Cajon Plans – Downward Facing Sound Hole and Inclined Tapa

How to Build A Cajon

This article describes the latest construction design and details of the Firebox Cajon ‘Open Hearth’ design.  It uses technical drawings, photos and text to help guide you through the process of building a cajon. Please see the related blog for a discussion on the features, pros and cons of this design.

This video has good audio recordings of two cajons with similar design. The main difference in sound seems to be due to the type of woods used.

Download the complete set of plans here

The plans are full of construction tips. Use the photos below to help understand the drawings and construction tips. The plans present two variations in the design: Option A uses long ‘bottom support’ pieces, Option B uses short ‘bass bar support’ pieces. The plans are free for your use but not for redistribution, re-purpose and not to be used commercially.

Assembled Parts

This design has an inclined front which makes construction a bit more difficult than a typical rectangular cajon. The side panels are based on ‘right trapezoids’ not rectangles.  However, because the pieces create a large rectangular sound hole, there is no need to cut and shape a large circular sound hole as with traditional cajon designs.

View of downward facing sound hole on a firebox cajon
Option A: cajon with downward facing sound hole using ‘bottom supports’. This cajon is a mix of Western red cedar and Sitka spruce
Option B: Short ‘bass bar support’ in place. This cajon is made from solid Sitka spruce.

The plans use 1/2 plywood for the cajon box. Many commercially produced cajons use plywood from hardwood trees, either birch or tropical hardwoods.  Many cajon makers use solid hardwood. Cajon builders usually recommend plywood with more plys and fewer in-sheet gaps. Marine plywoods can be used. Cabinet grade plywood produces a nice looking cajon. Specialty plywood stores can supply multi-ply hardwood plywood. I have also used 3/4 inch softwood plywood with good results, although the cajons are heavy.

Side Panels

The plans produce a cajon which is about 17 inches tall, which is short for some folks. Modify the plans to suit your height. I never cut all pieces at once. I start by cutting the right trapezoid shape to the height I want, then fit everything else to suit the sides. Cut the small notch at the bottom front after the bottom support/bass bar support is glued in place.

Left and right side panels with bottom support glued on.

It is important to get the cut on the front as continuous and clean as possible. You don’t want to have to sand the front surfaces. The tapa must seal well to maximize the bass, tone and volume. I typically make the top and bottom cuts, then the front cut, and finally the back cut.

Once  the sides are cut, measure the front angle.

Top Panel

Typically I’ll cut the top to the desired width, then cut the angled front.  When the front cut is perfect, I’ll make the back cut on the top such that the top fits the top of the side panels perfectly.  The front angle of the top panel must match the angle of the front of the side panels.

Bottom Supports

I prefer Option A, long bottom supports. I think they are faster to make and add strength. I glue them on before cutting the angled front surface.  This gives a cleaner joint. Option B, bass bar supports produces a bigger sound hole. I’ve not made enough cajons with these options to have an opinion on which produces ‘better’ sound.

Cutting the glued side panel and bottom support.
The dowels are added after the bottom support is glued on. The screw for a rubber foot will go between them.
Bass bar support with an additional thin strip of wood to reinforce the end grain. Not necessary if you are using plywood sides.

Bass Bar

The bass bar holds the bottom of the tapa and is the principle support element spanning the front. I use a 2.25 inch wide piece but it is worth experimenting with wider or even narrower pieces.  This piece is often used to carry the cajon so I typically round the edges in the back which is not shown in the drawing.  The front of the bass bar must be cut at the same angle as the front of the sides to ensure a tight fit of the tapa.

Bass bar with rounded back edges in place on bottom supports.

Bottom Panel

The simplest bottom panel is a rectangle.  The plans include a bottom panel with a cut out but I’ve not yet made one this way. I’ll update this when I can.  I round out the edges of where appropriate which is not shown in the plans.

The bottom panel is the lowest wooden piece on the cajon. View is from the back to the front. The bass bar spans the front of the cajon and sits on top of the bottom supports.

