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The Varigrind Jig Is Adjusted, Now What?

Dennis J Gooding

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John Murphy's thread on the geometry of gouge sharpening has grown long and has meandered into the issue of how to use the the jig after it is set. I would like to throw in another consideration that may draw it out even further. Glenn Lucas, in his DVD on Sharpening Techniques, points out that special problems arise when trying to grind wings on bowl gouges with V or U profiles, and that the parabolic profile is to be preferred when using jig sharpening. The issues that he describes sound a bit like what Grant Wilkinson mentioned. :personally, I have only parabolic flutes in my arsenal so I am not in a position to experiment. Have others faced this issue?
 

hockenbery

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I prefer the parabolic flutes. I use the Ellsworth grind or at least my version of it.
I have a Thompson v and don't like it.
I have a Jamieson gouge made by Thompson that I love.
The v has little usable shoulder and it does not do the flute up shear cuts well or have much of a sweet spot for the push cut.
The Jamieson is a dream to use flute up or in the push cut.

So I just use the Thompson v for roughing cuts and the pull cut which it works well for.
Then I use one of my parabolic gouges for push cuts and shear cuts.

I haven't seen much issue with sharpening a v gouge with a jig other than it is their nature to be pointy and it is easy to grind the tip too much.
 
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john lucas

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When I first started using jigs like the Ellsworth and Wolverine I had problems similar to what Glen Lucas describes. You get a small dip in certain sections depending on how the flute is ground. I quickly got over it. You don't grind with the same pressure from one side to the other. You grind where you need to and use light pressure where you don't. I've been using a Thompson V for many years and love it. I guess I just learned to use it.
 

Dennis J Gooding

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Everybody uses the term parabolic flutes, but I think that they are elliptical. :) I would like to see how they are ground.

By Jove, you are right Bill. The tips of all skinny ellipses start out as parabolas and visa versa. Then they slowly diverge. I expect that you could not measure the difference between the two over the small depth of a gouge flute!
 

Bill Boehme

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By Jove, you are right Bill. The tips of all skinny ellipses start out as parabolas and visa versa. Then they slowly diverge. I expect that you could not measure the difference between the two over the small depth of a gouge flute!

Dennis, I was sort of hoping and expecting that you are another engineer or mathematician would respond. My post was a bit tongue-in-cheek because the difference between the two is so slight. Depending on how the stock is milled, it might be easier to mill an ellipse. Of course, it is also possible that the flute shape is neither elliptical nor parabolic, but something else that is the result of polishing out milling marks. I think that if we delve deeply into mathematically precise descriptions for the various flute shapes, I would like to offer up three flute categories: skinny, medium, and fat.
 
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Aha Dennis, I found you <grin>. I take no blame for the diversion of the meandering of the thread.

I beg indulgence if I become hyperbolic, or use circular reasoning, but I fully concur with you and Bill on the matter of flute shape. The progression of conic sections from the perfect circle to the ellipse, then to the parabola, and finally to the hyperbola is continuous as the angle of the section to the cone progresses from the horizontal (assuming a cone axis oriented vertically). As I think of it I realize that a V is also a conic section, but a special one that is a hyperbola oriented on the axis if the cone and coincident with it.

As you and Bill both know, the circle and the ellipse are complete within the cone, they come to closure - and the parabola and hyperbola both exit the cone and diverge forever. To be exact, the parabola and ellipse are never the same - even at the skinny tip - but they can be so close there is no way to tell the difference.

A U is another thing, it is a half a circle then a couple of straight lines - it is a construction, not a conic section. OK, another chapter of Murph Says - but the real point is that you are both quite right that the exact nature of the curve of the "superflute" is undefined. Is it an incomplete ellipse or a parabola? Who cares. It is a smooth curve of gradually changing shape.

I think it is a parabola, looking at my tools and doing an eyeball extension - but it could easily be the end of an ellipse that I can't picture.

That said, may I add that I'm still convinced that the leg angle should vary to make the same side grind shape with different tip bevels. Dennis's two angles, A and B are not the only angles. I just haven't figured out how to describe it clearly. I started my thread on the geometry as a proposal, and that because I couldn't find a way to measure the angle of the grind on the wings. I think I've found one, but have to experiment more.
 

Emiliano Achaval

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When I first started using jigs like the Ellsworth and Wolverine I had problems similar to what Glen Lucas describes. You get a small dip in certain sections depending on how the flute is ground. I quickly got over it. You don't grind with the same pressure from one side to the other. You grind where you need to and use light pressure where you don't. I've been using a Thompson V for many years and love it. I guess I just learned to use it.
I have noticed that turners get used to whatever they grind. I have seen some really strange grinds, yet somehow the turner adapts to it... Beginners bring their tools to me to resharpen. Problem is, their set up is different than mine, so when they go back to their shop... I used to recommend the Don Geiger old system, but now he doesn't sell it anymore. He does have something new, that I dont know how it works. With the old one, was easy to reproduce the height and distance... To me it was harder to learn how to sharpen than to turn, we didn't have internet with you tube then...
 

