Thread: What is the best bit for machining a box joint?

19mm ply is pretty heavy duty for drawers. A downcut spiral bit will provide the best results, but you may still need a backer board to prevent tear out.

Thanks for the reply. I wanted the drawers to be strong enough to hold some heavy items I have in my workshop. The plywood I have isn't the top grade, and I think this is also making the tear out worse so perhaps routing isn't the correct way to make the joints in this cas. I have a backer board as part of the box joint jig, but it hasn't really helped.
I've used the jig and straight bit on some wood I got from a pallet, and this seems to work fine. It's the plywood layers which seems to be the problem.
I've made a dovetail jig, and will probably use my TS to complete the project.

Finger joints on the TS are dead easy. An incra box jig is magical.... but one can easily make a sled with the many instructions on youtube, etc.

The real art is the blade. A good dado set is excellent. Get one that is completely flat, without the tiny edge knickers. A regular blade can also be re-ground to a flat top by any sharpener.

It really does need a backing board even if using a TS. Nothing worse than tear out!

I've wanted a dado set for some time, but can't afford it at the moment.
As I said above, I've now made the drawers using dovetail jigs on the TS. The joints fitted off the machine, and I'm pretty pleased with the results. There is a small amount of wood between the tails to chisel out by hand, but the pin boards can be machined out completely.
You mentioned a dado set that is completely flat without tiny edge knickers. Not sure I understood what you were getting at, can you explain please?

Some of the dado sets available have ATB teeth on the outside blades. These can leave a not completely flat flat bottom. I've heard some woodworkers refer to them as bat's ears.

As I have a MiniMax combination machine, I can't mount a dado set. I bought a 4mm Torquata flat top blade from Timbecon. The 6mm with a 30mm arbor hole was (and still isn't) available. I separately bought a 6mm CMT blade from Amazon. They both cut equally well and leave an extremely smooth, flat, and square bottom.

Another option if you have a saw with a 5/8" arbor is this set from Freud for $118 (and change) including delivery with Prime from Amazon. These are specifically designed for this task. Freud saw blades are also excellent.

I recently bought a Freud 120 tooth super fine cross cut blade. It's promoted as leaving a polished cross grain edge; and it really does!

That Freud set looks excellent.

I bought the Forrest box blade set some time back and think its fabulous. It does the job beautifully.

(edit) I found this which is the kit, but I'm dead certain mine was metric with a 5mm blade that combines to a perfect 5, 8 or 10mm..... Forrest 8" 2-pc Box Joint Blade Set

Either way, Mark is right. These blades make it a breeze to do finger joints.

More... I also have this older kit, the SD208 from Freud. Freud 6 In. Professional Stacked Dado (SD206): Amazon.com.au: Home Improvement

It is excellent. The pictures on the product show the "bats ears". They are tiny, but annoying for boxes.

The Forrest kit leaves only a flat bottom, but its a sacrifice, as they are ONLY for boxes and small fingers due to the geometry. I would NOT use them to make dados. The Freud kit is for that.... for dados the Freud is/was my go-to kit (I'm out of the studio for a bit, so all this is based on my own shop experience).

I’ve made quite a few drawers using 19mm ply and box joints for my workshop. I have used a router table, a home made jig and a spiral up cut bit which was the only one I had. Aldav’s advice re a backing board is the answer. I always use one and have no tear out problems. My jig has replaceable sacrificial units in the backboard so they can be changed as often as needed with minimum cost

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Thanks for all the info. guys, really appreciated.
My box joint jig, like lappa's has a sacrificial backing board, but I don't have a really sharp router bit, and I think this, combined with the poor quality plywood, is the main reason for the tear out.
I'm now using my router table to slot the drawer sides and fronts, and will rebate the bottoms to form a really strong set of drawers.