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Arch
 
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Default Musing about up milling, down milling, shearing and scraping.

While facing a faceplate today I thought about how metal is milled and
wood is turned. In milling a rotating cutter is forced into sliding but
firmly held metal. In woodturning a sliding but firmly held tool edge is
forced into rotating wood.

Consider a horizontal milling machine doing what machinists term
peripheral milling and we term spindle turning. In climb or up milling
the work moves in the same direction that the cutter is revolving and is
forced toward the holding vise as it slides through the cutter. It
produces a smooth surface and wispy swarf. In the more conventional down
milling, the work is moved against the direction of the rotating cutter
and is forced away from the vise. This mode can produce a ridged surface
and chippy swarf if the work isn't held firmly.

Now consider scraping and shearing wood. In scraping the edge is moved
in the same direction as the revolving wood as the tool slides along the
rest. It can produce a smooth surface and delicate shavings. In
shearing, a tool edge is moved against the direction of the rotating
wood and if not controlled can produce a scalloped surface and distorted
shavings.

Machinists and woodturners know that there are other important
considerations that make down milling of metal and shearing of wood
usually preferable to up milling and scraping.

I know that my strained analogy is of no account and boring to many
(most) of you. I thought it interesting and I hope I got it right, but
if not somebody please correct my ups and downs for the sake of future
generations of woodturners.


Turn to Safety, Arch
Fortiter



http://community.webtv.net/almcc/MacsMusings

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George
 
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"Arch" wrote in message
...
While facing a faceplate today I thought about how metal is milled and
wood is turned. In milling a rotating cutter is forced into sliding but
firmly held metal. In woodturning a sliding but firmly held tool edge is
forced into rotating wood.

Consider a horizontal milling machine doing what machinists term
peripheral milling and we term spindle turning. In climb or up milling
the work moves in the same direction that the cutter is revolving and is
forced toward the holding vise as it slides through the cutter. It
produces a smooth surface and wispy swarf. In the more conventional down
milling, the work is moved against the direction of the rotating cutter
and is forced away from the vise. This mode can produce a ridged surface
and chippy swarf if the work isn't held firmly.

Now consider scraping and shearing wood. In scraping the edge is moved
in the same direction as the revolving wood as the tool slides along the
rest. It can produce a smooth surface and delicate shavings. In
shearing, a tool edge is moved against the direction of the rotating
wood and if not controlled can produce a scalloped surface and distorted
shavings.


Although I see a lot of it, anyone "forcing" an edge into rotating wood
deserves what s/he gets in the way of dismounts, flying shavings, broken
tools and maybe facial remodeling.

Scraping is more likely to cause those problems associated with forcing, and
broad scraping angles pick up and sometimes tear out fiber - a material
consideration the milling process doesn't have to worry about. One thing
that puzzles me about scrapers is that the best scraping technique, had it
been applied while using a gouge, would have made scraping unnecessary.

Shearing does not produce scalloped surfaces - chatter from an overextended
or dull tool, or a poorly supported piece of work produces a scalloped
surface.


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Bjarte Runderheim
 
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"George" george@least skrev i melding ...


Scraping is more likely to cause those problems associated with forcing,

and
broad scraping angles pick up and sometimes tear out fiber - a material
consideration the milling process doesn't have to worry about. One thing
that puzzles me about scrapers is that the best scraping technique, had it
been applied while using a gouge, would have made scraping unnecessary.

Shearing does not produce scalloped surfaces - chatter from an

overextended
or dull tool, or a poorly supported piece of work produces a scalloped
surface.



Well said, George!
I completely agree.

As a beginner I used my couple of scraping tools a lot,
including some special Sorby-tool.

As I learn more about sharpening and the different uses
of gouges I find that two bowlgouges and two oval chisels
and a roughing iron is all I need for bowls and spindlework.

But I can see that the inside bottom of small boxes would be a challenge,
if ever I tried one!

