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4K views 59 replies 21 participants last post by  murch 
#1 · (Edited by Moderator)
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#5 ·
Not to argue, but there are applications and there are applications.

Some of which will tolerate a touch of back bevel, and indeed function better with that…
I think of my adzes, where the mere touch of a back bevel allows the "wedge to ease itself up and out, instead of digging in.

The simplistic description of a wedge that steve describes implies that there are equal forces on both sides, but we all know there ain't…maybe a mere 1/10th of a mm on one side, but an inch or two on the other side. The physics of a simple wedge cannot apply without considering the forces directing that force.

OTOH, if yer paring edge banding off of a panel with a 1/32 veneer, yer back better be absolutely flat.

And if you have to pare a 1/4 mm off of a tenon on an expensive antique undergoing repair, you want to be certain that the first chisel you use has the same characteristics of the one you pick up next, and will cut the same. Easiest to do if the back is flat on each and every chisel. OTOH, you might be able to fuss around with variably sharpened chisels trying to find the one that will follow a flat…till you found one that worked.

Besides, when yer sharpening the chisels, how do you figure out how much out of flat the bottom is, so as to take off the wire edge.

I however appreciate and understand where yer coming from steve, some of my carving tools, adzes, and particularly the power carving tools have a deliberate back-bevel (unflattness) to allow them to defect up instead of down….And when I'm rehabbing old tools, sometimes I just do the best I can, and hope for flatness in future rehabs. There is a law of diminishing returns, and sometimes you just have to accept a less than perfect , but perfectly usable result.

Eric in Cowtown.
 
#7 ·
All my chisels have a flat back - never touch the back except for stropping to get the burrs off. Most of my chisel work involves cutting off tenons to the flat surface or cleaning out dado cuts etc… If I encounter work that needs to be done with a beveled edge, I flip the chisel over and use the beveled side. But whatever you do with your chisels or however you want to sharpen them is OK with me. As the old saying goes… "there's more then one way to skin a cat".
 
#10 ·
??

You make a post that you know damn well is going to be controversial…..

"…So I'm sure this post will ruffle some feathers with flat back worshipers "

One person 'slightly and politely' disagrees with you…and you snatch up your chisels and stomp off?

"I wish I had the time back for deleting this. So, I will not watch it no more…have at it."

Bizarre to say the least…..
 
#12 ·
Steve, the reason the back of any blade needs to be flat has nothing to do with a "wedge" or "geometry" or arguing.

It has everything to do with sharpness. Two points intersecting cannot produce a sharp edge if one of them is not flat. Simple. It's just a physical fact.

So, while you are certainly entitled to your opinion, know that making statements that refute scientific fact does not constitute "thinking outside the box".

Having said that, do whatever YOU thinks works for YOU. Be safe and enjoy….
 
#13 ·
Since it takes two surfaces to make an edge, I flatten and polish the back, but no more than about 3/8 to 1/2 inch. This is also my procedure on plane irons.

Lately, I've been sharpening the bevels at 25 degrees and then applying a slight micro bevel. This makes it easier to quickly "dress up" an edge by hand while I'm working.
 
#14 ·
"Steve, the reason the back of any blade needs to be flat has nothing to do with a "wedge" or "geometry" or arguing.

It has everything to do with sharpness. Two points intersecting cannot produce a sharp edge if one of them is not flat. Simple. It's just a physical fact."

That was always my understanding….it was tied to the sharpness. Same with plane blades.
 
#15 ·
That's basically what I was saying. It just needs to be flat just behind the edge so you can achieve the theoretical ) 0 radius edge. Take a look at Japanese chisels, there is only a small amount of flat surface in front of the ura and behind the cutting edge.
 
#16 ·
Sorry but your premise for a flat back as being a reference for choppin and paring is wrong from the start. Fred and lj have it right it all about sharpness and the meeting of two edges. If your back isn't flat your tool will never be as sharp as it could be. Like Fred said, no argument just fact.
 
#17 ·
Maybe I'm not so sorry that I contributed to this thread with my pragmatic contributions.

It's interesting to see what comes out of the woodwork!

lj61673 said…. Two points intersecting cannot produce a sharp edge if one of them is not flat. Simple. It's just a physical fact.

and just where does the physical fact that two points intersect produce a sharp edge (aka known as a line?)

No where in any geometry book that I own do to points intersecting produce a line/edge. Maybe you could point out the basis of that physical fact just to educate me.

and "know that making statements that refute scientific fact does not constitute "thinking outside the box".

"physical fact" has now been replaced with "scientific fact"

I don't understand at all!

But neither of them assertions are qualified with data points or references. andneither dogmatic assertions give any reference to pragmatic experience.

Yes there are needs for dead flat backs
Yes there are needs where a dead flat back is a drawback

This is one case where "two points interesecting" do not make a point, they make a Void which could be easily filled in with experiental data.

Cutting tool geometry ihas long been an evolving field, and while in theory it is simplistic geometry, it is so much more than that. The machinists handbook is full of tables of cutting speeds,lubricants, geometry of cutting tools for different materials, and a whack of info that is eye-opening to those that figure a simple saw blade or chisel geometry is "best"

Fact of the matter is that the more you know, the more you know what you don't know.

Granted you won't get to first base without the geometric knowledge, but then the application, ah the bottom line ..the application, and using your knowledge and experience to achieve that comes into play.
My clients don't pay for my theoretical knowledge, they pay for results. My results come from making it work. Therein is the bottom line.

Richard Feynman, a leading physicist in years gone by, seldom read the current literature-as it simply repeated dogma ad-nauseum.

Grasshopper, it's all in the experience.

Ducking….

Eric in Calgary
 
#19 · (Edited by Moderator)
Chuck - I think the fact that you are not sure makes you qualified to understand sharpening!

and no, you don't have to have a flat back when you sharpen - I myself stoop over most of the times when I am sharpening my irons - but I can't stay in that position for a long time.

to the OP - when you say "I" with quotation marks, does it really refer to you? or is that "you"...? ;)
 
#21 ·
Eric,
Do you think you could have said less if you had used a few more words?

In saying "two points intersecting" the obvious reference was two edges. I didn't think that had to be pointed out. Now I know better. By the way, that IS a physical fact as those two edges are actual tangible "physical" objects.

As for the "scientific fact", that would be a reference to it being proved by actual and trail and error experimentation and measurements. A flat back produces the sharpest straight edge. Period.

If you're still confused maybe a little more reading on the subject and a lot less pontificating might serve you better.
 
#22 ·
What is the point of this discussion? The OP says "I do flatten a 1/2" of the back". That is enough for sharpening purposes and is quick and easy to do. I don't think anyone is arguing against that.

If the point of this is that it is not required to flatten the entire length of the chisel back, I think that most people would agree with that too. I have done some work that was aided by a longer flat portion on the back. In that case, I just used a ruler against the backs of my chisels and choose the flattest one I found.
 
#23 ·
I agree Chuck, I'm not sure what this is all about. I have yet to find anyone that actually flattened the ENTIRE back of the chisel - most if not all people I know just flatten the leading (near the edge) portion of the back. the OP is a bit vague in what it's trying to present.
 
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