy turning practice makes perfect
     
  8. MichaelMouse

    MichaelMouse

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    Guess you can REALLY whip that blade through the paper, eh? That observation has more to do with inertia, if you think about it. Think of the vase and the tablecloth. Hopefully, the tool will be on the rest, not held free.

    If you believe in legal departments, I think that other chuck makers besides Teknatool may have suggested maxima. Some people will approach even that sort of like walking the dog. No matter how long the leash, he'll be at the full extent.
     
  9. hockenbery

    hockenbery AAW Advisor Staff Member

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    when I teach bowl turning I turn the bowl with my left hand and make a cut holding the gouge in my right. The students see how the tool contacts the wood the effect of a slight roll of the tool. this is at 2-4 RPMs. At a full revolution I hit my cut about 40 percent the other 60 percent I'm past it making a spiral. this cut is quite clean but not as good as the one made at a reasonable speed.

    I can also show catches at this speed with little danger.

    To understand the roll of speed put a 3x3 spindle blank on the lathe. use your bowl gouge and cut straight in from square to round and work down the piece. use the bevel riding push cut. this is the only cut you need for a bowl. others help but master this one and you will be fine.
    try it a 1200 rpm, 600, 300, 100, 600, 800, 1200, 1500. (when I was doing lots of Christmas ornament I pinned my 1018 for roughing because it took less time then slowed down for finish turning.)

    Ron,
    Taking a bowl class from a good instructor will make turning bowls more enjoyable. You'll have better surface, better shape, learn tool usage and sharpening, and get an understanding of the the process and the role lathe speed plays and overall be more confident and successful.

    if you can't get a class find someone to watch. Ohio has lots of AAW Chapters. Find one near you and ask about a good bowl turner to watch. some chapter have mentoring program for new members and workshops.

    Al
     
    Last edited: Apr 24, 2012
  10. robo hippy

    robo hippy

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    The only rule of thumb that I consider valid is from Pat's Fan on Woodnet Forums: "When sphincter tightening exceeds chuck tightening, you have a problem." That being said, we hope we all have enough common sense to know when our sphincters should be tightening. You can turn at about any speed that you are comfortable with, but higher speeds do make errors more spectacular, and dangerous. Like Al, I also like to demo catches in very slow motion. The pictures tend to be absorbed better by the students. Do take a turning class, or find a club, most of which have mentors. I find the teaching to be as much fun as the actual turning.

    Tear out most commonly comes from dull tools, and/or tool presentation. A scraping cut, no matter how sharp your tool is (scraper flat on the tool rest, or gouge held level with flutes all the way on their side) will pull the fiber with a tendency to tear them out before they cut. Shear cuts (scraper at 45 or higher degrees on the tool rest or several gouge cut variations) do a better job of lifting the fiber up gently as it cuts and slices rather than just pulling them out. Of course, some woods (I hate cotton wood and palm) will tear no matter what you do.

    robo hippy
     
  11. odie

    odie

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    Another aspect not mentioned yet, is that it's easier to get a nice flowing artistic curve to a cut when the speed is high enough to facilitate body movement that best coordinates the union of body to machine. This is not to say that body movement is better, the faster it can proceed through a cut.......only that there is a point that is fast enough, but not too fast. This perfect combination of variables may not be the same from one individual to the next.......

    Lathe rpm is that which controls the speed in which your body can proceed through the cut (assuming sharp tools, and good presentation). Even if everything is mechanically perfect, this does not mean the human aspect will be perfect......this is only one part of what makes individuality a great part of the outcome!

    ooc
     
  12. john lucas

    john lucas

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    Right you are Odie. I used to turn at fairly slow speeds compared to most turners. Gradually over the last 5 or more years my speeds have gone up and my forms although not necessarily better come a lot easier.
    It's all about what we call the turners dance. I use my body to control the tool and it's easier to make a nice flowing line when I move my body at a certain rate.
    You don't necessarily have to increase the speed of the wood, simply take lighter cuts. The lighter cuts not only remove wood more cleanly but let you move your body in a more flowing motion.
     