Bjarte


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George
 
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"Bjarte Runderheim" wrote in message
...



As I learn more about sharpening and the different uses
of gouges I find that two bowlgouges and two oval chisels
and a roughing iron is all I need for bowls and spindlework.

But I can see that the inside bottom of small boxes would be a challenge,
if ever I tried one!

Bjarte


Nope. They make 90-degree gouges for that which work well. They're called
hook or ring tools, and they allow you to pare the end grain of the
box/goblet the same way you pare the end grain working off the edge of a
bowl - with a shearing cut.


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Arch
 
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Thanks for responding, George. An orphaned musing is a terrible thing to
lose. A differing opinion is a good thing,
dogma isn't. I only meant to share and provoke other ways to think about
the cuts we make. We won't change the world as we know it, but future
generations of woodturners will be saved.

Semantics aside, we aren't far apart. I said, "... but firmly held
tool edge...." and "....in shearing...,a tool edge _if not controlled_
can produce a scalloped surface...." (emphasis added)

I considered a scraper edge as trailing the rotating wood's surface and
a shearing edge as opposing the rotating surface. This seemed to be
somewhat analogous to up and down milling. To cut rotating wood, a tool
must move toward and penetrate the wood's surface. The wood can come
toward the tool but not to it. This motion requires some force, even if
very little.

I'll suggest another possible analogy. Not as specific advice for good
woodturning, just as a brief respite from Jet vs Delta. I know (ok,
believe) from experience that cutting metal below center draws the bit
inward and produces a rougher surface. Cutting metal above center pushes
the bit outward and can avoid a rough interrupted cut.

If the analogy is partially valid, are there any useful parallels or
opposites for turning wood? I realize that this tortuous analogy is
only loose and general, since wood is cut and scraped with much more
latitude and with different cutting tools.

Who will be the first to post that a little knowledge is a dangerous
thing?


Turn to Safety, Arch
Fortiter



http://community.webtv.net/almcc/MacsMusings



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Bjarte Runderheim
 
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"George" george@least skrev i melding ...

As I learn more about sharpening and the different uses
of gouges I find that two bowlgouges and two oval chisels
and a roughing iron is all I need for bowls and spindlework.

But I can see that the inside bottom of small boxes would be a

challenge,
if ever I tried one!

Bjarte


Nope. They make 90-degree gouges for that which work well. They're

called
hook or ring tools, and they allow you to pare the end grain of the
box/goblet the same way you pare the end grain working off the edge of a
bowl - with a shearing cut.



Ah, well, _now_ I can throw away my scrapers!-)

Bjarte


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Ricky Dietsch
 
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Ah, well, _now_ I can throw away my scrapers!-)

Bjarte


Throw them my way. The ones I have made out of old files are not strong
enough to work at the depths I am getting to now. I need a "real" scraper,
but I just spent all my disposable income on a new accordion!
Ricky



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George
 
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"Arch" wrote in message
...
Thanks for responding, George. An orphaned musing is a terrible thing to
lose. A differing opinion is a good thing,
dogma isn't. I only meant to share and provoke other ways to think about
the cuts we make. We won't change the world as we know it, but future
generations of woodturners will be saved.

Semantics aside, we aren't far apart. I said, "... but firmly held
tool edge...." and "....in shearing...,a tool edge _if not controlled_
can produce a scalloped surface...." (emphasis added)


Don't know how far. For my examples, I use wood cut across the grain.

I view the work as the most flexible part of the turning combination. Any
tool presentation which applies more of a force outward - more or less
perpendicular to the surface - rather than downward toward the bottom, where
it cuts as the wood wishes to be cut, can cause chatter. This can be
because of the angle of the tool, or because the turner "rides" the bevel
into alternately harder/softer wood. If the turner is foolish enough to
give leverage to the piece, tool flex can begin to affect the process as
well.

