Nana's Dining Chair Rebuild, Part 4: Joinery

Today's entry will be all about the joinery, some of which was easy and some was tricky.  I started with the easy part - the front leg assembly.  Just two legs and a rail connecting them.  No angles.  But I did have to be aware that the side seat rails would have mortises that met the front rail mortise inside the leg.

Front rail mortise is 1/4" longer than side rail mortise

I blew out a little bit of the first mortise when I broke through to it while chopping the adjacent mortise.  That was due to unsupported fibers from the first mortise, but I don't believe this will cause any structural weakness in the finished chair.  To avoid that issue in subsequent adjacent mortises, I filled the first mortise with a block before breaking through on the second.

To avoid blowing out some of the mortise walls, blocks fill one mortise while chopping the adjacent one

The front rail is 18.5" long and the back rails are 13.5" long, so the side rails have to be angled.  I choose to chop my mortises perpendicular to the leg face and so the tenons of the rail must be angled.  So after knifing the shoulder lines at the proper angle, I pencil in the 3/8" thick tenon.

Tenon penciled in

I didn't get any pictures of it, but after sawing close to the lines I used a shoulder plane and chisels to fit the tenon into the mortise.  Here are the parts making up the work so far.

Front leg, front rail and side rails

With the front leg assembly complete, including having fitted the the side rails, the next operation was the on the back legs.  The back legs needed to be mortised for the side rails as well as for the back rail (and lower backrest rail too).  Recall from the previous post that to ensure the two tenons didn't cross in the leg, I offset them.  In the next picture, the back rail mortise is done, then it's filled with a 3/8" scrap to avoid any blow-out while chopping the side rail mortise.  Notice the top of the back rail sits 1/2" above the top of the side rail, and the tenons are staggered to avoid having them cross inside the leg.

Back leg mortises in progress - tenons staggered so as not to interfere with each other

The reason I couldn't just let the tenons intersect and miter the end of the tenons (like I did on the front legs) was that the back rail and side rail are only 1/8" and 1/16" from the edges, respectively, and this would have made the tenons too short.

When the back rail and side rails were fitted, it was time for a first look.

First dry-fit.  Starting to look like  a chair!

Now on to the lower rails.  These are angled front to back (like the seat rails, but at a slightly different angle), and they also need to fit into angled faces on both the front and back legs.  Actually they need to fit into curved surfaces on both front and back legs, but joinery is far easier and far more precise when working on flat surfaces.  So I created flat surfaces to mark out and cut mortises and will work the curves later.  For the back legs this was easier to deal with as the eventual curve shape is convex.  In the following picture, the location of the lower rail mortises is marked with the double lines, which are wrapped around the sides to maintain the location after the waste is cut away.  The straight line that I've cut to approximates the curve at the location of the mortise.

One flat cut - both will be cut and planed to provide a good reference surface for mortising

 Once the surfaces were prepared, the mortises were laid out and cut.  These were a little tricky as they are only 5/8" long and 5/8" (or maybe 3/4") deep.  It's tough to fit the chisel in to pry up the waste material, trying not to bruise the mortise perimeter.

Lower rail mortise laid out on back leg

Lower rail mortises

The mortises in the front legs are tougher as they are placed at a location of concave curvature.  I did my best at creating a flat spot, removing leg material with saw, chisel and plane.  I used a backer board clamped to the legs exactly at the line of a "flat" that I was trying to create.  This allowed me to plane across the grain and not get blow-out at the far end.

Front legs and backer board clamped in vise for planing a flat surface

You can see the curve and the flat line (blue line) that I'm trimming to in the next picture.

Layout lines for the "flat"

Eventually I got the surface prepared and the mortise locations re-marked and chopped.

Lower rail mortises marked on front legs ...

... and chopped out

For the lower rail tenons going into the front legs, it was a matter of getting my angles from the angle template I made (because I only have one bevel gauge) ...

Getting the angle for the vertical shoulder of the lower rail to front leg

... then marking the tenon angle with another angle from the template ...

Marking tenon angle

... then cutting and fitting the tenon into the mortise.

A satisfying fit

Before I could cut and fit the rear leg tenon, I needed to know the right distance for the rail.  This meant I had to fit the lower backrest rail ...

Lower backrest rail fitted

... and dry-assemble the chair.  With the chair clamped up, I need to find the proper length of the lower rail, but how best to do it?  This is a critical measurement if everything is to fit right!

Lower rail held to legs at approximate location

Tape measure is too imprecise to get the length measurement

I remembered that I had once made a gauge for internal measures, but it was too short and I didn't want to make a longer one.

Gauge for internal dimensions

I opted for a length of 1/8" dowel rod, sharpening one end to a point.  I placed that point at the lower inside corner where the lower rail meets the front leg, held it next to the similar location on the rear leg, and made a knife mark on the rod.

Marking the dowel rod with length of lower rail

With that distance known, the shoulder angles were marked and the tenons sawn and fitted to the mortises.

Another dry-fit with the lower rails in place

A side note here.  I had noticed earlier that both side seat rails had a little gap on the outside face where they meet the front legs.

Small gap where side rail meets front leg

So I trimmed the tenon shoulder on the other side to have this fit better and that worked well.  First time I've tried this.

Much better

This chair has a cross rail connecting the two lower rails, about 4 3/8" back from the front legs.  This was easy enough to lay out and mark while the chair was dry-fitted and clamped up.  I marked the shoulder line angles on the cross rail directly from the lower rails, cut one tenon cheek, butted it up to the lower side rail and marked the other end for length.

Fitting the lower cross rail

A small problem came up here.  When I fitted the cross rail with the lower rails, the assembly did not sit flat on the bench.

Mortises and/or tenons a little out of square

I tweaked the tenons and mortises a little and got it better, but didn't worry too much about it because this poplar is very soft and will easily bend to the chair's will.  Now here is the chair dry-fitted with all part so far completed.

Dry fit with lower cross rail included

The last thing to do was to fit the upper backrest rail.  I had to fiddle a bit with the shapes that I had laid out on that rail and on the rear legs so that the tenons won't show after final shaping is done.  Earlier I had made an angled saw cut on the upper portion of the back legs for the backwards lean of the backrest.  This became my new reference face to lay out the tenons.

Upper part of rear legs with tenons cut

Actually I had chopped the mortises first and used those locations to get the final location of the tenons.

Upper backrest rail with mortises, comparing locations to the tenons

There were a lot more details here than what I've reported, but all in all the joinery phase went well.

All joinery complete

OK, there is still one joinery issue to do - I have to fit a back panel between the upper and lower backrest rails.  But I need to wait until shaping is partially done for that.

Next step: shaping.