Tapering French (Sliding) Dovetail Joints for Ease of Assembly

Tapered French Dovetail 

French dovetail joints are a good way to connect critical cabinet parts.  A straight dovetail slot is easy to cut with a straight edge, router and a dovetail bit.  The mating male end of the joint takes a little more care to size right.  I've used a router table with the same dovetail bit and a tall fence to trim off the sides of the mating part end until the dovetail end fits snuggly into the slot. 

On wide boards a snug French dovetail joint can be difficult to slide in place. Friction builds up the farther into the slot you go. Getting the joint closed occasionally requires the pull of a bar clamp or tap of a wood mallet. The tendency of wood panels to bow or flex a little can add to the difficulty.  

Tapering the routed slot is fairly easy.  Make a straight slot against a straight edge first. Then move the leading end of the straight edge over 1/16" or so. Make note of how far you moved it.  A penny (1 cent) measures almost 1/16" thick (.06" rather than .0625"). I usually moved the straight edge end 1/16". 

To taper the male dovetail end a shim as thick as the amount the straight edge was moved is needed.  I tape a penny (tape + penny=1/16") or laminate sample chip to one edge of the mating piece just above the height of the dovetail bit. As the board slides past the router bit that shim holds one end away from the bit 1/16" to effectively taper that cut. The other side is cut parallel to the face. Care needs to be taken to match the side of the tapered slot to the tapered side of the joining piece. Cut on the wrong side the joint will still slide together, but be slightly crooked when in place.

CNC cut Dovetailed Side

CNC cut Tapered Slot

Using the CNC to cut the tapered slot is easy. The CNC cut tapered slot can be centered so there is no chance of ending up with a crooked panel. A vector that starts at the beginning of the slot, runs to the end of the slot, then returns to the beginning 1/16" away from the start point is all that is needed.  A profile toolpath centered on the line with cutting depth set for one pass for the dovetail bit will cut the tapered dovetail slot. 

Vector for Tapered Dovetail Slot (blue)

In the drawing above the green circles represent the diameter of the dovetail bit at beginning, middle, and end of the vector. The red lines show the width of the intersecting board. Blue is the vector needed and it is extended outside of the board by half the diameter of the bit.  This lets the bit drop down to cutting depth before it enters the edge of the board. To ease the stress on the dovetail bit I usually run a 1/4" down-cut spiral end mill on the same vector to clear out the slot before the dovetail bit runs to undercut it.

Vector for Male Dovetail End

Cutting the mating end requires being able to clamp the mating board vertically and level under the CNC spindle.   Making the toolpath to move the dovetail bit around the mating end requires some careful drafting.  You need to know the side angle, diameters of the dovetail bit, and depth of the dovetail cut.  Offsetting the male toolpath vector from the vector used to make the tapered slot will ensure a matching taper.  The amount you offset that vector is the necessary detail to come up with.  See the diagram above. One half of the bit tip diameter plus 1/2 the bit diameter at the cutting depth is how far you should offset the vector. I use a section view of the dovetail bit to find the cut depth diameter of the bit.  

Almost together. Still loose.

Alternately you could simplify the CNC toolpaths to duplicate the result of doing it without a CNC.  This is easiest if the dovetail isn't blind, and runs completely though the board. Two vector lines, with one slightly angled, to run the dovetail bit down on-the-line. To make the mating dovetail you'll need one pass down one side of the board, and an angled pass down the other side of he board. Placing the vectors for these requires knowing the bit tip diameter and the diameter at the cutting depth on the bit.  

With all CNC cut joinery the precision fit can be hard to nail down.  Using the same vectors and precise offset leaves no room for error or glue. I prefer to do the male side of the joint first, then the female slot.  I leave the slot side on the CNC to check the fit before unclamping it.  If the joint doesn't close completely you can reset your X axis (or Y axis depending on how you clamped up the board)  by a few thousandths and run the dovetail bit toolpath again.  I find .003" is a good amount of offset between sides of the joint for a good snug fit with room for glue. With sliding French dovetails it should take just a tap or two of a wood or plastic mallet to close the joint completely.  I tweaked my sample until my joint took two taps to close and will take the same two taps to free it up. 

