Finger joints --- again

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Forum topic by Tom posted 09-30-2015 10:51 PM 4325 views 0 times favorited 8 replies Add to Favorites Watch
View Tom's profile


127 posts in 483 days

09-30-2015 10:51 PM

After having tons of issues trying to do finger joints on my table saw I picked up a Porter Cable Dovetail jig (4210) for $40 on Craigslist. I finally got around to using it…and the cuts are an exact 1/2 apart using my router.

The problem I’ve run into…I’m using as test some HD “hardwood” plywood. It has a very nice veneer on it that is probably .000001 inch thick and when I carefully ran the router through…it blew out on the front and back sides really bad. I was cutting end with the veneer grain and when I rotated the piece it wasn’t quite as bad when cutting across. I’m 90% sure that most of the issue is the lumber…even on the table saw the cuts were kind of ragged. As for the’s a 1/4” bit Ryobi. I do have a 1/2” bit router as well (too cheap not to buy plus has a nice dovetail bit) and was wondering if I should invest in a good 1/2” straight bit and maybe some better quality wood to work with.

8 replies so far

View mahdee's profile


3465 posts in 1191 days

#1 posted 10-01-2015 12:42 AM

That happens with most plywood. To remedy it, use masking tape over it before routing.


View intheshop's profile


58 posts in 2262 days

#2 posted 10-01-2015 02:31 AM

I have a lot of experience with that jig. The best thing you can do for yourself is to use a backer board held tight up against the work piece. That will greatly reduce or even eliminate tear out from the back of the work piece. The backer board sits horizontally on top of the jig. To reduce tear out on the front of the work piece, better plywood, a sharp bit, and mrjinx007’s idea of masking tape should help.
As for the cuts being 1/2 apart, I’m assuming you mean they fit but the edges don’t line up by 1/2 the width of the fingers. If that’s correct, try this: cut one side of the joint with the offset stop held tightly against the work piece. Then loosen the work piece and slide it slightly to the side so the fingers of the work piece fit between the fingers of the jig and center the work piece fingers between the jig fingers. (You can eyeball it, or use a caliper. The more precise you are the better the joint will line up). Then tighten the work piece in the jig, loosen the offset stop and slide it over against the work piece and tighten it. Remove the work piece and put in the work piece for the other half of the joint tight against the offset stop. Cut the other half of the joint just as before and it should line up perfectly.
Good luck.

-- Fast is fine, but accurate is final. The real trick is learning to take your time when you're in a hurry. - Wyatt Earp

View Tom's profile


127 posts in 483 days

#3 posted 10-01-2015 02:58 AM

Sorry if I wasn’t clear…the fingers/spacing is pretty much spot on with the spacing. I used my calipers and they were off by about .002 on each one…which is close enough. I might have some scrap 1” pine to test with before I try some better lumber. If I’m still having issues it might be worth buying a better bit…the one I’m using is a Ryobi one from the $25 multi pack.

View Redoak49's profile


1824 posts in 1412 days

#4 posted 10-01-2015 12:49 PM

I have found that a different router bit of the same size will cut slightly different width slots.

Also, the same router bit will cut slightly different sized slots in different woods.

I have the best luck with a spiral bit in terms of consistent clean cuts. I do not have very good luck on plywood and have to put backer boards on both sides.

View TheFridge's profile


5682 posts in 909 days

#5 posted 10-01-2015 12:57 PM

A .002 gap between the fingers is just about right.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View bearkatwood's profile


1173 posts in 435 days

#6 posted 10-01-2015 01:48 PM

I have the incra setup and love it, it is easy to use and dial in.

-- Brian Noel

View CharleyL's profile


190 posts in 2788 days

#7 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.


View Jofa's profile


272 posts in 1261 days

#8 posted 10-07-2015 02:25 AM

I can also recommend the incra ibox. I use it quite a bit in building speaker cabinets along with a Freud dado blade and it works great. However it took quite a bit of trial and error but now its dialed in.

-- Thank you Lord for the passion and ability to make things from your creation.

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