First box joints

Hi Debbie... nice job...
1st time eh... what ever you say...
my favorite method is a cranked neck paring chisel... use a sweeping cut/shaving action...
Narex® Cranked-Neck Paring Chisels - Lee Valley Tools
a trim router w/ a modified base..
the PDF shows a mortise style bit.. I prefer a helix trim bit..
safer/cleaner/more accurate/less fuss/muss/less tear-out...
if there are minor irregularities in the wood or the fingers a really proud run the bit's bearing on a layer of tape 1st...
Freud Tools - Search Results for helix
use a hard backed flat/long sanding block to finish and sand w/ the long grain....

NOTES:
after using the trimming base I found that adding a piece of HPL (laminate/Formica) to the bottom and rounding over the edges I avoided marring and scratching finished surfaces...
also.. wax the HPL before using the trimming base...
..

Looking great so far, Debbie. . You have accomplished the hardest part of making boxes with box jointed corners, and that is getting the pin widths and pin spaces to fit together well..

Is your box glued together yet? Have you thought about how to attach the top and bottom? Solid wood tops and bottoms change size with humidity changes, and if just attached to the box sides with glue they will either split or break free with humidity changes. It's best to cut a dado into the box sides and fit the solid top and bottom into these dados without glue as the box is assembled and corner glued. This lets the solid wood panel change in size as the humidity changes without breaking the box.

Plywood is more dimensionally stable than solid wood, so in most cases you can just glue the plywood tops and bottoms to the box sides without a problem, but the disadvantage to this is that the box appearance will suffer, because the plywood edges will show, unless installed in dados like the solid wood panels.

Now to answer your question about trimming the pins to length after gluing -

I place a scrap piece of 3/4" thick wood on my router table top attaching it in place with double sided tape. This is a spacer to hold your box above the router table. I locate it about 2" from the router bit. A flush trim router bit with an end bearing is installed. I then place one of the assembled box sides on this board so that the pins of one side of the corner hang down between the board and the bit, and set the height of the router bit so that the bearing rides on the smooth box side above the pins to be cut. Use a piece of scrap as a pusher and feed the pins into the spinning bit. The pusher block should protect the last pin from chipping as you cut this last pin and cut into the pusher block. Then flip the box and repeat to flush trim the excess pins from each side of each box corner. A light to moderate sanding should make the pins perfectly flat and smooth. I usually use my Random Orbit Sander and 150 grit paper for this.

If you go with just gluing a plywood top and bottom to your box sides, make each about 1/4" larger all the way around.It's easier to position and glue correctly this way. Then flush trim this excess off on the router table with the same flush trim router bit. Sand the edges smooth with the ROS and 150 grit paper.

Charley

Hi Debbie. I like what Stick and Charley said best but if you have a block plane you could use that. Just giving you something else to think about.

I use my linisher (belt sander), it's quick, simple and gives a perfect finish. I just took this set-up shot to illustrate the method.

Thanks Dan for the encouragement! What do you guys use torsion boxes for? Not sure if this is relevant but we use box girders for curved bridges to help with torsion resistant. I posted a generic cross section of one. Very complicated to design, involves 3D modeling. I have never designed one but have designed repairs for several. I love this forum! Very interesting topics!

two layers of sheet goods w/ a honeycomb internal frame..
table top...
shelves..
seats...
anyplace you want strength and light weight..
many designs say to block the ribs but I've found halflap joinery to be far surpior..
Router Forums - Search Results
https://duckduckgo.com/?q=torsion+box+construction&t=ffsb&ia=images


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Debbie said:

Thanks Charley! I didn’t think far enough ahead about the top and bottom before I glued the sides. I was going to glue the top and bottom and use a flush trim it with a bearing on top. I initially thought I would use the same technique on the joints but I had lots of blowout problems on scrap pieces. I’m trying to picture the process the you described in your post but can’t quite picture it. Why does the box need to elevated off the table? Does this eliminate blowout? I can’t picture where the pusher block goes. Is it clamped to the box? Are you running the box along the fence? So sorry for all the questions!!

