Lost Art Press

Author: fitz

Woodworker, writer, editor, teacher, ailurophile, Shakespearean. Will write for air-dried walnut.

Wood Bending by Kerfing

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The following is excerpted from “Volume II: The Woodworker: The Charles H. Hayward Years: Techniques.” As editor of The Woodworker magazine from 1939 to 1967, Hayward oversaw the transformation of the craft from one that was almost entirely hand-tool based to a time where machines were common, inexpensive and had displaced the handplanes, chisels and backsaws of Hayward’s training and youth.

This massive project – five volumes in all – seeks to reprint a small part of the information Hayward published in The Woodworker during his time as editor in chief. This is information that hasn’t been seen or read in decades. No matter where you are in the craft, from a complete novice to a professional, you will find information here you cannot get anywhere else.

– Fitz

A kerf is a cut made by a saw, and a number of kerfs cut in a piece of wood will enable it to be bent to shape without steaming. This process is known as kerfing and can usefully be employed in all types of construction.

The principle of kerfing is simple. A number of slots cut part way through a section of wood, as in Fig. 1, reduces the resistance of the whole to bending. When a solid beam is bent the inner fibres are subjected to compression and the outer fibres to tension (Fig. 2). In the case of a solid beam this limits the amount of deflection possible without failure of the section, unless the fibre stresses are relieved, such as by steaming. In the case of a kerfed beam, however, these stresses are diminished and the beam will deflect fairly readily until the individual kerfs are closed up as in Fig. 3. Obviously the extent of the bending possible will depend on both the depth and spacing of the kerfs, and also on the material itself, which is still subjected to compressive and tensile stresses, although to a lesser degree.

Kerf Width and Spacing. Broadly speaking, kerf spacing and the width of individual kerfs is more important than the depth of the kerfs. The greater the depth the more readily will the material bend, up to the limit where the kerfs close. At the same time, however, excessive depth of kerfing will weaken the completed work.

Taking a semi-circular bend as typical (Fig. 6) there are three possible arrangements. (A) employs a large number of closely spaced kerfs of fine width. These close up completely with the bend and provide a smooth finish on both sides of the bend. (B) also employs a large number of closely spaced kerfs, but of slightly greater thickness. Bending is somewhat easier and there is less risk of the wood splitting, but the kerfs do not close completely and, if the inside of the bend is to be visible, the surface will require facing. Both of these methods, it will be noted, employ deep kerfs and thus the over-all strength is not high. (A) is stronger than (B).

(C) shows even wider kerfs spaced farther apart. This is a more difficult bend to make, but has the advantage of greater strength. The outside curve, however, will not be smooth and will require sanding down. Partly to allow for this the depth of the kerfs is reduced.

In both (A) and (C) where the kerfs are closed on completing the bend there is an additional advantage in that glue may be run into the cuts before bending. When this has set the curved form will be capable of holding its own shape.

In considering the design of a part to be kerf-bent, the three main factors are the radius of the bend, the actual widths of the kerfs and the spacing of the cuts. The depth of the cuts can be considered as an independent variable which can be adjusted to give the degree of flexibility required. Normally a minimum value of at least three-quarters of the thickness of the material is chosen, provided the thickness of uncut wood is not reduced below 1∕16 in. minimum (Fig. 4). Kerfing, therefore, is only logically applicable to wood thicknesses of from 1∕4 in. upwards. Best thickness for good results appears to range between 5∕8 and 1 in.

A simple test bend on a spare length of material can be used to determine kerf spacing. Make a single saw-cut in the wood and from this mark off the radius of bend required. Lift the material up as shown in Fig. 5 and measure the amount of deflection that can be achieved before the kerf closes or the wood shows signs of fracture near the kerf. This distance will give you the spacing required for the kerfs for that particular radius of bend in that material.

Limits of Bend. It is important to note which limits the amount of bend—the kerf closing or the wood splitting. If the latter, another test bend should be made with a deeper kerf to give greater flexibility. If the former, it is possible that a wider kerf can be used and greater deflection obtained, so that the kerfs may be spaced out more widely. This will depend to a large extent on which of the three original types of bends is required (Fig. 6). If the kerfs are not too close on the final work then this fact must be allowed for on the test bend, when naturally the required spacing will be closer.

Tables can, and have, been prepared giving data on kerf widths and spacings for different radii of bends, but these as a general rule have their limitations. Each thickness of wood demands a separate table and practical variations may also be introduced by the mechanical properties of the wood itself. Hence the test bend method is generally preferable.

