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Minimizing plywood tear-out in box joints

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Forum topic by DrPuk2U posted 08-03-2021 04:01 PM 389 views 0 times favorited 10 replies Add to Favorites Watch
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DrPuk2U

102 posts in 3505 days


08-03-2021 04:01 PM

Topic tags/keywords: plywood tear-out dado

I’m building a large (15×15x19) box out 3/4 baltic birch plywood. I have jig that makes it easy.

The dado set is a Forrest Dado King, almost new. Very sharp.
My problem is tear-out:

I’ve found that blue tape helps but doesn’t solve the problem. The wood being cut is fast against the jig so it already is kind of a zero-clearance.
Suggestions? Tia

-- Ric, Western Oregon, "Design thrice, measure twice, cut once... slap forehead, start over"


10 replies so far

View 987Ron's profile

987Ron

1967 posts in 529 days


#1 posted 08-03-2021 04:13 PM

Try using a back up board clamped. When using the jig as the backup board only one side or half of the board being cut is supported on the edges. Having a backup board supports both sides a bit better. Ply will tend to do this more than solid woods. Just a suggestion, hope it helps

-- Ron

View Madmark2's profile

Madmark2

3057 posts in 1801 days


#2 posted 08-03-2021 04:16 PM

Backer board. The jig doesn’t support the ply after the first cut. Also, orient the grain of the surface of the ply with, not across the grain.

Ply goods aren’t really the best for box joints.

You may get cleaner cuts from a router than the saw. I use a router for box joints and get perfectly flat square corner cuts.


Router cut box joints.

-- The hump with the stump and the pump!

View CaptainKlutz's profile

CaptainKlutz

4859 posts in 2707 days


#3 posted 08-03-2021 04:29 PM

+1 want outside veneer grain to run with fingers, not across the joint.

+1 clamp it down against the jig. Slower, but it helps.

- IME – Quality BB plywood is important. I can get inexpensive Chinese made 4×8 sheets where top layer veneer splinters and peels simply by walking past it on bench. It’s OK for shop cabinets, but not quality indoor furniture. Decent Nordic produced 5×5 sheets have a lot less issues with splintering. Russian produced BB is somewhere in middle between the two on quality. YMMV

-- If it wasn't for bad luck, I wouldn't have no luck at all, Doom, despair, agony on me… - Albert King - Born Under a Bad Sign released 1967

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DrPuk2U

102 posts in 3505 days


#4 posted 08-03-2021 04:31 PM

Hmm. But the tear-out is on the side AGAINST the jig so I don’t see how a backer board helps. Unless the backer board was as large as the work piece. I suppose a 1/4” pice of MDF might work?

-- Ric, Western Oregon, "Design thrice, measure twice, cut once... slap forehead, start over"

View Fred Hargis's profile

Fred Hargis

7185 posts in 3706 days


#5 posted 08-03-2021 04:37 PM

You could try the backer board and see if it helps, but I’m also not sure it will. I think the problem is Chinese plywood and the grain orientation mentioned above. If you’re up to trying things, cut a piece with the veneer grain running the other way.

-- Our village hasn't lost it's idiot, he was elected to congress.

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Madmark2

3057 posts in 1801 days


#6 posted 08-03-2021 04:45 PM

Yep, backer board runs full width, is clamped tight to, and moves with, the workpiece. 1/4” is may flex, stiffer is better. You need a fresh edge on the backer for each edge cut. Gang cutting wastes less backer and goes 4x faster. Only drawback is one mistake shows on 4 pcs.

-- The hump with the stump and the pump!

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DrPuk2U

102 posts in 3505 days


#7 posted 08-03-2021 05:04 PM

These are great suggestions MM, I note where the tearout is bad are the pieces with the grain running across the fingers. I suspect that working with the grain and blue tape will make a big difference. Learning, learning, learning…

-- Ric, Western Oregon, "Design thrice, measure twice, cut once... slap forehead, start over"

View therealSteveN's profile

therealSteveN

8640 posts in 1787 days


#8 posted 08-03-2021 07:07 PM

I seldom use a backer on solid hardwoods with my I Box, but I always do with plywood, those thin plies are easily chipped off. You’ll note the ragged edge proves there was ample glue, it’s simply the thin ply. Tape may help, a solid backer piece of even a cardboard box behind the back side of your plywood will help decrease all tearout.

Also make the lead side (where blade enters) your show side, and back up the back side (inside) of your box. Orienting the pieces like that will help with plywood, but as already said plywood isn’t the best finger joint material. Presents more issues to deal with than even softwoods. Again, simply due to thickness (actually thinness) of the individual plies.

Conventional wisdom, and practice when cutting plywood on a TS for appearance is good side up, where the blade goes through the face side as it rises from the cut. You however are slightly different because your blade is catching the face at the end of it’s arc. I would still orient the face to be cut first, but do a practice cut to see if it looks good. If you still don’t like it, then use a backer front, and back. Again, box joints and plywood aren’t a given, and sometimes a bit of work to get them right.

-- Think safe, be safe

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DrPuk2U

102 posts in 3505 days


#9 posted 08-03-2021 08:32 PM

I agree. I am a newbie and thought this would be a good learning experience. It has been, but the result is less than stellar. I can see now how I could have done a better job but finger joints weren’t a good choice. No problem – it’s shop furniture.

-- Ric, Western Oregon, "Design thrice, measure twice, cut once... slap forehead, start over"

View Axis39's profile

Axis39

536 posts in 810 days


#10 posted 08-04-2021 01:47 PM

A few have pointed it out, and I agree, your biggest problem is grain direction.

Even without a backer board, I would wager if you turned your boards 90 degrees, you’d get much better results.

-- John F. SoCal transplant, chewer uppper of good wood

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