Box (Finger) Joint Jig Made With Recycled Plastic (#2 HDPE)

Do a search of box joint jigs and you find several ideas. Some are for the table saw, band saw, jig saw, router...you name it. I love the idea of making box joints however, I have had no luck with box joint jigs (aka finger joint jigs) and dado stacks are expensive (in my opinion) for the weekend warrior like me. My problem is either the joints are too loose or they are so tight that I break the fingers. It can become very frustrating for something that looks relatively simple. That is why I adapted this idea for the table saw I will share in this Instructable. Hopefully it can provide an alternative.

HDPE Plastic (#2) to recycle

Oven (I used a used toaster oven)

A Mold to create your plastic blank (aka a solid bar)

Something to hold your mold together, i.e. glue, nails, brads, etc.

parchment paper (to keep the plastic from sticking to the mold)

Heat resistant gloves (to handle the hot plastic coming out of the oven)

Clamps (potentially a lot)

Respirator

Safety glasses

Saw

Cross Cut sled

Wood

Wood glue

Measuring tool(s)

soldering iron or heat gun

For some folks, a visual demonstration helps the concept click. In this video I briefly go over the initial HDPE recycling/forming, followed by the process to use the jig in creating box joints.

What is HDPE? In the USA, it is marked #2 and stands for High Density Polyethylene. It becomes pliable/workable at temperatures of 275F to 300F (~130 - 145C). It is widely used as milk jugs and bottle caps.

I actually purchased a small toaster oven from a local thrift store specifically to melt HDPE in my shop. Often, I wire it outside and melt in the backyard. I have seen folks online melting plastic in their kitchen oven and that personally is not for me. I also wear a respirator when in the vicinity of the fumes just in case. You really don't want the HDPE to burn and degrade because you will definitely release some nasties that way. Take the proper precautions.

I had some rough dimensions for the mold to make my blank. I recommend Not using MDF for your mold. While it does provide a relatively smooth surface, the structural integrity is lacking. Use something with some strength to it. Plywood is common and cheap.

I melt my plastic on parchment paper to minimize sticking. I set my oven temperature and generally give it 15 - 20 minutes. Depending on how you process your plastic will correlate to your end product. For example, Precious Plastic uses a shredder capable of producing small pellet sized pieces. The smaller the piece, the more uniform the melt will be and the less air pockets you may create. The other part of that is when your plastic is melted and ready to mold, Precious Plastic / Brothers Make use a press. Here, I am using a bunch of clamps because they are readily available and cheap. It's all about problem solving and using what you got or what you can afford/create.

Another thing to note is you may have to periodically pull your plastic out of the oven, twist it up together, and then reheat it a couple of times. You really want to go for a nice homogenous melt to minimize air pockets. Air pockets will lower the strength of your jig and since mine is made so small, I cannot afford to take the chance of it breaking.

When you are ready to pop it in your mold, wear protective gloves. It will be hot and you definitely don't want that sticky plastic on your skin! Clamp it down and wait a couple of hours for it to cool down. If you are using several clamps, your mold might break and that is fine if you are doing a one and done situation. Mine did and I can always make another one.

What you want to try to avoid (hence don't use MDF) is the mold warping. Your blank will look less like a block and more like a blob...IF that does happen, no worries. Just try your best to trim it up. As long as you can keep the grooves for use square, you should be in good shape.

Once I had my blank, it was time to clean it up. Likely, your plastic will seep out the cracks. I just lobbed those pesky pieces off with a saw (save them to recycle again later). I then cut it in half lengthwise.

The "first piece" will interlock with the "second piece." The process of cutting box joints will be covered in the next step. The idea is that the first piece will need a tab protruding to interlock with the second. The second piece will also need a tab to mesh with your cuts, hence, the tab will need to be equal to the kerf of your table saw blade (See pics above).

