Scribed Joinery – the Process

In this post I thought I’d cover more of the process of scribing. The process I will show here is aimed at joining two pieces of wood together when there are uneven surfaces involved, but essentially the same concepts apply when scribing wood to any uneven surface such as stones.

So for the sake of demonstrating scribing I cut up a couple pieces of scrap Japanese chestnut and layed out some joinery. Here is the layout for the mortise which is 25mm long by 10mm wide. The overall size of the piece is about 38mm square and after cutting the mortise I will turn it round on the lathe.

Here is the layout for the tenon, sized to match the mortise. The tenon will be a through tenon so I drew a shoulder line at about 42 mm from the end of the stick. That way the tenon will stick out about 4mm.

Below you can see the mortise after cutting. And some more layout for the piece with the tenon. When doing carpentry, one of the ways I learned to keep track of where the tenon piece should finally end up relative to the mortised piece, is to use a reference line. On a building the reference line is often laid out relative to a center line, but in this case I’m laying the reference line relative to the shoulder. Using a center line is usually more ideal on larger structures like buildings since most of the structure will be laid out relative to each post and beams center, vs western timber framing which often uses the surface of a given framing member as a reference. Centerline layout works great particularly when you’re working with uneven surfaces or material of varying dimension.

In the picture above I set a reference line at 100 mm from my desired finished shoulder. Like the bench that I recently built, I knew that the final size of the round mortised piece would be more or less consistent, so in this situation referencing the shoulder works just fine. On larger beams where the surface may be much more uneven, the shoulder might be an unreliable reference point, in which case working off the centerline is a much better way to go, and I’ll try to explain that process in more detail later.

The whole point of using a refence line is so that in case you’re layout gets cut away around the joint you still have some way of measuring the distance. Hence in the picture below I wrote the distance from the reference line to the desired finished shoulder location…. and the number I wrote is wrong! I read 200 off of my ruler, but the actual measurement is 100 mm. This is the kind of mistake that is really easy to make, and is what makes laying out a building one of the most stressful tasks for a carpenter. In your head you’re making all sorts of calculations, visualizing the layout and location of different joints, all the while keeping vigilant not to make simple mistakes. It’s not easy! In any case this reference line tells me that the finished shoulder should be 100 mm from this point.

Below you can see the mortised piece after turning round on the lathe and after a bit of handplaning to smooth out the surface. I also rough cut the shape of the tenon shoulder. Using a compass I drew a curve with the same diameter as the mortised piece (about 38mm) and then rough cut along that curve leaving 2 mm or so for safety sake. If the mortised piece you are fitting to has a really uneven surface, then precutting is not always an option. In that case you can cut just enough of the tenon to fit in the mortise, and proceed to use your scribing tool to layout the rough shoulder shape.

Here are the two parts together for the first time. Pretty gappy on this side.

On the opposite side you can see the joint is touching along the edge of the shoulder. I have about 3mm of space before reaching the final depth of the shoulder, and I set my scribing tool to a little over 2 mm for the first scribe. It’s best to set the scribe a bit less than your final depth, especially when doing the first rough scribe. Handling the scribing tool, and accurately transferring the shape of the round part takes care, and it’s virtually impossible do a perfect scribe right to you finished depth in one take. So the process usually requires a few rounds, gradually sneaking up on a clean fit.

When it comes to holding the scribing tool while you are scribing, the first thing you need to consider is the orientation of the tool relative to the imaginary center of the mortised piece. When you first start holding the tool, it’s important to maintain the same angle as you scribe around the surface of the piece. In this case I set the scribing tool so that the brass part is essentially parallel with the mortised piece, so I need to maintain that same angle as I move the tool along the round surface. If I was to shift the tool up, such that the brass piece was more like 45 degrees relative to the mortised piece, then location that the pencil will be marking onto the tenoned piece will also change (and become shorter). This effect is exacerbated the farther your pencil is from the scribing point. It doesn’t necessarily matter what angle the tool is relative to the mortised piece, what matters is that whatever angle you choose, you maintain that angle throughout the duration of the scribe.

