Philadelphia Stool Strength questions

Re: Philadelphia Stool Strength questions

William R. Duffield on the Cohansey

>I think ³/₁₆" is insufficient. I would decrease the height of the tenon to provide a ³/₈" high shoulder. I assume that your "1.25" thick tenon" is really a 1 ¹/₄" HIGH tenon, and the thickness is really nearer to ¹/₂" or ³/₈"?

Using traditional joinery, no shoulder is required on the bottom of the tenon, and a very minimal shoulder is required on the inside of the rail. Of course, this doesn't work with a loose tenon. To my way of thinking, you lose a lot more than you gain by using loose tenons, because the artistry of one of these stools is built on sound craftsmanship and on the understanding of the strengths and weaknesses of the materials and joinery.

Note that the difference in strength of a loose tenon vs. an integral tenon is all in the apron or seat rail, not in the leg post. All the failures I have seen in this type of construction is in the leg post, not in the rail. Therefore, using a loose tenon should not be a major issue.

I don't understand what you mean by a "butt joint". Do you mean the shoulders of the seat rails? If so, again, I would not be concerned. As long as the joinery is tight and sound, the only significant stresses on the shoulders is compressive. No glue is needed in this part of the joint. In fact, if you try to put epoxy between the shoulder of the rail and the side of the leg post, you will get a visible glue line, because epoxy requires a little room to make a strong joint. No reason to use epoxy, unless you are making a stool for outdoor use, or for the pool or shower :^)

A commonly used technique for strengthening leg to apron joints, in chairs, tables and cases on short legs, is glue blocks. These might give you an acceptable compromise and let you get away with using the loose tenons. I find it easiest to make two blocks per corner. The thickness of the first one you install is exactly the difference in thickness between the post and rail. Then, the second block is glued to the other rail, the leg post and the first block. Use hot hide glue to attach them. It makes it a lot easier to disassemble everything if you ever need to make repairs. Glue blocks also require that you trim the inside ends of the pegs flush with the leg posts.

Re: Philadelphia Stool Strength questions

Andrew Alcazar in Charlotte, NC

>I think you are thinking that this looks like a traditional seat. It doesn't. The legs actually attach with pegs into the seat. It is an oval stool made out of 4 pieces of mahogany 4" wide X 2.5" thick by 19 and 8.5" long. The tenon is 1.25" thick, 4 inches long, and 2.5 inches wide (it lays flat not verticle. The shoulders are on the top and bottom. The top shoulder is thin, and the bottom shoulder is thick. The force would be applied compressive to the shoulders. It is not like post and rail construction at all. The legs attach through a 7/8" hole drilled through the tennon and long rails . I wish I could scan and post an image from the book.. but copyright and all. Did I mention the four pieces get cut to oval shape?

- Andrew

Re: Philadelphia Stool Strength questions

William R. Duffield on the Cohansey

>Sorry, I did not intend to confuse you, Andrew. It seems I do not have the Taunton Press book that contains the plans for your stool. I have several Taunton books with 18th Century furniture in them, including Greene and Vandal, but I guess I'm missing that one. If the article is a reprint from FWW, and you know what issue, there is a chance I may have the original FWW.

In the meantime, from your description of the construction, I think loose tenons, approximately the same size as the integral tenons in the original, should do you just fine.

The only potential concern I would have with the stool you describe (or at least that I now think you are describing) would be if you undersized the round tenons in the top of the legs, or drilled into the legs to insert dowels, instead of turning the tenons in the top of the leg stock. Does your stool have round, turned legs, or carved, cabriolet legs.

Re: Philadelphia Stool Strength questions

Andrew Alcazar in NC

>I'm not gonna mess with the legs. Those will be original to the plan. It calls for ball and claw cabriole (I can't believe I can't remember how to spell that.. my dad drove one for years). The tennon on the leg is turned to 7/8 in. It fits into a hole drilled through the tennon on the long side of the oval. I guess it acts as a giant peg for that tennon. I was going to peg the short side of the oval... which on the plan is an integral tennon. I think that alot of the strength comes from the fact that the knee blocks for the legs spread across the joint. One knee block on the short side and one on the long. That probably takes much of the torsion on the stool. Based on this... do you think epoxy is still overkill? Thanks!

- Andrew

Re: Philadelphia Stool Strength questions

Bill Houghton, Sebastopol, CA

>The major difficulty with epoxy is that you'd better get it right the first time, because it's not reversible.

Bill, trying to type with a cat putting her nose in the keyboarjasfadgasd

Re: Philadelphia Stool Strength questions

Andrew Alcazar in NC

>My other choice would be PVA... so it doesn't really make a difference. I don't have a heated shop, so I've never gotten into hide glue. When its real cold in the winter (well real cold for Charlotte... hide glue isn't real useful). The 60 minute open time is really intriguing on epoxy.

- andrew

Re: Philadelphia Stool Strength questions

Mike Stallard

>Andrew,

I don't have a copy of the 18th Century Style book, but I assume that the stool you plan to build is the one designed by Randall O'Donnell and featured in the April 1999 issue of FWW (No. 135). I congratulate you on your taste. That is a very pretty little stool, and looks like it would be a blast to build.

I am a little concerned about the structural effects of some of the changes you are planning in the design, though. My concerns generally center around replacing the integral tenons in the frame with 1 1/4 inch thick floating tenons.

1) Why such a thick tenon? The stock the tenon is mortised into has a rough dimension of 2 1/2 inches, but is reduced to 2 1/16 inches by the seat rabbet. Using the common rule of thumb for mortise thickness (1/3 to 1/2 of stock thickness), the tenon shouldn't exceed about 1 inch in thickness. O'Donnell's plans (FWW #135) call for 3/4 inch thick tenons. The concern here is with the mortise wall strength, not the tenon strength. Post and rung chairs, which carry similar loads, are almost all built with 5/8 inch or 9/16 inch tenons without problem.

2) From your description, it sounds as though you plan to center the mortise in the rough stock. With the thick tenon and seat rabbet, that leaves only 3/16 inch of mortise wall above the tenons. The mortise should be centered in the stock that remains after the seat rabbet is cut. Using a 3/4 inch tenon thickness, that will give you mortise walls 5/8+ inch thick.

3) O'Donnell's plans show tenons with no shoulder on the inside edge of the frame. A floating tenon would be placed in a mortise that has no wall on that edge. Then he calls for removing an arc of stock behind the tenon to lighten the frame. Effectively, you would have a saddle joint. That may work out if the fit and glue are perfect, but I would feel better with a full mortise. There isn't room to move the tenon further in to the frame member. You could reduce the width of the tenon, but it is sized to resist twist in the frame. If you are going to go with floating tenons, don't remove the waste material. That will at least keep a fourth side on the mortise and help hold it together.

4) The reason you give for using floating tenons is economy. I am not sure how much savings you will realize. There are 4 tenons, each 2 inches long, and cut in 2 1/2 X 4 1/2 inch stock. That works to 90 cubic inches, or less than 2/3 bd ft. I don't know what you have to pay for mahogany, but here that is about $10 worth. Then you have to deduct cost of material for the floating tenons and the additional time required to form and reinforce them. Also with integral tenons, there would be no need to pin that side of the joint. The other side of the joint is already pinned by the leg tenon.

Hope this helps. Of course, if you are not building O'Donnell's design, then never mind. Good luck.

Re: Philadelphia Stool Strength questions

Andrew Alcazar in Charlotte, NC

>Nope you're right it is O'Donnel's design. Also, I need to check my measurements before I post them twice... The tennon thickness is 3/4". Not 1.25". With the size rabbet the chair requires it is still a top shoulder of 1/2" thickness. 7/8" original shoulder minus 7/16" rabbet. You are correct about 90 cubic inches. The problem was that I had a single slab that was perfect for this piece. It was nice and straight grained and would yield very consistent color. Unfortunately, the extra few inches of length would have denied me the ability to have all four pieces come from the same section of slab. 12/4 mahogany is tough to come by in my neighboorhood (takes a 2 hr drive). So the cost is multiplied by the fact I only get to the supplier twice a year.

You are correct about the tennon on the short side being more like a lap. The one wall does have to be open. I plan to peg this joint to provide racking strength. Honestly, I thought most of the racking will go to the leg. That joint should mostly be in compression, which is why I'm concerned about the 1/2" top shoulder and my large rear.

- Andrew

Re: Philadelphia Stool Strength questions

Mike Stallard

>Andrew,

I looked at the plan a little more closely, and I am less concerned than I originally thought. As designed, each member of the frame extends over supporting legs. Therefore each frame member functions separately as a beam supported at each end. As I said earlier, the beam cross sections are more than adequate to support the loads. The depth of beam is really there to support racking in the leg connection.

Your modification with the floating tenons substitutes a joint with severed wood grain and glue for continuous wood grain. Modern glues (PVA, polyurethane, epoxy, a variety of others) should handle the whole thing.

The only question I would have would be how long will the piece last? The original design should be good for several hundred years. With the glue, it should outlast me and you, anyway. How much longer, I don't know. Most glue joints that fail seem to fail in cross-grain movement. The joint we are talking about is long grain to long grain. I would expect it to be fine.

Re: Philadelphia Stool Strength questions

Andrew Alcazar in Charlotte, NC

>Thanks for the help. I think it'll be fine now that I get ready to glue up. I hope it outlasts me for awhile, but hey nothing last forever.. except newspapers in landfills.

- Andrew