I used super glue (commonly called CA glue) to place the tabs in place. However, HDPE doesn't really have an affinity for cooperating with certain common glues, CA being one of them. What I did to combat that, was I heated up a soldering iron and melted the tab in place. Another alternative is if you are really precise/careful, you can make the tab placement so snug (being HDPE helps due to its rigidity when cooled to minimize breaking, even being so "thin") that it stays in place.

The length from the tab to the point of making cuts on your table saw will be double the width of the wood. Again, see pics to reinforce this concept. If you make a large/thick enough set, you could feasibly use it for multiple widths of wood.

Now that we set the ground work, lets get cutting!

Just to reiterate, you will also need to make tabs for the jig to interlock into each other. The tab on jig piece #1 where jig #2 will lock into must NOT protrude past jig #2 on the saw. It will get in the way of your wood! See pic 1 - 3 above.

I attached a PDF to print if you want to build/use your own jig. Also, check out the video since you get a visual here, too.

I found myself having to view my own video so much to re-introduce the process in my mind. I wanted a quick reference as opposed to rewinding, watching, rewinding, watching...you get the idea.

I will discuss the instructions here since I have terrible handwriting. There are also times where I find myself living in a "box" so if you know of a computer program or app that I could have used in lieu of handwriting jigs & instructions, please leave a comment below :)

I felt it easier to label the jig pieces 1 & 2 and the wood pieces A & B

Cut #1B

Place Jig #1 up against the table saw sled

Place wood piece 'A' perpendicular to the tab of jig 1 on the inside (where the sawblade is, PIC 4)

Place wood piece 'B' butt up against A.

This will be your first cut

Remove A

Add jig #2 in front of #1 (facing the sawblade) and interlock them

Cut #2B

Slide B butt up against the inside tab of #2, PIC 5

Cut

Remove (i.e. cut) the rest of the wood inside the portion of the finger slot (I will refer to this as "cleaning up" the inside of the finger slot)

Cut #3B

Take your newly cut and cleaned up finger and butt its left side of against the tab on #2 (still attached), effectively "pushing it" to the right

Cut, PIC 6

Slide B over to your left. This will make the 1st finger butt up against the tab on #2, effectively "pushing it" to the left, PIC 7

Cut and clean up the inside

Cut #nB

(n = some repeatable number of cuts to achieve your finger joints)

Repeat the process of #3B until you reach the "end" of your wood piece, PIC 8 shows the joints

Cut #1A

With jig 1 & 2 still interlocked, take the last finger slot (not the protruding part but the recess) of B to the outside of the tab on #2, effectively "pushing it" to the right, PIC 9

Line up A butt up against B, PIC 10

Cut (this should be the edge of A to interlock into the edge of B)

Slide B to the left, now (the recess is) butt up against the other side of the same tab on #2

Slide A to the left with it, PIC 11

Cut and clean up the inside

Cut #2A

Line up the B edge against the outside of the tab on #2, PIC 12

Line up A along with it, PIC 13

Cut

Remove B completely and slide A to the left, PIC 14

Cut and clean up the inside

Cut #3A - #nA

Follow the same process now like you did for #3B - #nB

This will get you the first edges for A & B to interlock your finger joints

Now you will need to repeat the process from the beginning with the other sides and pieces of your wood

I know that was a lot but after seeing it and doing it (practice with some scrap first like I did!), you should get a better understanding of the process. Again, I had to print it out and get my mind in the right space to start over with the other edges. Marking the jointing pieces helps, in my opinion. I did this by marking the corners as A, B, C, & D. That way I had a method in place to follow in case I got interrupted and had to come back (I had a place holder, so to speak).

Once you have repeated all of your cuts and everything meshes together nicely (always dry fit prior to gluing), you can breathe a sigh of relief. I added a groove in the bottom of the side pieces so that my bottom piece would slide in tightly.

Again, the process may see like a lot, but if you are trying to batch these out for sale, gifts, small drawers, etc., then it does get easier.

Good luck and happy box jointing!!