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Old June 14th 04, 04:52 PM
 
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Default Surface finish



I was machining a bit of steel from hell the other day and
having no luck in achieving an acceptable surface finish. The steel
was of unknown origin (my local scrap dealer).It produced stringy
swarf and work hardened when light cuts were attempted. Heavy cuts
resulted in a uniformly rough surface. Cuts light enough to
potentially give a good finish skipped in and out of cutting leaving
random grooves more than 0.001" deep. I tried most of the usual tricks
but the only one that really worked was to machine oversize and take
the last few thou off with a fine single cut file. The penny then
dropped that, if the file works, then a file geometry variant of the
old skiving cutter might do the trick.

To cut a long story short - it does! The geometry I finished up
with was a 5/8 HSS blank with a corner to corner 45 deg flat ground on
the last half inch. In lathe tool terminology this is 45 deg NEGATIVE
side rake.

This was then given extreme front clearance by grinding 40 deg
clearance directly normal to the flat 45 deg cutting edge.

If the middle of this tool is presented square on to the OD
of the work piece, as far a the chip is concerned, it is a 45 deg
skiving cut with zero top rake. The cutting action is improved by
angling the whole tool holder about 15 deg anti-clockwise. This
results in a small effective top rake which allows the thin wide chip
to exit in long satisfying curls.

This tool produced consistently good surface finish with cut
depths in the range 0.0005" to 0.005. The finish and chip formation
seemed to be best when cutting in the 0.001" to 0.002" range. The main
drawbacks are that the tool shape makes it impossible to machine up to
a shoulder and the wide thin chip increases the tendency to chatter.
This means low surface speeds (sometimes backgear!). Power traverse is
advisable for uniform chip thickness.

The small included angle of the cutting edge (50 deg) is
well outside normal lathe tool parameters but is not a concern because
of the intended light cuts. It's perhaps worth remembering that the
normal cutting edge included angle of even a standard spiral twist
drill is close to 60 deg and that's happy with pretty heavy cuts.

These are results from a guesstimated set of cutting
angles on a very limited range of workpieces. The angles don't seem to
be very critical but I've no idea whether they are near optimum. I've
yet to discover how well the tool performs on other work materials.
It's hardly a tool for the professional because of the low metal
removal rate. But for an amateur, a tool that takes light cuts and
consistently leaves a good finish can be a pretty useful item.

Jim

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Old June 14th 04, 05:26 PM
Joules
 
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Default Surface finish


Thanks Jim, that was a useful, and well presented bit of information, I
have some suspect/unknown steel here to play with. I shall follow your
lead.

Joules
  #4   Report Post  
Old June 14th 04, 05:28 PM
Ed Huntress
 
Posts: n/a
Default Surface finish

wrote in message
...


I was machining a bit of steel from hell the other day and
having no luck in achieving an acceptable surface finish. The steel
was of unknown origin (my local scrap dealer).It produced stringy
swarf and work hardened when light cuts were attempted. Heavy cuts
resulted in a uniformly rough surface. Cuts light enough to
potentially give a good finish skipped in and out of cutting leaving
random grooves more than 0.001" deep. I tried most of the usual tricks
but the only one that really worked was to machine oversize and take
the last few thou off with a fine single cut file. The penny then
dropped that, if the file works, then a file geometry variant of the
old skiving cutter might do the trick.

To cut a long story short - it does! The geometry I finished up
with was a 5/8 HSS blank with a corner to corner 45 deg flat ground on
the last half inch. In lathe tool terminology this is 45 deg NEGATIVE
side rake.

This was then given extreme front clearance by grinding 40 deg
clearance directly normal to the flat 45 deg cutting edge.

If the middle of this tool is presented square on to the OD
of the work piece, as far a the chip is concerned, it is a 45 deg
skiving cut with zero top rake. The cutting action is improved by
angling the whole tool holder about 15 deg anti-clockwise. This
results in a small effective top rake which allows the thin wide chip
to exit in long satisfying curls.