Rubber Feet

The back feet and front feet are different sizes. The back feet should be low. The front must be high enough to level the cajon. There seems to be some complex interactions between the width of the bottom panel, the width of the sound hole and the height of the feet. A small difference in the height of the feet can make a big difference in the sound of the cajon. In general too high is better than too low. I try different feet to get the best sound.  Although the plans suggest 0.75 to 1 inch I find myself using 1 inch most of the time. Rubber feet can be screwed to a scrap piece of wood and sanded to desired height on a bench sander.

Glue-up

The bottom supports are glued to the sides first and the dowels put in. When this is dry all pieces are glued. First the bass bar support is glued and clamped to the two sides and immediately after the top and bottom are glued and clamped to the sides.

Gluing the bottom supports (Option A) to the side panels.
Gluing the bass bar. This cajon uses bass bar supports, Option B.
I believe in at least 4 clamps on each side. But not too tight!

Corner braces are glued in after the box is dry.

Gluing triangular shaped corner braces between the top panel and the side.
Gluing corner brace between the bottom panel and the side. Only necessary if bass supports are used.

The Back

Here is an earlier blog about attaching the back.

I use 0.093 inch thick Lexan for most of my cajon back panels which I attach with 2 part epoxy and screws. If I use wood I am now inclined to use thicker dimensions for the back. The original plans suggest 4mm but I’ve updated them to 0.5 inch thick. I  think that for many types of plywoods 4mm is too thin.  Commercial cajons use thin, dense hardwood plywood  for the back.  The awesome cajons made in Peru use thick solid wood on the back and have a great sound. But they are heavy. If I was making a cajon out of hardware store plywood made from softwood trees, I’d use 1/2 plywood for the back panel as well. If I had dense hardwood plywood I’d use 4 mm or thicker.

The Tapa

The tapa is the front of the cajon, the striking surface. It is the ‘drum head’ of a cajon, it needs to flex so 2.5 to 3 mm is typical. I use 3 mm, 3 ply ‘birch’ plywood from the local hardwood store for my tapas. I believe the inner ply is not actually birch, only the thin outer skin.  I’ve also used recycled hollow door skins successfully. Older hollow doors skins were made from tropical hardwood and produced excellent tapas.  Commercially made cajons often use 2.5 to 3 mm, 5 ply birch hardwood plywood for the tapa.  5 ply birch plywood can be found on the internet from specialty plywood suppliers.

Screws hold the tapa in place. I’m inclined to use a lot of screws. My current thinking is the tighter the box, the better the sound.  The gaps between screws leaves possible air escape passages. Some commercial cajons now glue the lower portion of the tapa and use screws on the upper parts. Screws should be countersunk.

Round the Edges

The plans don’t show it but it is important the round the edges and corners, especially the top. A cajon with hard edges is difficult to sit on for long.

Snare Wires

Many cajons include snare wires pressed against the inside of the tapa to produce a rattle. There are many possible snare wire configurations. I don’t include any in these plans. Check out the photos in this Pinterest board for ideas. I think any of these snare wire designs would work in this cajon design.

The Finish

I use 100% pure tung oil on the tapa. It is all natural and the players hands will have direct contact with the tapa.  The box can be finished with just about any finish you’d like. It’s a drum so it will need to take some abuse.

Completed cajon with a 100% pure tung oil finish.

Time to Completion

Folks often ask me how much time it takes to make one.  That’s a tough question for me because I usually have several cajons in the works at one time and life in general often creates work gaps. And plywood vs solid wood will also effect the time it takes to make one.  Plus there are four steps, 1) gluing the bottom support to the sides, 2) gluing the box together, 3) gluing the corner braces and 4) attaching the back, where the adhesive should dry overnight. Depending on the finish you use it will need to dry days to weeks. I think a skilled woodworker with a good shop could cut and shape the pieces from plywood in an hour or two.  Gluing and clamping after that should take a couple more hours. Sanding and finishing a couple more hours.  Add to this time the overnight glue drying and a week is about right. I often take two weeks start to finish.