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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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
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On Mon, 14 Jun 2004 14:52:01 +0000 (UTC), snipped-for-privacy@yahoo.com wrote:

I think I've got the other end of that bit of steel :-) A couple of questions - what machine are you using, and what cutting fluid?
ChrisH
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wrote:

I've been using it both on a Boxford ME10 (pretty close to a chinese copy of a 9" swing Southbend) and also on a Denford Viceroy 280 Synchro (11" swing).
Because of the low cutting speed and small chip size heat is not a problem and dry cutting is possible. More from habit than necessity I usually drip a bit of sulphurised neat cutting oil (Prestoil) on the workpiece but it doesn't make much difference.
Jim
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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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On Mon, 14 Jun 2004 15:28:51 GMT, "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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wrote:

finishing
knife
of
tool
can
right
The
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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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:

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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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On Tue, 15 Jun 2004 16:07:36 GMT, "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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Interesting. I'd always pictured skiving tools angled the other way, so that they cut the chip off to the right. And I want to say that some of them are angled even more, so that they're nearly perpendicular to the lathe axis...?
--Glenn Lyford
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wrote in message

gymnastics
it
front
result
comes
correctly.
cutters
of
brass
clean,
which
Yes, we have a language problem. <g> I wrote a lengthy article on skiving tools back in the '70s, which appeared in a McGraw-Hill textbook. Your skiving tools are not what we traditionally call skiving tools.

That isn't traditionally called skiving in the US.

I'll take a look.
Ed Huntress
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snipped-for-privacy@yahoo.com wrote:

Which Stellite are you using, please.
Ted
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On Fri, 18 Jun 2004 16:30:40 GMT, Ted Edwards

It's Stellite 100 - the lathe tool grade. It is difficult to find these days so I eke out my small stock by using small lumps as tips.
I used it on the mark 2 skive tool because of its wear resistance and fine edge capability - and because I had a spare lump!.
HSS is a pretty fair substitute - the high cobalt Cleveland MoMax or the M42 8% cobalt grades are the best choice because they are a bit more tolerant to overheating during brazing. Incidentally I may have been a bit misleading here. By brazing I mean silver brazing - I used Easiflo No2.
Jim
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On Mon, 14 Jun 2004 14:52:01 +0000 (UTC), snipped-for-privacy@yahoo.com wrote:

Excellent post!!!!!! Bravo!
Gunner (with several negative angle tools in his Box of Last Resort)
That rifle hanging on the wall of the working-class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there.         - George Orwell
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