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Obelus and Radical – part 2

The boy’s mathematical bookcase project continues…

In the previous blog I talked about the design of a bookshelf for my son’s bedroom that would have mathematical symbols hidden into the shelves. I had bought the wood and marked it all up, and was beginning the assembly. I tackled the most complex piece of work first; the shaping of pieces of wood to form a radical (aka square root symbol.) The other symbols were easier to assemble, and in theory the remainder of the project should proceed the same as for the tumbling boxes bookshelf I described in an earlier blog.

So, after staining the poplar to look like oak, the maths symbols came together to look like this:

The next task was to join these symbols together with the maple pieces to make a single unit. I call it a single unit. If you look closely at the design, it’s actually in two parts. I wanted to have a multiplication sign, but struggled to work out how this would connect easily to the rest of the work. I would need at least one 45 degree cut, and preferably two or more for rigidity. Then I would need to take some care over measuring up the joins, since having some pieces at 45 degrees would introduce irrational numbers into the calculations. I settled on having the multiply sign sit close to the rest of the unit but remain slightly separate and unattached.

The assembly continued on apace, following the same approach as for the earlier work. In the design I had taken care to plan the sequence so that I could always slot pieces together and was avoiding having to insert any pieces between other already-glued pieces.

When gluing any biscuit joints, you put sash clamps around both bits of wood and set them tight to create a good join. Keeping a joint perpendicular while the glue dries takes a fair amount of adjustment and patience. If either end of the clamp is off the centreline of the join, it will apply an uneven pressure and this can cause the pieces of wood to sit at a slight angle to each other. Even a portion of a degree could mean that other joints further down the wood would be out of line by several millimetres and you would have wonky shelves.

Of course, the square root symbol would once again make life tricky here. Fortunately, I spotted the issue in advance and prepared for it. The problem would be that, though the symbol itself would be perpendicular where it joined the rest of the shelf unit, the clamp would need to grip onto a sloping edge to pull it in tight. If I were to try and tighten the clamp onto this slope it would either slip off the wood or would bend the symbol to make a wonky join. My solution was to keep the triangle of scrap wood that I had sawn off when forming the slope in the first place – clamping the triangle against the symbol would restore the original square edge and take away the slope. First, I clamped a piece of scrap wood to the side of the symbol to hold the triangle in place. Then I could put the main clamp to this nice flat surface and make a good join between the symbol and the rest of the shelves.

With the unit complete, it was time to mount to the wall. I planned to use the same approach as for the end room unit, and mount onto concealed shelf supports. So out came the drill to make suitable holes in the unit and equivalent holes in the wall.

This time however, I was unable to source the same shelf supports as I had used previously. I found a set that I thought were similar, however these had a solid shaft rather than a thinner shaft with rubber rings around. Still, I progressed, fitted the pins into the wall and prepared to push the shelves on.

I quickly ran into a problem. It is not possible to slide the shelves evenly onto so many shelf pins at once. It is inevitable that one end of the unit would slide onto the pins a little further than the other, until the unit sat an angle. The pins, being solid, would not tolerate having the wood at an angle and so they wedged tight. Pushing the other end might alleviate this wedge a bit and allow the unit to inch further onto the mounts, until it wedged the other way. This twisting and wedging got progressively harder as the unit moved further onto the pins – it took less of an angle each time to cause the pins to lock into the holes. At the point at which I was about half-way onto the pins, the unit had locked solid. Then, as I stood back to scratch my head and think about how to release it, there was a loud crack, and suddenly I had two shelf sections instead of one.

Time to pull it off the wall and rethink. Inspecting the damage revealed that the wood had failed around two corner joints. The glue itself had stayed fast, but being close to the ends of planks, the wood around the glue was quite thin, and had split, taking small chunks off the ends of the shelves. I was fortunate of two things; first, the pieces slotted back together very neatly and could be rejoined. Second, joining them together again would produce a couple of ragged edges, but they would both be hidden, on the underside of a shelf and against the side wall.

So, what to do next? I took the pins out of the wall and re-filled their holes. I decided to go for the simpler, but less discrete and more noticeable approach of fixing strips of wood to the walls then sitting the shelves on these. I had room for some small brackets at the bottom to take most of the weight, then strips below each shelf would give enough support to hold the weight of the books. I painted the strips to match the walls, so that would minimise their visibility, and being tucked away within the shelf boxes on the undersides of shelves they wouldn’t be seen unless you went looking.

With that done, the unit was up and in place. The maths symbols stand out just enough to be noticeable and to prompt interest, while the maple matches the rest of the room nicely. The shelves themselves are sturdy and can withstand the piles of books they now hold. Another job done; another handful of lessons learned ready for next time.

And here are my Obelus and Radical…

By nickcnickcnickc

I spend my working life staring at computer screens, so in my spare time I look for things to do with my hands, preferably involving wood. It's a little ironic then that I've now starting writing a blog about my woodworking, and thus introducing computer screens to my main hobby..!