Blind Double Dovetail Joint

I used this joint once in the past for a student's project. I stumbled across a need to use it again while iterating on an end table design. It allows dovetailing a board into the face of another board with no obvious entry point. Drop in.  Slide over and the board won't pull out. Dovetailed sections with a slight taper do the job. 

The taper of the dovetail keys makes them easy to insert but wedge up tight when slid over. 

Cherry 4" wide into mahogany face 

I'm toying with a way to lock the joint in place when assembled.  Of course gluing them would work. This is to connect them without using glue.  First idea would be to use a spring loaded pin that would pop out and prevent the joint from sliding back to remove. Another idea is a pop-up plate that blocks the dovetail section from backing up once passed. Conical springs behind the plate would allow it to be pressed down out of the way when the board is first inserted, but then pop up when it slides over.  

Drop in, slide over for a snug connection.

Should I come up with a locking idea it'll mean the boards will need to be sanded and a finish applied before final assembly as they won't come back apart. The fit can be tested before the locking pin or plate is put in place. 

I used a 1/8" spiral end mill to removed most of the area before the dovetail bit undercut the edges. 1/2"d 14° dovetail bit.  On the cherry board end a 1/4" spiral end mill was use to pocket cut the areas between the dovetail section. 

As the joint is only 1/4" deep it could be done on the opposite side as well. You could simulate a board that passes through another board.  Using my angle clamping jig and some careful layout the board could intersect at any angle up to 15 degrees or so from perpendicular. Rotate the joint intersection for a compound angle appearance. 

4D

Unfolding Flat Pack Coffee Table Design

Unfolded

This design is loosely based on a folding workbench table I came up with 20ish years back. Reduced in scale and revised in detail the table is 16" high when unfolded, 42 inches long, and 15 inches deep. Folded up it is the same length and depth but flat and 2.25" thick. No tools are needed to erect it from it's folded state or to fold it back to its flat state. The base consists of two leg panels that aren't attached to the top, two flipping panels that are attached to the top end of the legs and the top, and two diagonal braces that attach to the top and near the center of the leg panels. A twist lever locks the flipping panels to the top when the table is unfolded.  This is so the table can be picked up without wanting to start folding. 

Underside View

For the folding action to work a precise connection point between leg panels and braces must be found. Through some iterative drafting steps I've been able to get within .001" of where it is. To fold flat both the leg panels and the braces have an offset end. The leg panels are one inch narrower than the  table top.  The flip panels and braces are tucked under the top to minimize their visual impact on the stance of the table. 

Leg Panels

Flip Panels and Braces

When folding up the flip panels flip over and out of the way so the braces can lay down flat against the top. 

Folded flat.

The pivoting connections are the only challenge left to resolve.  The flip panels connect/pivot from the top underside and have to flip 180 degrees over where they connect.  The braces connect to both the underside of the top and near the center of the leg panels. Some clearance between pivoting parts and where they pivot from is needed to prevent wood from rubbing against wood as they pivot. 1/8" (3mm) Baltic birch plywood could be used as the pivot brackets extending from the bottom face of the table top.  Glued into a slot they would be a challenge to remove/replace if they ever fail though. Plate brass or aluminum could be used but would need a mechanical connection into the table top.  This connection deserves an original design solution. 

The leg panels have openings for the flip panel and the braces.  Making each leg from three sections assembled to capture the other parts would be the obvious strategy.  I need a way to mechanically do this so they can be taken apart should any piece fail. My intersecting binding bolt trick I used on the last two TV tray tables might work.  I'd like to figure out a way to have the action of bolting them in also snug the joint tight. Some sort of cam or lever action. There is a double end cam solution already out there used for knock-down furniture that might work.   

It wouldn't take much to modify the geometry to side table dimensions. Longer legs and a shorter top might require one leg to fold over the other to keep the folded length within the length of the top.   

My house is overrun with coffee tables so I doubt I'll ever build this design. It is parked here for future reference so I can clear the idea from my mind and let it fill up with newer ideas. 