Click to expand...

Debbie,

You need to elevate the box that you are trimming with a spacer board because there are pins that extend downward in addition to the ones that you are about to trim off with the flush trim bit. If they have no space to hang down into, your box side pins being trimmed won't be at 90 deg to the bit and router table and the flush trim bit may trim off part of the box side, in addition to the pins. You don't need the fence because the box will ride against the bearing on the end of the bit, but if you have a vacuum connection to the fence, putting it close may get most of the chips. Do a kind of rehearsal by setting it all up and you will see what I'm talking about. Set it up both with and without the spacer board to see the difference before you decide to cut.

Are you cutting the box joints with a router bit or with a table saw? A router bit is cutting one side of the joint in one direction and the other side of the bit, because of it's rotation, is cutting the other side of the cut in the opposite direction. To prevent blowout/chipping of the edges of the cuts you need to place a sacrificial piece of wood or MDF on both sides to the board that you are cutting, so the bit has to cut through these as well. Like a cero clearance insert in a table saw, the fresh cuts in these sacrificial strips will hold the wood fibers next to the cut line and prevent the bit from breaking them out.

What kind of box joint jig are you using? An Incra I-Box jig contains a sacrificial strip that you need to move to a fresh spot just before you begin cutting your box joints. This strip only works for one side of your work and one box joint setup. You need a fresh position is you change any adjustments after you begin cutting your box joints. This is true for the jig adjustments as well as the saw blade height adjustment. If using a table saw, only the side of the work facing you needs this sacrificial strip, because the blade is cutting toward you and exiting the wood n the side facing you. If cutting them on the router table, even if using an Incra I-Box jig, you need to add another sacrificial strip behind the board being cut, because the rotary motion of the bit is coming out of the board being cut on both the front and back sides of the board.

I tried cutting box joints on my router table ONCE. I now only cut them on my table saw, and with the Freud SBOX8 box joint blade or with a Freud ripping blade that has the same flat tooth grind and cuts a 0.126" wide kerf (for 1/8" box joints). I also made copies of the I-Box sacrificial strip, so I have plenty on hand ( I make a lot of boxes) . Incra sells these at 3/ $10 plus shipping, but it's easy to copy the original and make about 14 copies in an hour from a 2' X 4' piece of 1/4" MDF.

Since you have already glued the corners of your box together, you will need to use plywood for the top and bottom, because it is more stable than solid wood. Baltic Birch plywood is available in many metric thicknesses, but close to imperial dimensions of 1/16, 1/6, 1/4" etc. Craft stores sell it in smaller panel sizes, but a full sheet is 1.5 meters square or about 5' X 5' and it isn't cheap, but there are no internal voids like the American construction plywood. Birch is a nice medium hard wood to work with too. Make the top and bottom about 1/4" larger all the way around, so if you don't get it glued on perfectly, it will completely cover the box. After the glue is dry, trim off the excess with the flush trim bit. You won't need the spacer board for this step.

Charley

Debbie, any time I can use a table mounted router rather than freehand, I'll do it. The exception is using a light weight trim router with a flust trim bit. You can use that bit in the table most of the time, but the slightly proud fingers will set the box slightly off 90 degrees, probably not very noticable, but it's off. You can elevate the box on heavy card stock about the thickness of the proud fingers and that will make the cutoff nice and square. Several layers of painters tape can also do the trick. That's one reason so many of us also own a trim router--saves unmounting the router from the table.

Once you trim one side's fingers, you won'd need the tape for the second side because the trimmed side is now flat. If you keep the tape in place, you introduce the same small error, but in the opposite direction. We're talking something like 1/32s, but that's enough to detect with your fingers.

Generally box bottoms are set into a dado, especially if they're made of hardwood. You don't glue that panel in, to allow the piece to expand and contract. Plywood is more dimensionally stable, but hardwoods move.