  13. robo hippy

    robo hippy

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    Some where along the line, I figured that higher speeds gave more cuts per inch than slower speeds, so the cuts were smoother.

    robo hippy
     
  14. odie

    odie

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    Yes, absolutely John.............:D

    You are right about the relationship the depth of cut will influence the speed at which one can proceed through the cut while maintaining a smooth flowing curve. The rpm speed is one factor, but not the only factor that allows a smoother body transition through the cut while maintaining a good clean cut. This is a good point you have made, and thanks for pointing it out.:D

    "The dance".........I'm not sure who first used that term, but the first time I heard that was in Richard Raffan's first video produced in the late 1980's. It really is a very good conceptual use of words to describe the "human element" of turning.

    ooc
     
    Last edited: Apr 25, 2012
  15. odie

    odie

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    Howdy Robo.........

    You are also correct about the point you are making here!

    It is evident from Dale Nish's recommended lathe speeds that there is a difference in the quality of the cut between the roughing speeds, and the finishing speeds.......and, this difference is plainly illustrated in a higher rpm recommendation for a finishing cut. If there were no difference in the quality of the cut regardless of rpm, then there would be no reason for a range of rpm recommendations when searching for the best cut........

    Until a piece of wood is brought to round, and the majority of the waste wood has been removed, the slower speeds for roughing purposes are the standard mainly because of the concerns for safety. The higher speeds recommended for a finishing cut are generally accepted as the standard because it will produce a finer surface. (Provided, of course, that all the other variables are also at an optimum.)

    ooc

     
  16. MichaelMouse

    MichaelMouse

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    Love it! Now while you guys are "dancing" and cutting a bowl, do you change the rpm to get the fpm you would get if you followed the experts? Only one zone can be optimum, no? The rest can only be a second-rate cut at best?

    I like slow dancing. In lathe work as on the dance floor, it gives me a better feeling and control over the curves. Tactile feedback is an important part of cutting. If you're rushing, you can't really modify your tool presentation comfortably as the curve changes, or you change the curve. That's one big source of tearout, especially to aficionados of the bowl gouge. Advance the tool rapidly, and what was slicing starts scraping pretty fast as the bowl narrows. I like to slow the progress of the tool and the passage of the work to tune the presentation to changes in the wood and the shape. It's a lot easier if I let the lack of pressure on my arm and the flow of the shaving be my guide. My feedback loop can't handle these changes at warp 3. Fortunately, there's no need to turn there.
     
  17. hockenbery

    hockenbery AAW Advisor Staff Member

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    Tool usage does more for the surface than speed. The suggestion that there is an optimum speed seems quite valid. That speed would vary from foot to rim on the outside and from rim to bottom on the inside.
    My goal on a bowl is to get a 220 surface off the tool,then sand with 180,220,320.., I change speeds as I go.

    My roughing cut brings the bowl to shape quickly. I cut leading a bit with the tip and lower wing slightly off the bevel. With a 5/8 dia bar ellsworth gind gouge I take 3/4" wide shaving and leave considerable tearout. I then switch to a bevel riding push cut amazing 1/4, 1/8, 1/16 shavings as I shape the foot, rim,and refine the outside curve. I sharpen my tool for the finish cut and take tiny shavings and examine the back side of the endgrain where there is always some tiny tearout because some of the cut is tangential to the bowl and there are no fibers supporting the cut in the tangential direction on the back side of the endgrain. On most woods I can shear scrape and sand.
    If I can see the tearout i try a flute up shear cut.
    If i still see tear out i switch to a handle down pull cut with wing slicing just below the nose at high angle.
    If there is still some visible tearout I spritz with water and try the shear cut again,
    If there is still tearout, I try a 1/4 bowl gouge This takes 95 percent of the bowls to a tool finish I can shearcrape and sand.
    If i have sometimg really bad i try a spindle gouge then a square scraper in a shear scraping position.
    I haven't found any good quality wood where these techniques fail to produce a quality tool finish

    On the inside it use the bevel riding pushcut taking smaller cuts as I approach finish thickness. I then use my 1/4 bowl gouge for a finish cut as far as it will go usually 2 inches, then use a flut up sheare cut to finish the rest of the inside. If I'm getting too much bevel drag I'll grind the heel off the bevel to shorten the bevel. On most bowls I can sand the inside with 180,220,320 May need 120 in few spots.
    If I'm having a bad day and don't nail the bottom, I'll use a round nose scraper to refine the bottom.

