Plane iron clearance angle.

[swagman]

Hi members. Came across this blog on an Australian Hand Tool Forum.
Topic starts off questioning the need to include a clearance angle on the main bevel of a plane blade. Below is feedback that was supplied on this subject. I thought it worth sharing.

swagman.




First answer on clearance angle.

As the blade wears, it wears unevenly. As a blade is used it wears from the perfectly sharpened edge you started with into a new shape called a wear bevel. The face of the blade that is pushing up the shaving (known as the upper wear bevel) wears approximately 3 times faster than the other face of the blade (the lower wear bevel). But they both wear.

If there was no clearance angle, even a minute lower wear bevel would cause the cutting edge to be lifted from the wood surface and reduce the cutting action. This is exactly what Terry Gordon was describing in the previously mentioned article. It doesn't matter if the plane is of bevel-up or bevel-down design. Both blades will develop a wear bevel as they are used.

The more the clearance angle is greater than zero, the longer the plane can continue cutting once the blade begins to wear because it will take more wear for the wood to rub behind the cutting edge.

Absolutely nothing to do with springing wood fibers.

 

[ivan]

Not at all sure this is the whole answer...

Take a nice flat piece of stuff, and a newly sharpened chisel. Place the chisel flat on the wood (zero degrees relief) and push. See any shavings?

 

[matthewwh]

I'm in two minds on this issue, I can see value in the wear bevel argument, but if that were all there was to it we would surely all be planing with razor blades.

Ivan has answered his own question with the words 'freshly sharpened' thereby eliminating the issue of wear. (no offence) but he still has a point. Do we have any business expecting performance from a worn iron?

From my perspective, honing angle is just one of the three elements that make up the sharpening triangle, the others being quality of steel and quality of surface.

Quality of steel is given and is a purchasing decision, correctly heat treated Japanese white paper #1 steel will give a better edge than anything else on the planet, the closer you get to that optimum the better off you are. O1 is very close indeed, 12c27 isn't far behind but nobody has had the wit to make stainless plane irons from it yet, A2 is miles off and D2 makes A2 look vaguely acceptable. Both of the air hardeners (D2 and A2) are acceptable for scrub planes but you wouldn't want them within a mile of your finishing surface.

Quality of surface refers to the degree to which you have polished the steel. I use 3M lapping film which goes down to 0.3 microns, about half the tolerance they use for the mirrors on intergallactic telescopes, so in woodworking terms it's 'kin fine!

Honing angle is the big question for me. I know global use completely inappropriate steel in their knives for gaining an edge. But they hone it flat at a ridiculously acute angle and achieve a workable edge. Sharpen one at a more obtuse angle and you will soon feel the disadvantages of those chunky carbides that it relies on for edge retension though!

I understand that sharpness is a triangular relationship between these three factors but I have no idea how long each edge of that triangle is?

There is little doubt that sharpness is a determining factor in finish, which is why, in my humble opinion, smoothing planes should always be equipped with deliciously fine, well (and recently honed), highly pure carbon steel blades.

I can also see an argument for low (primary bevel angles)* in terms of support for the cutting edge, which infer greater edge retention, but this would rely upon refinement of steel and degree of polish to achieve acceptable results.

*edit - should read 'clearance angles' - sorry!

 

[swagman]

I think the important issue to come out of this debate is the need for due care when resharpening. If honing by hand a small error in technique can determine whether you end up with a flat or rounded edge. Instead of removing the wear bevel you can end up creating a new one.

swagman.

 

[newt]

Matthew, agree with all you say except perhaps the last para, I thought that a higher bevel angle eg 30 instead 25 would provide support not lower; or are just refering to the clearance angle, in that case I agree.

 

[Vann]

Quality of steel is given and is a purchasing decision, correctly heat treated Japanese white paper #1 steel will give a better edge than anything else on the planet, the closer you get to that optimum the better off you are. O1 is very close indeed, 12c27 isn't far behind but nobody has had the wit to make stainless plane irons from it yet, A2 is miles off and D2 makes A2 look vaguely acceptable. Both of the air hardeners (D2 and A2) are acceptable for scrub planes but you wouldn't want them within a mile of your finishing surface.

Interesting opinion on steel types. Do you have any thoughts on M2 steel for irons? M2 being the steel that Academy use in their irons.

Aussie Woodworkforumites have recently combined an order for 200 odd irons out of M2 steel (salt furnace hardened). Initial feedback is glowing, though the Aussies probably tend to look for irons that will hold an edge with their tough hardwoods, rather than give top quality surface finishes.

