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Deep hollowing question

Discussion in 'Woodturning Discussion Forum' started by Albert Cwirko, Jul 20, 2019.

  1. Albert Cwirko

    Albert Cwirko

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    hello everybody.I have a problem with deep hollowing. the wood is green ash its 28% moisture content its 27" tall and 16" dia a vase.the cutter started to skip at 18" .i have tried all the carbide cutters 1/8' .3/8" square ,cupped and tear drop. I haven't tried the hogger cutter or pro forme cutter yet. I talked to my woodturner club got some ideas ,it still is a problem.. I have the keith clark hollowing system with folcrum tube.Thanks Al.
     
  2. hockenbery

    hockenbery AAW Advisor Staff Member

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    My only experience with this system is my friend has one and loves it.

    I’m assuming you have the big unit with the the big bar so bar wise you should be fine.
    You have an even wall down to 18” and it was cutting well at 17” deep.

    things to consider

    1. your bar may be off level ( front rest too low or too high) and you reached a spot where the cutters are no longer being presented properly. either not cutting or grabbing.
    The bar need to move parallel in height off the lathe bed.

    2. Could have a bunch of torn fibers that keep you from getting a cut going. You can make these by cutting the wrong direction. More common in face grain hollowing. Should be able to see them with a flashlight inspection light. Just have to slowly cut across the grain to get rid of them. These things can get fairly long so go slowly to clean them up and not keep tearing new ones.

    3. Could be shavings packed in that are keeping the cutter from engaging. Work them free with something ( your tool tip will do ) and then blow them out.

    4. Could be a foreign body in the wood. Metal, rock, concrete, some type of solidified resins, grown over knot.... Use a flashlight inspection light to see what you can see
     
    Last edited: Jul 20, 2019
    Bill Boehme likes this.
  3. Mike Johnson

    Mike Johnson

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    You have a large piece of wood on the lathe, maybe you found a foreign object buried in the wood that your cutter is hitting? Are you able to visually inspect the inside of the vessel wall where you are running into the problem? A hard knot can cause a problem when you have a hollowing tool extended a distance past the tool rest. You might try changing the speed of the lathe until you get past this problem spot, sometimes slowing or increasing the speed of the lathe will help overcome a resonant response from the wood. Are you using a steady rest on the outside of the vessel?
     
  4. Bill Boehme

    Bill Boehme Administrator Staff Member

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    That sounds very much like the boring bar overhang has reached the point where the flex is causing the cutter to go below center. When that happens the cutter will dig into the wood until the spring force of the boring bar causes it to bounce up. Try raising the height of the cutter about ¼" or more.
     
  5. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    At least with my Lyle Jamieson system, a micro-adjustment is all you need sometimes to get a perfect cut, you have to be right at the center. I use Lyle's tool rest, comes with a 1 in post and a nut so you turn it to raise and lower the rest. Every time I use it I'm glad I listened to Lyle and bought it.
     
    Bill Boehme and Dennis Weiner like this.
  6. john lucas

    john lucas AAW Forum Expert

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    You didn't say what size bar you are using. I get chatter beyond 12" with a 3/4" bar. A few things you can do to reduce chatter. A sharp tool is a must. A smaller diameter cutter. I use either a 3/16Hss or #1 Hunter. cut downhill with the grain Instead of hollowing straight to the side (I'm assuming end grain hollowing) or parallel to the side. Start your cut on the inside and as you go toward the outside pull the cutter back so its cutting the fibers at about 45 degrees. Another option if you have a large enough hole is to make or buy a tool rest that will go into the vessel. A larger bar like the big Jamieson rig will go deeper than 12".
     
    Bill Boehme likes this.
  7. Bill Boehme

    Bill Boehme Administrator Staff Member

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    One other thought: in addition to the boring bar flexing as you get really deep, the wood itself is also flexing more than the boring bar is flexing. A steady rest helps, but doesn't eliminate flexing. Do as John Lucas says ... use smaller cutter and adjust the angle. I have found that the guarded Rolly Munro cutter works very well in controlling the amount of "bite" into the wood. It also produces a very smooth cut.
     
  8. Richard Coers

    Richard Coers

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    The one real advantage of the Pro-Forme and Munro cutters is that you can really peel a thick curl. You turn the speed way down, and you'll hollow just as fast as the small cutters at higher speeds. So less vibration and chatter. I stopped using my Pro-Forme cutters because the curls were so difficult to remove from small openings. I was using a claw to dig all the curls out. Very time consuming!
     
    Bill Boehme likes this.
  9. Dennis Weiner

    Dennis Weiner

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    Hi Al. I also own Clark hollowing tools and have been able to achieve 25 inches of depth in a test with a 12 inch fulcrum but that was with straight grain cherry(knotless), not ash. For that test, My bar was about 60 inches connected to a D handle to prevent rotation of the bar. I had a second fulcrum about 3inches on a 2nd cross bar support followed by a trap for the Dhandle. My bar is not slotted so I may have more material on the bar and therefore possibly more strength with less tendency to flex. I don’t feel that you have reached the maximum depth at 18inches as there are utubes/videos that show up to 30 inches. For those unfamiliar with the system, the 1 1/4 boring bar rides inside a tube(like a piston inside a cylinder) whose outer diameter is about 1 3/4” effectively strengthening the boring bar reducing bar flex.
    Nevertheless, after 15-18 inches deep I could not be as aggressive removing stock.

