jointer blades - how sharp is sharp?

[Van Huskey]

Most everyone is half right regarding blades cutting. There are two parts to cutting pinching or cleaving and sawing. In simple terms the latter is what Brendan is describing and the former is what the others are talking about.

To the OP JP knives should be REALLY sharp but everyone sees REALLY as a different thing. Although I don't fool with knives anymore I always got a few nicks setting blades and never thought of them as being anything but "razor sharp" when I installed them. I never tested them by cutting paper but I will point out cutting paper is far from a universal constant since there is a technique to it and the "better" you are the duller the blade one can get to cut paper, even if we assume all "printer" paper is the same. Since there is no easy way for the average woodworker to quantify sharpness you just have to do your best to subjectively or objectively rate the results and longevity of the edge.

Now if someone wants to build a sharpness jig it seems possible and rather easy, you would need a consistent substance that could be consistently dimensioned and could cut by the blade in question relatively easily then a way to bring the blade in contact with, and guide it through, the material along with a force gauge for the imperical measurement...

[Dan Friedrichs]

I have an Xacto right here... can squeeze all I want... no injuries...

Your knife is dull. Think about how chisels and planes work - there is no "sawing" or "friction" motion.

[Ron Jones near Indy]

Have you contacted Global? I'd like to hear their side of this.

[Leo Graywacz]

It doesn't really matter all that much. That razor sharp edge goes away pretty quick. I mean in about 8' of wood quick. Then you are left with sharp.

[Brendan Plavis]

Your knife is dull. Think about how chisels and planes work - there is no "sawing" or "friction" motion.

Oh contrare.... your pushing the block along causes friction.....

[Dan Friedrichs]

Oh contrare.... your pushing the block along causes friction.....

How is that any different from pushing a knife into your finger?

[Chip Lindley]

Yes, back to the OP's question! Freshly sharpened planer/jointer blades should shave you! They are ground commercially on a grinding jig with round wheel, which leaves a slight hollow grind. After grinding they are stropped to remove the wire edge. Knives will then easily shave you! They will also cut flesh at the slightest provocation. A brand new knife duller than this is unacceptable IMO! You didn't get your $$$'s worth.

I grind planer/jointer blades in the shop with a holding fixture slid back and forth beneath a hollow cup wheel. This does not leave a hollow grind but a flat surface with minute crisscrossing swirl marks. The wire edge is still removed by honing on an oil stone. These edges will shave.

Any presumably sharp blade that will not bring blood by pressing down on it, is not very sharp at all. A dull blade will show a glint of light (especially nicks) when the edge is viewed straight on.

One night while waiting and waiting for our freight train to arrive, (what railroad crews get paid to do most) an office worker complained about his dull pocket knife. I spent fully a half hour sharpening his knife with a small diamond grit pocket sharpener. When I finished, that blade would slice paper with little effort! I handed it back to him, saying, "Be care.....ful" He pressed his thumb into the blade and sliced himself to the bone. He said that knife NEVER cut like that before; even when it was new! Oh, well, I tried to warn him!

[george wilson]

Maybe I'm another misinformed idiot,too,Brendan. I'm NOT going to squeeze razor blades in my hand either. I am well known for making a sharp edge,and I can guarantee you that I've pressed a sharp edge into my fingers more than once.

And,I think you are very arrogant. If your time is so precious,why are you hanging out on a forum???

[Brendan Plavis]

Maybe I'm another misinformed idiot,too,Brendan. I'm NOT going to squeeze razor blades in my hand either. I am well known for making a sharp edge,and I can guarantee you that I've pressed a sharp edge into my fingers more than once.

And,I think you are very arrogant. If your time is so precious,why are you hanging out on a forum???

Is this comment needed?

[Andy Sowers]

Well you can add me to the list of not super sharp jointer knives from Global... I went with them a year or so ago after reading some positive reviews here on the creek.

Granted, they were MUCH better than the dull OEM ones I took out, but they were no where near razor blade sharpness, and they would barely cut paper.

I guess the next time, I'll think about getting these resharpened vs buying new ones. Last time I priced it, the cost to get them sharpened is about the same as buying new ones for my little 6" jointer.

Andy

[george wilson]

It was just as needed as your arrogant comment about wasting your precious time. I see your precious time is still on the forum.

[Gary Click]

I don't use knives in my jointer and planer any longer but when I did I made sure that they were very sharp weather I had them sharpened or sharpened them myself.

I used my 6X18 surface grinder with a bevel jig to grind blades for a 6" jointer and a 15" planer. I also resharpened the blades for my DW735 even though they are "disposable" by lighty grinding the back flat side before switching to a Byrd head. If they were not run until the edge was turned taking .002" to .004" off the back would restore the edge.

I agree with the sharpness needing to be extreme and without getting into the mechanics of the cutting action, I will say that if you touch the edge of one of my blades when fresh you get a cut that starts off feeling like a burn and after a second or so starts to bleed.

I was taught what a sharp edge was by my father who ran a book trimming machine in a bindery for 40 years. He could hone an edge sharp faster than anyone else I have ever met.

gary

BTW - Chip what RR are you with? My day job is designing and building turnouts and diamonds for the Class 1's.

[george wilson]

When I was in college there was a veneer factory nearby. We went on a field trip to it. There was an old guy who sharpened the 10' long veneer slicing blades. He went up and down the blade VERY FAST. He had a stone in each hand. One was for the bevel,and one held on the flat to take the burr off. Both were manipulated in circles very rapidly. I must say,the guy made me nervous!! I had long been making razor sharp edges,but the size of that blade,and his quick motions did make me nervous,because one false move at that speed would certainly gone to the bone!!! He claimed he never cut himself.

[Nathan Palenski]

I have an Xacto right here... can squeeze all I want... no injuries...

Have any more half baked comments you would like to add?

Please dont go off half-cocked...... it only wastes my precious time...

Your blade is dull.

If you'd like to be educated on the subject go register on bladeforums and talk to cliff stamp. He's an expert on the subject. I'd ask him about his "worlds best sharpener" test.

Now if someone wants to build a sharpness jig it seems possible and rather easy, you would need a consistent substance that could be consistently dimensioned and could cut by the blade in question relatively easily then a way to bring the blade in contact with, and guide it through, the material along with a force gauge for the imperical measurement...

Cliff stamp has system he uses for testing sharpness. He's gone into excruciating detail on the subject on bladeforums.

Any presumably sharp blade that will not bring blood by pressing down on it, is not very sharp at all. A dull blade will show a glint of light (especially nicks) when the edge is viewed straight on.

Very well said. I've always used the thumbnail test. A sharp blade will "grab" a thumbnail at a steep angle. A very sharp edge will shave it a little. An extremely sharp edge will shave hair without being held against the skin. Jointer knives should come extremely sharp even if they dont stay that way very long.

[Keith Weber]

You can also squeeze a razor all you want, and not get cut... Inorder to be cut by something sharp, you friction must occur, such as rubbing your finger over the blade...

When I first read this, I thought that that somebody was grossly naive, until I noticed who posted it. Then everything made sense! It was... well... almost expected.

A sharp razor will cut you without drawing it across your hand. To the OP... "Sharp" planer blades should draw blood with ease, or shave the hair from your wrist. It sounds like yours are dull.

Keith