Building a Cajon

A cajon is a simple percussive instrument which resembles an innocuous wooden box. Despite it's simple appearance, it can produce a wide variety of sounds depending on where and how it's hit. An internal snare and a loose tapa (the striking surface) further adds to the variety.

I thought I would put my table saw to a good (and first) use, and try building one!

Design Guidelines

The information below has been compiled from internet searches and my own understanding:

Tapa thickness	Thinner gives better response (less force required and louder sound). Thinner gives better higher frequency tones.  General size range: 2-4 mm
Tapa material	Stiffer woods (eg. birch) are preferable.  3 ply plywoods are okay, with 5 ply plywoods being preferable. It can also be a solid wood, like a guitar soundboard. However, since the tapa behaves more like a drum membrane (for which the dominant factors that influence the vibration are the mass and the compliance (“elasticity”) of the material), it might not make a substantial difference.
Tapa attachment	The tapa can be either glued or screwed on, depending on preference. There is no agreement as to which sounds better. Often the top third of the tapa is often left loose to create a high pitched “slap” when hit. This can be achieved by adding shims, or sanding the top corners down slightly. Screws can be added to tighten / loosen the tapa to tune the sound. It is noted that a poor tapa seal will degrade the bass note.
Sides	Most cajons made from plywood. Material does not seem too important, providing it is strong enough to support the player.  High density is preferable to reflect the lower frequencies. MDF is not recommended as it will dampen the sound and resonances.  General size range: 9-16mm
Internal Supports	Corner blocks are recommended to reinforce the corners. Use hardwood.  A front frame is optional, but can provide additional rigidity and if screws are used, provides more room for creating a thread.
Back	Same guidelines apply as for the sides. It is generally made half the thickness of the tapa. Perhaps this provides some compliance to allow for resonance to occur between the tapa and the back. General size range: 4-8mm.
Snare	An internal snare can be made using guitar strings stretched over the tapa, or a drum snare pressed against the top half of the tapa. The snare action may be symmetric or asymmetric between the left/right sides, depending on playing style.
Exit hole	The exit hole allows the sound to exit. It also determines the natural resonant frequency of the cajon, which behaves as a Helmholtz resonator. The dominant factors for the resultant resonant frequency are the diameter of the hole and the internal volume. The exit of the hole (rounded vs sharp corners) will also have a slight influence. Rounding the edge is suggested regardless to prevent splinters from forming.  General size range: ½ to 1/3 the cajon width.
Joints	Traditional woodworking techniques apply – use glue and try a rabbet, dado, mitre or biscuit joint. Even a simple butt joint works well. Screws / nails can be used, or omitted entirely.
Finish	Any finish can be used for the sides and back (oil, varnish, polyurethane, shellac, etc). A thin oil finish seems preferable for the tapa, to prevent dampening of the vibration (although I do not know how true this is). An oiled finish does provide a good texture for the playing surface though.
Top surface	A non-slip, soft surface can be added to provide increased comfort for the player. Foam and leather are good materials to try.
Feet	Feet should be added to prevent damage to the bottom surface of the cajon, and to prevent it from slipping, as you will often have to lean it backwards to play. Rubber and foam are good materials to try.

Cajon Body

I used 10mm thick, Fujian Cedar marine-grade plywood (purchased from Mr Ply & Wood) for the cajon body. Marine grade ply is designed to survive the rigours of the sea, and has the best void-free veneers. This 1/2 sheet (1200x1200mm) will be enough to make 2 average-sized cajons (300x300x450mm).
(Update: I found out later from another plywood supplier that Fujian Cedar is currently a shortage of Fujian Cedar. No wonder I paid such a premium for it....)

Boards were ripped to size on the table saw. 

The side pieces measure 430 x 295mm. 

The top / bottom pieces measure 300 x 295mm. 

After taking the back board and the tapa into account, the final dimensions will be the target 300x300x450mm. 

2mm deep rabbets were cut on the top and bottom pieces to help with alignment during gluing. It would have been very difficult without these. My Router Fence helped immensely with this process.

I glued the sides of the cajon in three stages. 2 "L" shaped pieces were made using a side and a top / bottom piece. This ensures that I get two 90 degree angles. The 2 "L" shaped pieces were then glued together in the third step. This is the recommended procedure for welding a steel frame, and I assumed that a similar principle also applies for woodworking.

To glue the "L" shapes, I supported the upright board using a plastic stand that I salvaged. (These were made from solid blocks of what I believe is ABS). Two 11" spring-action C-clamps were used to hold the upright piece to this stand after it was lined up with the rabbet. Ordinary white PVA glue was used, and clamps used where required to close up gaps.

To glue the 2 "L-shaped pieces together, I used a plastic stand to constrain one of the joints. Two ratchet straps were put around the cajon loosely, and glue was applied to the joints. After maneuvering the pieces into place, the ratchet straps were tensioned, and clamps were applied where necessary to close gaps. In an ideal world, I would have clamped each joint in two dimensions, but the reality of the situation was that I didn't have that many clamps.

Back Board

The back board was made using 6mm marine grade plywood. I'm not too sure what species wood is, but it looks relatively similar to the Fujian Cedar. This was purchased at Mr Ply. Also shown are the 4 corner support blocks, made from 30x18mm Meranti (from Bunnings). A rabbet was cut all around the back board to provide strength (as it helps prevent the cajon from deforming into  a parallelogram) and to make gluing easier. Again, the Router Fence made light work of this. 

Next, the back hole for the cajon was laid out. The target size was 120mm (which is between 1/2 and 1/3 of the width of the cajon, 2/5 to be exact). It is located on the centreline and 150mm from the base (1/3 up). Very coincidentally (and conveniently!) this is the exact diameter of a CD! (I'm glad I didn't throw these out - made a robot and a template already!)

The back board was clamped to a sacrificial board and a 6mm pilot hole was drilled through both at the center of the circle. The circle was then cut using my Circle Cutting Jig.

I was only able to use a 6mm router bit for the cut, as any larger would've made a bigger circle. The minimum distance from the axis of the router to the M6 pin is 58mm.

However, I wanted to put a 6mm radius this edge, but the router bit required 12mm of clearance (for the bearing). I ended up creating the clearance required with freehand routing. 

After using the 6mm round-over bit, the results were better than expected, especially after some cleanup with sandpaper. Definitely better than routing freehand or using a jig saw followed by copious sanding. 

After this, the back board was glued and clamped to the cajon body. This was an easy process, as the rabbets prevented the board from moving. All my clamps are being used here, and even more clamps would have been convenient. This would've been a good use for a parallel bar clamp...

In quite a few build logs, I've seen the tapa being glued on before the back plate, as this allows the snare to be adjusted. I can't help but think that this is a bad idea - any warpage of the body that occurs later when the back board is clamped and glued, will be transferred as residual stresses in the tapa. The tapa was the last piece to be attached in this design. 

Corner Braces

The corner braces went in next. The pieces measure 30x18 for the side braces, and 30x12 for the backboard braces. Material is Meranti from Bunnings. (An interesting note - "Meranti" describes many species of hardwood rainforest trees that are common in South East Asia - hence why there is a smorgasbord of different colours, densities and hardnesses).

A good cabinet-maker trick to use when gluing the pieces is to slide it up and down along the joint to help "massage" the glue into intimate contact. Another trick is to size the backboard braces as close as possible, so they push the side braces outwards and help hold them in place. They were also held in place using 11" C-Clamps (that I normally use for welding).

Tapa

Two tapa materials will be trialed.
1) 6-ply Birch Plywood, 3.0mm thickness.
1) 3-ply Basswood Plywood, 1.5 mm thickness. These thin sheets of plywood are often used in RC planes, so a RC hobby shop is your best bet.
(Update: The basswood did not work out too well. I also found a superior alternative.)