Cheers, Vann.

 

[bugbear]

...tough hardwoods...

They're not just tough. IIRC many of them have extensive silica inclusions too!

BugBear

 

[PaulO]

I use 3M lapping film which goes down to 0.3 microns, about half the tolerance they use for the mirrors on intergallactic telescopes, so in woodworking terms it's 'kin fine!

How does the surface tolerance or deviation from parabola of a telescope surface have any relationship to the effectiveness of sharpening media? One is focusing light and the other is abrading a metal surface to be used for cutting wood. An abrading particle of 0.3 microns does not guarantee a surface roughness (or flatness) of 0.3 microns RMS. Even if it did we have no basis for assuming that that degree of roughness is appropriate or desirable for use cutting wood.

Don't get me wrong, I'm not suggesting that .3 micron lapping film isn't enough, I am just saying that your analogy with telescopes is a bit meaningless and sounds like the sort of comment I would expect on a home shopping channel.

I am also interested in your comment that A2 is "miles off". I have seen this discussion before with people claiming that they achieve a better edge with O1. There is no doubt that sharpening A2 is harder work, and I wonder if some of the evidence against A2 edges is from people who haven't sucessfully sharpened their A2.

I'd be happy to plane any sample you would like to provide with my "way off" A2 blades for side by side comparison with O1 planed surfaces. I'd be interested to see if anyone could feel the difference. That's before we look at the effects and requirements of finishing which would probably mask any effects of the differing edges.

 

[Anonymous]

Don't mean to appear rude, but all this is academic and pointless really. Plane makers determine blade angle and most sensible people grind around 25-30 degrees and hone a few degrees off this (except bevel up)

I played with angles for years and finally realized it is a waste of time and stops me doing what I enjoy (making furniture) whilst taking up loads of my time doing what is tedious (sharpening).
I now just grind all (A2) plane blades at around 30 degrees and hone just beyond this - I really found no significant difference in performance, wear etc. at any of the angles I tried and whether using scary sharp or waterstones also found no real difference.

I say quickly sharpen at the end of each job (as Chris Schwarz recommends) at any angle you fancy and then make beautiful things

 

[woodbloke]

I'd be happy to plane any sample you would like to provide with my "way off" A2 blades for side by side comparison with O1 planed surfaces. I'd be interested to see if anyone could feel the difference. That's before we look at the effects and requirements of finishing which would probably mask any effects of the differing edges.

I agree. I used to use O1 carbon steel blades and now use A2. Provided that you've developed a decent honing regime, an A2 blade can be honed equally as sharp. In the very recent chisel test I did for F&C, there was a huge range of different blades to hone, including two that were A2 and all were honed to the same degree. In my mind, it's not so much the steel, but the system of sharpening that makes the difference - Rob

 

[matthewwh]

This is what I mean about alloyed steels in edge tools, the more alloying elements you introduce, the more rubble you end up with at the cutting edge. I've just learned that a friend of mine has acquired a microscope that can take these pictures so I'll try and get the same sort of pictures taken with some other popular tool steels.

Carbides form a weak bond with the surrounding material and therefore the first bit of the edge crumbles almost immediately, then rapidly stabilises, although this still produces an effective and highly wear resistant cutting edge it doesn't have quite the same feel as a freshly sharpened edge. In many woodworking applications this is still perfectly acceptable. M2 HSS would fall into this category, and will stand up well to super hard Aussie timbers. If he isn't the originator of this scheme I'll bet Derk Cohen would be keen to try one out!

With a smooth steel that initial sweetness is much easier to achieve and lasts longer, but the edge will eventually wear beyond the point that the alloyed steel maintains and need a light honing or stropping to bring it back up again. It does however give you an extended period of 'freshly honed' sweetness, where the edge is extremely crisp. This is why professional Chefs usually prefer pure carbon steel knives, Barbers almost exclusively use pure carbon steel straight razors, and I recently learned that many Surgeons use carbon steel chisels rather than surgical stainless ones because they can get a sharper edge which causes less damage to the surrounding tissue.

Tony makes a very valid point that if you are using good quality, well maintained blades and follow the manufacturers recommendations, your tools will function effectively and you can get on with the business of making furniture. We are definitely in the realm of theory and personal preferance here.

