Fig. 1 Three of the six finished chairs 

I have been working on my skills and techniques for building this style of dining chair over several iterations, some of which are offered on this site.  Recently, I’ve had it in mind to do a complete matched set from good quality lumber, not from the cutoff bin leftovers of the previous learning builds.  I have also felt that I should dabble in if not learn inlay and marquetry techniques, and possibly learn those new skills while producing something practical.  All that came together one evening when scrolling through Craig’s List ads. 

 

  Fig. 2   Lumber as received. 32 long boards and 4 shorter boards.  One layer is resting cross-wise on a second layer while being acclimated for a couple weeks in my shop. 

From time to time I scan the local Craig’s List for lumber, just to see what can be had.  Every so often, some really nice boards show up.  One evening an ad for 60 board feet of Alaskan yellow cedar caught my eye.  After it had been up for several days, I dithered while I kept wondering if this nearly pristine clear straight-grained wood could serve well for my set of chairs.  Finally, after hesitating for a few days, I could not resist buying the wood for my matched set.  The wood itself has an interesting story as I learned when I picked it up.  A well-known Shoji screen maker here in the Bay Area had come across a Japanese style cedar hot tub that the owner (I think he said they were one of his customers) wanted dismantled and carted away.  Lucky for me, he took the trouble to pick out and clean up the salvageable pieces and offer them for sale.  As I received them (Fig. 2), they were planed square and straight.  Four or five still had some screw holes, with accompanying rust stains, but the rest were perfect.  All in all I had 32 boards 42.5” long x 5.5” wide and 1.5” thick, as well as four shorter boards of the same thickness.  I easily had more than enough to make a set of six chairs.

I began laying out just how I would make the set of matching chairs now that I knew what the lumber looked like.  As I contemplated the build, I realized that the lumber brought a certain elegant calmness with it, but the chairs would need something to offset that with a bolder statement.    For contrast I chose bright red seat upholstery to create a warm color palette of red with the yellow hue of the timber.    To balance the broad smooth areas, I also wanted to include some elements with visual interest.  I chose to include detailed inlay patterns on the front seat rail and back of the crest.  As seen from a distance the red upholstery first catches the viewer’s eye, but up close the inlay patterns draw more attention.  At least that was my plan.  I spent a couple days designing patterns and finally settled on some simple line designs that could be carved by the CNC.
 

 

 

Fig. 3   Scaled Line Drawing of Chair - front and side views 

 As with all my larger projects, I begin with a scale line drawing of the finished piece (Fig. 3).  The individual parts can then be pulled out as single drawings that feed directly into CAD software, Vectric VCarve in my case.  Only the back legs would benefit from this treatment since all the other components are essentially straight line elements or easily shaped on the band saw.  Indeed, I could have chosen to lay out the back legs by either directly penciling the shape on the lumber or alternatively with printed paper templates glued on.  But I wanted to try using a plywood template to band saw the blanks then without removing the template use a trim router bit to get to the final dimensions.  I quickly found out that a ½” thick MDF template is easier to work with than plywood in this situation.
  

 

 I loaded my original drawing for the back legs into VCarve, generated the CAM code, and cut the back leg template on the CNC router, including the mortise positions for the back rest and frame rails.  A 5” flat, designed into the leg where it joins the side rail, serves to align the template to the cedar stock (Fig. 4).  After clamping template and stock together (Fig. 4A) they are locked together using expandable plugs (Fig. 4B) and flat head wood screws.  The leg is roughed out on the band saw using a standoff jig against the template leaving about 1/8” to be removed with a straight trim bit in the router table (Fig. 5).  The template is released, but before the expandable blocks are removed, the mortise positions are traced around them.  Finally, the mortices are cut on a bench top mortiser.  If I ever do something like this again, though, I would use a spiral cut bit instead.  A spiral bit is more expensive, but worth it for the quality of cut and the control it gives while feeding the work piece into it. 

 

 

Fig. 5 Trim bit mounted in the router table, shown after transferring the final profile from the MDF template (bottom) to the back chair leg (top). 

Starting with 1¼” square stock, the mortise positions for the front and side rails are marked and cut with the bench top mortiser. A simple 1.5° straight taper on all four sides, extending from just below the rails to the end, is cut on the table saw using clamp-downs built into a sled.  A little touch-up on the easy-to-sand yellow cedar completes the operation.
 