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Here are a couple of significant statements made in this thread:
I have noticed that turners get used to whatever they grind. I have seen some really strange grinds, yet somehow the turner adapts to it...
It's true! The speed at which one learns is much determined by the time and effort one puts into "shop time"......multiplied by any one's ability to observe, comprehend, retain, and repeat little things that work well. Gather up enough of these little observations into a bundle of knowledge, and you have a turner who gets results that are noticeable, and distinctive.
the exact nature of the curve of the "superflute" is undefined. Is it an incomplete ellipse or a parabola? Who cares. It is a smooth curve of gradually changing shape.
Right.....who cares? Well, I guess some people do, but is dwelling on descriptive language that which produces physical results in the shop? I think not. To be correct, though......there must be some means of passing thoughts accurately between individuals, and it's difficult to do that without using the (most) correct words. In this particular case, Jon is right, in that the curvature of the flute need not be described accurately, because words are pretty useless means of describing how to use that flute shape, once the gouge is in your hands and applied to wood. Big differences might mean a lot, but here, we're attempting to describe minor details which make little difference in practical application. Not only that, but each piece of wood is different, cuts different, and we all need to learn to adapt on the go!......

-----odie-----
 
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hockenbery

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I have noticed that turners get used to whatever they grind. I have seen some really strange grinds, yet somehow the turner adapts to it..........

I used to recommend the Don Geiger old system, but now he doesn't sell it anymore. He does have something new, that I dont know how it works. With the old one, was easy to reproduce the height and distance... To me it was harder to learn how to sharpen than to turn, we didn't have internet with you tube then...
I see a a fair number of post beginner turners limited by their poor sharpening. They somehow learned to use a gouge with dips in edge with occasional catches. When I hand them a gouge with a Convex edge they improve their turning instantly.

I have Don's old vertical solution system works great.

I have been playing with his new "Evolution" system.
It is quite a slick way to set up the standard vee arm and varigrind to sharpen gouges.
Easy to relieve the bevel or add a micro bevel.

Don is a distant neighbor about 2 hours away. The guy is sharpening wizard.
 
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Interesting thread with information that I'm not familiar with as a newbie. Need to do some homework.
Would the variations in tool grinds be something that started out as an experiment- What would it do if I ground the tool this way?
I have learned that it is necessary to keep tools sharp in order to have a good finish and lees effort at cutting contours, angles, etc. My grider is about two steps from the lathe. I keep the Wolverine set for whatever tool I am using.
 

Emiliano Achaval

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I see a a fair number of post beginner turners limited by their poor sharpening. They somehow learned to use a gouge with dips in edge with occasional catches. When I hand them a gouge with a Convex edge they improve their turning instantly.

I have Don's old vertical solution system works great.

I have been playing with his new "Evolution" system.
It is quite a slick way to set up the standard vee arm and varigrind to sharpen gouges.
Easy to relieve the bevel or add a micro bevel.

Don is a distant neighbor about 2 hours away. The guy is sharpening wizard.
I couldn't remember the name last night, "Vertical Solution" I love it... I would like to try his new system, but I dont want to spend the money... He emailed me to tell me about the new system, I had recommended the vertical solution to 2 turners, both called me back to tell me he didn't sell it anymore! Don said David Ellsworth had call him too about it...
 
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Dennis, it seems meandering is generic to the Forum - I'll try to come back to your original question. I haven't seen Glenn's DVD, nor Grant's comments, but it seems to me that the continuously changing curve of the parabolic (or partial ellipse) flute shape is a match for the continuously changing grind angle when using a jig and making "long" wings. As I picture the process it seems that the wings on a V would change angle more rapidly that on a parabolic when using a jig. A hand grinder could compensate for that, but that is beyond my skills.

Oops, just saw John Lucas' post in this thread - what he describes is what I was guessing at. I guess the answer is that if you can compensate for the straight sides of the V then keep on using it, but if you haven't learned how to do that yet go with the parabolic.

I do have a couple of Thompson U gouges that I grind as "bottom feeders" - a tip bevel of about 80 dgs. and very short wings - but they are "specialty" tools for me.

I "tugs me forelock, respectful" to Odie. Adapt on the go, learn to use your tools in different woods. But it is a lot easier to do that if the curve of the grind changes consistently along the wings, and I think the parabolic makes it easier to maintain that with a jig.

The meandering has started with the mention of Don Geiger's vertical system, so I'll add to it. I had bought his original about six months before he came out with the improved one (the one with a pin to set four different fixed distances given the same setting of the main arm). Don was kind enough to sell me parts to make the conversion, and I made a few of my own modifications. It is a gem - I can duplicate my grinds in two passes (more if I've done a lot of honing). I've looked at the instructions on the Evolution and it appears it performs the same function for a lot less money - at least for most turners.

