Chris Schwarz's Blog

The Science of Rented Mules (aka Chisels)

Sometimes when I
write, I come to a question that paralyzes me – and I cannot continue
until I get it answered. As a newspaper journalist, I was trained to
“write my way around the problem,” which probably is one reason I don’t
work for newspapers anymore (the other reason: there aren’t any
newspapers).

A few days ago I was writing about chisel handles.
Blah, blah, blah. Tang, socket or Japanese. Yadda, yadda, yadda.
Ferrules, bolsters and hoops. Gabba, gabba, gabba. Best types of wood
for a handle.

And pause.

This is a good question: beech,
hornbeam, maple, white oak, hickory, something else? And what can one do
to delay a handle from splitting? Use a mallet that is softer than the
handle? A mallet that is harder than the chisel? Perhaps the mallet
doesn’t matter?

So every evening I’ve been diving deep into the
research available on the impact resistance of woods. What makes some
woods better for beating and others not so good? I have yet to turn up
much science that is really helpful to me. There is a lot of data out
there that relates to baseball bats, but most of that relates to woods
being beat on the side grain or face grain – not the end grain.

There is this article
from the Royal Society that has some interesting data, but it doesn’t
really answer the question when it comes to end grain impacts. Bruce
Hoadley’s book “Understanding Wood” deals with wood failure parallel to
the grain, but not about wood impacting wood. Where does the energy go
during the impact? What happens if the chisel handle is softer or harder
than the striking tool?

I must be missing something out there.

So if you are an impact nerd, could you drop me a line? I’d really like to get this sorted out so I can get on with my life. Thanks in advance.

— Christopher Schwarz

Resources Related to this Post
• David Charlesworth’s DVD “Chisel Techniques for Precision Joinery” is excellent. We need more information on chisels in the world.

• Here is one of the best free links ever. The Forest Product Laboratory has just released its 2010 “Wood Handbook.” You can download the whole thing for free from the U.S. Forestry Service’s web site. Go here. Bookmark it.

37 thoughts on “The Science of Rented Mules (aka Chisels)

  1. Mike

    In contrast to sweetgum, the FPL Wood Handbook lists black locust as "vedry heavy, very hard, very resistant to shock, and very strong and stiff". It was used as insulator pins (?) and wooden pegs in the construction of ships. It sounds like this wood has desirable characteristics for a handle or mallet.

  2. www.google.com/accounts/o8/id?id=AItOawnu9I56BuXyCbcAePPK3wHorYOPRzgJ_k4

    "Mark’s Standard Handbook for Mechanical Engineers" Chapter 6 Section 7 has decent discussion on the mechanical properties of different species of woods. Of particular interest would be Table 6.7.2 which has values for shear strength parallel to the wood grain. All the traditional suspects are there.

    The chisel handle failure mechanism is buckling delamination. The lecture notes on this particular area for Dr. John Hutchinson’s Fracture mechanics course at Harvard are available at: http://imechanica.org/files/Lecture11-12.pdf (see slide 5) if you want way too much detail.

    To roughly and poorly summarize:
    The impact of the mallet on the chisel handle imparts energy into the handle. This energy is stored in the handle in the form of strain energy. The handle gets hit, deforms (strains), and stores energy like a spring. If the strain energy stored in the handle exceeds the critical strain energy for the material then a crack will form. If a discontinuity (crack) already exists in the material then the critical strain energy for that material is lowered. Leather washers on handles absorb some of the energy from the mallet and prevent it from transferring into the handle. Hooping with metal rings would increase the critical strain energy by pre-straining the material (adding a sort of negative strain energy).

    Once a crack is formed it starts propagating through the material and the stored strain energy is released at a rate proportional to the rate of crack propagation.

    Of course the bulk of the energy should be transfered through the chisel handle to the blade, and from the blade to the work.

    regards,
    Matthew
    (who, unfortunately, studies this sort of thing for work these days)

  3. JOS

    I am new to woodworking and so if this out to lunch forgive me.
    One aspect of the wood that I have not seen discussed in this thread is moisture content. Are we letting the wood in our hand tools dry out to much?

  4. Steve

    I think the principal reason that wood handles fracture is that wood is an inhomogeneous material. When a stress wave from an impact propagates through the material, it is going to preferentially concentrate at stress risers, which typically occur at places like the end of a crack. In hardwoods, radial ray cells are often rather loosely held in the wood matrix, and so they act like cracks (this is most obvious in oak, where you can sometimes lift a ray in quartersawn wood with your fingernail).

    The way to avoid fractures is to avoid stress risers, and the way to do that is to have the handle material as homogeneous as possible, and without any sharp corners, etc. In fact, I think the real reason the acrylic-infused handles from Blue Spruce are so tough isn’t because the acrylic is itself phenomenally strong, but rather because it homogenizes the stress/strain characteristics of the wood, so that the impact forces are uniformly absorbed throughout the handle.

  5. Ron Herman

    Chris ; In the shop ,over the years,the three woods that can take the most abuse are black locust,hophornbean(ironwood),and center cut hickory.My limited typing skills dont suit me for a long answer ,but grain acclimation is very important as well as tight growth rings.Its easier for me to show the difference than try to write it. Great topic though. Ron Herman

  6. Jerry Curry

    Just to confuse the issue some more — Lie Nielsen’s handles are actually hophornbeam (Ostrya virginiana) not hornbeam (Carpinus caroliniana). They’re both pretty hard, but if you’re looking them up, you want to get the species right.

