Sunday, May 3, 2015

Table Progress - Part IV

This post is about the latest progress on the curved table which I have been working on. In this installment, we will be looking at the carving done for the apron, as well as the main point; steam bending some parts to form the curve of the top.

Channel carving

First of all, as I mentioned, I had to carve the channel moulding for the table apron. I wish there was a better term for this carving, but if there is, I cannot find it. "Channel moulding" sounds like some sort of moulding with deep grooves running down its length, and in fact this can actually be called channel moulding as well. What we are working on here, however, has deep grooves running at right angles the piece. If the shape were reversed, so the design stood proud the surface, then it would be called "nulling" and if the 'nulling' was curved, it would be known as "gadroons", but for some reason, there seems to be no other term for it when it is sunk. (If someone knows of one, please inform me, along with a reference source for the term.) All the books I have on English furniture, seem to call it "channel moulding", "channeling" or "carved channels".

setting out the distance from one channel to the next

In the old days, before folding rules, tape measures, and the widespread use of metal rulers for lay-out work, the compass was the tool of choice for doing this; in my opinion it still works the best in this case. Once you have established the number of repeats you will have in the pattern, and the width those repeats will be, it is a simple matter of setting the compass to that dimension, and starting at one end, walking it along, and making a mark with the point at each rotation of the compass. Once the end is reached, you will have the left side of each channel, (assuming you began from the left) marked off. Next, at the right end, you will make a mark with your gouge for the last channel;  this will establish its width. Now, using the compass, set at the right side of the channel, as marked by the gouge, walk the compass back the way you came; again, marking as you go. You will then have set all the channels, each exactly as the other, as you want to carve them. With the same setting on the compass, you can also set out the narrow groove between each channel; simply mark the point of the first one, and walk the compass along as before. (I just made one centre line for this.) No matter how many marks you want to make, using the compass in this manner, they will always be the same distance from one another.

Scribing the lines from the compass point marks

Once all the marks are set with the compass, use the square and a scribe (or a marking knife as you like) to mark the lines out; these will be your guiding edges for carving. It was actually more helpful to mark these lines with a knife, as that eliminated any potential for the grain to tear whilst carving across the grain; which is what is happening here.

Cutting sequence for the carving.

I showed 9 steps to cutting a channel in the photograph above. It is not hard work, but it gets a bit repetitive after making around 80 of them.  Once the channels were all cut, I went back with a smaller gouge and cut narrow grooves between each one, as can be seen in the first picture.


After the carving was finished, it was time to build the steam box. I mentioned in my last post on this table, that I can usually cross-cut timbers fairly close to square. Here is a three out of four go at it. These were the pieces which came off the ends of the cedar used to build the box. I did not want to make it any longer than I needed it to be, because the longer it is, the more time it will take to get the temperature up.

Number three is not quite square as it is leaning to
the left; it is square enough to stand, however.

On a more relevant note, I did not want to drill or screw anything together for this project, as steam bending is not something I normally do, and therefore do not want another contraption lying about. I will use the timber for something else once the project is finished. Therefore I came up with a (I think) clever method of fitting it all together, as you shall see.

The first step was to cut a bunch of small dowels to the same length and bore holes about 7mm deep on the inside of each side. I had to mark it out carefully so that when I drilled the holes and inserted the dowels, both sides would line up; they did.

the dowels will keep the parts separated as well as
establishing the width of the box 

Once the two halves were fit together (held with cramps) it was time to install the steam pipe. This was just some plastic water pipe from the local handyman shop. I just taped them together; as already stated, this is a temporary fixture.

end caps and a series of holes along the
top distribute the steam evenly

Once the box was sorted, it was time to make the forms to bend the parts to. This involved making a bunch of short blocks and attaching them to a piece of plywood. I used my original template for the top to establish the radius for each part. I then made a sheet aluminium template to shape the blocks to. It was a simple matter of marking the block, using a scrub plane to remove most of the material, and then smoothing it up nicely.

spring is a nice time to work out-of-doors

squaring the block

checking the curve

Once the blocks were all shaped and screwed down, and the whole thing screwed to the bench it was just about time to get some steam up.

forms for the inside apron, inside guide rails for the draw leaf,
and inside table top frame

For a water boiler, I used an empty tin for spirits. It was just about perfect, as it holds a lot of water, is made of metal, and has a hole in the top in which to fit a pipe. Once I had the tin filled with water and the pipe connected, it was time to have a go at it. Before I sealed the box up and put the parts inside I wanted to be sure the steam was working properly. It did not take me long to remember an important point of physics; steam will not go downhill!

water-boiler from a large tin

A few adjustments later, and things were working much better. I originally had the box on the ladder to the side of the fireplace, but I realised the hose pipe was too long, thus the steam was cooling off too much.

Insulation helps keep the heat inside.
wrapped around the boiler, it also helps the water to heat more quickly.

In this picture, you can see how the box is held together without screws. A roll of cello-wrap did a great job of holding everything together and keeping the steam inside. There is a wooden panel screwed to the other end, and it has two holes at the lower edge to let the condensed water flow out. I also made a small hole in the top edge at the join of the two pieces of timber, in which to insert a baking thermometer. 

The ideal temperature would have been 100 degrees Celsius, (The hottest you can get water without a pressure tank.)  but I was only able to get it to 97. Once it reached that point, I kept it there for about one and a half hours.

Steam bending is an amazing phenomenon which can be achieved with wood. A plank of elm 25 mm thick can be bent to a radius of about 50 mm without breaking when properly done. Part of that "properly" is using timber which is in the right condition to begin with. It should be air dried and have a much higher moisture content than kiln dried timber does. The reason is that once the wood is too dry, it has a fixed memory and it is difficult to change its mind. I have many times used very hot metal, stones, and bricks to "iron" bent timbers straight, even if they were kiln dried, so I know some degree of change can be achieved even with kiln dried material. Since my radii are not that severe, I was hoping for the best with this project. 

Normally I use air dried material for all my projects, as it has a much more 'alive' feel to it, but in the case of this table, I was not able to find suitable air-dried timber. I decided to bend the tighter radius parts to begin with, as I like to get the hardest part over with first. If I am successful with the hard parts, then I should be able to do the entire project.


The hardest part was the inside table apron because it is the shortest piece and is of almost the tightest radius; it is also wider, which creates more resistance. The channel carving is against the form, away from view in this picture.

Inside apron and inside table-top frame;
this will be split in half and serve for the top and the leaves

Steam makes the wood so it can bend without breaking, but it does not make it easy to bend. The state of the wood in the super-heated condition is termed "plastic", but it still needs a heck of a lot of force to pull it to the form. I had to use my biggest cramps to do the job. It may be 'plastic' but it definitely is not rubber.

parts 5 and 6 (guide rails for the draw leaves)

This is not the end, however, as I still have to do all the long parts. They should bend a bit easier though, as their radii are not nearly so tight at these parts. Once they are bent, I can begin assembling the top section of the table...

Videre Scire

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