Scribed Joinery – the Process 2

In last weeks post I talked about the process of using the scribing tool to mark the shape of an uneven surface followed by cutting/trimming. This week I will try to explain as best I can the process for calculating the the position of parts as it relates to scribing. A particularly difficult area is when you have a part sandwiched between two others, in which case the dimensions and spacing becomes a big factor in how much you can scribe. I’ll be completely honest and say I’m not an absolute expert on this process, but I’ll do my best to explain what I know gleaned from books and from work I did with Somakosha.

Reference Lines

When it comes to Japanese carpentry, centerlines and reference lines play a big role in locating and orienting parts relative to one another. Designing a building is often done by drawing a series of grid lines that designate the spacing between posts, with the grid lines intersecting the center of each post. The actual posts, beams and other structural members of the building will then have centerlines drawn on them, making it fairly simple to accurately locate and position members relative to one another. This system is particularly helpful when it comes to working with natural or hewn logs with uneven surfaces. By working off the centerline of a wonky beam or post it becomes possible to locate and fit joinery very accurately.

As an example, one of the first projects I worked on with Somakosha was to scribe and fit short posts to beams for the roof structure of the Little Lights Project. These short posts were going to be sandwiched between hewn beams below and flat, milled purlins above (moya 母屋梁 in Japanese). So the tenons on the top of each post consisted of a square shoulder and tenon, but the tenon shoulders on the bottom of each post required scribing to fit the hewn surface of the beams.

The critical issue here is that there is a specific spacing required between the beams and purlins in order to accurately build the roof structure. To ensure that we cut and scribed each post so that the final length gave the proper spacing, we used reference lines on the posts and beams to measure and check our progress while scribing. We also took advantage of the flat shoulders of the upper tenons to use as a reference surface.

Below you can see how I clamped a straightedge to the tenon, essentially extending the plane of the shoulder.

Each time we started the process by placing the post in place, and before scribing, we would take a measurement off of the straight edge, down to the reference line on the beam. That measurement would tell us how much material we needed to remove when scribing. (All of the posts were left somewhat long to accommodate this process.)

The distance from the straightedge to the reference line on the beam also needed to be equal on both sides. An equal distance ensured that the post was plumb relative to the vertical center of the beam. Likewise the side of the post had a reference line that corresponded to another vertical reference line drawn square off of the beam’s horizontal reference line. Those lines also needed to be in alignment.

Below you can see two of the vertical reference lines running down the center of the post, and you can see how they meet the corresponding reference lines on the beam as well. (The line on the topside of the beam is a bit fuzzy, but it’s there).

Another method that we used to check the post and beams centers were in alignment was to use an adjustable level. (Sorry the picture below is the only picture I have of this tool and it’s partially cut off. For more details here’s the manufacturers page.)

The beauty of this type of level is that the bubble level itself can be rotated and adjusted to reference surfaces that may not be plumb but are coplanar. When doing this kind of work the beam is usually set on a pair of sawhorses. If the ground, horses, or beam itself have an uneven surface it’s pretty likely that the beam will not be sitting perfectly plumb. In that situation we would use the yellow level above, hold it’s long edge up to the vertical centerline on the beam, and twist the bubble level until it was perfectly level. Then holding the level in the same orientation we could walk it over to the post and check it’s level the same way. Once the post was in alignment we could proceed with scribing.

The process for scribing these posts then essentially followed these steps:

1. Set the post on the beam/insert the tenon and align the centers of the post to the vertical centers of the beam
2. Measure the distance from the upper tenon’s flat shoulder down to the beams centerline. The difference between that measurement and the required distance between the purlin and beam (determined by the master carpenter’s drawings), is the amount of material to be removed when scribing.
3. Set the scribing tool to slightly less that distance, and draw the shape of the beam onto the post.
4. Cut to the scribe line
5. Repeat the process. Insert the post, checks the alignment again with the beam, and measure the distance between the flat shoulder and beam center. Scribe and cut until you reach the desired spacing between the purlin and beam.

Touge 峠

The process just described involved using the flat tenon shoulder as a reference point, but sometimes that may not be possible. The image below represents a cross section of a roof structure where a short post needs to be scribed on both ends. The process of locating and fitting the parts together essentially involves establishing a series of reference points. One common reference point when working on a roof structure is the intersection between the center of a post and the designated roof pitch. That point in Japanese is called touge 峠 (I’m not sure what the equivalent word might be in English so I’m going to stick with touge for this explanation). Touge is often represented in drawings by a dot with a v-shape on top (as in the drawing below). When you design a building the location of the posts, elevation, and the intersecting roof pitch allows you to calculate the location of the touge at the eves as well as at the purlins. When you do layout on the actual posts and beams, you can draw reference lines relative to touge on both the posts and beams, and you can use those lines to calculate how much material to remove when scribing.

In the above illustration a refence line has been placed on both the purlin and the post itself relative to the touge point. Usually a convenient distance is used to set a refence line from touge, such as 300mm. In reality the distance doesn’t matter as long as it’s convenient and you know what the distance is. Using those reference lines in combination with the building plans, you can then calculate the necessary distances between the post reference and beam reference when scribing. My apologies as I don’t have pictures of an example of this process, but it’s really very similar to what I showed previously. Just imagine that instead of the tenon shoulder, if I had used I reference line on the post to measure the distance between the post and the beam for scribing. Essentially the process is the same.

Check, check, check again, then scribe and cut

I realize this may sound pretty complicated especially to people completely new to Japanese carpentry which uses centerlines and reference lines heavily. It was pretty confusing the first time I did this work as well. But it’s also one of those processes where once you dive in and begin to understand how the parts relate to one another the actual process becomes much more clear. A lot of the work involves spending time to check all the centers and reference lines to ensure that the parts are in the proper orientation. From there you can measure exactly how much material needs to be removed when scribing, and proceed to cut some wood. Like I’ve mentioned in previous posts, scribing involves a few different rounds of rough scribing and fine tuning, and if you’re careful with your measurements, the closer you get to the final fit, the more the post and beam will start to naturally align with one another.

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Writing about this process has me thinking about different ways to demonstrate this process, as well as potential designs that could incorporate more hewn or natural log surfaces in furniture work. One very rough idea I’ve had is for a table with a couple of natural logs connecting the legs. Those curvy logs could then be joined with a short post between them using the same process I’ve described here.

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This kind of work is detailed and takes time. It’s a bit complicated and involved, but it’s also a pretty amazing process and learning the techniques involved really open up a lot of design possibilities, both in carpentry and furniture.

If you made it this far thanks for reading! I hope this offers a bit more insight into the process of using reference lines when scribing. I realize it all may still be a bit fuzzy, but I hope to do some kind of more thorough demonstration or process post at some point with more detail on the steps involved. In the meantime please feel free to share comments or questions.