CNC Cut End-To-End Finger Joint

I cut this joint as a sample to show my college furniture design class students. Originally just an idea I had to prove would work.   I haven't found a practical application for it yet, but that was true for every original joinery idea I came up with.  So far roughly 50% have found applications in student furniture projects. 

Stacked Sides

Each side was cut using the same vectors.  Cut inside the vectors for one half, and outside the vectors for the other half.  A -0.003" allowance was used for one side. This joint pattern can be extended for any wider width of  boards.  Done with a 3/16" diameter spiral upcut router bit.   The bit diameter was what specified the shapes and radiused corners..  From the outside the joint looks like simple interlocking fingers. 

Closed

Slid apart the secret begins to reveal itself.

Slightly Apart

When together the joint can't slide sideways or up and down.  Locked in two axes. 

Apart.

To cut the joint required clamping the boards vertically in my CNC frame.  

While the halves slip together relatively easy once glued the considerable surface area between the sides would make for a very strong joint. One option might be to make each side from a different wood. Split a table top making one half walnut and other half pecan perhaps?  

4D

 

Process. Steps to Making a TV Tray Table

These are the steps I took to get from a trip to a lumber yard to a finished project. How I got from a board that was 1.25" thick (6/4) to parts that are 5/8" thick and 7/8" thick and wider than the rough board was. 

It is important to know the final sizes of all the finished parts.  A cut list is helpful.  There are 7 wood parts to this project.  The top.  Two outer legs.  Two inner legs.  A top stretcher and a bottom stretcher.  Out legs are mirror images of each other.   Inner legs are also mirror images of each other. 

I started with a 6/4 thick board of cherry.  It was 8' long and averaged 8" wide. 

1" thick slices glued up.

I wanted to end up with a table top that was 7/8" thick, 14.25" deep, and 19 + 9/16" wide.  I cross cut two 20" long sections from my cherry board. 

On my table saw I ripped 1" wide slices from those boards until I had eleven pieces.  Those pieces I tipped 90 degrees to lay flat, and glued them together to make the panel you see above in my clamps.  Rip and tip. Knowing that glue is slippery until it dries and that boards clamped up may slip out of alignment, I added 1/8" to the thickness of the slices to account for that. 

Smooth both sides. Ends trimmed square,

Out of the clamp with the surface scraped down I fed the panel through my drum sander to smooth off both sides.  Flipped it over between each pass. I was hoping it ended up 7/8" thick. My ancient Craftsman radial arm saw had just enough depth of cut to trim the ends square. I checked the thickness between passes to make sure it didn't end up too thin. 

7/8" thick.

I repeated these steps for a longer board to cut the legs from and a thinner board for the stretchers. 

Legs. 1" thick, 31" long.

Stretchers. 3/4" thick strips.

The board for the stretchers was planed down to 5/8" thick, and divided to make a 2" and a 3" wide stretcher. 

Once I had the boards down to the needed thickness, the parts were ripped from them.  The 5/8" thick stretchers them were cut to length. I made sure to add length for the tenons on each end. I used my CNC with the stretchers clamped vertically to cut the tenons on their ends.

2" wide top stretcher.

3/8" thick tenon. 1/8" shoulder

3" wide bottom stretcher

3/8" thick tenon in progress.

Using a clockwise climb cut leaves the shoulders clean with no tear out. Next job is to round over the edges of the tenons. I used a 5/16"radius round over bit in my router table. 

All edges rounded over.

Take care to set the router height so the bottom tip is flush but not above the table surface, and that the fence face is flush with the bit's bearing. 

The inner legs of the table are what the stretchers attach to.  Using the same vector outline that I used to make the tenons I laid them out on the leg shape to cut the mortises on my CNC. 

There are several steps needed to complete the legs. Mortises came first.  A hole for the pivot bolt is needed.  The ends need to be rounded over.  The edges all need a 1/8" radius to remove the sharp corners. A slot for the tension strap and a hole for the binding screw are also needed.