This tool produced consistently good surface finish with cut
depths in the range 0.0005" to 0.005. The finish and chip formation
seemed to be best when cutting in the 0.001" to 0.002" range. The main
drawbacks are that the tool shape makes it impossible to machine up to
a shoulder and the wide thin chip increases the tendency to chatter.
This means low surface speeds (sometimes backgear!). Power traverse is
advisable for uniform chip thickness.

The small included angle of the cutting edge (50 deg) is
well outside normal lathe tool parameters but is not a concern because
of the intended light cuts. It's perhaps worth remembering that the
normal cutting edge included angle of even a standard spiral twist
drill is close to 60 deg and that's happy with pretty heavy cuts.

These are results from a guesstimated set of cutting
angles on a very limited range of workpieces. The angles don't seem to
be very critical but I've no idea whether they are near optimum. I've
yet to discover how well the tool performs on other work materials.
It's hardly a tool for the professional because of the low metal
removal rate. But for an amateur, a tool that takes light cuts and
consistently leaves a good finish can be a pretty useful item.

Jim


Congratulations! You've just reinvented the angled knife-tool, a finishing
cutter from the 1930s. g

I'm being only slightly facetious. This is nothing like a traditional knife
tool. It doesn't really have a standard name, but you'll see it called
several things, including an "angled knife tool." It should be called a
slicing tool, or something like that.

An angled knife tool typically is ground from round stock. It has a lot of
front clearance and a lot of top rake, which you get by grinding a groove
into the top of the bar, roughly half-way through. You must hone it very
sharp. The front of the tool is ground straight across, so that, if you
presented it square to the work, it would create an impossibly wide chip.

So you don't present it square to the work. You rotate the cylindrical tool
on its axis perhaps 15 degrees counter-clockwise, as you're looking from
behind the tool, into the work. The edge is now angled to the work so it can
slice, or shear, on an angle.

Then you start a very light cut. If you have the tool height set dead
on-center, you make only a fairly narrow, very thin, knife-slice chip, right
in the center of the cutting edge. Make sure you don't set this tool even
slightly below-center. Err on the up side, if you must err at all.

It's a near-last resort for getting a good surface on soft, gummy steel. The
last resort is a file. g

Watch out, it can grab if you cut too deep. This is a problem mostly on
long, thin workpieces that can flex and climb over the cutter. In that
regard it's like cutting brass with a lot of positive rake.

Ed Huntress


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Old June 15th 04, 10:08 AM
[email protected]
 
Posts: n/a
Default Surface finish

On Mon, 14 Jun 2004 15:28:51 GMT, "Ed Huntress"
wrote:


Congratulations! You've just reinvented the angled knife-tool, a finishing
cutter from the 1930s. g

I'm being only slightly facetious. This is nothing like a traditional knife
tool. It doesn't really have a standard name, but you'll see it called
several things, including an "angled knife tool." It should be called a
slicing tool, or something like that.

An angled knife tool typically is ground from round stock. It has a lot of
front clearance and a lot of top rake, which you get by grinding a groove
into the top of the bar, roughly half-way through. You must hone it very
sharp. The front of the tool is ground straight across, so that, if you
presented it square to the work, it would create an impossibly wide chip.

So you don't present it square to the work. You rotate the cylindrical tool
on its axis perhaps 15 degrees counter-clockwise, as you're looking from
behind the tool, into the work. The edge is now angled to the work so it can
slice, or shear, on an angle.

Then you start a very light cut. If you have the tool height set dead
on-center, you make only a fairly narrow, very thin, knife-slice chip, right
in the center of the cutting edge. Make sure you don't set this tool even
slightly below-center. Err on the up side, if you must err at all.

It's a near-last resort for getting a good surface on soft, gummy steel. The
last resort is a file. g

Watch out, it can grab if you cut too deep. This is a problem mostly on
long, thin workpieces that can flex and climb over the cutter. In that
regard it's like cutting brass with a lot of positive rake.