The rail tenons are milled using the horizontal adapter on the CNC router (Adapting a CNC for Horizontal Milling).  Since the back legs are splayed 4° outward and the side rails are angle another 9° outward, the back tenons have to be cut at a 13° angle to the rail at the back, and 9° to join to the front legs.  For the back rail, the tenons are cut at 4° each end.   This is easily accomplished by angling the board the proper amount relative to the router axis when cutting the tenon. To facilitate alignment at the desired angle, grooved lines were previously cut on the CNC table to make positioning the workpiece fast and simple.  If you look carefully at Fig. 6, you can see the board is angled relative the CNC table, a subtle 4° in this case.  While it is possible to do this same operation on the table saw, for me, the setup is easier on the horizontal CNC.  Once the work piece is properly positioned, and the CNC zeroed in x, y, and z, stop blocks are clamped to the CNC table to ensure exact repositioning of similar pieces.  Cutting the tenons then becomes a nearly push-button operation for the remaining five.  As a bonus, all the corresponding mortices can be cut square to their stock, making for a simple set up when mortising as well (Angled Mortises Blog).

Fig. 6   A 4° angled tenon on the back rail being milled on the horizontal CNC. 
 

Fig. 7  Routing the design into a curved front rail using a 1/16” bit.  Cut depth is 3/32”.  The upward bow of this piece is evidenced by the shadow underneath it.  After cutting, a thin nail file is used to remove the fuzzy fibers left inside the grooves by the router bit. 

The inlay patterns on the front rail and on the back of the crest required a great deal of experimentation.  I had designed the intricate inlay patterns for CNC cutting, not fully realizing that the surface to be cut is curved in the z-direction.  (On the finished chair the front rail curve bows forward, but once the rail is mounted flat on the CNC bed, that same curve becomes an upward bow.)  Generally speaking, a CNC can easily handle 3D work if properly configured, but mine is not.  A lot of dollars would have to go into upgrading the CNC with full 3D capable software.  The secret is to trick the CAM software into bending the design onto a curved surface.  Luckily, VCarve allows importing a single 3D model.  VCarve can also make an otherwise 2D cut conform to the surface of the imported 3D model – just as though the 2D pattern was ‘melted’ onto the 3D model surface.  (Think cheese melting on nachos, if you will.)  I found a suitable 3D model – a cylinder laying on its side – and stretched and resized it until a CNC test cut on some scrap pine boards replicated the front rail curvature I had already cut on the band saw.  Then, I superimposed the pattern on that surface in the software.  Another test cut proved all was ready.  The inlay cut bottomed uniformly below the rail surface along its full length.  Using stop blocks for repeatability I made quick work of cutting all 6 front rail patterns and all 6 crest rail patterns with the CNC (Fig. 7).  After some clean up sanding, the pieces were ready for pouring the inlay epoxy.
 

 Initially, I thought that pouring tinted epoxy into the routed pattern would be slam dunk simple.  Not so.  Test pieces showed that the tinted epoxy wicks up into the cedar grain, giving a fuzzy blurred look to the line definition.  After experimenting with various clear sealers, including shellac, I settled on clear Rustoleum spray paint from a rattle can.  The spray allowed me to easily coat the groove side walls.  Tests showed that there was essentially zero bleed-through of the red tinted epoxy.  Still, I was not impressed with the color and sheen of the tinted epoxy fill.  Finally, after more trial and error, I settled on a coat of red Rustoleum paint from a rattle can applied over the initial clear coats.  After three applications from different spray angles, I achieved a good coating of the bottom and side walls of all the grooves.  Backfilling with clear epoxy gave the desired look, with a bonus; an illusion of depth that tinted epoxy did not deliver (Fig. 8).  

I quickly found out that dripping epoxy into 1/16” grooves is tedious, sloppy, and leaves a lot of bubbles that are nearly impossible to coax out.   I resorted to vacuum outgassing the epoxy prior to pouring in order to minimize bubbling.  A syringe and needle facilitated dispensing directly into the thin grooves.
 

Fig. 8  Making the inlay.  From left to right:  Left: All six crests with clear coat sealer applied before the red paint coat – blue tape protects the tenons from overspray.  Center: A test board with red spray paint and epoxy ready for scraping.  Right: A finished front seat rail after scraping and sanding

After epoxy curing, the next step was to remove the overfilled epoxy and the coats of red and clear Rustoleum from the surface of the cedar, leaving bare wood everywhere except in the pattern.  A freshly sharpened card scraper made quick work of preparing the surface for final finishing.  But the card scraper left a dull surface on the epoxy.  Sanding it with progressive grits ending with 600 grit (Fig. 9) restored the epoxy surface to a nice reflective sheen.

 

Fig. 9   Close up of a completed inlay before final finishing.  No bleeding of the infill color is evident. 

 

 

Fig. 10   MDF measurement templates inserted in the crest rail bridle joint mortise during dry fit.  Careful measurement of the top and bottom lengths of the crest rail length produces accurate tenon cuts and a tight fit to the back legs.  Similar measurements are done for the lumbar rail. 
 

