Plunge Router and Fin Box

General Discussion
1

  1. how far down on first pass?

  2. start on the stringer, go straight down or route around the jig?

  3. 1/4-inch collet?

Any other suggestions?

BB

2

Router didn’t come with a cutting blade - what kind should I get?

BB

3

As my old Ethics professor used to say: “that depends”

On the jig, the router and some other details. If you’re using a collar and jig setup with a straight bit, then the size of the bit versus the size of the collar are important. There’s a little math involved.

First, a diagram

Now, we are gonna make a few assumptions.

  1. That you are gonna use a jig to rout in the box. Yes, you can do it freehand. But if you can do it freehand without buggering it up, you’re a better man than I am. You can make or buy a jig for this.
  2. That your router came with a collar setup or that you are gonna use a top-piloted bit*, though a collar is preferred. That’s the yellow thing in the diagram above.
  3. That you have a fin box handy, just like you’re gonna use, to take measurements off of. .

  • top piloted bit?? Wassat? Well , look in the Glossary under router_bit for that and more.

Okay, you have a jig slot width ( J in the diagram above) , a collar width ( L ) , a cutter width ( C ) and a cutter depth ( D) . You also have a fin box height H and a fin box width W. Get out some fairly precise measuring gear. Routers are capable of quite precise work, if you let them. From here on in, I am gonna use just the letters, to make my life easier.

D = H + the thickness of your jig … That is, set your bit depth ( or maximum bit depth on a plunge router if you have several settings for several passes ) to the thickness of your jig plus the height of your fin box.

J = W + ( L - C ) … this means that the width of the cut you make is gonna be smaller than the width of the slot in the jig by the amount that the router collar diameter is wider than the diameter of the router cutter. You can play this several ways - if you have a 1" wide fin box, a 3/4" collar setup and a 1/2" cutter, the width of the slot in your jig should be 1 1/4". Got the jig already? Okay, then measure the width of the slot in it. A 1 1/2" slot and a 1" fin box means your collar and bit diameters have to be a half inch different, for instance 1" and 1/2". Allow a little for the bedding goop, but not a helluva lot. The same sort of math goes for the length of the cut, though you can always put a stop somewhere on the jig.

Center your jig where the center of the box will go. When I make router jigs I usually scribe centerlines on them and work off of those, it’s easier.

You can, and should, get a set of collars like these shown at http://www.tools-for-woodworking.com/index.asp?PageAction=VIEWPROD&ProdID=2086 for working with jigs and plunge routers. You can use a top-piloted bit, but they can be a little unreliable ( they jump around and cut where they shouldn’t) if you’re using 'em with a jig that wasn’t made with a top-piloted bit in mind.

What kind of router bit? I’d reccommend an up-spiral straight carbide-edged router bit, of sufficient length to be able to handle the depth you need ( from above ) and the right width to work with your jig and your collar setup. Collet size? Well, what collet size(s) did your router come with and what size bits are available that’ll do the job - then, if you have a choice of collet sizes go with the bigger one for reliability and smoothness or the smaller one for price, though I usually won’t go with a collet/ bit shank size that’s larger than the cutter diameter - say a 3/8" cutter diameter and a 1/2" collet/shank size.

Yes, that’s all kinda confusing, but it’s the way to go. Hope it was of some use.

doc…

4

Boon, Make it easy on yourself. Use a pattern bit: It’s a straight cutting bit that has a bearing on TOP. (The top as you hold the router in the upright postion.) The bearing is the same diameter as the cutter.

Then make a template out of 1/2" plywood with a slot cut in it that is exactly the size of the slot you want to cut for the fin box. Lightly clamp the template onto the board, then set the router on the template. Slowly lower the bit until it starts to cut into the surfboard. By then, the bearing should be in contact with the inside edge of the template slot. As you run the router back and forth, the bearing will automatically run around the edge of the template. You don’t even have to look at the bit: the bearing and the template will do that for you.

I usually cut about 1/4’ deep, clearing out the entire area, then I lower another 1/4" and repeat til the hole is a hair over 1" deep. Key is to make your template accurate, because you will cut an exact copy of it.

The problem with collars is that the cutter may end up off-center and your cut won’t be as accurate as you intended. I think you’ll like using a pattern bit: quick, easy, accurate. Doug