Personally I like to understand the theory behind honing angles etc and the initial post challenges my previously held belief that steeper honing angles result in longer lasting edges and suggests the possibility that 25 degrees is an optimum rather than a compromise.

As long as we stick to one steel type or at least within the narrow bracket of high quality blades, we can assume that this edge of the sharpening triangle has little influence.

Similarly, the comment about telescope mirrors was merely meant to illustrate that it is possible to go beyond the pale in woodworking terms, in order to maximise (and therefore minimise the influence of) the surface quality factor.

Eliminating/minimising these two factors helps to isolate the subject of honing angles and their influence on blade life.

 

[bugbear]

This is what I mean about alloyed steels in edge tools, the more alloying elements you introduce, the more rubble you end up with at the cutting edge.

"rubble" - love it!

BugBear

 

[ivan]

I don't claim any wild expertise on the subject, but the length of time that LN blades last before sharpening is neded, still surprises me.

I'm sure Tony is right and exact choice of angles don't matter too much, but a quick thought about clearance angles is needed if you put a back bevel on a bevel up plane, to lower its effective bedding angle, to make it easier to push in end grain. Obviously if you grind too big a back bevel you remove all the clearance!

Incidentally a block plane with back bevel blade doesn't like cutting MDF without setting a large cut. The furry LD centre is "springing", I presume.

 

[bugbear]

Not at all sure this is the whole answer...

Take a nice flat piece of stuff, and a newly sharpened chisel. Place the chisel flat on the wood (zero degrees relief) and push. See any shavings?

If you start the chisel a shaving's thickness below the surface you might, which is exactly what a plane does...

And mitre trimmers seem to work well enough, and they normally have (AFAIK) zero clearance angle, or damned close to it.

BugBear

 

[monoman]

Hello all,

I am the author swagman quoted, but sadly did not acknowledge, in his original post. The thread this quote was lifted from can be found at http://www.woodworkforums.com/showthread.php?t=100194

Unfortunately, focussing on blade composition or sharpening regime without due understanding of rake angle, clearance angle and surface preparation of the cutting tool is not enough because cutting wood is a combination of all three.

Cheers

 

[DaveL]

Hi monoman,

Welcome to the forum.

Your link has been caught by our spam trap, this will stop once you have a few more posts to your name, here it is:
http://www.woodworkforums.com/showthread.php?t=100194

 

[aesmith]

The face of the blade that is pushing up the shaving (known as the upper wear bevel) wears approximately 3 times faster than the other face of the blade (the lower wear bevel). But they both wear.

If I read that right, it's an argument in favour of bevel-up designs, on the basis that you take more off the bevel than off the back when resharpening. Anyone agree, and does it matter in the real world?

 

[monoman]

The face of the blade that is pushing up the shaving (known as the upper wear bevel) wears approximately 3 times faster than the other face of the blade (the lower wear bevel). But they both wear.

If I read that right, it's an argument in favour of bevel-up designs, on the basis that you take more off the bevel than off the back when resharpening. Anyone agree, and does it matter in the real world?

No. Certainly not an argument in favour of bevel-up designs. Because it is describing the process that causes a cutting edge to dull. The laws of physics aren't going to change because you have a bevel-up plane regardless of what the bevel-up evangelists think.

At any rate, it's the wear on the back of the blade (the lower wear bevel) that determines how much you must grind of the bevel. i.e. Grind the bevel until it brings up a burr of the back of the blade.

Cheers

 

[matthewwh]

Now I'm really confuddled.

If the leading surface is taking three times the wear of the following surface then surely Tony's argument stands. Albeit on the cusp of pedantry as he acknowledges.

The biggie for me is whether a lower honing angle increases edge life as this appears to be contrary to my previously held beliefs. Surely if a more acute bevel angle produced a more durable edge and (logically) a sharper one, we wouldn't have stopped at 25 degrees when correctly heat treated carbon steel is capable of going as low as 15 without failing.

My previous understanding was that 25 gave us a balance point between shapness (more acute) and durability (more obtuse).

I humbly await luminary edification if this is not the case.

 

[bugbear]

No. Certainly not an argument in favour of bevel-up designs. Because it is describing the process that causes a cutting edge to dull. The laws of physics aren't going to change because you have a bevel-up plane regardless of what the bevel-up evangelists think.

Agreed - but given the different geometry of a BU plane (very low blade angle, lack of cap iron), applying the SAME laws of physics to a DIFFERENT scenario may well produce different effects.

BugBear (not an evangelist for anything)