How hard is that wood?

[jmischel]

Is there a scale that determines how difficult a wood is to carve? The Janka hardness scale doesn't do it.

For example, I regularly carve mesquite, which has a Janka hardness rating of 2,345. I've carved purpleheart once, and decided that it's way too hard to carve. But its Janka hardness is listed variously from 1,850 to 2,100. Granted, the purpleheart is kiln dried and the mesquite varies from green to decades-old dead. But I've never run into a piece of mesquite that's anywhere near as hard to carve as the purpleheart is.

Perhaps it's the difference in how the wood is dried, but I have my doubts. I have oak here that's dried the same way as the mesquite. Oak hardness is in the 1,350 to 1,500 range, but I find it much more difficult to carve than mesquite.

The Janka hardness test measures the resistance to denting and wear. But it says nothing about how difficult a wood is to cut. The best I've been able to find so far is on woodworking sites that talk about the difficult of sawing the woods. Purpleheart has "moderate cutting resistance" and mesquite has "low cutting resistance." But "low" and "moderate" aren't very precise measurements.

I thought maybe the wood's density was the deciding factor, but purpleheart at 54 lbs/cu.ft. is only slightly more dense than mesquite (50 lbs/cu. ft.), and yet it's a whole lot more difficult to carve.

I'd like to think that somebody somewhere has done the research to determine how much force it takes, with a slicing cut, to separate wood fibers in different woods at different angles, with different bevel angles on the blade. But if the research is out there, I can't find it.

Somebody please point me at the relevant research before I get crazy ideas about setting up a testing jig . . .

[Mike L Hasler]

Jim

No idea of a way to measure the wood structure hardness that effects what we do. But I do see where you mentioned mesquite, I've done a few pieces in Mesquitee ROOT, some of which is extremely hard to carve or cut, believe it has to do with how long it's been in the ground, any way if you haven't tried it I'd be willing to send a small piece for you to try. Also I find osage orange as hard or Later, Mike

[LBlake]

I've looked, and never found anything quite like what you describe, either. However, I have a "wood identification and use" book that gives a number of characteristics, and the characteristics I've found useful are specific gravity and bending and crushing strength. It sometimes mentions whether wood likes to split, too - very helpful. It also often lists blunting effect on tools, which is somewhat useful. I also like to look for any note that the wood is oily, as I haven't carved an oily wood yet that wasn't nice to carve (even if it was very hard). This book I have also lists typical uses - when it lists carving as a use, it seems to be pretty right about that.
Of course, "good for carving" is a VERY relative term - I carve small, delicate spoons with palm tools, and I like to sand my spoons, so all that factors in. Someone who likes to carve more in-the-round, and who finishes off-the-tool (no sanding) would probably look for softer woods than I would. Then, power-carving and large-scale carving would look for still other different qualities. So.... not sure what kind of carving you're doing, but I hope all that somehow helps.

[jmischel]

Thanks for the comments, Mike. I haven't tried carving mesquite root. I have a couple of stumps here with roots sticking out. I'll have to cut one of those roots and give it a try. Somebody told me to be careful, though, because of the silica content?

I have some osage orange here. The stuff is incredibly hard. I already have plans to make a few spoons and other kitchen utensils from some of it.

[jmischel]

That's good info, David. Thanks. I've seen some of those references that list different properties, and it's nice to know that there's some useful information. I'll make a note to pay more attention to those items you mentioned.

As for type of carving, it depends. I've been doing spoons and other kitchen implements, although not as delicate or intricate as the work you do. I'm also working on some stylistic carvings, all of which I'll sand smooth like the spoons. And, of course, there are my little animal caricatures, which have a little more detail.

All of my carvings are done with edge tools--knives and small gouges.

It's nice to know that I'm not the only one who has trouble determining if a particular wood would be appropriate for a given purpose. I'll just keep on muddling through. Experimenting is fun!

[AlArchie]

Jim, you may be asking for an indeterminate figure. Slicing force/bevel angle covers a huge range, and may be impractical to codify. A straight "push" type slice requires more force than a slice that uses a sawing motion. And there are so many bevel angles used by various carvers that this seems impractical too. Factor in the sharpness of individual blades, the polish on the surface, the thickness or tyhinness of the blade and the tool variable factor goes way high.

Then the wood......water content, age of the tree, heartwood, all have effects on the hardness of woods in the same species. Looks to me like that Janka scale gives a pretty decent "relative hardness" scale.

Even among basswood carvers you run into folks who have had hard basswood and even REALLY hard basswood, and some that is too soft be favorable.

Al

[Claude]

Jim:
This sounds like a really good project for writing a scholarly research paper.

Claude

[jmischel]

The problem is that the Janka wood hardness test clearly does not give a good "relative resistance to cutting" scale. As I said, purpleheart has a lower Janka number (by 20 to 25 percent) than does mesquite, and yet purpleheart is much harder to cut.

Al, whereas I'm beginning to believe that the information doesn't currently exist, I don't see it as especially difficult to obtain. Properties of the wood (thickness, moisture content, etc.) affect the Janka hardness scale in the same ways you describe, and yet that scale is well regarded. Granted, the other factors you describe (cutting angle, blade angle and thickness, metal smoothness, etc.) add more variables, but that's okay. You can specify most of those variables and hold them constant. The Janka scale, for example, assmues a 12% moisture content. We don't need a definitive number to know exactly how many ft-lbs it takes to cut a particular piece of wood, but rather a reasonably accurate relative force number. I'd like to be able to say, for example, "It takes approximately 50% more force to cut purpleheart than it does to cut mesquite."

[jmischel]

Originally Posted by Claude

Jim:
This sounds like a really good project for writing a scholarly research paper.

Claude

Exactly. I was hoping that somebody had done it. Eero Kivimaa did the initial research in the 1950s for a PhD thesis, "Cutting force in wood-working", but he only used one type of wood. There are other papers that discuss measuring wood cutting force, but most often it has to do with sawing. Some discuss chip formation when planing, which would probably be a better measure for carvers. But these papers typically work with one type of wood (or just a few) and vary other things. I've yet to see the cutting force equivalent to the Janka hardness scale.

Search for "kivimaa wood cutting" and you'll get quite a few papers that reference his thesis.

I wonder if I could find some PhD student who's looking for a thesis project and convince him to design a test methodology and a test jig. Hmmm . . .

[Claude]

Ought to be relatively easy to design. For example, use a new blade for a box cutter, clamped into a couple pieces of wood. Place a test block on a slanted surface, the blade on it, pressure applied by a spring that holds the blade against the surface. Pull the blade (slicing motion) and measure the depth of the cut at the edge of block. This presumes one could measure a cut depth of 10ths or 100ths of a millimeter or so...

Maybe Robson Valley knows some grad students in forestry who might want to try it...

Claude