THE SEAT AND KNEE PADS
The seat and knee pads are designed in two parts. The lower plywood sections are fixed to the leg base. The upper MDF sections are upholstered separately and then fixed to the plywood sections with M6 Tnuts and M6 x 25mm Stainless Steel, Allen Socket, Countersunk Screw Bolts. This allows the upholstered section to be removed for easy reupholstery or replacement, and gives a very neat edge detail.
MAKING THE CURVED SEAT SECTIONS
As both elements are to fit together in the final piece, I decided to make them together. Instead of making a former and then clamping the laminated sections to it whilst they dried, Malcolm suggested trying to just bend them over a piece of wood and clamp them to the workbench whilst they dried. This would obviously save much time.
From the CAD drawings I calculated the depth of the piece of wood over which to bend the laminates. This piece was then slightly curve on the top surface, and clamped to the desk.
I initially tried to bend a piece of 6mm plywood to this curve to ensure it would flex that much. It did, so I decided to make the lower seat out of 2 x 6mm sheets of Birch Faced Plywood, and the upper upholstered section from 1 x 9mm and 1 x 4mm MDF. The lengths were cut longer than the finished sheet as the end segments would tend to be straighter than the centre.
The MDF section called for a recessed lip into which the upholstery material could be secured to create a neat edge and a flat surface when the two segments were brought together. Because this would be difficult to achieve on a curved surface, this smaller section was cut from the 4mm MDF on the bandsaw and neatened back to the line on the disc sander, before the bending and laminating process.
A quick jig was made from scraps of the MDF to hold this section in place during the gluing process.
The sections were glued, layered together, clamped and left to dry over Easter weekend.
When the clamps were released the form sprang back a little from the postion I had envisioned in the design. In hindsight I should have anticipated this and made the section of wood over which I was bending the elements deeper.
I tried the form on the Prototype to see whether the curved seat looked better – which it did.
However I decided that the ellipse shape was not really working as well with the angled forms of the legs and thought this could be improved upon.
I experimented with a few different shapes bearing in mind that there needed to be enough space to allow a person to step over the knee pad and sit down. Playing with the shapes of the leg templates, I discovered a shape that seemed to work visually, as well as practically forming a hand rest at either side of the seat to give an enhanced feeling of security.
Having changed the seat design I had to repeat the process of making the MDF sections, by cutting the new shape on the bandsaw and sanding it back on the disc sander . . .
. . . making a new jig . . .
. . . and laminating it clamped with the plywood section as a former.
Once the sections were dry, I traced off the actual curve I had achieved . . .
. . . and used this to calculate the curve height above the centre line datum where the seat and side legs intersected. I used this imformation to alter the Sketchup drawing accordingly, so I could calculate the length of the corners of the side legs where they intersected the curved seat accurately.
Then using the Cardboard Template I marked the pieces with the final seat shape and roughly cut these out with a Jigsaw, sanding back to the line on the Disc Sander.
THE KNEE PADS
Being flat the knee pad was much simpler to make.
Using a cardboard template the shapes were marked on the plywood and MDF. The recessed piece was prepared from 4mm MDF and glued and clamped to a piece of 9mm MDF.
The final knee shapes was then rough cut on the Bandsaw before being sanded back to the line on the Disc Sander.
The plywood knee section was then marked up for screw holes and these were drilled and countersunk in preparation for assembly. The two separate elements were then aligned and clamped together to position the holes for the Tnuts and Bolts accurately by drilling through the plywood into the MDF. The Plywood was drilled at 6mm whilst the MDF required 7mm holes for the Tnut inserts.
THE LEG SECTIONS
I would have preferred to make these legs from thin veneer curved and laminated together with solid wood inserts to give the leg thickness of form, as this would have given me the most strength and would have allowed me to shape the legs more along their length. However this would have been quite costly so instead I opted to make them by laminated Plywood and Oak shapes together to give the width.
I drew an accurate full scale Master Cardboard Template of the leg sections, marking where the sections on each layer were to join. These were then marked out on 18mm Oak and 18mm Hardwood Plywood. The Plywood sections are sandwiched in the centre so the face is not exposed to chipping on the finished piece.