The top plank needs to be cut to final width and depth. The front and back edge need to be rounded over. The sides need to have their edges either chamfered or rounded a small amount.  I have both a 1/16"r and a 1/8"r round over router bit, as well as a 45 degree chamfer bit that can be adjusted to take of any amount up to 1/2".  1/8" may be just a bit too much. 

One challenge still to be resolved is that the pivot bolts that hold the inner and out legs together, and the outer legs to the top can unscrew or tighten in use.  There are a few possible solutions, but whatever strategy is used it should let the bolt end turn inside the top or inner legs. You don't want the bolt head to have to spin against the outer legs. 

My initial idea was to trap the pivot bolt in place with an intersecting binding bolt through the inner leg. This works as expected, but suffers with the head and upper shaft of the pivot bolt spinning inside the outer leg. 

Trapped bolt.

A perimeter grove around the pivot bolt would let it spin while still trapped by the intersecting binding post.  Another test is due.  My CNC has a radial axis and a 3-jawed chuck that can hold the end of extra long bolts.  The head can be held and centered by the tailstock center of this 4th axis. With a little trepidation I let the CNC give it a try, and this is the result: 

Pivot bolts with perimeter grooves.

The threads on these bolts will be cut off roughly 1/8" past the groove. The ends will be filed clean with a little chamfer.  Binding bolts will intersect and pass through the groove, keeping the pivot bolts from come out but still letting them spin as the table legs fold/unfold. 

Binding bolt for context.

Finished pivot bolts

The outer legs of this version have their offset hole at the top accessed with a bump at the top end of the leg. 

A short piece of cherry was glued to the longer leg at the end.  The shape and hole position will be marked and cut out/drilled once the glue dries. 

CNC cut ends

I used my CNC to cut the profile shape on the top ends of the outer legs. The bump shape is critical for the location of the point that connects to the table top. This offset point permits the design to fold up flat as well as unfold with either a slanted top or flat top to use. The angle at the top will align with the slant of the table top as seen in the drawing above. 

The bottom end of the outer legs, and both ends of the inner legs needed to be rounded over.  Again I used my CNC for that job.

Leg ends rounded over.

The same toolpath, and some fixturing on my CNC bed made this a quick task to complete. As the angle of the legs changes when the table changes from flat to angled, rounding the end over always provides a tangent point to touch the floor.  Legs are all now cut to final length, mortises cut on the inner legs, and offset bump added to the top of the outer legs. 

The holes in the legs require precise positioning. I have a CNC to use and since it is capable of that precision it is what I used to drill all the needed holes. The bolt heads have a slight fillet on their underside and using a trick I came up with I used the same endmill for the needed chamfer as I did to drill the hole. 

The trick:  Bolt hole chamfers

Intersecting holes and counterbores were needed for the binding bolts that keep the pivot bolt from coming out. Both on the inner legs and the back edge of the top. It is important  to line up the counterbore on both sides with the shaft hole.  

New 18mm long binding bolts showed up.  Verifying their head diameter and thickness, and the shaft diameter and length was important before I had my CNC cut the holes for them. 

Leg frame standing up

The recesses for the heads of the binding bolt and the shaft of the binding bolt ended up just a little too perfect. Pivot bolt cove is snug against them when inserted.  The binding bolts do intersect and pass through to keep the pivot bolts from coming out. The pivot bolts can spin, but not as easy as I intended.  A 5mm diameter diamond coated file came in handy for making the pivot bolts spin more easily. 

With the holes for the binding bolt done on the top the stand can now be roughly assemble to verify stance and folding action.

Standing up with top

The corner, close up:

Details for the top remain to be done. The outside of the outer legs still needs an edge detail.  This is the point in my process where I stop and reflect on the design. Let my mind process the look and come up with a set of final details I'll like. All the functional work is done and the geometry is verified. 

