Levelling Blank in Jig

Posted by Paul

Builders will tell you that even an hour spent on the boat is well spent and brings the finish that little bit closer. An hour was all I had time for today and it wasn’t a particularly productive hour but was worthwhile nonetheless. I found out today that I still have a couple of problems to resolve before I can shape the blanks.

The first problem is I am going to need to buy a new router bit. The sliding cross rail is made from the same material as the long rails, 16mm mdf. The drop from the top of the rail to the deepest router cut (the front and back edges to the centreline) is 50mm and the router bit only reaches 40mm at its furthest drop. I have access to 3 routers, my own battery one, a normal electric, both of which take quarter inch shank bits which I have but are not long enough and a heavy duty industrial model that takes half inch shank bits which I don’t have. There are very long bits for half inch shank and buying one will solve the problem but I hate paying for stuff like this for just one use. Anyway, they are only about fifty bucks so it is not going to kill me.

The second problem is that gravity is still having an effect on the shaping of the blanks. They are still flexible enough and long enough that being propped up at the ends it still falls about 20mm. The bench top is out by about 5mm in places so I don’t want to trust the shaping to the bench top. My solution is to suspend the blank by the ends in a cradle and then block the spine up in a couple of places to ensure it is level along its length.

I started by creating the cradles from the female mdf moulds that came with the kit (I made male mdf moulds to match in order to set the router to). I needed to trim them by about 10mm as they are too high as they came and by about 30mm on the length so they fit in between the rails. The male moulds are attached to the blank along the centreline and sit neatly in the female moulds. I set the top of the female mould (centreline) the same distance from the top of the rails at each corner (not up from the bench because it isn’t level) by packing out under the cradle at the corners. This takes care of any twist as now the corners are the same depth and level. I just have to secure the cradles to the bench now so they cannot move and with the male moulds attached to the centreline of this blank and then the other blank it will automatically fit in place correctly at the right height each time.

I then ran a stringline down the spine to ensure that the blank is level along its length. I set a spacer from the spine to the stringline at the ends (just touching) then ran it along the blank and you can see at the middle that the gap grows from just touching to 20mm, that is the sag in the unsupported blank. I will solve this with some blocks along the spine to ensure that the height is correct, being careful not to be too high and create a similar bow the other way. This is where stringlines are so easy to work to and easy to set up.

The cradle set up is handy in that once set up I can do all 4 runs on them (2 blanks x 2 sides) but because the male moulds (that fit the female cradles exactly) are attached to the blanks I have to finish both sides of one before starting on the other. Of course there is no reason why I wouldn’t do this anyway so this is hardly a problem. Once the blank is level I can secure the blank to the jig, run a quick recheck like I did when building the hulls on the strongback and then there is nothing else to do but run the router over the job and shape them. I just have to bite the bullet and go buy that router bit!

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