Building Cajons – Attaching the Back Panel Without Clamps

This method is a fast, easy way to get a tight, fully sealed back attached  to your cajon build.  It works well with any back material. No clamps are used. Here’s what you need:

  • camping pad foam material, must be compressible foam of some type, about 1/2 inch (1.3cm) thick
  • a small amount of hot glue or similar to attach the foam to the back plate
  • 12 small blocks of plywood or wood,  (about 4x1x1 inch {10×2.5.2.5 cm} depending on the thickness of your back material)
  • a drill and screws for screwing down the blocks
  • some heavy objects to weigh down the box, total about 100 lb (45kg)

If you use epoxy, this method works well even with surfaces that are not quite perfectly flat.  Clean-up is minimal. Glancing at the photos below might be enough information to get you going.

In these photos I use polycarbonate for the cajon back but the method is the same for plywood. There’s some polycarbonate specific info at the end of the blog for those of you who want to walk on the wild side.

I cut and sand the back slightly larger than the box by about 1/16 in (1.5mm), then, after it is attached to the box, sand the edges flush.

Hot glue secures foam to edges of polycarbonte used for cajon back.
Foam is hot glued only around the edge of the OUTSIDE surface of the back.

Cut the foam strips to be an 1/8 inch larger than the box side thickness.  Usually about 1/2 inch to 1 inch (1.3 cm to 2.5 cm). Use hot glue to attach narrow strips of camping pad foam along the edge of the OUTSIDE surface of the back.

The camping pad foam strips serves two purposes. It prevents the back from moving and it provides a compressible surface to distribute pressure evenly throughout the attachment zone. Do not use a full sheet of camping pad foam, it will cause the back to bow in the center and you won’t have a ‘stretched’ back. Turn the back over, the back goes face down with 6 small blocks offset from the edges. The foam supports the back. It is the only thing supporting the back.

Mask the inside of the back with tape such that the masking comes within about 1/8th inch of the sides.

Jig to hold cajon showing the first tier of blocks standing off from the polycarbonate back.

Prepare the jig that will hold the cajon box to the back while the adhesive dries. The first tier of blocks must not touch the edges of the back. Leave them back about 3/8ths inch (1 cm). If you use wood blocks you’ll need to pre-drill holes for the screws or the blocks will split. I prefer plywood blocks. The first tier must be high enough that the second tier never touches the adhesive oozing from the joint.

Put the back in the jig foam down and exactly where it will go during the actual glue up. Carefully put the cajon box exactly on the back where you want it to sit when it is glued.  Carefully place and screw down 3 second tier blocks without moving the box, two on one side and one on the top. They should just touch the sides of the cajon. With the 3 second tier blocks screwed down, remove the box.

Jig to hold cajon with 3 blocks having 1st and 2nd tiers of blocks in place.

Prepare your favorite adhesive, in these pictures I’m using clear 2 part epoxy but for a wooden back I’d probably use Titebond III (not hot glue, that’s just for the foam to back temporary bond). I usually take the back out of the jig to put the adhesive on.

Apply a layer of epoxy to the edges of the polycarbonate.
Apply a  layer of adhesive to the edges of the back. This back does not have masking around the inside edge and will require a lot more clean up than a back with masking.

Put the adhesive on the back around the edge where the box will be attached. Once your adhesive is on, put the back in the jig and make sure the back is perfectly lined up with the  appropriate 3 second tier blocks. Turn the box over and carefully place it exactly on the back, with the appropriate side and top against the second tier blocks.

For 5 to 10 seconds, slowly press the box down while slightly shifting its position to spread the adhesive evenly. End with the box pressed tightly against the 3 second tier blocks.

Place about 1/3rd of the weights on the box to keep it stationary. Place and screw down the remaining second tier of blocks. They should be snug against sides. After they are in place the box should not move at all.

The lower tier of blocks is offset so as to not contact the adhesive. The upper tier of blocks hold cajon box in place.
The lower tier of blocks is offset so as to not contact the adhesive. The upper tier of blocks hold the cajon box in place.
Turn the box over and carefully place it exactly on the poly, aligning the edges.
The box and back with the lower and upper tiers of blocks in place.

Once all the blocks are in place add the remaining weight. I use a lot of weight. Some of my early attempts with less weight were less successful.

Using big blocks to weighing down the cajon box while gluing the back on.
Weighing down the box. These are sitka spruce blocks which are fairly light. Total weight is about 110 lbs (50kg)

With the weights in place, reach inside the box, through the sound hole and remove the masking tape. If necessary scrape out the access adhesive that oozes from the joint. Then wipe the area with the appropriate solvent. The more excess adhesive you can remove the easier clean up will be after it has set.

Next day remove the weight and sand the edge to make the back flush. If necessary sand the inside of the back to remove residual adhesive.

The edge of the back sanded flush with the sides and top of the cajon.
The edge of the back sanded flush with the sides and top of the cajon.

A tight box is critical for good sound and durable construction. While building my first cajons I used clamps to attach the back. They worked but it was messy and a bit complicated. I’m sure there are other ways to do this. And plenty of improvements are possible to this method. Please comment below. Thanks, Bill

Firebox Cajon with polycarbonate back, red cedar and yellow cedar sides top and bottom.
Firebox Cajon with polycarbonate back, red cedar and yellow cedar sides, top and bottom. Made in Alaska, USA
Firebox Cajon with luaan tapa, red cedar and yellow cedar sides, top and bottom. Tung oil finish. Made in Alaska, USA
Firebox Cajon with luaan tapa, red cedar and yellow cedar sides, top and bottom. Tung oil finish. Made in Alaska, USA

Attaching polycarbonate as a cajon back:

Polycarbonate gives a cajon a lot of resonance. I cut the poly with a ‘ceramics’ blade on a jig saw which is much, much safer than other methods I’ve tried. Then I sand both sides to ‘frost’ the sheet with either a belt sander or an orbital sander. After cleaning the inside edges with alcohol the polycarbonate it is ready for ‘gluing’ with either 2  part epoxy or AquaSeal as the adhesive.  Critical is an epoxy that is not runny. It needs to be a bit thick to fill and adhere.  If you are using epoxy you must work fast to coat polycarbonate and position the box before the epoxy starts to set. Any end grain must be sealed with a thin layer of epoxy the day before glue up. Many epoxies are heat soluble, if you make a cajon with an epoxied back don’t let it get to hot. And it’s a good idea to put a few screws in to help ensure it stays put.

Updated Feb. 4, 2020

2 Replies to “Building Cajons – Attaching the Back Panel Without Clamps”

  1. Nice! I really appreciate this post. I’ve been looking for different methods and ideas lately. Is your soundhole on the bottom/front?

    1. Thanks, I’m happy with this method so I thought I’d share it. Yes, the sound hole in on the bottom. It is ‘offset’ in that the back of the sound hole is lower than the front. This, in theory, sends the sound out the front. Beating Cajon in Japan makes cajons with the sound hole on the bottom but the hole is just a standard circular hole. There is a difference in tone depending on if the cajon is on a hard surface or on a rug. I actually prefer the rug, but perhaps just because I’m used to it. Next post will be show I build them.

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