The Varigrind Jig Is Adjusted, Now What?

Discussion in 'Woodturning Discussion Forum' started by Dennis J Gooding, Feb 23, 2017.

  1. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Rebate is a British term that means the same thing as the American term "rabbet" not to be confused with "rabbit", a cute fuzzy animal. The origin comes from French word "rabbat" which more closely resembles the American English spelling, see this Wikipedia article.

    The difference between a rabbet and a mortise is that a rabbet is cut along an outer edge of a board or other piece of wood while a mortise is a recess cut within the piece of wood. Examples of a mortise are a slot in a piece of wood that accepts a tenon such as rail and stile joinery; a round hole in chair legs that mates with the tenon on a stretcher spindle; and a round recess used for holding a piece of wood for turning. An example of a rabbet would be a step along the edge of a board for cabinet joinery. You could also use a rabbet joint in woodturning ... a two part hollowform for example. You've probably seen rabbet planes that are used to create a step along the edge of boards.

    On this side of the Atlantic, a rebate is money back on a purchase. That's also the most common usage on the other side of the pond.
     
  2. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Books can be wrong. If the editor didn't know the subject and changed the spelling. Or, believe it or not, sometimes authors don't know how to spell. Dictionaries have been clear on the difference in meaning of the two words for at least a couple centuries

    When I was still working, I thought that engineers were the world's worst spellers, then I decided that photographers were even worse, but maybe woodworkers have taken that honor.
     
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  3. Emiliano Achaval

    Emiliano Achaval

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    Sorry if I was the one that mentioned Don Geiger and got the thread meandering... I have read some old books, included John Holtzapffel from the 1800... Did not see the word tendon in there... One of the oldest ways of turning the bowl around is what we in Hawaii call the Jack Straka plate... Very effective, but I prefer the vacuum chuck or a simple jam chuck...
     
  4. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Getting somewhat back on track, Dennis mentioned different flute profiles. That seems to be a perennial thread topic and often someone will comment that they prefer a particular brand of bowl gouges because the flutes are parabolic ... implying that the flute on other bowl gouges aren't. I suspect that this is a common mistaken assumption that a parabola is a particular shape that varies only in size such as would be the case with a circle. I have at least four brands of bowl gouges and I would call the flutes on all of them parabolic, or at least something closely resembling a parabolic shape since nobody would be able to just look at a flute and say that it has a parabolic curve as opposed to an elliptical curve or a curve that is neither. I was thinking about posting a little tutorial about parabolic curves that didn't get mired down in the math when I came across this nifty little video that does a great job of describing parabolic curves. It is only necessary to view the first three minutes as the discussion after that doesn't add anything for our needs.


    View: https://m.youtube.com/watch?v=1HRhm2T9-3U
     
  5. John Torchick

    John Torchick

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    Why don't we just say a tool has a U-shaped flute? That covers a lot of territory.
     
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  6. Bill Boehme

    Bill Boehme Administrator Staff Member

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    You can get a bowl gouge with a U shaped flute, but it is clearly different in that it is basically a half circle like a spindle roughing gouge and doesn't lend itself well to producing swept back wings without becoming a little harder to control.
     
  7. John Torchick

    John Torchick

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    Bill, thanks for your answer. Never really considered the profile in regards to the shape at the end of the tool.
     
  8. Dennis J Gooding

    Dennis J Gooding

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    Bill, this an interesting intro for the non-mathematically inclined. However he does not point out the fact that aside from rotation or movement, all parabolas are identical in that you can change any one of them into any other of them by magnifying the skinnier of them by a suitable factor.

    More germain to the sharpening problem is the fact that the parabolic shape is a very smooth shape. Not only are there no discontinuities (jumps) in the curve, but the rate of change of the slope is constant. The V and U curves have instantaneous changes in the rate of change of slope in two places. (Note that the V-flutes actually have a slightly rounded bottom rather than a sharp cusp.)
     