I'm sorry Don has dropped the VS, and seems also to have dropped his "centering solution". I agree with Hockenberry, he is a genius in devising devices for sharpening - but also for other things. The Centering Solution is a drill guide that screws into the threading of your chuck or face plate so you can drill a hole (I think about 3/8 or so) into the tenon of a bowl. With it is a plug that fits into the hole, and rides in a revolving tail stock center. Actually only tapered one's like Oneway, but I got a discount from Don to not take his plug and made my own out of aluminum stock. Using the plug shoved about 7/16' deep into the tenon I can recenter a bowl on a jam chuck that isn't a close fit in about two minutes, and sometimes it is perfect the first try. It used to take me a half hour using a pin center. And I no longer have wish I had space and money for a vacuum chuck.

OK, digression - but I wanted to support Hockenberry's compliments to Don Geiger, whose devices save money and time and actually work.

Best, Jon
 

Bill Boehme

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Dennis, it seems meandering is generic to the Forum - I'll try to come back to your original question. I haven't seen Glenn's DVD, nor Grant's comments, but it seems to me that the continuously changing curve of the parabolic (or partial ellipse) flute shape is a match for the continuously changing grind angle when using a jig and making "long" wings. As I picture the process it seems that the wings on a V would change angle more rapidly that on a parabolic when using a jig. A hand grinder could compensate for that, but that is beyond my skills.

Oops, just saw John Lucas' post in this thread - what he describes is what I was guessing at. I guess the answer is that if you can compensate for the straight sides of the V then keep on using it, but if you haven't learned how to do that yet go with the parabolic.

I do have a couple of Thompson U gouges that I grind as "bottom feeders" - a tip bevel of about 80 dgs. and very short wings - but they are "specialty" tools for me.

I "tugs me forelock, respectful" to Odie. Adapt on the go, learn to use your tools in different woods. But it is a lot easier to do that if the curve of the grind changes consistently along the wings, and I think the parabolic makes it easier to maintain that with a jig.

The meandering has started with the mention of Don Geiger's vertical system, so I'll add to it. I had bought his original about six months before he came out with the improved one (the one with a pin to set four different fixed distances given the same setting of the main arm). Don was kind enough to sell me parts to make the conversion, and I made a few of my own modifications. It is a gem - I can duplicate my grinds in two passes (more if I've done a lot of honing). I've looked at the instructions on the Evolution and it appears it performs the same function for a lot less money - at least for most turners.

I'm sorry Don has dropped the VS, and seems also to have dropped his "centering solution". I agree with Hockenberry, he is a genius in devising devices for sharpening - but also for other things. The Centering Solution is a drill guide that screws into the threading of your chuck or face plate so you can drill a hole (I think about 3/8 or so) into the tenon of a bowl. With it is a plug that fits into the hole, and rides in a revolving tail stock center. Actually only tapered one's like Oneway, but I got a discount from Don to not take his plug and made my own out of aluminum stock. Using the plug shoved about 7/16' deep into the tenon I can recenter a bowl on a jam chuck that isn't a close fit in about two minutes, and sometimes it is perfect the first try. It used to take me a half hour using a pin center. And I no longer have wish I had space and money for a vacuum chuck.

OK, digression - but I wanted to support Hockenberry's compliments to Don Geiger, whose devices save money and time and actually work.

Best, Jon

I am very pleased to see that you used the correct terminology "jam" vice the homophone "jamb" with reference to chucking. The use of the word "jamb" seems to have gained an unbreakable toehold and like "normalcy" may eventually become regarded as normal. Saving the English language from such kinds of mangling is probably a lost cause, but I shan't cease fighting on. I apologize for the diversion and return this thread to its prior direction.
 

john lucas

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I'm still confused about Rebate and Mortise. Is there a difference. Is it a mortise if it's flat work and rebate if it's a turning.
 
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Arrrrgh! :D

Yes, tenon vs. tendon is another lost cause. :(

BTW, I pulled a tenon in my elbow while putting a tendon on the bottom of a bowl. :rolleyes:
Bill according to Capt Eddie Tendon was in the old text books and yes I agree with you ,but maybe it is just the way it is.
 

Bill Boehme

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I'm still confused about Rebate and Mortise. Is there a difference. Is it a mortise if it's flat work and rebate if it's a turning.

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.
 

Bill Boehme

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Bill according to Capt Eddie Tendon was in the old text books and yes I agree with you ,but maybe it is just the way it is.

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.
 

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...
 

Bill Boehme

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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
 

Bill Boehme

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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.
 
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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.
 

Dennis J Gooding

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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
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.)
 
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I am very pleased to see that you used the correct terminology "jam" vice the homophone "jamb" with reference to chucking. The use of the word "jamb" seems to have gained an unbreakable toehold and like "normalcy" may eventually become regarded as normal. Saving the English language from such kinds of mangling is probably a lost cause, but I shan't cease fighting on. I apologize for the diversion and return this thread to its prior direction.

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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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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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
 

Bill Boehme

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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.

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).
 
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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).
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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