Fitting Box Joints

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Forum topic by swied posted 06-23-2009 07:37 AM 2848 views 2 times favorited 10 replies Add to Favorites Watch
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74 posts in 3785 days

06-23-2009 07:37 AM

Topic tags/keywords: box joint finger joint jig router

I have a question about using the box joint router jig that I recently bought (see below). It is a great little jig, and is easy to use. The only problem is that I always have to file down my cuts by hand afterwords. Otherwise, the fit is too tight. I have a project in mind that is going to require a lot of these box joints, and I am not looking forward to filing them all down. The directions aren’t much help. Does anyone have any ideas on how to get the fit dialed-in, or do you always have to follow-up with a file?


-- Scott, San Diego

10 replies so far

View a1Jim's profile


117115 posts in 3600 days

#1 posted 06-23-2009 07:44 AM

How about a larger router bit.

-- wood crafting & woodworking classes

View swied's profile


74 posts in 3785 days

#2 posted 06-23-2009 07:45 AM

Here is an example of some box joints from my last project. I had to do a lot of filing to get it all to fit together.


-- Scott, San Diego

View swied's profile


74 posts in 3785 days

#3 posted 06-23-2009 07:47 AM

A larger router bit? I don’t understand. If I am making 1/2 inch fingers, then don’t I need a 1/2 inch bit?

-- Scott, San Diego

View patron's profile


13606 posts in 3364 days

#4 posted 06-23-2009 08:18 AM

is your bit new ?
if it has been sharpened it might be a hair smaller than 1/2 ”
or the registry rail might be a hair over size .
caliper them both , and either get new bit or size indexer .
i.e. sand it down carefully or pad it out with some pacaging clear tape .
i hope this helps.

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View Chiefk's profile


163 posts in 3794 days

#5 posted 06-23-2009 02:23 PM

Sweid, I suggest you give Rockler’s Technical Shop a call and describe to them the problem. It most likely is a minor problem, they can help to correct. pkennedy

-- P Kennedy Crossville, TN

View coloradoclimber's profile


548 posts in 4091 days

#6 posted 06-23-2009 04:05 PM


Let me see if I can explain this without pictures. The problem is really pretty simple and the adjustment for your jig is simple too. I have a jig similar to this and here is your problem:

The gap between the index key (the metal bar that you index off) to the router bit is not EXACTLY the same as the diameter of the router bit. If your finger joints are tight or loose that IS your problem.

Think about it, the finger size (width) is set by the gap between the bar and the bit, the cut out space between the fingers is the diameter of the router bit. If the fingers are not exactly the same size as the router bit they wont fit right.

The router bit you’re stuck with, whatever size it came from the factory is the size it is. So you have to adjust your gap to be the same as the router bit. If your joints are too tight that means your fingers are bigger than your cuts between the fingers so you need to move the index bar closer to the bit to shrink the fingers.

If you have a set of calipers measure the bit diameter and the finger size and I bet you will find a difference.

The rockler manual for the jig is saying the same thing, maybe not as clear, or maybe more clear, but here’s the paragraph from the manual saying the same thing I just tried to say above. Note specifically the word equal, when they say equal it doesn’t mean almost equal, it means as close to equal as you can reasonably get.

5. Adjust the gap between the bit and the indexing key to equal the diameter of the bit. 1/4” box joints, for example, would require a 1/4” router bit, a 1/4” indexing key, and a 1/4” gap between the two. To adjust the gap, move the jig side-to-side in the miter track. Then lock it in place with the 5-star knobs. Note: Use a test piece to be sure your settings are correct and your joints fit tightly.

I find some trial and error works best, cut some test fingers on a couple pieces of scrap, test the fit, tweak the spacing, test again, perfect fit.

And the part in the manual where they suggest using a brass setup block to set the spacing, don’t count on it. The brass bar will get you close but I have yet to find a router bit that was EXACTLY the diameter it says it is. Even a thousandths or so off adds up to stuck fingers after you get ten or more fingers in a joint.

Use the brass bar to start off but expect to do some test cuts and tweak it.

All of these finger joint jigs are pretty much the same and pretty simple, the table saw or router types. A gap and a cutter. The spacing between the indexing pin / bar / rail / whatever to the cutter needs to be the same as the cutter width (diameter). Whether the cutter is a router bit or a dado stack. The cutter you are stuck with, whatever size it is it is, so you tweak the spacing to match the cutter.

View brad's profile


136 posts in 3927 days

#7 posted 06-23-2009 04:18 PM

I don’t have the Rockler jig, but the principle is the same with any finger joint….coloradoclimber is right ion.

-- Brad,--"The way to eat an eliphant is one bite at a time"

View patron's profile


13606 posts in 3364 days

#8 posted 06-23-2009 04:34 PM

i never used one of these jigs ,
so didn’t notice the side adjustment .
i’m sure it is as colorado says ,
got to get me one !

enjoy the rest of the work ,
and show us the results .
and a big smile for not having to file for the rest of your life !

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View Divotdog's profile


68 posts in 3466 days

#9 posted 06-23-2009 04:35 PM

I agree with Brad and Coloradoclimber. I am no expert put I have logged some hours pushing my box joint jig.

The size of the bit doesn’t matter (as long as it doen’t change) but the size of the spacer, gap, index key or whatever you call it really needs to match the cut size. If your first cut (on the edge) exactly matches with the size of the spacer, then you have have it made – just hold on tight and be safe!

Also, the height of your spacer really does not matter as long as it is no higher than your bit height.

-- David, Dallas,Tx - golf weather is warming up but it's cool in the shop.

View swied's profile


74 posts in 3785 days

#10 posted 06-23-2009 04:53 PM

Sounds like good advice. I’ll give it try. Thanks!

-- Scott, San Diego

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