  9. Jon Murphy

    Jon Murphy

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    I jammed my tendon in the door jamb as I was carrying my jury rigged tenon making device into my jerry built workshop. One of my pet peeves (Wm. Safire, pundit, and maven of the English language, said he wanted to buy a dog and name him Peeve - so he could say "this is my pet, Peeve") is the current use of the phrase "jerry rigged". Jury rigging is an old sailor's term for making do with the materials available to repair a broken piece of equipment - it implies ingenuity. Jerry built implies rushed and careless construction and comes from WWI and the English reference to Germans as Jerrys (it was also used in WWII). It was meant as an insult. Sad to say (not sadly) our language is being mangled.

    I don't object to changes in language, how could I when our language has changed so much over the many centuries. I do object to the loss of meaning that comes with mangling. I must infer from your comment that we agree, although should I have misread the implication I apologize. Another pair of words, imply and infer, that are being merged into one (imply) and losing the difference of meaning. If the "news person" says "the President implied" such and such it is quite different than if he says "I inferred from the President's words" such and such. They all now seem to be mind readers, as they all now just use imply. Best, Jon
     
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  10. Jon Murphy

    Jon Murphy

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    Dennis and Bill have discussed the mathematics of the parabola and ellipse. May I remind you that the ancient Greeks were geometricians first. I'm going to send a link, it is but one page and is pictorial. To justify my comment on the Greeks may I point out that the "Golden Ratio" was defined (forgotten which Greek) geometrically, not in numbers. The geometry came first, the math later.

    http://math2.org/math/algebra/conics.htm This is as "cut and pasted" - I think you will get it if you type math2.org/math/algebra/conics on Google. I'll give it another try to see if it comes through as a link on our Forum www.math2.org/algebra.conics. If none of these come up on the Forum as a "clickable link" I'll ask you all to tell me how to send the link.

    I think the pictures of the conic sections will make the differences clear to all. One thing is left out, the cones themselves could be of different ratios of height to radius as pictured - but that doesn't change the principle and basic shapes of the sections. One should realize that the cones extend infinitely, the pictures, necessarily, are finite.

    From these pictures you can see that the circle is a special case of the ellipse, each is a complete section of the cone. The parabola is an incomplete section of the cone, as it never gets closure even if extended to infinity. The hyperbola is a special case of the parabola, a pair of curves opposing each other that occur when the cone is doubled with its own inversion and the section intersects both.

    I realize I'm rambling on, but I think the "picture in the mind" is important. The parabola, and the hyperbola, are continuous curves when the cone is extended to infinity. There is one conic section that is not a curve, it is when the section is exactly on the axis of the cone (or doubled cones). It is a V, or if the cones are doubled, an X - a conic section is never a U.

    A straight line is a continuous curve with no curve, it is an end case of a conic section.
     
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  11. Jon Murphy

    Jon Murphy

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    This is relevant to the matter of flute shape, and straight from the lathe. I offer an experience I had tonight.

    I'm an Ellsworth grind fan. My basic bowl gouges for years were the Henry Taylor Ellsworth Signature. About six months ago I bought the Crown PM Ellsworth Signature (5/8" shaft) as my Henry Taylor 5/8" was getting a bit short. I still am using my HT 1/2". I'd been doing some other work for a time, including small bowls where I used the HT 1/2". Recently I've had problems entering the hollowing cut - I was getting "kick-back". I assumed it was my handling of the tool, or mistakes in grinding it, or the wood I was using.

    Tonight I was getting near the rim in hollowing a 6" hard ash bowl, and getting the kick back no matter the angle I held the tool for the "Ellsworth roughing cut". I resharpened my Pro PM, being very careful to match David's shape. BTW, he is not a god, we all have our ways to use our tools, but as I was having a problem I wanted to go back to basics. I tried my HT 1/2", and got better results - but was that the diameter of the gouge for the size of the bowl? I broke out the old HT 5/8", which was badly shaped. I reground the shape, using the same settings on my Wolverine and my Geiger VS as I used on the Pro PM.