    Jerry Curry

  7. John Walkowiak

    I am not so sure the problem is with the handle material. The most obvious issue in the photo is the flat on the top of the handles. As has been brought up, they should have a slight crown so the impact is only in the center of the handle. Antique English mortise chisels had a thick leather washer between the bolster and handle. This would absorb shock as well as even out the shock, as it would be nearly impossible to guarantee that the bolster and handle would be perfectly flat and mating exactly. I don’t see this on your chisels. The split mallet looks to be a glue joint failure. A correctly glued joint should be stronger than the wood.
    The next question is why so much force is needed? Is the workpiece secured directly on top of a bench leg, and is the bench leg sitting on a hard surface? If not, the workpiece will bounce and the mallet strike diminished and you have to strike harder to get the job done. I assume your chisels are sharp, but some woodworkers think mortise chisels don’t have to be as sharp as a bench chisel because you can just hit them harder. It should be just as sharp. The chisel bevel in the photo looks quite short, shorter than on my antique chisels. It may be too short for the amount of waste you were trying to remove, necessitating harder blows. Good luck!

  8. Christopher Schwarz

    Sometimes from the comments I can see that the original intent of my post has gone awry. Maybe I should go back to working for newspapers for a refresher in writing clearly.

    I don’t blame the chisel handle for breaking. Mortising is violent stuff. I don’t blame the mallet for breaking. It beats *all* my chisels.

    I also don’t blame myself. I take care of my tools.

    I am interested in the factors that make a good chisel handle. I’ve gotten a lot of great data off-line from this post, so I’m happy; but I don’t want people to get the impression that I hit my chisels with a framing hammer. Or I don’t sharpen them. Or I drive metal wedges with a mallet. It’s just not the case.

    Chris

  9. Al Navas

    Correcting a typo…:

    On the other hand, if we don’t have sufficient *data* yet, could it be that in a few years we will learn that impregnating resins was *not* the thing to do?

    Corrected: "date" => data, and added the word "will" (…we will learn…)

  10. Al Navas

    Resin-impregnated woods in woodworking – a question:

    QUESTION:
    Has anyone reported failure of these when pounding with a mallet?

    If not, is the technology so new that we don’t have a broad enough data base to date?

    If handle breakage *is* an issue, as Schwarz’s post states, maybe the answer lies in simply applying the resin impregnation technology to *solve* the problem.

    On the other hand, if we don’t have sufficient date yet, could it be that in a few years we learn that impregnating resins was *not* the thing to do?

    Al Navas

  11. Ken

    Just as another data point, the Japanese Woodworker catalog lists chisels with handles turned from oak branches so that the rings are concentric around the axis of the handle to increase durability. This intuitively seems to make sense and I have made a few handles from magnolia, sycamore, and oak using this technique, but haven’t noticed any particular difference in impact resistance. The obvious answer to handle breakage is the Stanley Everlast (or later incarnation No. 60) chisels with solid steel from the striking end to the cutting edge. Personally, I just don’t like the way they feel or sound when being whacked.

  12. Devin

    I agree with George, no matter what the task is, if I find myself either working too hard or breaking tools its time to re-evaluate how I am performing the task. Is there an easier way? John mentioned it as a safety valve I think thats a good way of thinking of it. When a tool breaks it is either do to excessive force just prior to its breaking, or excessive force accumulated over time until it fractured.

  13. George Walker

    A couple of thoughts. Burl was commonly used for traditional mallets. It’s not uncommon to find them worn to an hour glass shape from millions of blows, but seldom split. Is there a point where it makes sense to use a drill to horse out material rather than a chisel? I use a chisel for most work, but turn to a drill when the work is deep or wide. Unless the handle was defective, I’d guess you are pushing it (and yourself)too far. Most of the time when I break a tool, it’s me not the tool.

    George

  14. John Snyder

    At a certain point we need to recognize the value of wood splitting. It’s an important "safety valve" mechanism that protects the steel. I’m a woodworker, not a blacksmith…I can repair handles.

    John Snyder
    newadventuresinwoodworking.blogspot.com

  15. Sean

    - the seating of the tang (i.e., if the fit is too tight, you have a internal wedge issue)
    - the the fortuity of the grain of the handle (as with plane totes and saw handles, it matters a ton as far as duarability)
    - the presence of any cushion at the bolster (soem have leather washers which I always assumed was to keep the wood from impacting the steel bolster as hard)
    - the type of mallet used (i.e., a wood is good urethane mallet is not as likely to break a handle as a hickory one)
    - how the chisel is being used (i.e., trying to take too large bites in too hard a wood by beating the living crap out of the chisel – multiply if the chisel is dull)

  16. Al Navas

    Chris,

    Your question is important from a life-cycle standpoint. I wonder if the development of using resins to impregnate wood, such as used by Czeck Edge and Blue Spruce, might be a long-term solution?

    Maybe these wood-polymer composites are the answer to durability of the chisel handles.

    Table 19-5, page 442 of the PDF file provided by Forest Products Laboratory shows increased wood strength and hardness.

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