The lumbar and crest rails present two challenges: 1) cutting the tenons at the proper compound angle and 2) achieving the right length for the part, as measured tenon shoulder to tenon shoulder.  Slight variations in the back rail, when joined to the back legs, resulted in small but noticeable differences in the lengths of the lumbar and crest rails.  For that reason, the rails had to be individually hand fit on each chair.  To do that, I cut some MDF templates having the same arc as the rails.  By fitting them into the mortices and marking them (Fig. 10), I could transfer the dimensions to the unshaped rail stock pieces.  When zeroing the horizontal CNC, I set the depth of cut to produce tenon shoulders that resulted in the desired length.  That solved the length problem, but I still needed a solution to the compound angle problem. 

It took more than a few minutes to figure out how to set up for the compound angle formed by the splay of the back legs (4° for each leg) and the fact that the tenon is rotated to match the tilt of the chair’s back, 12°.  It finally dawned on me that in addition to the 4° angle used to mount the stock on the horizontal CNC, I also had to rotate the stock by that 12° while programming the CNC to cut the tenon at that same 12° relative to its own vertical axis.  To accomplish this, I made a cradle that rotated the stock around its long axis 12° from horizontal (Fig. 11).

 

 

Fig. 11   Left: The 12° Tilt jig.    Right: holding a finished tenon (clamps removed).

 To recap: To make a tenon cut for the lumbar and crest rails, I first clamp the stock to the 12° jig, producing the necessary rotation.  Then, I swing the jig 4° off the CNC center line in the horizontal plane.  Finally, I program the CNC to cut the tenon tilted at the same 12° from vertical.  The depth of cut and stock center are acquired by zeroing the CNC depth of cut to the center of the tenon at the shoulder.  I know this all sounds complicated, so I don’t blame you if you just look at the picture (Fig. 12) and move on.  But it does work, and with a little planning, all similar tenons can be cut with, yes I will say it again, a nearly push-button operation as well.  In my opinion, it is easier than laying out a compound angled tenon by hand followed by hand cutting or cutting on the table saw, especially when making multiple identical pieces.
 

 

Fig. 12   Dry fit of the crest and lumbar rails. 
 

Before any assembly, I sand each part progressively to 220 grit and polish with 0000 steel wool.  For final assembly, I first glue the front legs to the front rail and set them aside.  Similarly, I glue up the back legs to the back seat and lumbar rails, with the top crest dry fit while the glue dries.  Then, I remove the top crest, glue in the back slats to the lumbar rail, and finally glue the crest to the back legs and three slats by slipping it down the bridle joints.  The bridle joints make the back rest assembly easier, since they do not require the crest to be glued into mortices simultaneously with the lumbar rail while juggling the slats into place.  At this point I hand saw the bridle joint flush with the legs and crest (Fig. 13).  Finally, I glue up the two sub-assemblies, namely the front and back leg assemblies, to the side rails (Fig. 14).
 

 

Fig. 13   Removing the excess material at the bridle joint.  Tape protects the crest during cutting.

 

Fig. 14   Joining the front leg sub-assembly to the back leg sub-assembly

Corner blocks are cut on the miter saw at the same angles that the side rails make with the front and back rails.  One face of the front blocks is rounded slightly on the belt sander to match the front rail curvature.  They are all marked and drilled to accept 10-32 machine screws, for later mounting the seat, and glued in place (Fig. 15).

 

Fig. 15  Adding the corner blocks for strength and mounting the seat pan. 

I retained the CNC code for the slip seat base from earlier builds of this chair style, so cutting more from ½” plywood was straight forward and painless.  The CNC cuts the outer edges as well as five evenly spaced slots (Fig. 16).  The slots add some flexibility to the plywood as well as allow air to escape when the foam cushion is compressed.  Each seat pan is individually dry fit to a chair and the locations of the corner block bolt holes marked for insertion of 10-32 tee-nuts.  Layers of foam are glued up using spray adhesive, and the covering upholstery stretched and stapled to the underside of the plywood base.

 

Fig. 16  Plywood seat pans ready for upholstery.  The 10-32 tee-nuts are visible in the corners. 
 

The finish is three coats of hand rubbed shellac to seal the grain, followed by three coats of hand rubbed Minwax Tung oil varnish. The varnish adds a slight yellow tint to the cedar that actually enhances the look of the wood.  A light sanding between varnish coats knocks off the dust nibs.  Lastly, the seats are bolted to the frame.  The chairs are primarily used as indoor dining chairs, but I put them in the bright outdoor springtime sun for their photo shoot (Fig. 1).