    I had little room left on the bowl, I was almost to the rim width I wanted. I took a couple of entry passes in the same way I had been doing. I got no kick back. Perhaps I was more careful, perhaps it was something else - I can't say until I try more bowls. But an "eyeball" inspection of the flutes of the two gouges suggests a slightly flatter bottom curve on the HT than the Crown PM. Other measurements, taken some time ago, show the HT to have a shallower flute than the Crown.

    Not evaluating the tools or the makers - the Superflute shape is attributed to Peter Childs and is the continuous curve. I think the HT has a different parabolic than the Crown, a different conic section. And I found it made a difference. That doesn't mean I can't use the Crown - but as all here have said, it is a matter of using the tool. I'd been using the HT for years and am new to the Crown. They are the same, to the best one can "eyeball" - but they are obviously subtly different. The Crown is a deeper flute (by millimeters, I've measured the thickness of the remaining metal under the flute). That indicates that the curve at the bottom of the parabola is more open on the HT.

    That is so detailed I'm embarrassed to say it, but If I don't understand the "why" I don't know how to learn the "how to".

    I concur with Dennis and Bill, the tool and the turner have to work together - the details don't matter. But I make the point that even using the same grind and a similar tool that shape of the flute can make a difference. A minor difference in flute shape can involve a learning process. Nothing wrong with that, I'm not getting rid of my Pro PM - I'll just learn how to use it, and perhaps a bit of difference in the shape of the grind.

    Best, Jon
     
  12. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Those are the kind of illustrations that I had in mind when I began my Google search, but when I saw the family of parabolas in the video, it was even more to my liking although potentially boring to those who have an aversion to math (the missus being one).
     
  13. hockenbery

    hockenbery AAW Advisor Staff Member

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    The difference between a parabola, ellipse, and hyperbola is velocity
    Gravity invented all of them long long ago
    :)

    If you want your shavings to return to earth use a parabolic flute!

    :)
     
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  14. Dennis J Gooding

    Dennis J Gooding

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    Well said, Al, but if you want easy disposal of the chips use a hyperbolic flute.
     
  15. hockenbery

    hockenbery AAW Advisor Staff Member

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    :). Might be an answer to combat global warming.
    Enough wood chips blocking sunlight should be like Krakatoa ash blocking sunlight.
    :)
     
  16. Jon Murphy

    Jon Murphy

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    Thanks Bill,

    I think I'll drop out of this thread - but not form the Forum. With all due respect to Hockenberry gravity also invented the special case of the circle. I don't want to get involved in discussions on the nature of that very weak force that we call gravity. Newton was right, but not quite right. Einsteinian physics advanced the concept, the curvature of space by the weak but fully invasive force of gravity. Modern physics, and experimentation, has confirmed that - and added to it.

    But as a normal human being I'm comfortable thinking that the mass of the earth pulls on the mass of the heavy object in my hands that slips out of them and lands on my toe. Whether it is the curvature of space by the weak but fully invasive force called gravity, or the direct attraction of the massive earth on the less massive object, the pain in my toe when the object lands on it is the same.

    Gravity didn't invent the conic sections, Greek geometers did. They described what we later came to see when we started to understand the orbits of comets and planets - and the occassional "one timers" on parabolic or hyperbolic trajectories. The shape of the curve is defined by the masses involved, the relative velocity, and the angle of attack. The parabolic, and hyperbolic are "one timers" as they come from a distance and don't return. OK, comets or such might yet be elliptical by returning in a few billion years (our galaxy is about 120 light years in diameter - but objects from outside the Milky Way might come in from infinity and return to it.

    I come back to the conic sections, they define the shapes we deal with.

    Best, Jon
     
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  17. Mike Johnson

    Mike Johnson

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    Greeks invented yogurt, olive oil and wrestling.....it was ancient aliens that taught them geometry. :)
     

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