14 August 2006
Day 3: More on bending; mortising; slat theory
For bending, you’re looking for straight grain, it doesn’t matter if it’s flat-sawn, rift-sawn, or quarter-sawn. The face (front) of the bent leg should be flat sawn.
For cherry and walnut, rung precision is critical if you’re also using them for rungs. For rungs, Boggs said soft maple is too soft, but hard maple’s okay. For one-species chairs, he recommends: oak, hickory, ash, cherry, hard maple. He’s never tried exotics for rungs.
Boggs uses Titebond 2 as an end grain sealer on wet wood after cutting off any visible checks. He used to use anchorseal, but it can be VERY slippery on (and is difficult to remove from) concrete floors, and was just too hazardous (because of the slipperiness).
About the sample one of his chairs brought in, in curly hickory: “This chair is a gnarly, difficult wood and I thought it was just perfect for its owner.”
Tenons at the end of each seat slat are 22-30 degrees off the chord line between the mortises. Both tenons need to be the same angle. If your seat slat is unevenly bent, try moving the center of the slat to the left or right to adjust the curve (this is one reason why he cuts these oversize — to givee som play room).
If the curve of the slats are too deep, it pinches the shoulder. If you have to err, err on the side of making the curve too shallow. For a post-and-rung chair, 25 degrees works well. Oh, and to straighten out a slat: push down. That’s all, really. Be careful NOT to push down before the chair’s assembled.
Cut the slats to different curves so all four tenon angles are the same; otherwise each slat would have different angles.
Boggs trims the back rungs after assembly to get a drop in the rear seat as that is more comfortable than a seat that’s straight across.
For the chair back above the seat, 95 – 100 degrees from the seat is typical.
18-19″ above the seat is as tall as you can support with slats. Thus, the useful height of a dining chair is 36″, but this height is perceived as “not majestic enough” for dining room chairs. When he added a third rung (and 2″ overall), Boggs started selling more chairs at a time, because they weren’t just settin’ chairs (as he calls them), but being used as dining room chairs.
As Phil from South Africa put it: a shorter chair is less formal.
After that, we all got busy chopping our leg mortises. Boggs did (in about 10 minutes, and giving explanations at that) what it took me two hours to accomplish.
2 p.m. “Time for Chapter 14. Put all your weapons away and allow me to be the center of attention.” We all put our mortising chisels down. 🙂
And then a long lecture that I won’t do justice to commenced. I’ll do my best, and I’m writing this down as much as for me as for you, because I need to know how to reproduce it when making additional chairs later.
Note before proceeding through numbered steps: Cut and fit your top slat before starting on your bottom slat. If you don’t do it that way and make an error in your top slat, you can’t easily make it into a bottom slat unless you’re making multiple chairs.
So how do you know what angle your seat slats are?
1) Draw a line that’s the spacing of the chair rungs. Put the slat on there until it bisects the line at the same length as the rear tenon. Draw the first bit of the inside curve, then measure the angle of that curve at the point at which it meets the line (on each side; they may be different). Note that it’s got to be the beginning of the curve, because the angle gets more acute as you get further from the line defining the chord.
1a) Now, measure that angle. For our purposes, it was 25 degrees, so when you see that number in the following instructions, you can substitute your own result. Draw it onto the paper, then also draw a right angle outward. This should be where your legs fit.
2) Next, make a jig. It should hold the legs so that they’re splayed at the correct angle (in our case, 25 degrees) so that the leg mortises will come in square to the leg. (If you look at the [url=http://flickr.com/photos/muhe-e/sets/72157594229899329/]paper diagram in my flickr photostream[/url], you’ll find it easier than if I explain it) Clamp the rear legs in the jig, and then you’re ready for the following half-billion steps.
3) Measure inside depth between legs at the bottom front of each mortise. Easiest using a folding extension rule. Don’t measure the depth of the mortises at this point, just how far it is between them.
4) Transfer the lengths onto paper.
5) Draw 25 degree angle (facing the starting one) for each slat length.
6) Find a good location on each slat for the curve and mark the slat ends from that line. This is just a starting point for layout.
7) Draw a line through each mark and perpendicular to slat bottom. These aren’t final lines, but the bottom corners of the slat will be somewhere along these lines.
8) Mark each mortise with a letter and carefully measure its height and depth. For depth, make sure you get the minimum depth. Since the leg is 1-5/8″ wide, in an ideal world, the mortise would be between 1 and 1-1/4″, however most of us didn’t get that far in the stated time (especially me, I was the slow mortiser).
9) Determine where on the slat you want the top and/or bottom. In part, this will depend on how much “belly” curve you want (Boggs uses 3/8″), and how much curve you want on the top (Silly me, I used 3/8″ there, too. Bad me. Tired me.)
10) Mark the starting point on the line where the slat bottom begins or ends. The slat top will depend on the mortise height; there is no shoulder on this mortise (so you’ve got to be very, very good).
11) With the legs in the jig and a slat centered on marks made in step six, and holding the slat lined up with the bottom of the mortise, have a partner mark each side’s bevel angle on the back. Set your bevel gauge to one and check the other. If they’re the same, you’re golden. If not, re-set your bevel gauge to split the difference between the two angles.
12) Take that bevel gauge and mark from the bottom of each slat upwards the mortise height on each side (I didn’t ask what to do about uneven mortises, btw. Chop better.)
13) Then mark the mortise depth on the outside of those lines to form the tenon.
14) Take two spring clamps and a flexible stick (like a dowel) and lay out the top and bottom of the slat profile. The profile for the top should extend to the end of the mortise. For the jig that occurred at the tenon’s bottom, it should be removed because it can break off and possibly cause a worse split.
15) Cut the slat using the method of your choice (bow saw, coping saw, band saw, drawknife, alien space ship…). For Boggs, the drawknife is faster than the band saw….
16) Shave off about .002″ off the tenon face on each side. Remember that curve? The tenon isn’t flat, but the mortise is. Shaving the face will help correct for that. When I say “shave,” I mean using a block plane, not a spokeshave. If you’re really good with a spokeshave, you certainly could do it this way.
17) Shave down the back until the slat fits into the mortise (and trim the tenon tops and bottoms if required, too.). You fit slats from the back for the simple reason that convex surfaces are easier to plane. Repeat with other tenon.
18) Repeat with other slat.
19) Now they should fit into the rear legs and when the tenons are seated to the depth of the lines drawn in step 12, your rear legs should be as far apart as the rungs want them to be. However, chairs being chairs, they may no longer be at the 25 degree angle you started with. This is fine. From this point on, we’re dealing with reality, not theory. As Boggs said when another classmate faced this problem: “According to the chair, the jig is wrong. But you can’t trust those chairs,” he added with a grin.