    This is all can seem daunting to a beginner. In a beginning bowl class when the students master the bevel riding push cut they are going to get good tool finishes.
    Mastering that one cut leads to success.

    Have fun,
    Al
     
    Last edited: Apr 25, 2012
  18. odie

    odie

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    For those of us who've been turning and participating on the forums for some time, there is a realization that it's common to see that experience doesn't necessarily result in the same conclusions. This thread is just one more example of that!.......:D

    From my perspective, speed is a factor in the quality of cut that just can't be denied. It isn't the only factor, for sure. A master wood carver can get a high quality cut with a hand held tool and mallet......but this isn't generally at a time where there are alternating patterns of end grain/long grain to deal with.....as it is with bowl turning.

    As others have pointed out....sharp tools, tool handling, species, grain pattern, MC, depth of cut, and so on, and so forth......all of these things are contributing factors that can't be considered in any more capacity than "in concert" with all the factors.

    Sometimes a visual inspection of the results is not as indicative of the quality of cut, as it is at other times. It is frustrating to bring a wood surface to some degree of sanding.....and, only THEN discover that the original tool finish wasn't as good as you thought it was.....:mad:

    There is another very good indicator of how good the cut was......other than what a visual inspection of the surface reveals. That other clue resides in the shavings themselves. We've all seen the demonstrations of turners producing those long endless shavings.......very impressive, but not really an indicator of how good the cut was. The only thing for sure is the wood had a high MC (moisture content), and is generally meaningless for finish turning.

    Anything but nice shavings is an indicator of a finish that isn't as good as it should be.......but, the best tool finish is going to produce shavings that resemble "angel hair", or very fine shavings that clump together, sort of like a wad of steel wool......but, very delicate, light, and fluffy.

    ooc
     
  19. MichaelMouse

    MichaelMouse

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    OD is sorta right about the shavings. You can get continuous shavings a lot easier with wet wood, because it doesn't split apart going over the end grain portions. More important to the surface finish is the form of the shaving. These are a gross example, taken from dry maple. Their size makes things easier to understand.

    You'll notice that one side is thick, the other side feathers to a ragged edge. That feather is made as the gouge exits the wood, making only the slightest of cuts, therefore causing the least strain on those necessary up grain portions where the shaving normally breaks apart and people see rough areas on their bowl. These are made with a broad sweep gouge, my preferred finisher. If you have a narrow sweep, you will still be after the same straight/feather shaving, but with a narrow sweet spot, you can't get one as broad, since width is more a function of skew than depth. Which also means that you will have to narrow up where the curve is fairly rapid, transitioning to more of a pure shear than a shear/skew.

    Staying sedate in your rpm will keep you from having to restart the shaving if you lag the feed too much. Since you will have to raise the pitch angle to get back underneath and start the shaving, you leave yourself open for ridging at that point. You can see the restarts here pretty obviously, though since the bowl is not yet true, the old fool can be given a bit of slack for running the gouge out of the cut. If the rpm were high, the cut would run out ahead of the gouge.

    http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=35mmGougeRounding.mp4

    Get things going right, and you can almost get hypnotized watching the shavings.

    http://s108.photobucket.com/albums/n28/MichaelMouse/?action=view&current=CherryPeelIn.mp4
     

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  20. Ron Robbins

    Ron Robbins

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    Wow. I thought my little question would not get much attention. You guys are extremely helpful. Thanks!

    I read all of the replies and won't pretend that i understood all of it but I did a couple of bowls again this weekend and really spent some quality time with my grinder and bowl gouge. I think this is the sharpest my gouge has been. It worked much better.

    Also increasing the speed (after roughing) seemd to product a smoother cut as well. I had a couple of catches but that is due to my inexperience. I think I am on bowl number 10 now and I am still not entirely satisfied with my work yet but I am getting there.

    Thanks so much for all of the help!!
     

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