The chair is sitting on a level worksurface, the front legs are raised on blocks to achieve the proper tilt to the seat. The seat is then leveled from left to right with shims (if necessary). Measure from the front of the seat to the worksurface – 17-3/16″ in this case. Decide on a final seat height – 15-11/16″ in the illustration. Subtract the seat height from the overall height to determine how much wood you need to cut off – 1-1/2″ in this case. Cut a block of wood to that length. Use the block to scribe the legs with the help of a Half Pencil™.
The following is an excerpt from “The Stick Chair Book” by Christopher Schwarz.
Cutting down the legs of a chair so they rest flat on the floor is another one of the “great mysteries” faced by most beginning chairmakers. If you’ve never done it, it seems impossible. But if you’ve done it even once, it seems completely obvious.
If you’ve never done it, know this: Leveling the legs requires no special talents in geometry or math. The only skill required is being able to saw to a pencil line.
There are about five or six ways I know of to level the legs of a chair. I have tried them all. The following method is the one that is the easiest to teach. Most students grasp this method with ease, in my experience. If, however, you read through this technique and say: But why don’t you do …? My answer is simple: Try your theoretical method and see how it goes. There are lots of ways to do this operation.
Set the Stage
To trim the legs of your chair you need a flat worksurface that is level. This can be a piece of plywood that you have shimmed with wedges, the top of a table saw, your workbench or that one patch of floor in your shop that is inexplicably level.
To level the worksurface, I use a spirit (aka bubble) level and construction shims. I test the surface in the X axis and Y axis and add wedges until the surface reads level according to the spirit level. Then I gather the tools necessary for laying out the cuts on the legs: a handful of small wedges, the spirit level, a modified carpenter’s pencil, a tape measure, a 6″ rule and some scrap wood.
The Modified Carpenter’s Pencil
One of my favorite layout tools is a carpenter’s pencil that has been planed to half its thickness. I call it the Half Pencil™, and it is a useful thing to have around. It allows you to make pencil marks in the same way a spear-point marking knife works.
A marking knife works well for joinery because you can run the flat back of its blade against one surface (such as a try square) to accurately mark another surface below. The same principle applies to a half pencil. You don’t have to tip the pencil to make a perfectly accurate mark.
If you are skeptical, plane a carpenter’s pencil in half. This is easily done by placing the pencil against a planing stop. If you don’t have a planing stop, stick it to a scrap board with carpet tape and plane it in half with a jack plane. Once you own one, I suspect that you will find uses for it outside of chairmaking.
Plane a carpenter’s pencil in half with a jack plane until you get to its lead. This pencil (turned upside down in this photo) can scribe your legs without accounting for the pencil’s thickness.
The Two Big Ideas
Put the chair on your level surface. I’m sure that (like my chairs) it will wobble on the flat surface and look a bit awkward. This is how all that gets fixed.
The goal is to prop up the legs so the seat is:
1. Level from left to right.
2. Sloped from front to back so that the chair is ideal for either dining/ keyboarding or lounging.
Getting the chair level from left to right is straightforward. Place your bubble level on the seat and shim the legs so none of the legs wobble and the seat is level to the floor left to right.
Use shims to level the chair’s legs from left to right. Make sure the legs don’t wobble.
Now you need to set the “tilt” of the seat. How much does the seat slope downward from front to back? A seat that is level from front to back isn’t ideal. The sitter will feel like she is being pushed forward a tad. The seat needs to slope backward.
But how much?
The system I use is based somewhat on the way chairmaker John Brown worked. The seat should slope backward by “one finger” for dining chairs. And “two fingers” for lounging chairs.
Place the level on the pommel and the back of the spindle deck. Place one finger under the level at the back of the chair. Does the bubble level read level? If yes, then your chair is pitched correctly for a dining chair.
Usually, most chairs need to have their front legs propped up on scrap blocks to be sloped two fingers or three fingers back. If your chair (as built) is pitched at “one finger” and you want it to be “two fingers,” then you need to prop up the front legs by “one finger.” My fingers are about 3/4″ wide. So, I’ll cut scrap blocks about 3/4″ wide and place them under the front legs and any wedges.
I have put blocks under the front legs of the chair until the bubble level reads level from front to back with two fingers under the rear of the level. This pitch is ideal for lounging.
Then I check the slope from front to back. If I can put two fingers under the bubble level at the back of the chair and the bubble level reads level, then I’m where I need to be. If I want more pitch, I’ll add taller blocks at the front legs. If I want less pitch, I’ll use shorter blocks.
Mess with the blocks and wedges until the seat is level from left to right and pitched like you want it in real life. And make sure it doesn’t wobble on the block and wedges.