Introduction: R Credenza
The R Credenza is a compact hutch that takes shape out of a single sheet of plywood, cut by a CNC router. Made of just seven pieces, it snaps together with precise mortise-and-tenon joints and fat yellow ratchet strap. Sliding doors conceal three linear feet of storage, neatly compartmentalized by two nylon dividers. The name derives from both the fastening system and the side pieces, whose form echoes an abstracted "R."
I don't own a CNC machine. This project was prototyped at the Fab Lab Baltimore, a makerspace that provides low-cost machine time as a community service. If you don't own a CNC, search for a Fab Lab near you; there are 74 in the U.S. and over 400 worldwide. I am hosting the actual cut file as a download on my site, or you can gather enough from this Instructable to draw your own in CAD.
The R Credenza is an open-source object: its source code is available, and I encourage anyone who downloads or copies the design to hack it, improve it, and post the results here! Post your work on social media using the hashtag #OSOdesign to follow other makers creating open source objects.
You will need these tools:
- ShopBot PRS Alpha or similar CNC router that can accommodate a 4' x 8' sheet of plywood
- 1/4" end mill bit
- Drill/driver (unless you have a vacuum bed for the router)
- Orbital Sander
You will need these materials:
- 1 5/8" sheet of plywood (I used construction grade)
- 2" wide by 12' endless ratchet strap (I used this one)
- Handful of drywall screws
- Finish of your choice
Step 1: CNC Routing
Open the cut file in AutoCad or similar program to double-check that all the lines are closed and assigned to the right layers. Measurements here and in the file are in inches, but you can easily switch to metric in AutoCad preferences.
The red layer is for Profile, or cutting and the blue layer is for Pocket, or routing only 1/4" deep. The white layer should not be set to cut; it merely represents the outline of the 4' x 8' sheet. This file is optimized for nominal (U.S.) 5/8"-thick construction plywood. It is actually 19/32" thick, or 15.9 mm. I did this to make this build as light and inexpensive as possible.
While the 5/8" works, it is prone to some bowing under the pressure of the ratchet strap and the legs are not as strong. I would recommend changing to 3/4" plywood if cost is not an issue, in which case you must modify the depth of each tenon and the width of the door grooves to match.
Secure the plywood to the spoil board in with drywall screws or a vacuum table, taking care to keep screws out of the eventual tool path. Secure a 1/4" end mill bit in the router collet. Set the home position, Z height, and spindle speed on the router.
Import the cut file into the G-code program of your choice. Set the red layer (Profile) to a cut depth of 22/32", or slightly deeper than the thickness of the plywood. Set the tool path for all the piece outlines to cut outside (to the right) of the line. Set the tool path for all the interior tenons to cut inside (to the left) of the line. Set the tool paths for the four 2" lines inside the top and bottom pieces to cut to on top of the line. Set the blue layer (Pocket) tool path to cut 1/4" deep in between each set of parallel blue lines. If not using a vacuum bed, set at least four tabs per piece for safe cutting.
Run the cut file!
Step 2: Finishing
Use an orbital sander to clean up the pieces. Start with 80 grit, then 100, then 120. If using better-grade plywood, you can skip the 80 grit. Use a small piece of sandpaper roleld into a tube to chase out the finger holes in the doors and clean up the edges of the mortises and tenons.
I painted each door yellow to match the strap; since the doors are easily reversible, you can always turn them to natural wood if you end up hating the color. This step is optional.
I finished with an over-coat of flat lacquer, then sanded with 220 grit and added a coat of wax. This minimal finish is durable enough for a utilitarian piece.
Step 3: Assembly
Despite all the photos in this step, assembly is quite easy!
There are seven pieces. The long piece with 10 tenons is the back. Insert the tenons on each end into the corresponding mortises in each side piece.
Snap in the bottom, with the door grooves facing inward towards the eventual top. Add the top and doors, trapping the doors in the grooves as you do so. Squeeze the sides together and lift onto its feet.
Starting with the ratchet strap inside the body of the cabinet, feed the free end down through the left-hand slot in the bottom. Pull it through the slot that is part of the bottom mortise on the left side piece, then up around the entire credenza, through the opposite side mortise and back into the body of the cabinet. Bring it up, through the right-hand slot in the top of the credenza, over 12" and down through the other slot.
Feed through the ratchet mechanism and tighten until sturdy, but not so tight it bows the wood or keeps the doors from moving fluidly. Keep straps centered on unit to evenly distribute force.
Set it up and put it to use.