For my second skateboard attempt, I made some changes based on the issues with the first attempt:
- I used more glue – about 4 ounces per sheet of veneer – my first attempt used about 1-2 ounces per sheet.
- I smoothed-out and sanded my mold, intending for a more flush fit.
- I made my press more robust to handle the load better.
Also, I only used 5 sheets of veneer this time (not the standard 7, which is what I used for my first attempt) because I ran out of glue – I wasn’t very concerned, though, since 5 would still be a good indication of the effectiveness of the changes, would probably be at least ride-able (just not very durable), and would help minimize wood lost if the board failed.
The result:
I did a couple of ollies and kickflips and the board cracked in less than 10 minutes. I also noticed a very “soft” feel – like there wasn’t enough rigidity. Granted, the board was thinner than it should’ve been, but some additional research unveiled an issue with my mold I didn’t notice:
I did not “offset” the top mold and bottom mold to account for the thickness of the skateboard. I discovered this in some reading on the silverfish longboarding forums, and, to be honest, was confused by the concept at first, but it makes total sense. I designed the molds by basically making puzzle pieces of the contours, which makes them fit together perfectly when they are directly in contact. For example, let’s look at the detail of a 30° kicktail (the part of the board that angles up):
Let’s call the slanted sections (the kicktail) of the mold section “A” and the flat sections section “B”, as I did in the above diagram. Now, I looked at this and thought it looked like a pretty good fit, so the veneers will be nice and snug for an even force distribution from the press…
However! When you stick something between them, as I do with the skateboard veneers, you get a problem… Let’s say I put 7 veneers in between the molds – 7 veneers at 1/16” thick each results in a skateboard thickness of 7/16”.
Do you see the problem here? I didn’t at first. Here’s a hint – what’s the gap between the “A” mold sections? Vertically the gap is 7/16”, but the veneers are now slanted 30°, so we need to look at the gap with that in mind. Using a little basic trigonometry, and if we say that the gap is “x”, we find out:
cosine(30°) = (x) / (7/16)
x = 0.379 (rounded)
7/16 = 0.4375
So, 0.379 < 0.4375. This means that there’s less room in the “A” sections than in the “B” sections for the veneers. For demonstration purposes*, let’s assume that the veneers don’t compress at all when under pressure, so that they can never be less than 7/16” thick (*in reality, the veneers do compress a bit). Using the same diagram above, we can substitute 7/16 for x, and solve for the vertical distance (labeled as 7/16 in that diagram):
cosine(30°) = (7/16) / (vertical distance)
vertical distance = 0.505 (rounded)
This means that the smallest gap I can have is 0.505”, due to the slant of the “A” sections. Thus, the “B” sections would not be compressing the board at all since 0.505 > 0.4375.
In other words, by separating the molds to make the board fit between “A” sections, I’ve now created a larger gap between my “B” sections, meaning no pressure is applied there!
Now, my next step is going to be redesigning and rebuilding either the top half or bottom half of the mold to account for the offset, or investing in and testing boards made in a vacuum press. Vacuum presses compress veneers by sealing the veneers in a bag with half of a mold, then removing all the air inside so that atmospheric pressure (~15 psi) compresses the board. Right now, with a 5 ton (10,000 lb) press and a mold surface area roughly 306 square inches, I can compress the board up to ~32 psi. So I’ll be doing some cost/benefit analysis to figure out which way I want to go right now. Some other potential additions will be to place thin sheets of cork between the top and bottom molds to eliminate small irregularities on the mold surfaces and adding a second 5 ton bottle jack to help spread out the force. Also, I need to finish my 3D printer (which I’ve placed on hold while setting up the skateboard equipment, supplies, and plans), because I have some cool ideas involving 3D printing that may help…
If ___________, then venture. 