routing fin boxes in concaves?

So I have this problem.  3/16" concave (15-1/4" wide tail).  When I put the FCSII routing jig on the board it sits really high.  The front is much higher than the back.  If they were the same I could just adjust the depth so the edges of the box sit flush with the glass but because they are different I’m flummoxed.  Anyone had this issue.  Solution?


Ah- now, some time back there was a discussion on how useful a framing square can be? Or a fairly rigid straightedge, like a builder’s level. We use 'em in boat work too, to deal with flat stuff on curvilinear surfaces. 

In this case, use it across the board to get numbers for some shims, front and back. Foam scraps would prolly be adequate if they are wide enough that you dont squash them. Fiddle them some , imagine a plane ( flat) surface paralell to the bottom right there, that’s what you want your jig to be on.

Now, your box has a face that’s flat. It’s not gonna be perfect on the curved bottom. As long as the basic alignment is good, you can and will have to sand that flat face to conform to the bottom. 

heh- option B, glass-on fins. 

that help any?

doc…

double post

Thanks Doc. Helps.  I cannot do anything permanent to the jig as it’s borrowed.  Maybe inner tube rubber, if I can find some around the house, so the jig doesn’t move…

 

My option B was touse the old FCS round plugs…

Ummmmmm, yeah, inner tube rubber or some new amber color rubber bands with some heft to them? The latter don’t tend to slip much, have used them as quick and dirty anti-skid on commercial fishing boats to keep small gear from skating around. 

If you put those around the jig and the foam shims (say, as you might do with rings cut from bike inner tubes, like you’d cut squid rings for fried calamari) your shims are held in place but easily removed and flipped to do the other side? You prolly already thought of that.

Though I think I would still break out the good 3" masking tape and tape it down to a fare-thee well.Router bits are jumpy, untrustworthy little bastards, y’know? 

 Hope that’s of some use

doc…

I decided on using a cork backed ruler.  It evened it out (about 2mmm thick). The jig has a plastic mesh backing so it shouldn’t move.  No stringer involved so it should cut easy.  But yeah 10,000 rpms always scares me a bit…

Umm, bear in mind that your jig is no longer in full contact with the bottom. And tape is cheap. 

Used to use a big plunge router on timber framing. It was a little surprising, how such a heavy thing could and did jump. Now, I get nervous even around little laminate trimmers.