Ed Huntress


Sounds pretty similar to the sort of tool I've been playing with.

I'm a bit puzzled by your emphasis on the importance of tool
height because skive tool cutting action is not affected by this.

I normally set the tool height so that it cuts over a narrow
region roughly in the centre of its 45 deg edge i.e. halfway down from
the top of the tool. However any part of the whole cutting edge can be
brought into play, cutting with the same cutting geometry, by simple
adjustment of the tool height. This is quite useful because, as soon
as the initial cutting edge region starts to dull, a fresh region can
be brought into play by a making a small change in tool height.

Jim




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Old June 15th 04, 06:07 PM
Ed Huntress
 
Posts: n/a
Default Surface finish

wrote in message
...
On Mon, 14 Jun 2004 15:28:51 GMT, "Ed Huntress"
wrote:


Congratulations! You've just reinvented the angled knife-tool, a

finishing
cutter from the 1930s. g

I'm being only slightly facetious. This is nothing like a traditional

knife
tool. It doesn't really have a standard name, but you'll see it called
several things, including an "angled knife tool." It should be called a
slicing tool, or something like that.

An angled knife tool typically is ground from round stock. It has a lot

of
front clearance and a lot of top rake, which you get by grinding a groove
into the top of the bar, roughly half-way through. You must hone it very
sharp. The front of the tool is ground straight across, so that, if you
presented it square to the work, it would create an impossibly wide chip.

So you don't present it square to the work. You rotate the cylindrical

tool
on its axis perhaps 15 degrees counter-clockwise, as you're looking from
behind the tool, into the work. The edge is now angled to the work so it

can
slice, or shear, on an angle.

Then you start a very light cut. If you have the tool height set dead
on-center, you make only a fairly narrow, very thin, knife-slice chip,

right
in the center of the cutting edge. Make sure you don't set this tool even
slightly below-center. Err on the up side, if you must err at all.

It's a near-last resort for getting a good surface on soft, gummy steel.

The
last resort is a file. g

Watch out, it can grab if you cut too deep. This is a problem mostly on
long, thin workpieces that can flex and climb over the cutter. In that
regard it's like cutting brass with a lot of positive rake.

Ed Huntress


Sounds pretty similar to the sort of tool I've been playing with.

I'm a bit puzzled by your emphasis on the importance of tool
height because skive tool cutting action is not affected by this.

I normally set the tool height so that it cuts over a narrow
region roughly in the centre of its 45 deg edge i.e. halfway down from
the top of the tool. However any part of the whole cutting edge can be
brought into play, cutting with the same cutting geometry, by simple
adjustment of the tool height. This is quite useful because, as soon
as the initial cutting edge region starts to dull, a fresh region can
be brought into play by a making a small change in tool height.

Jim





I think I had the geometry of your tool clearly in mind yesterday, but I
lost it, and I have to run now so I can't go through the mental gymnastics
again.

But, to answer your question: a skiving tool usually is a form tool, and it
can be -- and often is -- cut with no top rake and no front or side
clearance. Form tools don't cut to the side, they plunge-cut. To get front
clearance, they're run below center height, which has the additional result
of making the cutting angle negative.

They also can be given front clearance all around the form, which allows
them to be set on-center or even above-center. My experience with them comes
from two places, the primary one being a turret-lathe form-cutting
operation, and we used our tools set above-center to get effectively
positive top rake, with hand-ground front clearance.

Your tool actually sounds like something else, if I understand it correctly.
My comment about the top rake refers to the fact that these shearing cutters
I'm describing, or whatever you want to call them, typically have a lot of
positive rake and a lot of front clearance, which is the combination that
can cause a tool to grab and dig into gummy or "grabby" material, like brass
or hot-rolled mild steel. But the positive rake is intended to make a clean,
sharp slicing cut, and it's considered to be part of the tool design.

I haven't experimented much with finishing tools. I just take old designs
off the shelf and try them. I consider myself fortunate when one actually
works. d8-)

The thing that made me think of those shearing cutters when I read your
description was the idea that it was shearing chips off like a knife, which
is what the cutters that I'm familiar with actually do.

