Ok, here we go. This was actually not a difficult project to make; however, it WAS a difficult one to design and I truely believe that I could not have done it without the help of the CAD drawing, except by making many prototype pieces. In the end I only made one extra of the bottom and top legs and these never were truly completed.
I started with one board of Soft Maple 5/4 and laminated two pieces together for each lower leg. The original block was roughly 4" x 4". The was to be tapered to 80mm at the bottom and 50mm at the top (excuse me while I jump between metric and Imperial!). As well as the taper each leg had a compound angle at each end. 8 Deg in one direction for the length spread and 2 Deg for the width spread.
The design called for a recess to be made in the top to receive the hardware that attached the leg to the lower frame. The recess needed to be perpendicular to the compound angle face.
After prepping the blocks square and over length I then cut the compound angles on a mitre saw. Making sure that the vertical height was correct and even when sitting at the compound angle and this height combined with the vertical height of the upper legs would set the final table top height 30"
I then cut the recesses before tapering so as to have the largest surface on which to rest the router jig.
The recess 15mm deep. And was made with a simple jig.
All four legs were done this way. At the same time, using an insert inside the original square to ensure concentricity, a hole was drilled to accept another part of the hardware.
The size prevented me from tapering on the table saw as I had done in the past because the depth of cut available was not enough. I could have done this on the bandsaw and then finished by hand, but the subsequent steps needed precision surfaces so another method was required. I built a simple sled style jig for the planer on which I could taper all 4 sides without too much effort.
Shows the leg held with side wedges at the correct angle on the front wedge.
coming out of the planer. After two sides were done the wedge was adjusted for the other two.
One the tapering was done, the corner was removed where the cherry veneer is applied. This was done by hand and then a very light pass over the jointer to ensure a perfectly flat face for the veneer.
That completed the lower legs.
Each top half is supported by 4 upper legs each at a compound angle of 25 Deg in each axis. These were cut and tapered in the same way and in the same jig as the lower legs.
The top of each upper leg has a square mortice that accepts the through tenon you see in the final photos, coming through the table tops. Cut with the same jig as before to produce a 25mm square recess
The bottom of each has a 3/8" threaded insert embedded in it, again perpendicular to the compound face. The hole was drilled and the inserts threaded in, by setting the table of the drill press such that the face was perpendicular to the axis of the drill bit. It was vital that the angle of the top recess and the threaded insert were perfect or the faces would not lie flat against the joining face when assembled.
The middle 4 legs had a smaller 5/16" threaded insert, but the method was the same
That completed the 8 upper legs. The difficulty here, other that the recesses and thread angles was getting the vertical height exactly right to give me the correct dim between the table top and the lower frame (110mm).
The lower frame is simply a square frame with a recess for the glass. I used two #20 biscuits in each corner. The glass weighs about 40 pounds so the frame was a full 1" thick. the weight of the glass is transferred to the lower legs by the short rails.
This is were it gets a bit tricky! So we have 8 upper legs that join to the lower frame and though tenons in the table top. Now anybody that's ever tried to get even two parallel dowel holes to line up perfectly can imagine what it would be like to get 16 joints perfectly aligned. Here is how I tackled it.
I drilled the lower frame directly from the CAD layout with very tight holes for 3/8" and 5/16" bolts. The 8 upper legs were attached to the lower frame. The corners using threaded rod and the centre legs using 5/16" cap head bolts. A jig was made that fitted the 8 top mortices using 8 25mm x 25mm tenons to locate it. This jig would ensure that the 8 through tenons were all facing the right way!! and I could also use it to transfer the exact position of the tenons to the table tops.
Hope you are all still with me!
Sooo...
On to the table tops at this stage. This is made up of a ply core with a 1/8" thick curly maple veneer and a maple frame. The frame is underneath the top veneer so can only be seen from the side. This gives me a robust top approx 30mm thick. The full story behind this construction method is a long story! However, the ply core does remove the wood movement problem between the table tops and the captured upper leg tenons.
Anyway.... The tops were laied down on the floor, upside down with a dummy piece of glass (MDF) laid between the two tops. (the glass sits in a rabbet cut along each inside long edge). So the final position of the tops
with respect to themselves and to the glass is now being established. Any mistake here and either the tops will not be aligned to each in length or the glass will be too tight or too loose.
The lower frame assy, complete with the jig mentioned above is laid on top of the table tops. The jig was, in fact, four jigs, representing the four pairs of upper legs. The jigs were pinned to the table tops with brads and double sided tape and the lower frame assy removed. This left 8 square holes sitting in position to receive the tenons from the upper legs.
Now these needed to be routed, I needed to route them from the other side of the table tops using a bottom bearing trim bit. But with a bottom bearing bit you can't plunge cut. And I couldn't use a top mounted bearing bit because I would be cutting the wood before the bearing made contact with the template with the possibility of cutting off the side of the mortice. Soo... I plunged all the way through the centre of each square hole with a 3/4" cutter, then turned the tops over and continued with a 1/4" bottom bearing cutter to route the holes with the bearing riding inside the jig. The left me with 1/8" R corners that I chiselled by hand to exactly fit the tenons I had already made.
The tenons were cut for a cross wedge of maple before being glued into the table mortice, wedged and then cut flush.
So now to the corner joints. The lower and upper legs share a common centre line for design reasons but this makes it difficult to attach and clamp the legs.So the method I used was a threaded rod that is screwed in to the insert in the upper leg. This passes through the lower frame and secured with a washer and nut, which is locked in place with thread lock at the last assembly. Now the tricky part. As the lower leg has no apron there are a lot of loads that could be applied to the leg and therefore a fairly substantial fixing was required. A tenon in to the lower frame assy was not an option because of the fixing for the top leg and visa versa, hence the threaded rod. Inside the recess in the lower leg is a simple "T" nut. Now when you thread something like this on to a rod you have no control over the rotation angle the two surfaces meet. The is the top of the leg and the under side of the frame, other than by changing the thickness of the frame. even then, I needed the leg to snug up firmly to the frame and be in the exactly perfect rotation so that the edges of the leg were aligned correctly. My solution to this was to make the "T" nut linearly adjustable. Using two screws to fix the "T" nut in place I could tighten or loosen the screws to move the Nut towards or away from the top surface of the leg. By moving the nut in or out I could adjust very accurately the point in the rotation when the leg snugged up against the frame. This was my clamping method when it came to gluing the legs to the frame. So although it appears to be almost an end grain glue joint between the top and the frame, the strength actually lies in the threaded rod. Racking is reduced by the addition of the braces.
To make transport easier the through tenons in the tops are not glued in to the mortices in the upper legs. They are simply a very tight fit over all 8 joints and you can lift the table using the top only without the tenons coming out.
I think that about covers it, as I said, simple woodwork, complicated engineering. The end result I think hides the engineering well and leaves a very simple looking table that holds the interest of the onlooker at all levels, from the curly grain and wedged through tenons in the top, then down to the 8 cantilevered upper legs and then down to the wonderful, but hidden piece of glass in the lower frame and on down to the cherry veneer on the corners of the lower legs.
I like it, and I hope you do too.
11-03-2009, 02:15 PM
.gif "Drogar Laugh(DBG)")
just that I did need to laminate the legs
beuwtifull design and execution.
"By the waterside I will lay my head......"
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