Help with finger joint jig

[shane lyall]

Iv'e pulled out what little hair I have left! The blade width and stop have been set down to a couple thousandths. Everything starts perfect but over a 48 inch (I know that's alot for box joints) it seems to get of. By the time I'm at the other end of the board, I'm off 1/2 an inch. What am I doing wrong?

[glenn bradley]

I will be interested to hear if there is a cure for this in working with woods. Cumulative error. 1/2" of error over 48" is a very small error per move. Lower end tablesaw arbor runout could amount to 1/2" of accumulated error over 48" depending on how wide the fingers are. I have improved my success on long runs of finger joints by:
1 - Using a hardwood for the backer that the key fastens to for strength at the key junction.
2 - Using a metal key to minimize deviation due to flex.
3 - using a lighter touch like at the golf or shooting range; controlled grip, not a death-grip.

[shane lyall]

The key is 3/4 ply and is glued solid to another piece of ply. I made a micro adjust and its all tight and solid. The first 2 feet is perfect with about 005 for glue space. It just slowly gets off along the way. If I were making drawer boxes I'd never notice but over the 4 foot span it shows.

[Dave Richards]

How wide are the fingers?

Being off 1/2 in. over 48" amounts to .0104 per inch.

[Phil Thien]

I imagine for that wide a joint you're going to need to use a template.

[shane lyall]

The fingers are 3/4 wide. I just went back to the shop, ran it half way, flipped it end for end, and ran it back to the middle again. One finger was off where they meet but, with a little clean up, it fell in place. Thanks for the help guys.

[Alan Schwabacher]

One way to deal with accumulated error is to clamp together the two pieces to be joined, offset by the width of a finger. Cut the joints in both at the same time. Though it may drift a little over a long distance, the two parts will match well enough to fit anyway, as mating parts are always cut close together.

[John TenEyck]

What Alan said, or you could build Woodgears finger joint "machine". It will make joints that fit perfectly, no matter how wide the wood.

John

[Alan Schaffter]

Box joints require that the dado (router bit) kerf, guide pin, and spacing between the guide pin and blade to be within a few thou- < .004). Notice, I say "kerf", not blade. 48" is a big joint to expect a good fit.

Accumulated error can happen with standard and screw advance box joint jigs. With a standard jig it is function of loose tolerance somewhere (fence, miter bar, guide pin) that allows the stock to move sideways, or the kerf is wider than the guide pin . . . . and a side force is applied so the guide pin registers against only one side of the kerf. In both cases each cutting can move the finger geometry further and further in one direction. If the side force is alternated the joint can be all messed up. Side pressure and a flexible guide pin can also cause the same problem. Those problems in themselves will not necessarily cause accumulated error- the second board needs to be cut without error or with more/less error. If both boards have the same amount of accumulated error they will still mate properly.

A screw advance jig can have accumulated error if the lead screw is not accurate, e.g. a 32 TPI lead screw moves the follower more or less than 1/8" with every 4 rotations.

Note- don't use calipers to match the width of the guide pin to the dado blade! Match the width of the guide pin to width of the actual kerf! When you make the guide pin, make is so it fits a kerf made in the jig with the dado blade you will be using. Do not get it close and file jig's kerf to match the guide pin!

With the I-BOX, you set the guide pin width by adjusting it using the actual kerf made by the dado blade or router bit. At the same the I-BOX also automatically sets the proper blade to guide pin spacing. It does not suffer from accumulated error. You can, however, use the micro adjustment to intentionally mis-adjust or de-tune the I-BOX to create gaps at each pair of fingers where you put contrasting colored inserts. This isn't accumulated error however, since the gap is the same on the first pair, last pair, and every pair of fingers in between.

[glenn bradley]

The i-Box is extremely cool, I have made several small items with it and the joints are flawless. The fingers on the i-Box will flex under pressure however so, even with all the great features it will still suffer under a heavy hand. I think some of your challenge is a large 48" panel of 3/4" ply having a fair amount of mass. You could be mis-aligning things without realizing it as the mass of the board hides the force being applied(?).

[Todd Burch]

Box joint jigs can be out a bit and still be perfectly serviceable. If you are actually trying to do 48" wide joints, then it can't be out that much!

In your case, over 24", you're out 1/4". Over 12", you're out 1/8". That's a LOT! Even over 6", you're out 1/16", and in oak, that's a NO GO.

The variable here is the indexing pin. The blade isn't cutting any narrower or wider on on either piece, so your index pin spacing is off, or, your index pin itself is too narrow, allowing slop in positioning.

You don't need to purchase someone else's jig to get a perfect fit @ 48". Get your pin the right width, and set it the proper distance from the blade, and start making trial cuts on scrap. Go for 6" first, then 12", then 24", etc. You really don't even need two pieces of wood to test this for getting it close. Cut one end of a board, then use your tape to see if the cut is adhering to the expected markings.

Todd

(I have successfully cut 36" joints with a jig I made out of scrap in about 15 minutes)

[Lee Schierer]

The key is 3/4 ply and is glued solid to another piece of ply. I made a micro adjust and its all tight and solid. The first 2 feet is perfect with about 005 for glue space. It just slowly gets off along the way. If I were making drawer boxes I'd never notice but over the 4 foot span it shows.

Don't leave any space for glue. The finger and the slot have to be identical in size to stay matched all the way across a piece that large. Making the slots slightly larger allows the pieces to shift slightly at each cut and the result is the joints don't line up after 40+ cuts. If you need to you can file the fingers slightly after cutting them identical to the slots to fit the pieces together.

[Joe Hillmann]

I would suggest cutting both parts of the joint at one time with one part offset by the width of one finger. That way as long as your finger and kerf are even close to the same size the joint will fit together.