    I will add to the other posters above with issues relating to our system. Perhaps, this may help you troubleshoot specific to your tooling.
    The 3/4 cutter bar holder should be as short as possible. The one I used is about 6 inches long and sets into the boring bar about 5inches.
    (I use the 3/16 HSS and the 6mm hunter/Jamieson cuter. Teardrop when I can control the depth of cut)
    Check for play in the bar bearing and tighten if present.
    Check that the fulcrum cross lower bar support has no play as well as fulcrum and boring bar play.
    The joint that connects the upper and lower part of the fulcrum should be tight.
    Any one of these conditions, can’t induce vibration which may reduce your depth.
    One of my crossbars was a few thousands too small of 1 1/4 which caused this condition. Keith replaced it.
    The Blank attached to 10inch faceplate with all screws holes used. I use 3inch lag bolts.
    Speed of hollowing: Personally, with a 27x16: 350-500 rpm.

    As you go deeper, the less aggressive you should be. Cuts should be lite and cutter angle should be skewed. Less is more.
    That can be difficult because as turners we tend to push too hard.

    On a metal lathe cuts are sometimes setup to take a few thousandth of an inch per pass making many cuts to accomplish a depth that could not be made on one pass. To emulate this concept of limiting the depth of cuts on the Clark system, you will need three 1 1/4 shaft collars and an $8 box of HFT body shims.
    Install and tighten 2 shaft collars on the crossbar right and left of the fulcrum to restrict side to side movements.
    Install the other shaft collar on the boring bar After the 12” fulcrum. Do not tighten. Move the boring bar cutter to the highest point not hollowed and move the shaft collar along the boring bar down to the fulcrum and tighten the shaft collar. Your initial cut depth is locked. Swing the boring bar from the center to the rim for endgrain to make sure it clears the surface. Turn lathe on and pivot the bar back and forth making your first cut. Turn lathe off. Without moving the boring bar, loosen the boring bar shaft collar and insert 1/64 shim between the fulcrum and the shaft collar. Tighten the shaft collar and remove the shim. Your now set up to cut an arc from the center to the rim of exactly 1/64”. Test with 1/32, 1/16 etc.
    It goes rather quickly but I built a threaded advance out of a threaded rod which made the hollowing endgame(when your nearing maximum boring bar cut limits) a little more effective using the limit depth cut method. I know that the proforme, exocet and tools limit the depth no mater how hard you push, but at these distances you want to avoid pressure on the wood to prevent vibration. To repeat what everyone else said, you want to be on or slightly above center and the cuter setup angular to shear.
    If you can’t get to 27 inches, perhaps you should consider the 18inch fulcrum.
    Hope some of these ideas help you work through your problem. Let us know how you progressed.
     
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  10. Albert Cwirko

    Albert Cwirko

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    I want to thank everybody for responding to my question on deep hollowing.It was helpful, I still have some hopping, So i tried the 3/16 little hogger cutter, it was slow on wood removal it wood skip some times so I tried the soren berger 5/16"end grain cutter, Wow'' that is the only cutter that don't skip. very smooth, Why is there such a difference? This without a fulcrom tube. update on my post from july 23
     
  11. Richard Coers

    Richard Coers

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    It's called cutter geometry. Try to sharpen a wood pencil by introducing your pocket knife perpendicular to the wood and then rotate the pencil. Next lower the pocket knife to a slight angle to the pencil and rotate it. One scrapes the other cuts.
     
  12. john lucas

    john lucas AAW Forum Expert

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    Could simply be the cutter bevel is too steep and you got below center. The bevel then limits the cut and it bounces on the wood. Try sharpening at a more acute angle and make sure your cutting at or above center. A 3/16 cutter HSS cutter will really hog wood if sharpened and used correctly.
     
  13. Jason Van Duyn

    Jason Van Duyn

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    Im not sure if this will be any help as I turn alot hollows and use Hollow Pro tools to do so. But what I can say is that by turning without a captured system at the extremes of the tool range I have experience much of what you are talking about just on a different scale. Couple of things I wonder about... the type of grain orientation, facegrain vs end grain. End grain turnings I expect to get more chatter, especially when they are drier. Given the depth of the piece I would say you are more likely to be end grain turning? If that is the case then keep grain orientation in mind. For hollowing end grain you will be hollowing with the grain by starting at the bottom and moving back towards the opening until you reach the widest part of the hollow. If your cutting against the grain expect more chatter. The grain changes in hollowing end grain so its from the opening to the widest point and bottom up to the widest point.

    Bevel angle of the cutter makes a big difference as well as type of cutter. More aggressive cutters in terms of the size of the cutter as well as its angle are more difficult to control. Think of it in terms of how much wood is removed on a single pass. The more wood removed, the greater the force to control, and thus the higher degree of possible chatter.

    Further out from the tool rest I prefer cutters with a negative cutting angle. That is the angle created by the top of the cutter and the centerline of the piece. Really far off the tool rest I am typically using a M4 cutter, and if I use a carbide one it needs to have a negative rake otherwise I get shooken to death trying to control the tool by hand. I often rotate the angle of the tool to vary this angle while I am hollowing. You might not be able to do that with a captured system. You may also want to position the cutter below center as well irregardless of what the cutter angle is, that can lessen the degree of chatter.

    The last thought is, wall thickness. If your wall thickness is thinner closer to the bottom of the vessel that can cause the hollowing of material higher up in the vessel to be more unstable. So its better to keep your wall thickness at the thinnest parts higher up and work down towards the base. For the most part this isn't a really big issue until your wall thicknesses start to get fairly thin. However on a really deep piece I imagine this effect may be more amplified as the forces are compounded further away from the headstock.

    JVD
    www.vanduynwoodwork.com
     

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