Oak Dining Chairs
Wednesday 04 March 2009
Chris Tribe makes the chairs to go with his table
The client requested flat solid seats and that the table be fairly chunky. A chunky table requires a fairly solid chair, provided by the 45mm-section legs and solid seat. Only five chairs? I also made a bench as part of the suite so the sixth chair was not required.
An important part of the design process is the creation of a full-size drawing and dowelled and screwed mock-up in pine. This enables you to try out the seating position for comfort.
If you have to change the mock-up, reflect these changes in the drawing and templates.
To discourage the sitter from sliding off the solid seat it is set at an angle of about 2 1/2 to 3 degrees below horizontal.
Using the full-size drawing, create a cutting list and machine the components.
Shaping back legs
Making templateThe back legs are shaped using a curved template. Form the template in 9mm MDF using a router and trammel, photo 2. For long distance trammelling I replace the fence rods with long 8mm threaded bar.
At the pivot end I use a spare router fence slider drilled to receive a 3mm pin. I also use another slider fixed midway to provide added stability.
Unless you have a ridiculously large workbench you will have to work on the floor at this point. Fix the template material to the floor with some waste material below to protect the floor, and establish the centre point for the inner curve.
Cut the inner curve, photo 3, then reset the trammel length for the outer curve. As the leg is tapered the centre pin will also have to be moved.
Cut the template to length, making it about 50mm over length, so allowing for fixing the template to a waste area of the leg piece and also providing a lead-in when machining the shape.
Create a short flat area at a tangent to the curve where the rail is mortised into the leg; this will save having to curve the tenon shoulder and will not be noticeable. Mark onto the template the position of the rail mortises.
Using templateFit the template to the leg piece by screwing into areas that will later be mortised, or into waste areas at either end.
Shape the leg using a bearing-guided template trimmer on the router table, photo 4. The extra length on the template allows engagement with the bearing before cutting. Stress is also reduced by bandsawing the majority of the waste away; the router is just for final trimming.
Before removing the template transfer the markings for the mortise positions.
Now cut the waste from the ends of the back legs. Mark off the positions of the top and bottom by measuring from the side rail mortise. Use a simple jig on the saw table to ensure that the ends are cut at the right angle, photo 5, this being ascertained from the drawing.
The 3 degree angle at the top of the front legs could be cut now, but I left this until I could mark it off when dry assembling.
Overcoming complicationsThe 3 degree slope on the seat requires a slightly angled front leg mortise.
To simplify jointing into the back I arranged the curve of the back legs and the flat on the curve for the rails so that the rails are at 90 degrees to that flat.
The leg mortises are complicated by the shape of the back legs, necessitating a number of jigs. All the mortise and tenons are 9mm. Although the side rails are splayed the mortises are cut square to the leg faces, the splay being created by angling the tenons.
Legs to side railsFor mortising the back legs to the side rail make a curved block to support the piece in the mortiser. Fit the block to an L- shaped backing piece that can be clamped to the mortiser carriage, photo 6. By shifting the leg on the support block an angle of 90 degrees between the flat and the chisel can be achieved.
Mark off the back-piece mortise position as a reference for subsequent legs and set the stops on the mortiser so that the mortise is repeatable, photo 7. Remember to reverse the settings between the left and right legs.
For mortising the legs for the back rails a concave curved block is used, again attached to the L-shaped backing piece. To ensure accurate positioning of the leg, thus enabling the mortiser stops to be used, drive a couple of screws into the curved block so that they fit really snugly in the rail mortise. To cut the side rail mortises in the front legs at 87 degrees, use a sloping packer under the leg, photo 8.
Side to back railsFinally, mortise the side rails to receive the forward-positioned back rail.
As the through tenons are on display, some care must be taken when mar king out and cutting the mortises.
Using a sliding bevel, square the marking round both faces, and mortise from both sides to prevent breakout. A sloping packer is required under the piece to angle the mortise, photos 9 & 10; pick the angle from the full-size drawing.
When laying out the mortises ensure that the spacing between them matches the width of a router cutter as this will make cutting the tenons much easier later.