What I consider the worst possible presentation is the one where force is
radial - stuffing the edge of the tool into the rotating work at close to
90/90. This is the classic scraping presentation, where the two portions of
grain are lifted and/or torn and the piece departs the holding device if a
bit too thick a scrape is attempted. If a gouge edge, which is
wedge-shaped, and therefore drawn into such a cut, enters at a scraping
angle, it's called a catch.

To me cutting is what drops a shaving, scraping or stabbing what throw one,
and Newton's third law applies. It is, of course, an elastic "collision,"
(Leo) with the piece deforming.

As to shearing, consider the gouge. I have to use slashes, though it is
curved in length and in width. After the edge lifts the shaving to begin,
the gouge is rotated so the bevel can rub at a shear angle ( \ ) if looking
directly in toward bottom, and (\) to cut, if you could look through the
piece from above. It then takes a shaving along its engaged length as the
wood is transported past the edge. As we know from other woodworking, even
a sharp edge benefits from shear. Plane end grain without shear, or try
cutting straight ahead versus drawing a knife along the piece if you need to
refresh your memory. Shear is the optimum presentation for cutting wood
because it exerts the least force which might develop into chatter or
"scalloping."

I'll suggest another possible analogy. Not as specific advice for good
woodturning, just as a brief respite from Jet vs Delta. I know (ok,
believe) from experience that cutting metal below center draws the bit
inward and produces a rougher surface. Cutting metal above center pushes
the bit outward and can avoid a rough interrupted cut.


My bet is on compression of the material by the bit. Same as scraping.

If the analogy is partially valid, are there any useful parallels or
opposites for turning wood? I realize that this tortuous analogy is
only loose and general, since wood is cut and scraped with much more
latitude and with different cutting tools.


Are you referring to spindles? There the work squirms up in wood cut below
center, and is burnished down from above. It's that elastic collision
again. The best spindle cut, as we know, is produced by a tool named by the
angle at which its edge is presented - a skew.


Who will be the first to post that a little knowledge is a dangerous
thing?


Alexander Pope


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Leo Van Der Loo
 
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Hi Rick

No, what you need is a tool rest that goes close in, so you do not have
the overhang.
How do you think I hollowed out bowls 8" deep 45 years ago, bowl gauges,
ring tools, and etc., where not invented yet!

Yes I know the answer, I used to say that too, : "that was then this is now"

Have fun and take care, also, make music G
Leo Van Der Loo

Ricky Dietsch wrote:

Ah, well, _now_ I can throw away my scrapers!-)

Bjarte



Throw them my way. The ones I have made out of old files are not strong
enough to work at the depths I am getting to now. I need a "real" scraper,
but I just spent all my disposable income on a new accordion!
Ricky




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George
 
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"Leo Van Der Loo" wrote in message
news
Hi Rick

No, what you need is a tool rest that goes close in, so you do not have
the overhang.
How do you think I hollowed out bowls 8" deep 45 years ago, bowl gauges,
ring tools, and etc., where not invented yet!

Yes I know the answer, I used to say that too, : "that was then this is

now"

A bit of exaggeration there. The ring / hook tools are older'n Arch!




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Leo Van Der Loo
 
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Hi George

Ring tools older than Arch ??
Hey Arch is that right ???
Maybe you should have come and told me G
Anyway if one does not know of their existence, that's almost the same
as not invented, kind of.
Still I do assume you would agree with keeping your tool rest as close
as possible in any case, as someone said here, don't give leverage away,
or something to that extend.

Have fun and take care
Leo Van Der Loo


George wrote:

"Leo Van Der Loo" wrote in message
news
Hi Rick

No, what you need is a tool rest that goes close in, so you do not have
the overhang.
How do you think I hollowed out bowls 8" deep 45 years ago, bowl gauges,
ring tools, and etc., where not invented yet!

Yes I know the answer, I used to say that too, : "that was then this is


now"

A bit of exaggeration there. The ring / hook tools are older'n Arch!



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