The slanted side of the top requires a projecting ledge near the front end.  I used a 3/16" diameter spiral upcut bit in my router table to make a 1/4" deep slot, just a hair more than 1/2" from the edge. The slot was centered from the ends, and 17.25" long.  The strip of wood used was 1/4" thick, 1/2" wide, and 17.5" long.  I trimmed 1/32" off each side, 1/4" high, so the wood strip would fit in the 3/16" wide slot. I used my bandsaw to notch 1/8" off each end so the ledge strip would fit lengthwise into the slot. 

Bubinga ledge added.

Made from what I believe is bubinga wood.  Machining it produced a vague cinnamon smell. 

Glamour shot.

Next comes rounding over the front and back edges of the top,  I had considered a fancier
detail but opted for simplicity.  In use this table is not for displaying something precious, but rather a utilitarian life of service to the owner.  The top should be easy to clean, with no unnecessary grooves or coves to deal with.  My router table and a 7/8" diameter roundover bit will do the job nicely. 

All set up to round over the top edges. 

Rounding over the edges remove considerable visual mass. Softening the side edges take away the sharp corners to make it more user friendly. 

Next comes the slots for straps and holes for binding bolt to hold the straps in. One thing that is not unusual when making a project is that mistakes can pop up up nearly anywhere along the path.  Wood is forgiving though and relatively easy to patch.  This is what happened when I cut the slots for the straps initially. 

One slot too low.  Patched

The nice thing about working with wood is that it can usually be repaired/patched/re-cut to fix a mistake.

Recut.  Slots now align.

Only a minor offset due to being removed then replaced on the CNC for the recut. Next come intersecting holes for the binding screw that will keep strap ends in the slots. 

Strap ends have a grommet hole through them. 

Grommet in strap end. 

With the binding bolt holes done, and set of straps made that hopefully will be just the right length are made and installed, it is time to stand the design up and see how the well straps work. 

Test straps tested.

When standing up the front to back distance of the legs should be the same as the depth of the top.  The first set of straps were close, but left the legs spread just a bit too far apart.  Straps are relatively easy to make so a second set was made just a little shorter from end to end.  The needed length is difficult to determine, even with accurate CAD drawing to consult.  The distance the straps are down from the pivot point was determined from where the top front edge falls when the stand is folded flat.  

The straps bend sharply out of the slot.  To keep them from wearing down I've came up with a CNC cut toolpath to round over the edge of the slot to remove the sharp edge.

Slot rounded edge.

Results of the moulding toolpath are nice smooth edges for the straps to lay over as they leave the slot.

Smoothed slot edge. 

This Cherry TV Tray Table now has every machining step complete. The center leg frame has been glued together, stretchers into the inner legs.  All leg edges have a 1/8" radius round over. The legs have all been sanded to 220 grit.  The top still needs sanding down smooth before the table gets 3 or more coats of Danish Oil. As the top will see the most abuse I'll sand it down to 300 grit and give it a minimum of 3 coats of Danish Oil. 

With 2 coats of  Danish oil on them they are waiting for their 3rd coat: 

Top. Slanted Side Up.

Inner leg frame and outer legs.

This is the payoff. Back together. Standing up to show off. The reward. The reason to weather through all the mistakes and steps needed to get from rough wood planks to a finished, working piece of furniture. 

Slanted top.

Flat, level top.

When the design parts come together, verifies that all the holes were in the right place, that the bolts and straps were just the right length, and that the geometry proves itself, you get a big smile on your face.

Out side out.

In side out. 

From the rear of the table, lift the top, pull the top stretcher toward you, then set what had been the top of the top down onto the stretcher.  You've now turned the table inside out.  

Choosing a strap color required some premonition and faith that when finished the wood would look good with the straps. 

Green webbing straps held in with binding bolts. 

Finally, when a project is done the tools can be put up, the messes cleaned up, and in this case the project can be folded up to rest until needed. 

Put to bed. 

This design is patented.  

For info on licensing the design please contact:

Sarah Nolting
Licensing Associate
Kansas State University Innovation Partners
(785) 532-3910
snolting@ksu.edu
www.k-state.edu/innovation-partners