Finger joints

[greatviewcsc]

I'd like to join 5/4 boards end to end and am wondering about making finger joints. There are many router bits available and I'd like some input of their use. How easy is it to get the board surfaces to be in the same plane? Lots of adjusting and shimming or does it just happen? Can it be done with a hand held router? (I've got a router table but I'll be working with lengths of 8 feet or so which may be difficult to handle on a router table. (The end result will be painted, so some blemishes are acceptable)

For the curious, I'm replacing some of the trim boards on my house that have rotted at the end(s) because no caulk was applied when installed 15 years ago and I figure it will be cheaper to cut off the rot and splice a new section than to replace the entire board. 5/4 stock is not cheap! Of course, a scarf joint is a possibility but it seems as if a finger joint may be the way to go.

Thanks in advance.

[dadiOH]

Scarfs would be easier and much better.

[StephenM]

With finger joints, it's not about the bit, it's about the jig.

Building a well-adjusted BJ jig is very fussy. The alternative is using a DT jig with a BJ template (you might as well buy the stock).

I have had good luck end-joining painted exterior trim with but joint, biscuit joined and water-resitant glue. Over a hal-dozed years the latex paint has kept a continuous skin over the joint.

If you have access to a buiscuit joiner, I'd go that way, otherwise a more traditional scarf.

A BJ is labor intensive if you're not set up for it and it would look just plain weird when it telegraphed through the paint.

I'll take end-priming before caulk any day,

-Steve

[greatviewcsc]

[dadiOH]

When you said "finger joint", I was thinking of this type...

I started to think you meant this type... you meant the first type, a scarf would be stronger length-wise against bow, just as good or better cross-wise, very unlikely to open due to seasonal/humidity reasons and consequently fewer potential areas for water to enter and start rot.

If you meant the second type, I guess it would be OK. So would one or more biscuits.

[J. Clarke]

With finger joints it's at least as much about the bit as the jig. A dovetail you can adjust by changing the depth. With a finger joint bit and a jig either the bit's the right diameter or it isn't. It might be possible to design a jig that allows the spacing to be altered easily by 1/100s of an inch but I've never seen one.

Might actually be interesting to design.

What's a "BJ" in this context?

[Upscale]

different applications though. The first is for box type construction and the second is dedicated to joinery along the lines of panel construction. Naturally, both benefit from substantial glue surfaces.

[StephenM]

You're right; it is possible, but...:

Might actually be interesting to design.

By box-joint jig I meant something like this:

have found that the best way to adjust this type of jig is not to move the reference pin, but instead, to move the blade by adding or subtracting stacked dado shims.

Box joint (a.k.a. finger joint) implemented on a "dovetail" jig.

"dovetail jig::

"finger joint template" for a "dovetail jig":

hope that clarifies,

Steve

[J. Clarke]

was thinking in terms of doing it with the router, not the table saw.

I just had an epiphany--with a stacked dado set in the table saw the kerf can be adjusted in tiny increments, and an Incra jig works as well on the table saw as it does on the router.

Next time I make box joints I think I'll do it that way--not really used to having a table saw in the shop yet so it tends to be the last tool I go to.

> "finger joint template" for a "dovetail jig":

> I hope that clarifies,

I've made box joints with the router before--it's always been a crapshoot--I had one 1/2" bit that cut just right, my other bits are either a tiny bit oversized (loose joint) or undersized (no way that joint is going to go together). Then I managed to break the good 1/2" and the replacement (same brand, same part number) was a little bit under.

[Sonny]

If you are repairing facia trim, then, additionally, no matter what jointery you use, put a galvanized or aluminum sheet behind the joint to prevent moisture from getting to your rough framing. If you don't have galv. or Al. sheeting, felt will do.

Sonny

[Upscale]

My experiences with a dado blade also tell me that it will be faster, produce less dust, permit easier adjustment and most certainly quieter then the router. Win, win all around. Go for it. I think you'll be pleasantly surprised.

[dpb]

Upscale wrote: ...

But I've never had a single case of even the factory finger-jointed material not failing quite early in exterior applications. If they can't make 'em work under controlled conditions of fabrication, chances of any better luck for diy'er are even less imo.

I'd go w/ the scarf option meself; how one would make the cut w/ the material already in place as I gather is the OP's intent w/ the router is beyond my ken.

That said, if try it, the only thing that would have any chance at all of holding more than a season or two at the outside would be epoxy or resorcinol. I don't know what they use for the commercial stock but I'd think it also would be something similar for exterior application but whatever it is certainly isn't up to the task. My opinion is the joint is unstable to shrink/swell and that causes the failure but that's just that--I've not tried to make some for testing but just observed they don't last. In fact, I just noted the other day there's repair work Dad had done just before we moved back that have precisely that problem that's needing replaced around the basement door frame... :(

--

[Morris Dovey]

(sample joint at botom of page)

(also does dovetails and M&T and ...)

:)

[J. Clarke]

Your creativity never ceases to amaze me.

[Hoosierpopi]

I'm replacing some of the trim boards - splice a new section - a finger joint may be the way to go.

Take a look at Lock Miter Bits.

AND SIMILAR

These cut a 45 which may prove best for your application since the pieces are long an unwieldy.

This is definitely a router table solution IMHO, but if you've a 3.5HP monster and strong arms, maybe you could do it hand-held.

If the two surfaces do not mate up exactly, a little sanding should do nicely as you are painting the results with 2 coats primer and three coats exterior paint.

They also have bits that do something similar with a 90 cut - something akin to a double tongue and double groove.

Think SOLID supports and HOLD DOWNS for cutting the ends of an 8 footer!

[Phisherman]

For trim, your best bet is the scarf joint. It is much more forgiving than a finger joint when you need the two to line up perfectly. Sure, I'd pick the finger joint if I had a jig to help out. Tip: Spend some time inspecting the joint with a work light at various angles and feel the joint, making sure any step is gone before applying the primer.

[dpb]

...

And even those don't last worth anything in exterior application ime...

--

[Jack Stein]

Finger joints like this are mostly done in factories where they have specialized equipment to do it right. I don't think you can do it in this situation.

How are the trim boards joined now? Not sure what trim board you are speaking of.. fascia probably not joined at all, just abutted end to end?

5/4 stock is expensive for a couple of reasons, usually it is select, clear lumber so it starts out expensive. Wolmanized decking is 5/4 and it is cheap, and not prone to rot.. If you had a source of quality treated lumber, perhaps thats an option? Or something like this:

real expensive, but maintenance and rot free I'd expect.

Of course, a scarf joint is a possibility but it

I guess it would be easier than a finger joint but only a boat builder would think a scarf joint easy:-)

My advice is if you are removing the whole piece of trim, then just replace the whole thing, forget messing with it, the cost of 5/4 just isn't that big a deal in the scheme of things... My lumber yard, before they closed, sold 5/4 but didn't carry it. You would pick out what ever 2by lumber you wanted and they planed it to 5/4 for you. The price was whatever the 2by price was...