Uniformity. In cutting the kerfs, great care is necessary to ensure uniformity. Width is controlled by the thickness of the saw blade, or rather, its set, but correct and uniform depth is of considerable importance.
Any individual kerf which is either too deep or too shallow will result in corresponding weakness or excessive resistance to bending at that point and deform the finished curve.

Kerfing by means of a handsaw, therefore, is an intricate business and must be carried out with extreme care and patience. With machine tools suitable stops can be arranged to ensure uniformity.

For equal curves, equal spacing is required, but where compound curves are attempted, such as a large radius curve rounding into a small radius curve (Fig. 7), spacing should become progressively closer. Ideally, of course, spacing should be directly related to the curve radius, as with the test bend. It is usually sufficient, however, to design for the smallest radius bend and open out the kerf spacing uniformly on either side of this bend. Alternatively, the problem can be dealt with more scientifically by reducing the whole bend to a series of adjoining circular arcs and determining the appropriate spacing for each radius. There is yet another method where the same spacing is retained throughout, but on the shallower curve the depth of the kerfs is reduced.

Normally once the kerfs have been cut the curve can be bent “dry,” In the case of acute curves, however, bending should be tackled in stages, bending first to say twice the required radius and clamping the work in that position for an hour or so. It also helps to sponge the wood down with warm water to prevent splitting.

There is also another form of lengthwise kerfing which is sometimes used where the kerfs are cut endwise in the wood (Fig. 8). The sawcuts are filled with veneer or similar material, the whole steamed or soaked in hot water and bent around a suitable former and clamped in position until dry. It can then be unclamped, the slots and veneer glued up and re-clamped.

Resources for Making a ‘Jennie Chair’

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A chair made by Jennie Alexander, author of “Make a Chair from a Tree.”

We’re proud to offer two resources for making Jennie Alexander-style chairs: The third edition of “Make a Chair from a Tree” and the how-to video Jennie recorded in 1999. We’re also fans of a new video from one of Jennie’s long-time collaborators and friend, Peter Follansbee.

MACFAT: The Book
In 2014, Jennie Alexander somewhat reluctantly agreed to a third edition of her 1978 seminal book on green woodworking, “Make a Chair From a Tree” (MACFAT) – a book that launched the careers of thousands of woodworkers and helped ignite a green woodworking movement in this country.

Her reluctance wasn’t due to a lack of passion for the book’s subject – the simple but gorgeous object that we now call a Jennie Chair had been an obsession of hers for decades. She simply didn’t know if she was physically and mentally up to the task of essentially starting from scratch on a new book – she had learned so much since the first two editions were published that this is an almost entirely new book.

Thus, “Make a Chair From a Tree: Third Edition” is the culmination of a lifetime’s work on post-and-rung chairs, covering in detail every step of the green-wood chairmaking process – from splitting and riving parts to making graceful cuts with a drawknife and spokeshave, to brace-and-bit boring for the solid joinery, to hickory-bark seat weaving.

With the help of Larry Barrett, one of her devoted students, she worked on this new version of the book until just weeks before her 2018 death. Larry polished Jennie’s final manuscript, then built a chair in Jennie’s shop using her techniques and tools as we took many of the photographs for this book. Nathaniel Krause (another of Jennie’s devoted students), wove the hickory seat for this book. Longtime friend and collaborator Peter Follansbee helped to edit the text into the intensely technical (but easy to understand) and personal (but not maudlin) words that ended up in this third edition.

We know Jennie would be delighted by the contributions from the people she taught and who, in turn, inspired her. (Though we also suspect she’d say we should just start rewriting the book at the beginning…. again.)

MACFAT: The Video
The 1999 video “Make a Chair from a Tree” is based on Jennie’s book and the courses that she taught. Jennie taught post-and-rung chairmaking for 25 years at her shop in Baltimore and around the country. During that time she refined and simplified the process, and those improvements are incorporated into this video. The construction of the chair incorporates wet and dry mortise-and-tenon joinery and interlocking joints that result in an elegant but simple chair that will last for years. 

This two-hour video, produced by Jennie, Anatol Polillo and ALP Productions, takes the viewer through the entire chairmaking process, beginning with a log, then riving and shaving the posts, rungs and slats. You will see all of the techniques and methods needed to create a chair, including steam bending, horizontal boring and seat weaving. The entire chair is built using only hand tools and traditional techniques.

It also comes with a pdf (delivered at checkout) that shows many of the jigs and part sizes discussed. Lost Art Press offer the video via streaming; a DVD is available through ALP Productions.

‘Making a Jennie Alexander Chair with Peter Follansbee’ Video
In his new video, “Making a Jennie Alexander Chair with Peter Follansbee,” Peter takes a deep dive into all the details of making a Jennie Chair, and shares some of his personal history with Jennie. Peter has been making and teaching these chairs for decades, and has developed his own approaches and touches to the form Jennie popularized, including some improved techniques. Plus he shows how both how to harvest and process hickory bark, and how to use it to weave the seat. It’s almost nine hours of detailed video instruction.


