This office armchair is a simplified version of Hans Wegner model 502 chair. I used oak, leather, steel and a nice base from a stool found in a yard sale.

Step 1: Tools, Supplies and Plans

Supplies :

5mm plywood for the seat
threaded insert for wood
2m of 12mm steel round bar (easier to bend without tool)
about 50cm of 3mm thick 3cm large steel flat bar
Vegtan Sheep Basane leather
foam: 50cm x 50cm, 7cm thick, 35kg/m3 density
large sheets of paper to draw and cut templates
nice base from a old stool found in yard sale
enough wood (I used oak) to cut the following pieces:
the arm back rest is made with 10 layers of 25mm thick oak.
- going from top to bottom layers i used (size in cm):
  1. 35x10
  2. 60x15
  3. this layer has 3 pieces (two arms and the back) 2 times 43x20 and one 35x15
  4. this layer has 3 pieces : 2 times 43x15 and one 45x15
  5. 55x15
  6. 43x15
  7. 35x10
  8. 30x10
  9. 25x10
  10. 20x10

Tools I used :

band saw (a jigsaw would do the job too)
router
planner
angle grinder with carving blade, sanding disk and polishing wheel
sander
metal handsaw
leather tools :
stiching awl
sewing awl
cisors

Plans and templates :

See the pictures for the top and front view of the arm and back rest. I used it to draw and cut full size patterns.

Step 2: Back and Arm Rest Block : Arm Part

I started by gluing layer 3 and 4 (those are the larger ones, they contain the arms)

Before gluing it is important that:

all planks on a layer have the same thickness (same pass in the thickness planer)
planks have a perfect edge for clean gluing (I used a router and a straightedge with a trim router bit with bearing)

Glue all parts in contact and clamp. (you can see on the photo that I added a cut on the arms as my larger clamps were not long enough)

Once glued, I used the template to draw the egde on the wood then cut with the band saw.

Step 3: Back and Arm Rest Block : Upper Part

I added layer 1 and 2 (well actually I added a last layer as my planks were thinner than expected).

Glue, clamp, let it dry and cut (it is still doable with the band saw).

Step 4: Back and Arm Rest Block : Lower Part

For the lower part I pre-cut each piece (use the template for drawing on wood and mark the center of each piece for a good alignment when gluing).

Step 5: Back and Arm Rest Block : Prepare Shaping

Create temples for a nice upper and lower curve.

Draw those curves on the wood block.

Step 6: Back and Arm Rest Shaping and Sanding

It is time to shape, get prepared for some dust...

First a rough shape with the carving disc then a heavy duty sanding disk, progress with finer grain.

I started by shaping the "horizontal projection" of the back rest, following the line drawn in the previous step (see second photo).

Once this is obtained, you should draw a line on the upper and lower part of the back rest (they will be the top and bottom outline and will guide you for the next shaping).

I proceed with the shaping, sanding, up to the desired form and wood finish.

Step 7: Back and Arm Rest Supports Upper Part

The back rest block is linked to the seat by three supports made from polished 12mm steel round bar.

Each end of the supports has a plate to allow attachment to the back and arm rest.

I made diamond steel plates:

Two 4mm holes are drilled for screw attachment.
One 12mm hole is drilled in the center (the support rod will go through this hole, rod and plate will be epoxy glued at a later step during final assembly).
the plates are polished (sand up the 1000 grain and then use a polishing or bluffing wheel).

Step 8: Back and Arm Rest Supports Bending

I decided to use rod instead of tubes as they are much easier to bend.

12mm can still be hand bent.

The angle is a little bit more open than a right angle (I used about 95°). I made this little piece of wood to bent with a consistent curve and angle the three supports.

The rods are polished first, then bent in a bench vise.

The two arm supports have only one angle, they are about 28cm long between the arm and the angle and 14cm for the other part.

The back support has two angles separated by about 26 cm and 11cm for the part going under the seat.

I decided of those lengths after trying the back and arm rest block.

The two arm supports can be cut on both ends for adjustment.

The back support can be cut only on one end but you have some error toleration with the place of attachment to the back rest.

Step 9: Back and Arm Rest Supports : Seat Attachment

The seat attachments are not visible and are much simpler.

They are rectangles with two 5mm holes for seat attachment, welded to the support rod.

They are polished after the welding.

Step 10: Seat Plywood Support

I wanted to have a curved seat so I used two layers of 5mm plywood glued together.

I used the paper template to draw on the plywood, however I made it one centimeter smaller (I wanted the foam to be larger than the plywood for comfort)

The photo shows two pieces of plywood for the second layer, this is only because I used left over.

Although I clamped with some curve, it retracted much more than what I expected....and it ended up almost flat...

Step 11: Foam Cut

I bought a square piece of foam and had to cut it.

I draw the shape using the template and cut with the band saw (it was surprisingly easy to have a nice cut)

You could also ask the vendor to cut from your template of course....

Step 12: Sewing Pattern and Leather Cut

Before deciding to go with leather as I had never worked with it before I followed the instructables leather class, it is very well done and convinced me I could do it....

The seat cover is made with two parts sewed together:

the main seat disk (horizontal part)
the vertical part which will wrap the foam and be stapled under the seat. This piece is about 150 cm long, so I made it from two shorter pieces)

I drew and cut a disk using the foam as template.

The sewing will be at 1cm from the edge (so you end up with a cover smaller than the foam), it will allow a tight wrapping.

For the border I drew and cut two pieces of 80x11cm (we have 1cm to sew with the top cover, 7cm for the foam which leaves us with a few centimeters to wrap bellow the seat)

Step 13: Sewing, Sewing, Sewing...

The overall progression is :

The first sewing is to make the 150cm long border from the 2 smaller pieces.
Next is the the border to the cover
Finally, close the border.

It is not difficult...but required patience....It took me about 4 hours to sew everything...but the result is rewarding.

I used two tools (stitching awl and sewing awl) and strong waxed thread for leather.

I sew the pieces together inside out with the flesh sides facing out. I drew the sewing line on the inside (1cm from the edge) and followed this line.