Thursday, February 3, 2011

Preparing the core

So now its time to get into making some progress on the core.....

I traced the outline of the board (CrazyFly Raptor) onto butchers paper and cut it out. This made it an easy job to rescale the baord size by placing folds along the centre lines of the paper. I took 2cm off the length and 3cm off the width.

Remember to account for the width of the rails (10mm) and reduce the board core size accordingly.

I transferred this across to a plywood template of 1/2 the board. I made a half template to help make sure that I will get the board exactly symmetrical. A plywood template will also be more robust and reusable. In the BroKite video they make their templates out of 3/4" marine ply and cut out the entire board. This lets them run a router around the outside of the template and cut out the board and the inserts very precisely.

To make the upper deck of the board I used exactly the same template but shifted it in from the ends about 15cm and traced the top deck out in quarters using the same profile.

The long holes marked near the tips are where the reinforcement for the fins will go. These will be cut out and filled either with epoxy thickened with q-cell. The fins I measured up have a hole spacing of 47mm.

While fin insert locations look close to the tips, remember that the rail will add another 10mm.
Its useful to draw the center lines on the board to help with the later alignment on the rocker table.

You can then just cut the PVC foam out with a Stanley knife.
Working surface

I used waxed baking paper over the surface that I was working on to make sure any runaway resin didn't stick and it worked like a charm. I hammered nails into the surface when I was holding the rails in place while the glue cured so the surface needs to be pretty solid and expendable.
The Rails
As I mentioned earlier I made the rails out of 1/4" ABS plastic sheet. I traced the outline of the board onto the sheet and cut it out with a hacksaw blade.

This was a real pain in the arse to do. It was slow and ultimately the outline was shaky and so the rails' thickness waxes and wains along the length of the rail which makes it look like crap. On googling around it seems that what might have been a better idea would be to cut straight strips 10-12mm wide as you can do this with a jigsaw and a guide so that the width is perfect. Then flame the surfaces of the strips over a gas burner (stove or other exposed flame is fine).
This does 2 things. It oxidises the surface of the plastic which makes it a better surface for the epoxy to stick to and it softens it so that it can then be formed to the required curvature without the with thickness varying along the length. There is a brief snippet in the Brokite video of them doing this.
Again, I used the slow curing epoxy and thickened it with Qcell. This turned out to be trickier than I thought. In the first batch I added too much qcell and waited too long to start using it and so it went off in the pot. It went from workable liquid to a solid mass that couldn't be used in the space of about 2 mins (after having been mixed for 20mins or so). I'm guessing the the QCell might shorten the pot life as well?

Then, in the second batch I put too little in and a fair amount of it flowed away when it was put on the rail edge and board. In hind sight I think the best approach would be to get it to honey consistency and put it in a squeeze bottle (like a tomato sauce bottle) and squeeze it onto the rail. Using shorter cure time resin would be far better but I was no where near quick enough to use 5 minute epoxy which goes off in 2-3mins. Alternatively using polyester resin might work well as its typical curing time would be short enough but not too short.
When the glue was in place and the rails positioned I held everything together by using old kelgecell offcuts that I nailed in place to squeeze the rails in all the way around the board. and then placed weights around to stop it from bulging out. The idea of standing the nails off from the board is that if the nails were butted up against the rail then any resin that flowed away would then flow through the hole in the wax paper underneath and stick the board to the working surface.

Fin positions

Cut out the fin slots to get them ready for filling with epoxy. Firstly cover the top and bottom of the opening for the fins with masking tape. Then cut the tape away from the top side. This will keep the resin inside the fin support area and make removing the excess resin that will inevitably get on the board as easy as peeling the tape away.

I used the epoxy resin with about 20% (by volume) of QCell. It seems that one of the keys to getting this to work is to mix it really thoroughly and air on the side of adding a little more hardener than required. If it takes too long to start curing (10 min seems too much) the qcell separates and floats to the top of the resin which is not the desired outcome at all. Qcell apparently makes the resin a bit stronger but I did it mostly here to save using expensive resin.
!!!!!! After you pour the resin in make sure you give it a good stir to get any air bubbles out as these will weaken the cured resin.

Footstrap Inserts

I used 1/4" stainless steel T-nuts for the footstrap inserts and filled around them with 5 minute Araldite cause I didn't want to have to wait for too long to get things moving along.

The holes for these are made all the way through the board and then filled with resin. I just used a softdrink bottle top to trace out the hole size. I've made the distance between the instep bolts 18" and the spacing between the bolts on each foot are 6.25". Because I stand with my feet splayed out like a duck I've put more angle on each footpad. I put the instep bolt 3/8" above the centerline and the outside one 1" below. The other boards I've looked at have the instep bolt on the centreline.

You want the top of the t-nuts to be just below the surface of the board and you want the bolt when screwed all the way(through the footstaps and pads to not damage the setting by cracking the resin below the nut. To get the t-nut sitting in exactly the right spot I used the following approach - I thinned out a Klegecell offcut so that when the screw was passed through it into the t-nut and flush to the surface of the klegecell, the threaded end of the screw poked out a millimeter or so. The kelegcell piece cahn then be sat across the hole to make sure the t-nut is in exactly the right spot. The hole around the t-nut can then be filled with 5 minute epoxy and the screw/nut connected to the klegecell can be inserted into the epoxy. Leaving the screw in prevents epoxy getting inside the thread and making it difficult if not impossible to screw the bolt in.

Although I am probably going to use 12mm bolts in the final board, I worked with 16mm bolts so that I could make sure that there is enough of a gap between the end of the screw and the epoxy in the insert to avoid damage from over tightening or using long screws later.

Make sure to work the t-nut up and down to avoid any air bubbles being opened in the epoxy when you insert it. If air bubbles do open up it would be wise to grind the resin out from around the bubbles and re-fill them.

In the photo below I placed some baking paper under the klegecell but that was a bad move as it creased and then caused a crease to be set into the top surface of the insert. Its also not necessary as the klegecell is easy just to sand back once everything is set. Afterwards I topped it up with 5 more 5 min epoxy and sanded it flush with 50 grit sandpaper.

Filling around the edges of the top deck

As I understand it, fibre glass doesn't deal well with sharp corners and so its necessary to fill the sharp corener that exists between the edge of the top deck and the lower deck.

Apparently this is right up QCells alley. It needs to be thick like 'peanut butter' according to Trev so you can spread it on and it will fill out the sharp corner made by the join. I made mine too thin and so I didn't get the filling effect I was after (see the photo). So my plan is to redo this before glassing. I sanded off the edges to help.
Final core
So after a bit of sanding with 50 then 150 grit sandpaper and the dremel tool to clean the rails up here is the final core ready to be glassed. the rail thickness is still all over teh shop and so I will try to fix that after its been glasses as I may be able to use the liquid abs to fill out the rails a bit once. I haven't quite worked out how to do this yet but I've got Trev working on it.
At this stage the core wieghts 750gms. So with c. 1.3kg of resin plus the fiber glass the board is looking like it will come in at around 2.4kg before accessories. If I can hit this weight for the first attempt then I'll be pretty happy. I haven't tried my hand at vacuum bagging and fibreglassing before so its likely to come out a bit heavier. Time will tell.


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