The Sheets were rough cut into their individual segments.
Each element was then carefully cut out on the Bandsaw.
The Oak could be cut quite close to the line, but the plywood tended to splinter more so couldn’t be cut as accurately.
The Inner curves were quite difficult to cut, so had to be carefully nibbled away in stages
The outer curves were easier to cut in short segments.
The inner curves and lengths were then carefully sanded back to the line on the Drum Sander . . .
. . and checked that they aligned.
The long outer lengths were sanded on the Belt Sander, the outer curves on the Disc Sander. The pieces were then checked against the Master Cardboard Templates to ensure they had been cut correctly and the ends were adjusted to give an accurate fit.
The pieces were then layered together as they would be glued.
Checking closely it was apparent that the Oak and Plywood were slightly different thicknesses.
So with a Hand Plane I carefully planed the centre pieces of Oak so they would fit closer together in the stack.
Laying a piece of cardboard on the bench surface to ensure the pieces would not stick to it, each leg segment was then glue laminated together. Pieces of scrap wood were clamped front and back to align the pieces and stop them from slipping when they were clamped down to the desk to dry.
The glued legs were then sanded on the Drum Sander to even out the surfaces.
They were then compared again to the Master Template to ensure that they aligned accurately.
The position of the mortise joint on the central section was marked off (taking into account the slight discrepancy in the widths of the sections) as was the heights of the central legs, so they could be sanded back to the correct angle.
The lengths and angled cuts of the side leg tenons were likewise transferred from the Master Template. It was decided that the tenons would only go 36mm into each side, so the cental strip of Plywood would carry through.
The marked Mortise and Tenon joints were checked against each other for discrepancies.
and double checked that they were marked at the correct mirrored angles.
The centre of the mortise joints were first drilled out using the Bench Drill Press to remove most of the waste, and then finished using the Hollow Chisel Mortiser
The machine cut mortise was then cleared of any remaining waste with a Hand Chisel. The mortise joint was not quite as wide as I had initially marked, so I adjusted the Tenon measurements accordingly.
The Tenons were then cut with Japanese Saws . . .
. . . and neatened back to the lines with a Hand Chisel.
The ‘Red’ saw was used for cutting across the grain and starting cuts accurately, the ‘Grey’ for ripping along the grain. These are Pull Saws ie. they cut on the pull stroke rather than the push, and cut very finely. They are the only Hand Saw I can seem to cut accurately with!
The sections were compared against each other . . .
. . . and the tenons shaped to fit the mortise angle with the Japanese Saws.
Using a Hand Chisel the Mortise and Tenons were eased gradually . . .
. . . to a snug fit.
The top of the side legs were then marked with the lengths taken from the sketchup model altered to the actual seat curve.
These were double and tripled checked to ensure the angle was in the right direction before cutting!
Because the side legs are meeting a curved surface at an angle this cut was slightly bowed not flat. It was cut as accurately as possible in from each corner using the Japanese Saws. Once most of the waste was removed a Hand Chisel was used to smooth the surface as much as possible.
The legs could then be assembled. The bottom of the central mortise was marked up and drilled to take 4 screws to hold the tenons in place whilst the glue dried. The tenons were then glued and screwed in place from below, and the top of the legs held in tension with tape whilst the glue dried. I think this latter was a mistake as the legs were tensioned too much and bowed the sections, opening up the joint slightly.
The assembled leg sections were then given a good sanding using a Hand Sander with 280 and 400 grade papers.
I printed out the CAD drawing of the seat shape and marked the position of the rear leg and the line along which the corners of the side legs lay, then I transferred these measurements to the seat itself.
When the curved seat was rested on the legs, I found the curve had relaxed more since the shape had been cut from the sheet as well as the legs having been forced inwards in the assembly process so the two didn’t fit. Kevin suggested that if we weighted the seat it might bend it enough to force it to meet the legs so we could mark the exact position of the side legs. Whilst Kevin held a heavy weight on the seat above my head, I crawled under the chair, carefully aligned the seat and and marked the position of the side legs
Paying attention to how the side legs sloped and taking care that I would not be screwing into the lamination joints, the fixing holes were drilled and countersunk into the plywood seat.