Mechanical Library: Miscellaneous & Translations

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We sure have a lot of sundry/miscellaneous/mixed/jumble shelves. Could be worse; at least we don’t shelve books by color. (We do, however, shelve a few by size; we’ll visit that lower-right corner soon).

On the far left of this bay is most of our “Mortise & Tenon Collection,” both some issues of the magazine and the two books M&T has published (between us, we have all the issues, just FYI). When Joshua Klein first mentioned to us he was thinking of starting a magazine both Chris and I independently told him not to do it, then shared with him everything we could think of about doing it right. Good thing he didn’t listen to us (about not doing it; he’s certainly doing it right).

And then comes the jumble. We have two copies of Edward F. Worst’s “Coping Saw Work” (First published in 1927 by Bruce Publishing). This book is, according to Chris, a “cool representation of the humidor or turned pens or epoxy river table of the early 20th century.” I other words, it’s a slice of a small point in history when coping saws were the hottest thing. And I gotta say – the Minecraft-looking animal patterns are pretty cute.

But if it’s toys you’re looking for, the compilation book “Sunset Woodworking Projects 1” (Lane Publishing, 1987 – first published in 1975) is chock-full of pre-computer-chip offerings for the little ones and simple projects for around the house. Chris has it not for the contents so much as for the layout – it is easily identifiable as of a certain period in book design history, which is always useful when thinking about how things should/could/shouldn’t look.

Next to that is Yannick Chastang’s “French Marquetry Furniture: Paintings in Wood” (Wallace Collection, 2001). Chris met Chastang in London (he thinks it was London) and was impressed with a talk he gave, so of course he bought the book. In between that and another marquetry book is a “hidden” copy of “Grandpa’s Workshop” (I think it’s our last copy, and we don’t want it at easy grabbing level). The other marquetry book is Richard Mühlberger’s “American Folk Marquetry” (Museum of American Folk Art, 1998); it reminds us of a lot of Kentucky furniture (for which both Chris and I have a great fondness).

Michael Dunbar’s “Federal Furniture” (Taunton, 1986) is an ex-library copy and was too good a buy to pass up (and though neither of us are huge fans of the style, it does deserve representation in any woodworking library). Then we’ve a gifted copy of “Strait’s Chinese Furniture: A Collector’s Guide,” by Ho Wing Meng (Times Media, 1994).

Then we’ve Graham Blackburn‘s “Traditional Woodworking Hand Tools” (Lyons, 1998) and “Traditional Woodworking Techniques” (re-published by Blackburn Books in 1994). These are “must-haves” for the hand tool woodworker; Graham was writing about hand tools and teaching their use when just about no one else was. (An aside: Graham is among the nicest guys I’ve met in woodworking, and is also an excellent musician and dancer!)

Thomas Day: Master Craftsman and Free Man of Color,” by Patricia Phillips Marhsall and Jo Ramsay Leimenstoll (North Carolina Museum of History/UNC Press, 2010) is a landmark book. It’s the first significant book on a Black furniture maker and his contributions to the craft, the first to acknowledge that Black people didn’t historically just work in menial positions in the United States, but were skilled tradespeople with important jobs.

From there, we move into the tool section of our collection (for the most part) – and it continues on several other shelves still to be covered. First up is a trio of books edited by Jane and Mark Rees: “Christopher Gabriel and the Tool Trade in 18th Century London” (Astragal, 1998) and both editions of “The Tool Chest of Benjamin Seaton” (Tools and Trades History Society 1994 and 2012).

Then we have “The Tools that Built America,” by Alex W. Bealer (Bonanza, 1976). “Someone told me I had to have it and gave it to me,” says Chris. “I flipped through it and was like, meh.” (But not so “meh” that we’ve passed it on to someone else, yet.) Alongside it is “The American Patented Brace, 1829-1924: An Illustrated Directory of Patents,” by Ronald W. Pearson, D.O. (Astragal, 1994) and “The Rule Book: Measuring for the Trades” by Jane Rees and Mark Rees (Astragal 2010). I dipped into these books that are ostensibly for collectors quite a lot when I was editor of The Chronicle, Chris loves them not only for the history of toolmaking, but because “they show things that are missing from the written record – it’s written in tools, not in words.” And that holds true for the rest of the tool-related tomes in this bay.

