Monday, July 23, 2007

More Seat Carving

Here is Doug Roper working on his high back sack back (It's actually done now but this is the first chance I've had to write!)
We had a great week working on the finer points of chairmaking and designing a chair together, I look forward to seeing the chair painted!

I've had a number of questions lately about travishers and inshaves. Elia Bizzarri is making travishers based on the one that I made and use. The most frequent concern about these tools seems to be the curvature. When we look at a windsor seat, we see curves. The seat is scooped, the sides are curved and the underside is rounded. This gives us the impression that the seat is extremely shapely and dished. This turns out to be an illusion. Most windsor seats are merely a number of gently curved planes intersecting. The flatter seat allows more movement and therefore is more comfortable. Most important to the shape of the seat is that the deepest part is correctly located in relation to the back and that the front is properly relieved to prevent cutting off the circulation of the legs. I have found that a 5" or 6" radius on a travisher is fine for achieving both the tightest radius and the flattest areas of a seat.

The key to working the flats and curves with a single tool is the angle at which you hold it. Above is a photo showing the curve at the bottom of my travisher. I use it to carve the flatter areas by making a series of strokes in one direction and then going over the same area in a different direction, much as you would while using a scrub plane. As I approach the desired shape and depth of curvature, I lighten and overlap the strokes to get a finer surface. This is a fine time to mention that nearly every stroke that I take with the travisher is across grain and with the tool at least slightly skewed. While cutting cross grain, I make sure to skew the tool in the direction that will cut cleanest (normally towards the deepest part of the seat).

Below is the same travisher pictured while skewed. You can observe that the curvature of the bottom is tighter. It is in this position that I am able to cut the curved at the back of the seat. When cutting this area, I simply stop when the tool won't cut anymore. The problem that I most often see with students is that they are trying to cut the deepest part of the curve without cutting the areas around it. It is a good rule of thumb that if the tool won't cut the desire area, try cutting the surrounding material. Think of the way that a handplane cuts, always riding the high spots. A travisher works the same way, if all the surrounding areas are high, it stops cutting. This is when folks start reaching for tools with a tighter radius. I think that this is a mistake. The idea behind the shape of the travisher is similar to the handplane. With either, the idea is to "map" the shape of the tool onto the workpiece. With a handplane, the desired shape is flat, and when flattening a board, we reach for the largest practical tool because it will take in more information (high spots) and more easily do the job. The same goes for the travisher. The information taken in by the increased contact of the flatter curved tools will yield a more consistent result.
Working the tighter curved shaves is the equivalent of using a small block plane to flatten a plank. Yes is always cuts but it will be difficult to get the surface uniform.

There are other advantages of using the travisher in the skewed position. Below is an image of a chisel blade showing the direction that the shavings travel across the cutting edge when used straight and skewed. As you can see, the path of the straight cut shavings will go directly up and have to deflect the full angle of the bevel. This makes cutting more difficult and yields a coarser cut when cutting crossgrain. The path followed by the shaving when skewed is less extreme. The benefits are many. Less force is required to cut, the tool cuts cleanly cross grain (while pointing with the grain), the bevel angle is reduced (actually acting as a thinner, sharper blade) and the ease of slicing while cutting is increased. With the travisher, you also add being able to adjust the curvature of the bottom. Most of what I've touched on here also applies to the inshave, which is why I also advocate for the flatter ones. I would like to note that I have no financial involvement with Elia. I just want to see a good tool made by a good craftsman.

No comments: