Reply by CharleyL

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Posted on Finger joints --- again

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223 posts in 3847 days

#1 posted 10-02-2015 02:16 AM

Finger joints (box joints) are difficult to make in plywood using a router unless both sides of the board being cut have some kind of sacrificial board against both the front and back surfaces to prevent the veneered surfaces from chipping. Cutting the joints in the end grain of solid wood should have less chipping, but even then a sacrificial board on the back side will be a benefit. Think of these as “zero clearance inserts” where they hold the wood fibers adjacent to the cut line to prevent them from breaking.

I frequently make boxes from Baltic Birch plywood in thicknesses between 1/4” and 3/4” on my Unisaw using an Incra I-Box jig and I place an un-cut portion of the MDF backer in the jig in line with the new saw cut, so a fresh new cut is made in the MDF that is exactly the height and with of the blade. This fresh cut will help prevent chip-out of any following cuts in the plywood as long as the saw height and blade width are not changed. This similar principle is what you need to do when using a router to cut the joints, but since the router bit is spinning it exerts force on the plywood fibers in both directions during the cut. This is why I recommend using the sacrificial boards on both sides of the work.

I’m tired, had a long day, so I hope this makes sense.

Here is a photo of some box joints that I made with my Unisaw, a Freud SBOX8 box joint blade, and my Incra I-Box jig. I can be done in plywood if the right precautions are taken.


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