First thing was to decide on the orientation of the balsa planks. Although I am trying to minimise the stiffness of the core as its thicker and the tapered tips will also be stiffer running the grain on the tips at 90 degrees to the center section just looked wrong so I decided to stagger them like below. I'm going to thin the balsa down to maybe 3/4 mm to reduce the stiffness of it so hopefully it shouldn't matter.
To avoid having the foam core showing through under the balsa when looking at it from the side I trimmed about 5 mm of foam away from the edge and glued balsa spars along the edge. Sewingpins are ideal for holding basla in place.
In this picture you can also see the footstrap inserts that I've made instead of trying to track down the stainless t-nuts. I used the smallest hole cutter (25mm) and cut out 4 washers out of ABS plastic offcuts. I opened the hole in the middle so that the nut could be pressed into it and then I used 5 min epoxy to glue it in place. I also drill 2 x 1/32" holes on each washer. These holes were to make it easier to set the inserts in place. I'll provide some more detail below.
Then I mixed up 60 gm of epoxy and q-cell and wet out the surface of the foam. Placing the pre-cut balsa pieces on the surface I laid the 4mm ply wood that I used for the rocker table surface over the top of the balsa and weighted it........ and waited for it to cure. I was a bit worried that the epoxy would be enough as I had sanded the surface with 60 grit paper pretty hard before in order to clean it up and rough the surface for a better bond and I thought that this may have opened up some of the otherwise closed cells in the foam which would suck up the epoxy as might the balsa. But in the end it seemed all ok. 60m was just enough to wet out the surface and the balsa stuck nicely.
|Core with balsa veneer in place|
Next I marked out the insert holes and the areas to reinforce for the fins. Both of these I cut right through the board and cleaned the holes up. I placed masking tape on both sides of the holes. The bottom layers will hold the resin in when its poured in and the top ones will be cut open with a super sharp Stanley knife and be used to keep mess of the board when the resin gets poured in.
|Footstrap and fin insert holes taped and ready for pouring the epoxy in.|
I mixed 72gm of resin ( 60+12gm resin/hardner) and this was about 15gm too much for not only filling all the insert and fin holes but also for using as filler where the rail and the balsa meet ( to get rid of the right angle corder that the fibreglass won't be able to conform to).
I only used a small amount of c-qcell in the mixture for the inserts because it was mostly ofr coloring that I used it. However, for the filler I heaped the q-cell in until it was like peanut paste. It barely flowed at all. this is the consistency that you want for the filler because any thinner and then you put it in place it needs to stay bridging the right angle and flow away.
Setting the inserts
As mentioned earlier the 1/32" holes help make it easier to set the inserts in place. The idea when setting the inserts in the board is to drill the 25mm holes (same size as the ABS washers that the nuts are pushed into) for the footstrap inserts and open the holes a bit so that the ABS/flange nuts are a snug fit. I drilled these holes right through the board, placed marking tape on the underside of the holes and filled the footstrap insert holes with about 2mm of 5 min epoxy to assist in preventing an over tightened screw from pushing into the lower laminate and delaminating it.
I used some 6 x 25 mm stainless bolts and screwed them through the flange nut and out the other side so that the end of the bolt touched the 5 min epoxy barrier and the washer sat about 2-3mm below the top surface of the board.
When the 5 minute epoxy was cured I 2/3 filled the each hole q-cell/ epoxy (not 5 min). Q-cell is only for lightening the mix and colour so I didn't use much at this stage. With the holes 2/3 filled I pushed the inserts into the resin. Because they are a snug fit, the 1/32" holes let the resin run through and over the top of the ABS washer. I then filled the rest of the insert hole with resin so that it sat just above surface of the board.
Its important to make sure that there are as few air bubbles as possible here so its work getting a toothpick or the like and stirring and poking around to get any out.