My 18th Century Workbench in progress 9

25/5/2014

I managed to chip away at preparing the aprons a little bit each night during the week. Here you can see how I squared up the shoulders first by chopping down using the prop to improve accuracy and finally some horizontal pairing to get it perfect.

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Then I moved on to a plan I dreamt up to deal with wood movement. Windhoek has some of the most challenging seasonal changes in humidity of any location in this part of the world. In my shop the ambient humidity changed from 20-25% in winter to 75-80% during the rainy season last year. This means that the Equilibrium Moisture Content of the wood (which average around 6%) can fluctuate between 1-11%. This represents a major challenge when building a bench that needs to stay as flat as possible. This is the main reason why I designed my bench with a split top to allow the two parts (which will be fixed to the legs by means of the Roubo through tenon and sliding dovetail joints) to move into the space between them, rather than trying to pull the leg-apron joints apart. Please note the elongated hole for the 20 mm threaded rod that will fix the top to the apron, which is meant to allow for the mentioned horizontal wood movement.

In order to deal with the vertical movement of the top, I use the following strategies: 1) chose quarter sawn stock for the aprons, 2) cut relief gaps (for a lack of a better term) into the aprons. The holes drilled into the aprons in the pictures below, was the start of that process. Hopefully the pictures will do a better job of explaining.

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Here I did a quick check to make sure the 20 mm threaded rod is able to move freely in the elongated slot.

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I then grabbed the router to remove an area to allow the bolt to be countersunk below the bottom surface of the apron. This is important to ensure that a clamp can still find a nice flat surface in future, then using F-style clamps through the split top.

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The router also assisted with the initial 20 mm (depth) or so, of this 6 mm (width) relieve groove, before I took to it with the drill press, a chisel and finally a bed float.

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I used the bandsaw to cut two 3 mm wide relieve grooves in each tenon which extends and overlaps with the aforementioned relieve groove. This will hopefully mean that the tenon (which is about 115 mm (about 4½”) wide) will be able to expand without destructing the joint and for the apron to have minimal effect on the top when it inevitably moves throughout the year.

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The three ‘fingers of each tenon were then cut to length. You will notice that there are a short haunch at the top (which stops short of the main tenon at the top of the leg in order not to weaken it) a through tenon in the middle (which will be wedged at the front of the leg) and a normal tenon at the bottom (which stops about 1″ short of the front of the leg). Both middle and bottom tenons will be pinned with a custom made 10 mm Assegaai dowel.

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Then I turned my attention to the end (short) stretchers. I followed the same process as with the aprons. I only difference was that I found it a bit tough going with the carcass saw cutting the shoulders so I tried the tenon saw and it worked like a charm. The teeth of the tenon saw are set for ripping but I can honestly say that it worked as well as the carcass saw, only better because of the bigger teeth and extra weight and length it brought to the party.

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The waste was removed by chisel.

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The shoulders were squared up by chopping followed by horisontal pairing.

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Seeing that it is insignificant whether the stretchers move slightly, I only cut the relieve grooves in the tenons to protect the integrity of the join.

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As with the aprons the three fingers of each tenon were cut to different lengths. In this case the two outside tenons were shortened and the one in the centre were left long to become a through tenon to be wedged. Both the outside tenons will be pinned too. I plan to do the opposite with the side stretchers where the outside fingers will be through tenons and the central one shortened.

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On to the female parts of these joints. I used the same setup as before to mark out exactly where to cut the mortises using the actual aprons and stretchers. Again the position of the legs were referenced off the inside shoulders of the through tenon at the top of the legs (see first picture).

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