Snug and Tight when completely together.

Comments and questions I encourage and welcome. 

4D

Woodworking Education: Tiny Table 4

Tiny Table 4 is another example for the final beginning workshop class project.  This charging table example is also 24 inches tall with a 9" diameter decagon (ten sided) top. It is also another example of detail continuity throughout the design.  These four Tiny Table projects are made as initial diverse examples to show and inspire the students. The creative projects designed and built by students of this class will become the new examples to show and inspire and challenge the next class of students. Rather than have the next class start from scratch with this project each class year sees a new baseline to challenge them to improve upon.  

Top face of decagon top.

The top started out 1.157" thick.  The top side has a 45 degree chamfer around the edge to help the edge look thinner.  The chamfer also catches light differently than the top, adding visual complexity/interest to the simple top. 

Decagon Top. Bottom view.

The bottom face of the top is carved to emphasize the ten sides as well as reduce the visual thickness of the wood slab. 

This project teaches how to chamfer the perimeter edge of the table top.  It also teaches a simple way to make an interesting 3D shape on the bottom face of the table top using the CNC software.   With the CNC software installed in the college computer lab, students are assigned to design their own 9" maximum table top, then save the toolpaths for it to cut on the small CNCs.    

There are 5 legs. Five legs provide the most efficient and stable footprint for the nearly circular top. All legs start at 3/4" thick.  The legs taper inward to just a bit thicker than .5" at the top inside corner. 

Beveled Edge and Tapered Thickness

The outer edge of the legs has a double chamfer that matches the corners of the top above them. The top edge of the legs is shaped to echo the carved underside of the top above them.

Top End Profile

The leg elevation view tapers down to 1.25" wide at the bottom. 

One of 5 legs.

The legs are a complex shape and require several processes to produce. This project teaches the logical sequence and tools used for the steps needed to produce it:

 

1. While the board is still a rectangle, cut double bevels on the straight outer edges of the legs. Use the table saw with blade tilted 18 degrees.

2. Cut out duplicate parts (5 legs) using a template. Use double sided tape or a few spots of hot melt glue to attach the template to the leg board.  Rough cut within 1/16" or so with the bandsaw and use a flush cut pattern bit on the router table to trim flush to the template.

3. Layout and cut mortises.  Use the mortising machine or the drill press with a Forstner bit followed by chisels to clean out the mortise. Or potentially start with a slot cut using an end mill in the router table followed by some chisel work to square the mortise.  A CNC configured for clamping boards on edge could also be used.

4. Taper both sides of the leg thickness from outside to inside edges.  Use a sled with shims to tilt the leg slightly (2.7 degrees) and run through a planer or the horizontal drum sander. Repeat on the other side but shim the board 5.4 degrees. Shims can be cut using the bandsaw. The digital angle gauge can check the shimmed angle. Hot glue in place before running through either machine. 

 

Short 1/2" thick stretchers are mortised into the inside edge of the legs.  The inner end of the stretchers dovetail into one face of the 5 sided center hub sections. Top and bottom edges of the stretchers have a small double 18 degree bevel to perpetuate the table top corner angle detail.

Top Stretcher Dovetail End

Top Stretcher Tenon End

There are two center 5 sided hub sections.  One is 2" tall underneath and connected to the top. The second is 3" tall and roughly halfway down the legs. The hubs connect all the legs together. Stretchers from the legs dovetail into these hubs.  Exposed ends of the hub sections are shaped to a shallow peak to contribute to the overall design aesthetic. This project teaches how to make a 5 sided block, and how to cut dovetail slots perfectly centered on the sides of a 5 sided block.

Top center hub.

There are 3 other Tiny Table examples that along with this one will show students the level of detail and range of creativity they are expected to achieve. The continuity of details in this project add to the story that can be told about it. It is the story about the table details more than a quick glance at it that make it memorable and more desirable.   

More related educational projects are here:

Tiny Table 1.

Tiny Table 2.

Tiny Table 3.

The Lathe.

The Joinery Box.

Tool names and Functions.

Material Sample Blocks

Making Samples vs Making Projects.

Learning Actual Project Costs.

Calculating