Skew irons for wooden planes

[Stewie Simpson]

http://www3.telus.net/BrentBeach/Sharpen/bladetest.html

[Patrick Chase]

D2 is kind of a weird choice, also for a chisel. It is actually less tough then O1 or A2. The large carbides tend to break out of the steel matrix. It does have very good wear charactaristics but might indeed not get as keen as O1. It is more work to sharpen too.

I don't understand why they choose D2 for those mortise chisels. Maybe in practice it is all half as bad as I imagine.

D2 isn't terribly tough as measured by Chapy C-notch, but it is very wear-resistant. As you say, the negative is that the carbides are huge, and much more problematic even than A2 in that respect.

[Patrick Chase]

http://www3.telus.net/BrentBeach/Sharpen/bladetest.html

The words say "the resulting edges were just as sharp as blades made with other steels" but his pictures sure don't look that way. Those shiny spots along the edge look like carbide voids, which is exactly what one would expect from D2.

EDIT: Of course it's pretty hard to determine anything from Brent's pictures. Sort of like reading tea leaves IMO.

[Stewie Simpson]

http://www3.telus.net/BrentBeach/Sha...ercy%20D2.html

[Oskar Sedell]

Not that you asked for any advice, but just FYI, my own attempts at very low angle + skew have not been terribly happy…the low sharpening angle needed makes the edge more fragile, the skew makes it still more fragile, and end grain shooting is one place where the edge can't be fragile. If you make it 35°-38°, you will constantly be fighting to get an edge that won't chip and still provides the requisite clearance angle.

Hi again, I've been thinking about this. Why is it that the skew makes the edge more fragile? I'm thinking that the bevel angle is what it is, and the skew would rather make less stress on the edge, since it enters the wood gradually. How are you thinking about this?

I got a reply from Phil at philly planes, he is grinding his blade with 25 degree bevel, making for a 13 degree clearance. If there is a problem with the edge, one could increase the bed angle with a couple of degrees, still having 10 degrees clearance.

Best regards,

Oskar

[Stewie Simpson]

Oscar. 12* clearance angle is generally considered the minimum target to aim when determining the primary bevel. Based on a 38* bed angle; 25* primary bevel =13* clearance angle; I cant see a problem. Phil Edwards is a well respected traditional plane maker. I see no reason to doubt his recommendations.

[Kees Heiden]

Once I did some tests with the clearance angle on a shooting board. Until then I hadn't ever seen evidence that a long clearance angle made a difference on long grain. But that experiment showed me that below 10 degrees the effect was weird. it looked like the edge was skipping spots. I was planing some hard maple. And that was with a sharp blade. Use creates an additional wear bevel on the underside of the edge which only makes the ssituation worse

When you have made a plane with 38 degree bedding angle you are very close that limit. 25 degrees bevel angle seems very low to me for an end grain situation. Especially with steels like A2 or D2 that are known for their chipiness with shallow bevel angles. I sharpen freehand and I am not "counting the degrees". I'm afraid I wouldn't like it, too little leeway.

[Kees Heiden]

When you think about it, a skew lowers the angles. In the planing direction it is as if you are walking up a hill not straight on but at an angle. This lowering of the angles not only goes for the cutting angle, but also for the bevel angle. So I guess the edge becomes even more vulnerable when skews are involved.

[Stewie Simpson]

When you think about it, a skew lowers the angles. In the planing direction it is as if you are walking up a hill not straight on but at an angle. This lowering of the angles not only goes for the cutting angle, but also for the bevel angle. So I guess the edge becomes even more vulnerable when skews are involved.

Kees; you may want to give the highlighted area of your post a little more thought.

Stewie;

[Patrick Chase]

Kees; you may want to give the highlighted area of your post a little more thought.

Stewie;

Stewie is right as usual. If anything the edge becomes less vulnerable as the plane cuts a narrower track, so the pressure (force per unit cutting edge area) is reduced. That reduction is very small at the sorts of angles we're talking about here, though (it's proportional to cos(skew_angle)).

Also the *total* cutting force may be reduced because skew lowers the effective cutting angle, but I don't know what if anything that actually does for the wear zone on the edge.

[Patrick Chase]

Hi again, I've been thinking about this. Why is it that the skew makes the edge more fragile? I'm thinking that the bevel angle is what it is, and the skew would rather make less stress on the edge, since it enters the wood gradually. How are you thinking about this?

Gradual entrance slows the rate of change of cutting force and makes the cut feel smoother to the user, but it doesn't do anything to reduce the forces at any given point along the edge. One way to think of that effect is that it staggers *when* each bit of edge is cutting but does nothing to change *how* it cuts.

With that said, skew can make the edge more robust for a different reason: It spreads the cutting forces out over more of the edge and thereby reduces cutting pressures. You really need to be at a skew angle of ~30 deg or more for that to be a significant effect though.

In theory the acutely angled leading corner becomes a weak spot as it isn't as well supported as a square one, but that's a nonissue for shooting (the topic of this thread) as that corner never sees wood.

[Patrick Chase]

Oskar, rather than grind a skew on the blade, use a straight blade on a skewed bed. For example, the Stanley/LN/Veritas shooting plane beds have a 20 degree skew. The advantage of this design is a blade without weak areas.

Sorry to double-reply, but... How would "weak areas" matter in a shooting plane?

[Kees Heiden]

Hmm, maybe I shouldn't do this from my head, so I could certainly be wrong. As I see it, everything gets stretched out when you put it on a skew in the plaining direction. Starting with a 45 degree bed and a 14 degree clearance angel, so the bevel is 31 degrees. Skew at 14 degrees. Cutting angle becomes 36, bevel 25 and clearance 10.
Just a quick calculation on my phone while sitting in the train, so please correct when neccessary.

[Patrick Chase]

Hmm, maybe I shouldn't do this from my head, so I could certainly be wrong. As I see it, everything gets stretched out when you put it on a skew in the plaining direction. Starting with a 45 degree bed and a 14 degree clearance angel, so the bevel is 31 degrees. Skew at 14 degrees. Cutting angle becomes 36, bevel 25 and clearance 10.
Just a quick calculation on my phone while sitting in the train, so please correct when neccessary.

All else being equal (cutting pressure etc) the durability of the edge is determined by its true geometry, not the cutting angle. The bevel is 31 deg in the example you give, period. We're not bending space-time here :-).

Of course all else isn't equal because as you say "everything gets stretched out", but that helps edge durability for two reasons. First, as you point out the cutting angle is reduced, which lowers total cutting force. In addition, that cutting force is spread out over 1/cos(14) = 1.03X as much edge, so even if the total cutting force were the same you would still see a 3% reduction in cutting pressure (as I said in another post, you need more like 30 deg of skew to realize significant benefits from this effect).

[Steve Voigt]

Hi again, I've been thinking about this. Why is it that the skew makes the edge more fragile? I'm thinking that the bevel angle is what it is, and the skew would rather make less stress on the edge, since it enters the wood gradually. How are you thinking about this?

Oskar,

I'll leave it to others to debate the physics; I can only tell you what I've experienced in practice. Skew edges are more likely to break down faster than non-skew edges. A 25 degree bevel will work fine for most things, but for shooting it is hard on a lot of blades. If you find that a blade doesn't hold up at 25, it's nice to have the option of increasing the bevel a little. If your blade is bedded at 38 or lower, it doesn't leave you a lot of room to play with.

I do think you are on the right track getting a blade from Phil. He has dealt with this problem and presumably he's found the right temper and hardness to make it work. And it's O1; I definitely would not screw around with D2 or A2 for this.