Struggles with First Time Box Joints

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Forum topic by gauntlet21 posted 12-04-2018 02:55 PM 586 views 0 times favorited 6 replies Add to Favorites Watch
View gauntlet21's profile


69 posts in 818 days

12-04-2018 02:55 PM

Topic tags/keywords: bat ears dado stack box joints box joint issues box joint troubles flat tooth blade uneven box joint slots

As I’m working on a pretty standard paper towel holder that hangs from the underside of a kitchen cabinet, I planned on using my Dowelmax jig to join the 3 pieces of white oak. As I laid the pieces out after cutting them to size, I realized how boring the but joints would look on such an uninteresting design. I’m making this paper towel holder more out of necessity than style and flair. I’ve been meaning to get to box and finger joints but hadn’t yet and thought this project was the perfect size and design to expand my joinery skillset.

I own a contractor’s table saw that is capable of fitting my Oshlun 8” dado blade set (only some of the blades fit due to a short arbor). I’ve measured my workpieces out and decided on the dado blade width so that I have an even number of pins. Workpiece is 5-1/2” wide so I set my dado stack to 11/32” using the two outer 1/8” blades, a 3/32” chipper, and a couple shims between until my dial calipers indicated the correct kerf thickness. Everything is going smooth at this point.

I set up my box joint jig according to a few videos I watched that accounted for blade height and pin adjustments based on how tight the test cuts I made fit together. While making my test cuts, I discovered that my dado stack isn’t what I thought it was. I watched a few online videos that explained why my box joints weren’t panning out how I wanted them to. The outside blades aren’t flat teeth like I assumed they would be since a dado is ideally flat. I’ve cut dados with this stack before and had a minimal cleanup of the dado itself.

After all of the nitty gritty setup, I made my final test cuts and despite the joint fitting nicely, all of the slots that are cut out are not flat. They’re bat ears on the outsides and slightly raised in the center. I’m guessing that I’ll need to purchase a new flat top dado set if I’m gonna make a habit out of box joints but in the mean time, is there a method that seems to work well for cleaning these areas up? The only method I have is to possibly chisel them flat or sand them but if I sand them, I’m at risk of compromising the joint’s fit. I did order the Incra I-Box jig and have a Forrest 10”, 1/8” kerf, flat tooth saw blade that I’m not certain will work with the IBOX jig yet. So far in the manual it states to use flat tooth dado stacks/sets but doesn’t mention the use of a single blade to cut out a dado by making multiple cuts and sliding over the work piece. I’m not certain how the IBOX jig works yet so I’ll see.

I could use my router table with the IBOX jig as well but I don’t believe I have an 11/32” bit so even that would require some manipulating to achieve the correct slot width. I am a fan of the Forrest 4 piece flat blade set considering my contractor’s saw only permits a few blades on the arbor but regardless, I’m going to have to purchase something else or clean all of the slots by hand.

I’m certain I’m not the first or last person to encounter this issue so I thought I’d ask the woodworking community for some tips, tricks, or advice as to how to go about achieving flat box joint slots after using a traditional dado stack. I was curious to see what would happen if you only installed some of the flat tooth chippers AFTER you’ve cut the slots. That sounds like a hassle but perhaps it could get me through this project until I’m able to make a purchase of the Forrest FJ08243 set.

Lastly, I’m even considering the purchase of a small shoulder plane (1/4” blade) from Lee Valley that would allow me to get in between those slots easily for some minor cleanup.

Thanks for the input,

Dan, Germantown, Wisconsin

6 replies so far

View Steve's profile


1702 posts in 1190 days

#1 posted 12-04-2018 03:15 PM

I would try with the flat top blade first before going and buying shoulder planes and other dado sets.

I don’t see why you couldn’t use the flat top blade with the Ibox jig. Or you could make your own box joint jig and use the flat top blade.

View bondogaposis's profile


5605 posts in 2959 days

#2 posted 12-04-2018 03:20 PM

Make your joints 1/4 or 1/2”, so they come out even.

-- Bondo Gaposis

View HokieKen's profile


12003 posts in 1746 days

#3 posted 12-04-2018 03:22 PM

You can use any size you want with the iBox. The jig itself is excellent. Not the simplest setup process but it more than makes up for it in speed and precision.

But, it’s really odd that your Dado stack doesn’t have flat ground teeth on the outer blades. I can’t see any good reason for that. My Freud set has alternating grinds on the teeth but still gives clean corners. I’d definitely look into replacing your dado set.

For the current project, I’d simply pare the corners clean with a sharp chisel.

-- Kenny, SW VA, Go Hokies!!!

View Rich's profile


5145 posts in 1197 days

#4 posted 12-04-2018 03:35 PM

The Incra jig can do the 1/8” joints but it’s going to be really busy looking. You’ll likely get some breakage too. The jig is not designed to do multiple passes per finger so cutting wider fingers with that blade won’t work.

I’ve been planning to do a blog post on setting up the jig for joints. Their KISS stuff all looks so easy on the video, but there are subjective factors that make it hit-or-miss, at least in my experience. Getting the guides set for the right width to fit the kerf is easy, but setting the distance to the cutter is a challenge. With a set of calipers that can measure thousandths, I can get the jig set up with just a kerf cut and a second one to measure the finger, then adjusting the silver wheel which is calibrated in thousandths to correct the finger width. Of course I always do a test joint before the real thing, but this technique has always produced a perfect joint first time.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View hairy's profile


3004 posts in 4140 days

#5 posted 12-04-2018 06:07 PM

Cutting the workpiece to final size before cutting the box joints is the hard way to do it.

Make the workpiece over size, cut the joints, then trim to final size. Usually a whole pin on the top and bottom looks best, but it’s all up to you.

-- Genghis Khan and his brother Don, couldn't keep on keeping on...

View a1Jim's profile


117909 posts in 4185 days

#6 posted 12-04-2018 07:04 PM

Hi Dan
I have an Ibox also but I haven’t had a chance to try it out. Is it possible for you to cut off the box joints you have now and start over or even use another piece of wood and use a size box joint you have a router bit for?


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