Bandsaw & tableFor a batch of chairs cutting tenons by hand would be tedious, so I remove most of the waste from the sides using the bandsaw, then trim them precisely using the router table. If the shoulders are not very wide they could be cut on the router table alone, but if you need to take two settings on the table I think it is quicker to remove the waste with the bandsaw.
This approach requires that the rails are cut precisely to length; bear in mind that the side rails are inclined 3 degrees from horizontal so the front ends of the rails need to be cut at that angle.
Bandsaw...Saw the majority of the waste from the tenons on the bandsaw approximately 2mm oversize. The tenons on the front and back rail are straight, but on the splayed side rails they are angled; this angle is picked from the full-size drawing.
Use an angled packing piece against the fence to hold the rail at the correct angle when sawing the tenon sides; a stop on the packing piece helps to hold the rail in place. The tenon should also be sawn to width. A shoulder is not required on the lower edge of the rail as a bead is applied which will act as a shoulder and cover any imperfection in the mortise. The front rails should have a haunch at the top.
... then router table...I have a rather primitive router table setup, made from a kitchen worktop without a sliding fence. It is possible to trim tenons by using a board against the fence to slide the rail piece at the correct angle to the cutter, photo 11, and this system could be used for the front and back rails, but would be difficult for the compound-angled side rails, involving various angled packing pieces, so a jig is required.
... and jigThe jig consists of a baseboard with a recess to allow for the travel of the cutter. Fit two angled blocks to hold the piece at the correct angle for cutting the tenons to thickness. Fit another block to hold the piece at the correct angles for the shoulder, 87 degrees at the front and 90 degrees at the back. Toggle clamps are used to hold the rail in place, photo 12.
Using jig & tableTo trim a tenon, offer the jig up to the fence and locate the end of the rail piece against the fence, photo 13. After cutting the shoulder the jig can be moved away from the fence to remove the remaining waste towards the end of the tenon.
If your router table has a sliding fence this can be used to set the angle to the shoulders, but a similar arrangement will be required to cut the tenons to the splayed angle. Now for the through-wedged tenons.
Laminating and jointing back slats
Slot calculationThe back slats are tenoned into 9mm mortises in the back rails. Cut the mortises using the router guided by a collar, photo 16, running in slots in a template. To determine the length of the slots calculate: length of slot = length of mortise - diameter of cutter + diameter of collar
A similar calculation can be made for the width. Blocks on either side of the template allow the rail piece to be clamped firmly in the jig, photo 17.
LaminationsThe 12mm-thick back slats are laminated from eight 1.5mm pieces. These can either be bandsawn or made from purchased constructional veneer.
The three curved slats are laminated as a single piece on a former, the tenons cut, then the piece is cut to width to give a set of slats with continuous grain.
When the back comes out of the press clean up the edges and cut it to length. The length can either be read off the drawing or marked from the dry-assembled back.
Cutting tenonsCut the short tenons on the ends of the slats before they are separated. This is best done using a simple jig on the router table to hold the back at the correct angle as it is passed over the cutter, photo 18. On the bandsaw cut the back into separate slats and clean up the edges with a plane, using the former to assist in holding the pieces. Form the shoulder on each tenon.
Seat rails & seatFit 8mm-wide bead pieces to the underside of the seat rails. The seat is fitted using expansion brackets similar to those used on the table, see last issue.
Dry assemblyDry assemble and clamp to ensure that all joints go together satisfactorily and that you have enough clamps and blocks. Check the top of the front legs to ensure that there will not be too much work to level them to the rails. Check the through tenons - I have them protruding though the rails about 2mm. Mark off the line of the tenon protruding through the surface and, after disassembly, chamfer the tenon to this line. This is easier done now rather than after glue up.
Get stuck inPrepare the wedges for the through tenons, about 30mm long, tapering from 4mm to 0.5mm.
There are a lot of joints here, some of them to be wedged, so glue up must be in two stages. Assemble the back and front separately, then later glue up the side rails and the back rail with the wedged joints.
If your through-tenon wedges are identical, driving them in the same depth should make them the same width when trimmed to the top of the tenon. I used hammer and block so that I was driving them both in at the same time.
After the clamps come off, trim the wedges back and clean up the top and edges of the tenon.