Sharper the better?

[Rotate]

I was just wondering. With things such as knives and woodworking tools, the sharper you can make the cutting edge the better they'll cut. Is this always true with cutting metal? The reason why I ask is because I've been studying both my HSS lathe bits and carbide inserts under a microscope (my wife just bought one for the kids and guess who's hogging it), and I'm surprise at what feels and looks to be very sharp doesn't look nearly as sharp as my Swiss Army knife's edge under the microscope. While the knife's edge looks perfectly straight and uniform, my HSS lathe bit looks like it's been scrapped along the edge and the edge is not very uniform (under x40). I also checked the carbide inserts and although they are much better, the edge did not look dead sharp.

I'm begining to think that my bench grinder for the lathe bits is not nearly as good as I thought. Any thoughts on this matter?

Albert

P.S. I'm having a field day with this microscope. I can study the geometry and the cutting edges of endmill, drill bits, and other tools and see how it dulls...fantastic learning tool.

[shorty]

Well, I'm REALLY new at this but I read that using a honing stone to final sharpen your bit is the way to go. I would assume that this would leave a cleaner edge than just grinding.
Like i said, just a rookie, just a thought...
Shorty

[bdarin]

I've got dull plain mills and new sharp ones. The ones that feel razor sharp cut the best. As they get dull, the edge feels dull (a sharp cutter feels dangerous) and doesn't cut as well. Instead of cutting, it's more like scraping the work away, leaving less of a nice chip and more of a chunk. When it's really dull, it leaves dust. I've made HSS lathe tools sharp enough to shave with and they cut beautifully. Use inserts for carbide work, they're unsharpenable. In short, sharper is better.

[Rotate]

I do touch it up with a honing stone and for carbide I use fine diamond file. Although it helps, it still does not produce a uniform edge, perhaps because it's a manual operation. I've just finished examining a brand new Niagara endmill under the microscope and it's amazing how sharp the edges are and how uniform the finish is. May be I'm asking too much, but it kind of bugs me that my lathe tool is not even sharp as my pen knife.

Albert

[Uncle Dunc]

I think it affects the finish on the work more than it affects anything else, like the power required to peel off a given amount of material. A rough tool will get dull faster than a smooth one, but that's probably more important in a production environment than in a home shop.

The sharpness potential of carbide tools is limited by the size of the carbide grains. Carbide is a composite of grains of very hard carbide in a matrix of a softer, tougher material, typically cobalt. Every sharp edge looks rough and scratched if you magnify it enough. As the abrasive gets finer, the scratches get smaller. When the scratches are about the same size as the carbide grains, you can't get much sharper than that because the sharpening action just pulls the grains out of the matrix, leaving little notches in the edge.


[This message has been edited by Uncle Dunc (edited 05-22-2002).]

[Thrud]

Albert
Try a 600x CBN "file" or small stone for hone the edges on steel. Another neat trick is to hone the newly ground edge of HSS after using it for a while - they will stay sharper longer this way.

When the bit cuts the metal it actually is bisecting the metal with a wedge until the chip is forced away from the main material block. Temperatures as high as 1800*C can exist just past the tool edge. Fortunately the chip carries most of the heat away with it, the remaining 50% or so is shared between the workpiece and the tool. This is information from Sandvik Coromants "Modern Metal Cutting" - they also show actual etchings of the plane of cleavage that the tool generates. It is really facinating stuff.

It is also interesting to note that most inserts actually have a more "rounded" edge than the expected knife like edge. In wood working and food use we need a sharp edge to cut well, and even then the best knife edges look fairly rough under a microscope - they tear more than cut.

Diamond actually fractures (in steels) the cutting edge and ends up shortening the life of the edge rather than increasing it. This is a strict no-no with laminated (sword steel) Japanese carpentry tools - besides the water stones produce a far superior edge.

[Indexer]

Albert

The notion of 'sharp' is somewhat different for metal cutting tools than it is for your pen knife. The first concept is how wide the wedge angle is. Metal cutting requies a wide wedge angle for strength and heat removal, but the wider angle then takes more power to force it into the work. So there is a compromise to be made in selecting the right angle for the material to be cut. Angles of 60 to 90 degrees might be used.

But in addition to the wedge or cutting angle, the edge must be relieved by a clearance angle to allow it to enter the metal being worked without getting in its own way. Without relief, the edge will rub rather than cut. A very fine edge doesn't last long in use, but becomes slightly rounded. As the tool is used and wears, the rounding increases and the edge has less and less relief or clearance. Sharpening is a process of restoring the clearance or relief of the cutting edge.

Honing the cutting edge must support the relief created by sharpening and not just smooth out the roughness. Otherwise, it becomes the initial wear of the new cutting edge. Proper honing, tho, is an important part of getting a fine finish, especially on softer metals.



------------------
Rich Kuzmack

Pi = 355/113 . . . to
<85 parts per billion

[metal mite]

In metalworking, a razor sharp edge will quickly chip.

A slightly radiused edge, while taking more power , will hold up longer in an industrial setting.

Most carbide inserts are radiused (Honed) to a certain extent.

Most homie machines will not handle much of a radiused edge, because of the lack of power and rigidity.

In some heavy interrupted cut situations (steel and stainless castings) only a tool with a radius up to .02 with a heavy feed will hold up.

It is all a compromise.

Power vs strength of the edge.

A tool with a lip and a flat of .002- .005 at the edge cuts like a drean and gives me exellent service.

Check the instructions on your saw blades.
They say feed gently until broken in(slightly radiused) than feed the hell out of them.

mite

[SGW]

For HSS, the sharper the better, I think. I use a 120-grit grinding wheel and then hone the edge a bit on a white Arkansas stone.

Getting the angles correct is important, both for good cutting action and surface finish and tool longevity. Buy yourself one of those swing-arm protractors ($10-$15, or whatever they are) and check the angles on your toolbits.

See if you can find a copy of "The Design and Use of Cutting Tools," by Leo. J. St.Clair. It's 350+ pages about single-point tools (i.e. lathe tools, boring bars, etc.)


[This message has been edited by SGW (edited 05-23-2002).]

[L_Miller]

There are times when to sharp is a bad thing.

Realy sharp tools will tend to chater more, but mellow out after a few passes.

A few degrees negitive cutting edge works mutch better in bronze and brass too. the chips just spray off the tool bit.

Bronze and brass will grab and stuck a realy sharp drill bit. I have an extra set of drill bits that I made just for bronze and brass. I put a very small negitive cutting edge on the flutes.

Just my .02

Later

Larry Miller

[SGW]

I don't think "too sharp" is the problem; you're really talking about incorrect cutting angles for the material. It's my experience that brass needs a wicked sharp tool for best results...but you're right, you need to reduce the cutting angle or it will grab.

[Thrud]

SGW:
Aluminum also cuts well with a sharp HSS "hook" - almost like wood (easier than Ironwood!)

[L_Miller]

You are right SGW. They need to be sharp.

I was talking about cutting angles as were a few others.

I just wanted to share a way to keep brass and bronze from grabbing tools.

And to show that a positive cutting edge like a swiss army knife would be on one end of the chart. And negitive cutting edges( still being a sharp edge) can be better used in some places.

Later

Larry Miller