This is a place where a picture is worth 10,000 words, I think.

Ed Huntress


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Old June 15th 04, 07:12 PM
Grant Erwin
 
Posts: n/a
Default Surface finish

Is this the once-infamous "Contrary Ground Finishing Tool" as described by
Frank Burns in the article "Grinding Tool Bits for a Smooth Cut" in
the July/August '97 HSM magazine?

Grant Erwin
Kirkland, Washington

Ed Huntress wrote:
wrote in message
...

On Mon, 14 Jun 2004 15:28:51 GMT, "Ed Huntress"
wrote:


Congratulations! You've just reinvented the angled knife-tool, a


finishing

cutter from the 1930s. g

I'm being only slightly facetious. This is nothing like a traditional


knife

tool. It doesn't really have a standard name, but you'll see it called
several things, including an "angled knife tool." It should be called a
slicing tool, or something like that.

An angled knife tool typically is ground from round stock. It has a lot


of

front clearance and a lot of top rake, which you get by grinding a groove
into the top of the bar, roughly half-way through. You must hone it very
sharp. The front of the tool is ground straight across, so that, if you
presented it square to the work, it would create an impossibly wide chip.

So you don't present it square to the work. You rotate the cylindrical


tool

on its axis perhaps 15 degrees counter-clockwise, as you're looking from
behind the tool, into the work. The edge is now angled to the work so it


can

slice, or shear, on an angle.

Then you start a very light cut. If you have the tool height set dead
on-center, you make only a fairly narrow, very thin, knife-slice chip,


right

in the center of the cutting edge. Make sure you don't set this tool even
slightly below-center. Err on the up side, if you must err at all.

It's a near-last resort for getting a good surface on soft, gummy steel.


The

last resort is a file. g

Watch out, it can grab if you cut too deep. This is a problem mostly on
long, thin workpieces that can flex and climb over the cutter. In that
regard it's like cutting brass with a lot of positive rake.

Ed Huntress


Sounds pretty similar to the sort of tool I've been playing with.

I'm a bit puzzled by your emphasis on the importance of tool
height because skive tool cutting action is not affected by this.

I normally set the tool height so that it cuts over a narrow
region roughly in the centre of its 45 deg edge i.e. halfway down from
the top of the tool. However any part of the whole cutting edge can be
brought into play, cutting with the same cutting geometry, by simple
adjustment of the tool height. This is quite useful because, as soon
as the initial cutting edge region starts to dull, a fresh region can
be brought into play by a making a small change in tool height.

Jim




I think I had the geometry of your tool clearly in mind yesterday, but I
lost it, and I have to run now so I can't go through the mental gymnastics
again.

But, to answer your question: a skiving tool usually is a form tool, and it
can be -- and often is -- cut with no top rake and no front or side
clearance. Form tools don't cut to the side, they plunge-cut. To get front
clearance, they're run below center height, which has the additional result
of making the cutting angle negative.

They also can be given front clearance all around the form, which allows
them to be set on-center or even above-center. My experience with them comes
from two places, the primary one being a turret-lathe form-cutting
operation, and we used our tools set above-center to get effectively
positive top rake, with hand-ground front clearance.

Your tool actually sounds like something else, if I understand it correctly.
My comment about the top rake refers to the fact that these shearing cutters
I'm describing, or whatever you want to call them, typically have a lot of
positive rake and a lot of front clearance, which is the combination that
can cause a tool to grab and dig into gummy or "grabby" material, like brass
or hot-rolled mild steel. But the positive rake is intended to make a clean,
sharp slicing cut, and it's considered to be part of the tool design.

I haven't experimented much with finishing tools. I just take old designs
off the shelf and try them. I consider myself fortunate when one actually
works. d8-)

The thing that made me think of those shearing cutters when I read your
description was the idea that it was shearing chips off like a knife, which
is what the cutters that I'm familiar with actually do.