2) Carbon Fiber Reinforced Polymer (CFRP), 1.0mm thickness. This will be an interesting material, as has ideal properties for a tapa (on paper at least) - high stiffness and low mass. I hope to get a good bass tone from this. I bought a matt finish 500x400x1.0mm sheet from eBay at a very reasonable cost.

Tapa Version 1 - 1.5mm Basswood

The tapa was made from a lamination of 1.5mm Basswood plywood (purchased from Kellett's Hobbies) and Walnut Veneer. This came as a 900x300mm sheet, which I cut to 450x300mm using a utility knife. Basswood feels quite flexible. I was intending on using it by itself as the tapa, but later found out that applying an oil finish was not recommended - as it will just soak up oil like a sponge.

The top surface will instead be made from Walnut veneer (that I bought from Trend Timbers). I bought 2 bookmatched veneers, which are overiszed for what I require. These came with a very neat edge, so all I needed to do was tape it together. I used 3M Scotch tape, as it will be easy to remove later on. 

(Update: Only veneering one side can potentially lead to warpage problems due to different rates of moisture absorption and expansion)

Cross-linking aliphatic "yellow" PVA glue was used for higher strength and water resistance. A rubber roller was used to squeeze out the excess glue, which was applied using a paint brush. 

A 6mm aluminium plate forms one platen, which will help press the substrate and the walnut veneer together during gluing.

The walnut was laid down first, glue applied, and the basswood placed on top. This is contrary to the typical veneer gluing procedure, during which glue is applied to the substrate. I did this deliberately though so I could accurate center the parting line of the veneer on the basswood. Otherwise I would not have been able to see where the basswood was under the veneer. A 30mm laminate table top offcut was put on the top surface, and every corner and edge was clamped. A put a heavy weight in the middle for good measure. I didn't take any photos of the gluing process, as it was messy, and the glue was drying. Instead, the final setup is shown below.

After unclamping it, I got a nasty surprise. There wasn't enough pressure in the center of the veneer so I got a furrowed surface, under which there are voids between the veneer and the substrate!

Shown below is the result even after judicious sanding with 120 grit. Note to self: make a big "parallel bar" clamp. 

Tapa Version 2 - 3.0mm Birch

Tapa version 2 is 3.0mm, 6 ply Birch Plywood from Finland! (Purchased in 1/2 sheets at a very dear price at DMK Forest Products). Here it is cut to the requisite 446x300mm.

Up to this point, I had not decided whether to glue or screw the tapa in place. I finally decided to use screws for the first iteration - as it can be glued later if desired. A template was created in CAD to locate the screw holes. The holes are 14mm away from the edges and on a 68mm grid. The top third is left loose, which allows the tapa to slap against the cajon body and create a "crack" noise.
(Update: I later decided to add a screw in the top corners - this allows for the "crack" noise to be removed completely if desired. It also helps protect the tapa - if someone's pants get caught on the corner, and they pull, then they can bend and even crack the tapa corner!)

A bradawl was used to mark the hole positions. The tapa was placed in position over the cajon body, and a 1.5mm drill bit used to drill pilot holes. The 1.5mm holes in the tapa were then drilled out to 3mm and countersunk. Brass coated 4G x 20mm cabinet screws will be used to hold it in place.

After the tapa was attached, the side corners were rounded with a 10mm round-over bit, and the front / back were rounded with a 3mm round over bit. 

Snare

A 14" drum snare was cut in half to form the internal cajon snare.

The drum snare will be held against the tapa using a cross-brace that runs horizontally across the front of the cajon. It was made from 19mm Tasmanian Oak Dowel. 4mm holes were drilled at regularly to allows the position of the snare to be adjusted. (I took a shortcut by using the milling machine).

The 4mm hole was enlarged to 5.5mm using a drill bit. This allows M3 nuts to be pressed into the wood. This removes the need to use self tapping screws (which tend to become loose of adjusted too often). Here is a snare assembled to one of 7 possible positions. The final cajon will use two snares.