Screwing the seat to the legs proved as difficult as marking the position because of the slope. It took 3 pairs of hands in the end. Kevin again held the weight, I laid on the floor and held the seat aligned from below, whilst Sean drilled pilot holes and screwed the seat in position.
Stressed by the weight we achieved a good fit with the side legs.
Sean sat on it, it seemed sturdy enough, so after such a struggle getting it together, we decided not to take it off to glue it in position. Sean commented that he liked the extension of the seat to the side because it gave you a feeling of additional security as a hand rest. The knee pad was similarly screwed into position.
To make the upholstery sections removable I used M6 Tnuts and M6 x 25mm Stainless Steel, Allen Socket, Countersunk Screw Bolts.
I had noticed before assembly that the MDF section of the seat was easing to a different curve to the plywood, so I clamped this section firmly to the plywood seat . . .
. . . and turned the whole chair upside down and clamped it to the bench. With a 6mm bit I drilled pilot holes through the fixing holes already drilled in the plywood seat to accurately position the fixings.
Whilst the chair was in this position I sanded the underside of the plywood.
The sections were unclamped and a 7mm drill bit used to drill into the pilot holes to create the holes for the T-nuts. These were firstly pressed firmly into position on the upper surface of the MDF sections then hammered into position.
The accuracy of the fit was checked . . .
. . . by temporarily fixing the components together.
The seat needed to be clamped because of the discrepancy between curves.
The MDF sections were removed to be upholstered, whilst the rest of the chair was given a final sand with the Hand Sanders using 280 and 400 grade sandpaper. The sawdust was cleaned off with the air gun.
With a clean rag I applied a coat of Danish Oil, allowed it to sink in for 10 minutes then wiped away the excess.
This brought out the grain patterns in the legs very nicely.
Several more coats will be needed for a final finish.
The first layer of the upholstery is a 35mm upholstery foam. The forms were laid out on this foam and drawn around. A 30mm margin was marked all around the edge to allow compression of the edge to create a soft rounded appearance.
The foam was cut with a serrated bread knife (an electric bread knife is supposedly neater, but I don’t have one!) and ‘spraymounted’ to the MDF.
A layer of wadding was then applied to cover the foam shape and pull it into the softer rounded form. This was fixed by pulling the wadding taut and stapling into place with a staple gun using 6mm staples. On the flat knee pad the centre of each long side and the corners were stapled first and the rest of the wadding eased along the remaining length.
On the curved seat, I worked from the centre of each long side out halfway to the corners before stapling the corners in position, in an attempt to keep the wadding flat to the form. Then I eased the remaining wadding into position. When the wadding was secure I cut away the excess.
I was slightly worried that the wadding might beard through the cover fabric, so I repeated this process with a stockinette fabric. Stapling it into position . . .
. . . and cutting away the excess to neaten the edge.
For the cover I decided to use a microfibre loop chenille fabric. I couldn’t find anywhere on the internet that sold it by the meter, but eventually found bathmats made of it at Dunelm Mill. They were slightly too small for the seat so I had to carefully seam two together for the seat.
I laid the material face down on the table and placed the prepared pad face down on top. I made sure that the fabric was aligned squarely to the form and pulling it taut started stapling it into position.
Longer staples would have been preferable because of the thickness of the fabric.
I cut away the excess with scissors . . .
. . . then neatened the edge further by removing some of the bobble pile along the turned edge so the two forms would lay flat together for assembly.
I added more staples as I worked so that there was at least two rows holding the cover fabric secure. In hindsight it would have been better to create a deeper recess, perhaps by using 2 x 6mm MDF sheets.
The finished upholstery pads were aligned with the plywood bases and secured using the M6 x 25mm Stainless Steel, Allen Socket, Countersunk Screw Bolts.
The curved seat again proved awkward and had to be clamped in position to curve the piece whilst it was secured, but the flat knee pad was easy to fit.