Thread: Geometry of woodturning

Hmm, not a lot around as you say.



Rob Wallace, Woodturner Info | Facebook
https://www.lylejamieson.com/tools/d...r-Winter06.pdf
Tips [at bottom of page]

If you are referring to the Golden Mean there is a start to the maths side of it at Golden ratio - Wikipedia, the free encyclopedia and the Golden Angle at Golden angle - Wikipedia, the free encyclopedia

Hope this helps
Mike

Thanks Mike, it has helped... to make me realise that I was not clear at all.

What I meant is an explanation of how a blade of any particular shape cuts depending on the angle it is presented to the surfaces of the wood cylinder and why.

Experienced woodturners would find "how" empyrically for each blade they have used for a long enough time, but because there are endless variations of shape and each woodturner has specific preferences, there are endless discussions because the "why" is not known in a systematic way.

I have a gut feeling that such an explanation, if it was ever developed, is now lost in the mist of the ages.

And I would bet at even better odds that there is not much interest in it anyway, because anybody can become proficient enough in a few tools to get reasonable results without needing to know whether the same results would have been obtained more effectively with different tools. As said, you can turn with a sharpened shovel. And probably become quite expert at using it, I suppose.

But I look forward to being proved wrong by the umpteen thousand members of this forum.

Hi all

With my (old) Mechanical Engineer's hat on, I would have to reckon there was some basic physics at work when we are playing with different shapes and angles of cutting edges.

As an obvious example, it is easy to see the difference in the quality of cut, and the forces involved on the tool tip, between a skew used in planing mode and in scraping mode.

Nearly 40 years ago I did my undergraduate thesis on forces involved in metal machining, based on different tool angles etc. Obviously in metal machining, optimizing cutting forces saves time and money. I've never had any reason to go looking for people who might have studied similar things in wood machining, but I would imagine the designers of router cutters would have looked at it.

Maybe I'll spend some time thinking about it ..... then again, maybe I won't.

cheers, Colin

That's it Colin. Now that you have shown your hand, I know whom I am going to pester to test my theories!

Mike Darlow's books like The Fundamentals of Woodturning go into the science of cutting more than any other books I've seen. He seems to have a love of the technical elements of turning, which bores some people witless.

I like it anyway.

Originally Posted by bookend

Mike Darlow's books like The Fundamentals of Woodturning go into the science of cutting more than any other books I've seen. He seems to have a love of the technical elements of turning, which bores some people witless.

I like it anyway.

True, I have his "Woodturning Methods" and agree that he goes in some technical detail. Still not the sort of thing done for metal machining, as Colin was saying. Maybe the technology was already obsolete before the capacity to document it had reached an appropriate level. After all we do not have manuals for pyramid building!.

BTW, Colin, am I underestimating the complexity of such an analysis, if I say that the principles of solid geometry and physics involved do not exceed what most people would have picked up in high school?

What are we talking about? Cutting angle, clearance angle, cutting edge support etc?

+1 to Darlow's Fundamentals for a basic treatment.

The flatware folk can also shed some light; eg. Hoadley's type 2 and 3 chips and how and why you get them.

Thanks Ern, yes, that's what we are talking about.

Quoting from Hoadley:

"The way in which a sharp blade interacts with wood to form a chip or shaving is classified using a system developed in the 1950s by Norman Franz for his doctoral thesis on machining wood. Dr. Franz described chip Types I to III. Later research by William McKenzie described an additional Type 0 chip.
The only account I have been able to find of their research is in Appendix I of Leonard Lee’s book The Complete Guide to Sharpening"

is evidence that some research of this kind was done and is now difficult to retrieve. . Hoadley picked up a few relevant concepts for his testing of planes. Range of variables: thickness of the blade, angle of the bevel, angle of the blade, direction of the grain, hardness of the wood. In woodturning there would be other variables: angle blade/ direction of rotation, direction of rotation of the grain. All of this for each one of the dozens of blade shapes, but for simplification let's say just straight or curved.

Will try to put my hands on the book you recommend to see how much is covered.

The beginnings of another philosphical dissertation.!!

I think Darlow's books are excellent and good enough to work your way from his beginnings to a knowledge and technique of your own that will only come from experience and practice.

To in anyway compare this sort of work with what happens in metalwork is a bit eronious (spelling?).

Metals of known composition have known properties and are easil scientifically tested for the ideal cutting angles and speeds etc,

Timber is a different kettle of fish (&chips).Timber is mighty variable, even within the species. The only thing we can aim for, at best, is a line of best fit approach. I doubt the Europeans had Our hardwoods in mind when they drew up general conclusions as to what did and didn't work.

Still, it's an interesting - albeit esoteric - diversion.

The Science and Engineering of Cutting, By Antony Atkins

Covers various materials, including wood.

.....

Wow - this discussion is really going to stretch the brain!

I agree our preferred medium is far more variable than metal or plastic, and so is our freehand tool handling technique, compared with tool mounted rigidly in a toolpost and fed via a handwheel or a feedscrew.

But ultimately what we are trying to do is apply enough force to the timber (or metal or plastic) to peel off a layer of the surface. The efficiency of that process is going to depend on the size and shape of the point of contact between the tool and the workpiece and the angle in which the force is applied.

I'm not any kind of expert in this stuff, but things do make more sense to me when I can understand the underlying physics.

I need to go and think about all this some more.

cheers, Colin

Thanks guys, we are getting there.

Artme, with these people on board, the interest of the discussion is assured!

Neil, getting very close indeed, you are a research powerhouse.

Colin, we are exactly on the same page.

Even if nothing specific to woodturning comes up, with these two sources (Atkins + Darlow) it should be possible to extrapolate a lot that is relevant to it.

For example, test hypothesis #1:

"whatever the shape of the blade, first contact with the wood is one dimensional (at the tangent point) and the cut is bidimensional (along a linear section of the edge)."

As posted elsewhere, we face partic. challenges compared to flatware woodies. With a bowl we are cutting long grain, cross grain and end-grain and mixes of these. With one tool where they might use several or several set-ups. We almost inevitably will be cutting long grain but against the way it lies from time to time as well with resulting tear-out.

Just out of interest last night I was comparing the performance of a fettled Stanley 6 handplane against that of a Veritas bevel-up jointer. Both had A2 blades with both bevel planes polished to 8000 grit and 45* cutting angles. Both mouths were wide open to simulate a turning tool.

What was surprising was that cutting against long grain both tools performed well, the Stanley leaving visible but really rather minor tear-out in the early growth sections. At a guess there are two reasons for this: the blades were far sharper than the tools most turners use and the speed was crawling pace by comparison. The very small blade projection may also be a factor.

Anyway, I only offer this tale to indicate that while the geometry is clearly important there's other factors in the mix.