Finger/Box Joints are too tight

[wd4lc]

This was a first for me. I've always wanted to try and make something with box joints or dovetail joints (really any joint that doesn't require screws/nails...I've always enjoyed watching them make those on PBS). This project also required me to make a round cut. Both were done with my router which was pretty much the most I've worked with my router (except for using it a bit on the router table that slides into my bt3 rails).

I felt pretty good about the results since it was my first go and also because they were done using homemade jigs. However, the joints were way too tight. I had to pound (not tap) them together with a mallet and doing so caused much chipping away in the fingers as the ends forced together. Would this be a result from my cutting or from my homemade jig being made slightly incorrectly...or perhaps the offset was slightly off? Thanks.

One note. After much searching on how to make a simple round cut, I finally found a what I needed from a video posted by a kid. Everyone is a teacher and everyone is a student. You got to love it.

[pelligrini]

I felt pretty good about the results since it was my first go and also because they were done using homemade jigs. However, the joints were way too tight. I had to pound (not tap) them together with a mallet and doing so caused much chipping away in the fingers as the ends forced together. Would this be a result from my cutting or from my homemade jig being made slightly incorrectly...or perhaps the offset was slightly off? Thanks.

Sounds like a very slight adjustment needs to be made between your registration point and your cutter. I believe you have it a hair too far away.

[Daryl]

I always make trial cuts until I get it dialed in. Sometimes during humid days the fingers will swell for an overly tight fit. Also make your slots a bit deeper so you can sand everything flush, looks neater in my opinion.

[LCHIEN]

on a box joint, the slot width is fixed usuallly by the cutter and the finger width is set by your indexing system.

When the fingers are too tight its not an offset problem but that your fingers are too wide, you need to move your indexing a bit closer.

[wd4lc]

Meaning the gap between the indexing pin slot and the cut slot are too far apart?

[pelligrini]

Correct, If it's too far from the cutter the pins will be wider.

[wd4lc]

Thank you. I'll check the measurements to see how far off they are and then I'll re-do the jig and test it.

[pelligrini]

It's probably very small, like a 1/64 or so.

I've found it better to make longer test cuts. It's better to have several fingers and slots to check the fit.

I prefer to do mine on the table saw. I picked up a Freud box joint set a while back and it's great. Perfectly flat and clean 1/4" or 3/8" slots. I use a Woodsmith jig. It works well, but I should have saved the money and just built one of my own.

What's that piece you're working on, with the big hole? A speaker box?

[wd4lc]

It's probably very small, like a 1/64 or so.

I've found it better to make longer test cuts. It's better to have several fingers and slots to check the fit.

I prefer to do mine on the table saw. I picked up a Freud box joint set a while back and it's great. Perfectly flat and clean 1/4" or 3/8" slots. I use a Woodsmith jig. It works well, but I should have saved the money and just built one of my own.

What's that piece you're working on, with the big hole? A speaker box?

I've read a few discussions and articles on Dado vs. Router for box/finger joints. Everyone seems to have their own preference.
This was my first crack with the router and I've never tried the dado way.
I'm still up in the air on whether my next purchase will be a dado set or (as mentioned in another thread here) a Porter Cable router jig that has templates for both box and dovetail joints.

The piece is a little bed box for a cat.

[LCHIEN]

Thank you. I'll check the measurements to see how far off they are and then I'll re-do the jig and test it.

The usual method is measure to get close then make microadjustments to get the fit you desire. Usually a fit where they slip together with just a little hand effort and do not come apart by themselves is perfect. Needing to use a hammer is too tight and falling apart under their own weight would be too loose.

If you measure to the nth degree, then you will be close but then you will get whatever fit your errors dictate, because all measurements have some sort of errors and the softness or hardness of the wood also comes into play.

Also as someone pointed out, you want to make the depth of the pins just a hair (like .010 to .020" or around 1/64th") deeper than the boards are thick, then you can sand the pins to give a nice flush finished look. Again, make the final setting of depth by fit results rather than measurements of the depth and thickness.

[Stytooner]

You might also try a different type of jig.

http://www.leestyron.com/newlynnjig.php

This is fairly cheap and easy to build. It takes a few test runs to get the hang of it, but once familiar with how it works, it will eliminate accumulated error and will allow you to get the fingers as tight or loose as you want.
Loring's description is correct. That is how I like mine to fit.

You can also cut all four parts at once with this jig.