The other side of the dowel was flattened using a sharp end mill (again, using the milling machine).

Many designs allow the pressure of the snare against the tapa to be adjusted using an external knob. It also allows the snare to be turned off, by moving it away from the tapa. I think this is a great feature to have, but didn't like the way the external knob interrupted the shape of the box.

Instead, a friction fit cross-piece was used. A ~22mm hole was drilled in the front braces. (It was probably closer to 23mm, as hole drills tend to cut oversize). Two 2.5mm thick, 18mm ID O-Rings were slid over the ends of the crosspiece and pressed into this hole. This is quite a snug fit. The contact surfaces were covered with chalk (the type you use on blackboards, and the type used by rock climbers and weight lifters) before everything was fitted to increase the grip. A hole was also drilled in the middle of the cross-piece and a short section of 8mm dowel was glued. This allows the player to reach through the bass hole and adjust the position.

The crosspiece was finished with Danish oil before it was glued in place. The ends however, were not oiled, and scotch tape was used to help prevent this.

Both side-tapa braces were glued into place at the same time. The crosspiece is permanently affixed to the cajon body. Hopefully it doesn't slip over time....
(Update: The chalk probably was not necessary, as it takes a substantial amount of force to adjust the snare.)

Feet

The cajon feet need to support the weight of the player. When playing, the player often leans backwards slightly, so most of the weight is placed on the rear feet. It is highly desirable for these feet to be grippy, such that cajon does not slide around, or worse - slide from under you!

I had a lot of black rubber rollers from a photocopier I took apart in the past. These seem to be made from a very high quality rubber. I have used this in the past before for feet (for a car seat lounge chair) and they perform admirably.

These already have an 8mm hole through the middle. Using a 12mm HSS end mill, I bored this out further to a depth of 11mm so it could accept a 6G countersunk wood screw and a washer. I cheated and used a lathe, although this was still a trick process. Since the rubber is highly compressible, if you hold it too tightly, the resultant hole will be more trochoidal in shape than circular. Hold it too loosely however, and it just slips away from the end mill. A new and sharp end mill helped immensely here. 

For a touch of flair, I made some 3mm thick brass washers with a 1/2" OD (so that it is a slight press fit into the rubber), and a countersink (to match the 6G wood screw). These were turned out relatively quickly on a lathe with a 6mm drill bit, a countersink and a parting tool. Brass is amazing to machine, as you can crank the surface speed very high and still get a fantastic finish. I was able to use the same spindle speed for all operations because of this. The surface finish seen below is "as machined" (although I did polish the outer diameter, which was unmachined, using some Brasso).  

The photo does not do the brass any justice, but it adds a nice dash of contrast to the black rubber and silver screws. It also matches the screws used on the tapa.

Finishing

All surfaces were sanded with progressively finer grits of sandpaper (up to 800 grit) before applying finish. The tapa was finished with Danish oil. It was supported on a bed of nails while it dried. 

The finished result can be seen below. It was further sanded with 1500 grit sandpaper to achieve a silky smooth playing surface.

The cajon body got a coating of satin clear polyurethane, both inside and out - to prevent issues of differential expansion due to different rates of moisture absorption / drying of the inside and outside. Here it is with the first coat.

A total of 3 coats of polyurethane were applied to the exterior surface and 2 coats inside, sanding in between coats with 800 grit sandpaper. The final coat was finished with some 1200 grit sandpaper for a silky smooth surface.

Final Result

After some assembly work, a cajon emerges from the mess:

A thick foam mouse pad makes a nice seat.

A final touch involved adding a custom logo to the inside.

The cajon resonates nicely and gives a strong bass tone. The snare position and pressure needs some experimentation to make it more independent of the bass note. The body feels solid and was easily able to support the weight of two people.

Still to come ...

• Cajon #2 with carbon fibre tapa and guitar string snare