At the same time you need to be mindful of the angle of the scribing tool relative to the tenoned piece. The pencil tip and scribing tip want to be inline with the center line of the tenoned piece. And again the farther your pencil tip is from the scribing tip the greater your error will be if you don’t hold the scribe in the right orientation. Say I angled the scribe toward the top of the photo below, then the tip of the pencil would be marking to the left of whatever point the tip of the scribe is referencing, giving me an inaccurate line.

Here is the main tool I used for cutting a scribed joint, a Japanese incannel gouge or soto-maru-nomi. You can use a standard gouge as well, but this kind of chisel makes the work much easier. The back of the chisel is round, which allows you to cut along a curved line. These chisels come in a variety of widths and curvatures, and if you do a lot of scribing it’s handy to have a small range of sizes.

Below you can see where I’ve started to trim the shoulder to my scribe line.

After a bit of chisel work here is the 2nd test fit. Still plenty gappy. I think I was a bit too conservative with the first scribe.

So on to the next round. This time I brought things closer to my final shoulder line, leaving the pencil just a touch short of the shoulder line.

Here is the inside of the joint after a bit more cutting. One tricky area to cut, especially on small pieces like this, is the intersection of the tenon and the shoulder. That area is really tricky to clear out so I tend to just undercut that area using a skinny 3mm (1/8″) chisel. It’s a pretty heavy undercut, but really simplifies trimming the inside of the joint by not having to fuss around and trim right up to the tenon.

I also tend to undercut the inside of the shoulder ever so slightly. It’s pretty faint in the picture below, but there is just a slight bit of shadow line visible under the bottom edge of my square. Undercutting is particularly helpful when the surface you’re scribing to is really irregular, such as a stone, natural log, or hewn log. In those situations it’s helpful to use chalk or some other method of revealing what areas inside the joint are making contact, then you can trim accordingly. The important thing is to make sure that no areas of contact on the inside of the joint are preventing the joint from fully closing. Contact itself is fine as long as the seam of the joint also closes up tight.

Here’s the fit of the joint after the 2nd scribe. Much closer, with just a bit of fine tuning left. One tip is to remember to erase any remaining pencil marks before test fitting each time. If you leave the pencil line, it sometimes makes it harder to clearly see where you have gaps, and also makes it harder to clearly mark for the next scribe.

For fine tuning I use a regular pencil to simply mark the areas of contact, and trim the shoulder every so slightly in those areas.

Here’s the joint after a touch more trimming. Getting closer but there are still a few faint gaps.

Scribing is one of those processes where you can really get lost trying to make things perfect. And sometimes you have to just do your best and move on. It’s also important to consider the angles at which the joint will be seen. In architecture, often the joint is being viewed from below, in which case shadow lines are often much harder to see. For furniture visibility becomes much more variable, depending on the type of furniture, it’s size and location relative to a viewer. One other consideration is that final clamping, or if you’re using draw-bored pegs, will tend to close up hairline gaps a bit, especially if you’ve done a little bit of undercutting.

I plan to use this joint as a model that can be taken apart to show customers how a scribed joint works. So I ended up doing a few more rounds of very light trimming and fine turning. Then I hand planed the tenoned piece and this is where things ended up. Not absolute perfection, but good enough for this situation, and if this was being clamped and glued I’m pretty confident the hairline gaps would largely disappear.

A few people have asked about the process of scribing when you are fitting a piece that gets sandwiched between two irregular surfaces. For example a short post that might be fit between two beams in a roof structure. To be honest my experience in those kinds of situations is limited, but I did have an opportunity to do a bit of that work with Somakosha. So next week I’ll share some pictures from that work and do my best to explain the process as well. Thanks for reading, and let me know if anyone has any questions. This is the kind of work that has lots of different nuances and would probably be easier to explain with a video or perhaps a class. If anyone is interested in that kind of thing I’d love to know.

Thanks for reading.

-Jon