Tucked into those is Robert Wearing‘s “Hand Tools for Woodworkers” (Sterling, 1996). That should probably be with Wearing’s other books. Then it’s “Source Book for Rule Collectors,” by Philip E. Stanley (Astragal, 2003), “The Art of Fine Tools” (Taunton, 2000) and “Tools Rare and Ingenious (Taunton, 2004), both by Sandor Nagyszalanczy. If you like nice-looking tools, these are must-haves. Peter C. Welsh’s thin but invaluable “Woodworking Tools 1600-1900” (Smithsonian, 1966) is a must for the tool-history lover.

David R. Russell’s “Antique Woodworking Tools” (Conti, 2010) documents an insanely good collection (in a well-made wrapping), and serves as inspiration for our own tool and book making. It’s just gorgeous to look at. “A Dictionary of American Hand Tools,” By Alvin Sellens (Sellens, 1990) is among our many tool dictionaries – books that we as teachers and toolmakers count as must-haves. The same holds true for Aldren A. Watson’s “Hand Tools: Their Ways and Workings” – but with a grain of salt, as he recommends everyone have a jack rabbit plane. Huh. But he’s masterful illustrator; the drawings are an excellent look at the way tools go together. “Codes & Symbols of European Tools,” by Laurent Adamowicz was too intriguing to pass up. “I’m always interested in tools, and that was an aspect that I’ve never seen covered. But it didn’t quite live up to the title,” says Chris.

And finally, we have the German and French editions of several of our books, and one of Chris’s earlier books for Popular Woodworking. (The right side of this bay was covered in an earlier installment.)

– Fitz

This is the eighth post in the Covington Mechanical Library tour. To see the earlier ones, click on “Categories” on the right rail, and drop down to “Mechanical Library.” Or click here.

Dust Collection in the Horse Garage & Bench Room

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One of our two portable “Vortex Cone” Jet collectors. These are due for a good cleaning on the first nice spring day. That means taking them apart and sucking out the filter folds with a smaller vacuum. Not my favorite job.

From time to time, we get asked about our “professional setup” for dust collection. It’s not what you might expect from a (semi) professional shop…or maybe it is exactly what you expect, given that we’re known for hand-tool stuff.

But we do use machinery – particularly for stock prep and rough cuts. Plus, there’s a fair amount of sanding in making stick chairs. Our machine shop (aka “The Electric Horse Garage“) is, however, quite small; there’s no room for 6”-diameter piping or a large cyclone. Instead, we have two Jet Vortex dust collectors for the large machinery. One is hooked up to the jointer and planer, with manual gates that we open/close to direct the suction to the machine in use. The other is for the table saw. I don’t know about Chris, but before I turn on any of these three machines, I first poke the bag to make sure the dust level isn’t too high. Trying to birth an overfull garbage bag full of fine dust between the uprights is a bear, especially on the table saw (that dust is heavy!).

An off-the-shelf solution (we had no room for another large portable unit).

For the spindle and belt/circular sander, we have a small Ridgid unit hanging on the wall (which reminds me – I’d best check it given last week’s chair class; the collection bag doesn’t hold much).

We are very bad about checking the collection bag in this one. My one New Year’s resolution is to never again have to pry out impacted dust and shavings. At least not in front of anyone next time.

On handheld machines such as the random orbit sanders and the Domino Joiner, we hook up Festool dust collectors (I bring mine in from home during classes so that we can set up two stations). Key is the addition of a 98-cent hose clamp to keep the collar from slipping off the machine’s dust port. (You can just see it in the image above.)

In the bench room, the only stationary(ish) machines we have are two bands saws. One, Chris’s venerable Delta/Rockwell, predates dust collection ports, but on the new Jet band saw we hook up the shop vacuum if we’re making anything more than a short, quick cut. Might as well go through the teensy bit of trouble of hauling out the vacuum, given that we use that same vacuum to clean up the machine and floor after using the band saw. Every time.

For larger stuff, let it never be said you can’t find a broom around here. We have plenty – all from Berea College in central Kentucky.

– Fitz

P.S. This is Chris, chiming in. This year we are going to add an electrostatic air scrubber to our machine room. We are serious about keeping the dust down. Also, the only sanding on the stick chairs is on the saddle. I sand about 5 minutes per chair. Beginners have to sand more. So it can seem like a lot when six beginners build chairs.

Summer 2023 Storefront Open House

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Typically when we announce an open house I get three or four complaints along the lines of “why didn’t you let us know sooner.” So for those three or four people:

The 2023 Summer Open House at the Lost Art Press storefront is on July 29. We’ll open the doors at 10 a.m. and lock them behind you at 5 p.m. We will probably have a special gift, guest demonstrations, etc. But I don’t know anything more than the date at this point. (And we will of course let you know more when we figure it out.)

– Fitz