I’m in a bit of a quandry. I’m about to sink the fin box into my first build and wanted to check I’m no going to bugger it all up. It’s a 10.5" FCS centre box for a longboard. It has a small lip around the top which I presume needs grinding off once it’s set. It also has three plastic bridges spanning the width of the box, thus stopping the fin being inserted at this stage. Do I need to set the box high enough so that I grind through both the top lip and also the plastic bridges (opening up the box), or can I snap these bridges out first, meaning I don’t have to grind the box down so far. In my mind I would have liked a dummy fin in the box during the resin hardening stage, so that I can see if it’s aligned properly.
You can grind that lip off before you install the box.
Cut those cross tabs out, stick a fin in, and eyeball it for vertical placement. Make sure you scuff up all the outer surfaces of the box and cover the base of the fin/slot with masking tape. Make sure the bottom of the board is level fore and aft in the tail so the resin fills the hole evenly. Run tape from the fin tip to the rails to keep it vertical while resin sets.
The reason I say to grind off the flange/lip is that it is damned easy to grind in to your bottom glass around the box as you are grinding off that lip. You don't need it anyway if you are using the fin as a vertical reference.
It's also a good idea to scuff around the box area and laminate a patch over the box seam.
I remember taking a lot of crap early on from a couple of notable posters on this site because I told them boards with thin tails that were routed thru the deck were $#!t looking and poor craftsmanship (ie unaccepteble). I don't think either one of them ever got over it.
For many years I never used a router... with a metal ruler, sharp knife and chisel I was able to cut the hole for all my box installs. The square holes at the corners were practically my trademark.
Now that I have a router I use a 'framed' jig that guides the base of the router instead of the 'slot' jigs that rely on a bearing bit to follow the hole. It all works but any method can wreak havoc on a board if you slip up.
I've seen plenty of boards with a telltale ding repair 'graphic' on the deck to disguise where the router cut clear through or warbles around the box when the jig slipped. I've even heard of boards flying clear off the racks when something went haywire.
A couple of things that have worked for me. And let me start off by saying that there are lots of guys on sways with 10x the experience as me here but this might help.
I use the 10.5 bahne box. The way you describe the FSC box it sounds similar, it has a built up ridge/dam and guides that allow it to rest on the glass so it hits the right depth.
I have had luck using a router without a jig and luck with a jig but not yet have I had one come out spot on perfect. The problem I ran into with a jig is it slppped when I got kick from hitting the stringer, and that includes several passes at increasing router depths. To correct this problem I now tape off the projected box placement with just enough width 1-2mm on each side and not using a jig I eliminate the stringer down to the depth of the box. The rest of it is easy to route with a jig in place. On my short boards I have used probox and now the futures. I built two quads and it was like butter but on my 5 fins and thrusters and that jig also slipped slightly when I did the tail/stringer box.
As far as the ridge/dam goes, I grind that down so I don’t have to do it when its in the board. I also might add that I don’t place my box in until I hot coat. I do this so I have a little bit of forgiveness when I sand. In my experience, trying to sand off all that ridge has problems, it really heats up, takes too long, and can be a pain to deal with if you go too deep. Also, if you drop a black box in a white board the fine black grit can get in the sanding swirl if you don’t hit it perfect, so the less to sand the better. Not all of it, just some of it.
By the way, Stingray is Stingray, a nice guy, very helpful, and I’m not just saying that because he gave me a great deal on some 6oz. And I realize that Kensurf said to just leave the ridge, he knows more that me but until I possess his skill this is my 2 cents.
One thing that I have found is that instead of dealing with the resin spill over I fill the hole so that when I place the box it comes up short. I then take a 5cc syringe full of my filling resin with an IV catheter tip and fine tune it so it’s nice and clean and easier to sand.
I've installed a 10.5 inch box and after I sanded the lip down and got the box shaped nicely I cut the center bridge out. The moment I cut the bridge it pulled away from the sides a fraction of an inch.
question 1 is: should I cut this little bridge before I install or should I cut it after I lay cloth over it? for future installs
question 2 is:should I fill this gap with resin during the glass over or fill gap with resin then place a bridge inside during glass over to keep the box pinched? for the current instal problem… I want the fin to fit…
I sanded the box with 60grit, bathed it in acetone, and set it with poly sanding resin, pigment, and chopped fibers
Rather than the multiple stir sticks mentioned above I had an old high aspect windsurfing slalom fin kicking around when i first started. I simply cut 2 slots into the fin base and removed the pin so I can easily slide it into. The box over the the bridges without cutting them out. I also cut the top of the fin off level With the base so it's easy to support it vertically with masking tape and you get nice even downward pressure when you pull the tape down at the rails. Sorry cant show you a pic of the fin jig as its currently sailing to aus in a shipping container…