This is a place where a picture is worth 10,000 words, I think.

Ed Huntress



  #9   Report Post  
Old June 15th 04, 10:14 PM
Ed Huntress
 
Posts: n/a
Default Surface finish

"Grant Erwin" wrote in message
...
Is this the once-infamous "Contrary Ground Finishing Tool" as described by
Frank Burns in the article "Grinding Tool Bits for a Smooth Cut" in
the July/August '97 HSM magazine?


I wouldn't know. I haven't read a hobby-machining magazine since around
1982.

All I know about hobby machining is some old stuff. I still don't use
carbide cutters on my South Bend, unless I'm machining fiberglass. d8-)

Ed Huntress


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Old June 16th 04, 01:07 AM
[email protected]
 
Posts: n/a
Default Surface finish

On Tue, 15 Jun 2004 16:07:36 GMT, "Ed Huntress"
wrote:

wrote in message

I'm a bit puzzled by your emphasis on the importance of tool
height because skive tool cutting action is not affected by this.

I normally set the tool height so that it cuts over a narrow
region roughly in the centre of its 45 deg edge i.e. halfway down from
the top of the tool. However any part of the whole cutting edge can be
brought into play, cutting with the same cutting geometry, by simple
adjustment of the tool height. This is quite useful because, as soon
as the initial cutting edge region starts to dull, a fresh region can
be brought into play by a making a small change in tool height.

Jim





I think I had the geometry of your tool clearly in mind yesterday, but I
lost it, and I have to run now so I can't go through the mental gymnastics
again.

But, to answer your question: a skiving tool usually is a form tool, and it
can be -- and often is -- cut with no top rake and no front or side
clearance. Form tools don't cut to the side, they plunge-cut. To get front
clearance, they're run below center height, which has the additional result
of making the cutting angle negative.

They also can be given front clearance all around the form, which allows
them to be set on-center or even above-center. My experience with them comes
from two places, the primary one being a turret-lathe form-cutting
operation, and we used our tools set above-center to get effectively
positive top rake, with hand-ground front clearance.

Your tool actually sounds like something else, if I understand it correctly.
My comment about the top rake refers to the fact that these shearing cutters
I'm describing, or whatever you want to call them, typically have a lot of
positive rake and a lot of front clearance, which is the combination that
can cause a tool to grab and dig into gummy or "grabby" material, like brass
or hot-rolled mild steel. But the positive rake is intended to make a clean,
sharp slicing cut, and it's considered to be part of the tool design.

I haven't experimented much with finishing tools. I just take old designs
off the shelf and try them. I consider myself fortunate when one actually
works. d8-)

The thing that made me think of those shearing cutters when I read your
description was the idea that it was shearing chips off like a knife, which
is what the cutters that I'm familiar with actually do.

This is a place where a picture is worth 10,000 words, I think.

Ed Huntress



I'm not sure where you're located but I think perhaps we have a
language problem. In UK parlance a skiving tool is a tool used to
slice or pare a thin slice of material which is just what this tool
does. It continuously slices off a thin ribbon of swarf which exits as
a long thin spiral. There is no way that this tool could be used as a
form tool. The cutting action is not infeed - it's longitudinal
traverse towards headstock.

As I noted in the original post it's not a new concept - I'm
pretty sure it originated in the early days of lathes with carbon
steel spring tools and swan neck tools when almost any outlandish
shape was tried in efforts to improve the process.

To help visualise the device I've posted a couple of pictures in
the drop box titled "Skive Tool". The tool shown is a Mark 2 version
with the business end formed by a lump of stellite brazed to a mild
steel shank. The stellite is brazed high on the shank so that when
mounted in a tool holder set for normal centre height the cutting
action occurs near the middle of the 45 deg skiving edge. The bilious
purple colour is my colour code for a stellite tool.

The pictures show clearly the 45 deg location of the cutting
edge, the 40 deg clearance behind the cutting edge is visible but less
obvious.

Jim


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