Making a Keystone Sextant Case

17 12 2009


The preceding post covers “A C19 Sextant Restoration”

Looking around me at my collection of nautical sextants I see that about forty percent of them came to me as homeless refugees from the internet. Eight now have cases with dovetailed corners and two with rebated and pinned corners. Recently, I attempted to reproduce a keystone case, that is to say, one that looks as if it has been cut as a wedge from a round cheese. Two problems distinguished the task from the making of a square case: making the bowed front of the case and cutting dovetails with narrow pins at angles other than 90 degrees.

Seven cases with four corners each amounts to a lot of practice at cutting dovetails, so I felt that I could overcome the second problem relatively easily. Everything hinged on making the bowed front, so I tried that first. I had a ninetenth century case as a model to guide me and the thickness of its walls is about 8 mm. A plank of that thickness, 150 mm wide and 350 mm long cannot be bent easily, but some guidance from the internet suggested I had only to steam it and it would be as putty in my hands. It wasn’t. Half an hour steaming in an improvised steamer resulted in warping of the wood, but bend it would not.

Having a 50 mm plank of Sapele (an African wood similar to mahogany) reduced to three 10 mm planks had left me with some of only 2.6 mm thick, so I tried steaming these and then clamping them in a mould while they cooled down. The mould is made from slices of thick tri-board board cut to (nearly) the same shape and glued together, with aluminium foil glued to the surfaces to protect them from the steam and water. It took quite a while to smooth the large concave surface to shape with a spokeshave, so long in fact that I contemplated buying a compass plane – until I discovered the price. The convex surface was large enough to make attack with a bench plane quite feasible.

The thinner wood was much more amenable to steaming and bending, but it didn’t stay very bent on removal from the mould, though it did acquire interesting three dimensional shapes, combined with splits. My third attempt was much more successful. I simply applied glue to the boards, laminated them and squeezed them to shape in the mould. Once the glue had dried, there was only minimal “spring back”. Once I had planed the edges, it was not possible to say that the proto-front had begun as three pieces.

I did not anticipate much trouble with the front dovetails as, except for the spring-back, the angles were about 90 degrees. However, I had never made any with the traditional narrow pins (the bits that fit between the tails). I don’t know why the pins were made narrow, rather than about equal in width to the tails. They do seem to be more elegant, but perhaps that is just a matter of taste.

This isn’t a woodwork site so I won’t describe how to make dovetails. There are several blogs that will give you a little video on how to cut dovetails. Usually, the demonstrator shows how to mark out and then makes rapid saw cuts that miraculously result in joints that instantly fit together perfectly and without any need to pare with a chisel. I do need to use a chisel, sometimes quite a lot, and sometimes I need a putting-on tool, something that does not exist in the engineering field but which in the woodworking  field is called mahogany paste. I find that it does help when marking out to use a marking knife for all cuts and then to cut out little 90 degree triangles into the waste wood using a chisel. They seem to help guide the saw and also provide a clear guide for the paring chisel that follows.

The rear corners are at an obtuse angle and I relied on my model case to give me the angles to mark out the dovetails. Sometimes my saw went a cut too far, but I was able to disguise this with mahogany paste after gluing the bits together. Clamping them cannot be done with any clamp that I possess, so I resorted to the old trick of using webbing. The blocks of scrap wood seen in the next photo serve the dual purpose of tightening the webbing as they are slid towards the corners as well as concentrating the clamping force near the corners. It is best to sit the carcase on a flat surface and “persuade ” it to sit flat before the glue dries. A sextant can be seen in the background, watching its new home being built.

I didn’t have any wood wide enough for the top and bottom of the case, so I glued pieces together edge to edge. When doing this, it is of no use to hope that glue will do the job without proper preparation. It is essential that the edges be both straight and square before rubbing them together with a layer of glue between and clamping them. It is I think  fair to say that the joint I made is invisible. Once formed and cut to shape I attached the top and bottom to the walls with glue and brass pins that were then punched below the surface, and the heads buried under mahogany paste. The next photo shows the new case in the rough, sitting atop its model.

The whole was now rigid enough to be put in the vice and the edges and corners finished by planing and sanding. It then had to be cut into two parts. I now use a hand saw for this and a Japanese dozuki saw is hard to beat. Those who have a bench saw might use this, but I have had one or two near disasters and now rely on the Mark I eyeball and hand. After dovetails, fitting hinges and producing the simple cut outs for the various pockets inside the case feels very easy. The sextant is looking very interested…

so in he jumps…

and closes the lid.

I finished the case by careful sanding, followed by a coat of red mahogany stain and wax. Given another hundred years of waxing and polishing, it should have acquired a beautiful patina. Now, if I ever sell this sextant, should I say that the case is not original? It is not exactly a fake, nor is it an exact copy. Is reproduction the correct word?

My comprehensive book on the nautical sextant has a chapter on cases, Christmas is coming and, as my grandchildren would say, I really, really need to buy a new sextant. Won’t you help me towards this goal by buying a copy of my e-book?*

* Now available only as a revised and extended print version (see Buy the book)




2 responses

21 10 2010
Jim Van Zandt

Congratulations on a very fine case. I’d say you made the right choice going for a glued lamination instead of steam-bending. For what it’s worth, I believe the springback can be estimated like this: If rf is the radius of the form, r the radius of the final part, and there are n laminations, then r/rf = 1 + 1/n^2. So, with 3 laminations, r/rf=8/9.

“Reproduction” sounds appropriate to me. I also suggest you find a way to sign your work.

12 01 2015
Jack Edwards

Slight adjustment of the math as !+1/9 = 10/9 which now makes sense as the former radius has always to be tighter than the desired radius
ie former rf 9 final r 10.

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