Monday, July 6, 2015

Galbert's Sightlines In Sketchup

For the past couple weeks we've been building a small staked desk, much like the one Schwarz built recently.

We like this style of furniture since it shares much of the DNA of chairmaking, and we also like it because its fast to make, completely functional, and supremely durable.

I usually do my rake and splay on paper with a quick sketch, then use Galbert's sightline square to find the numbers. But I thought I'd try and work everything out in Sketchup so I could spin the piece around in perspective view and taste the final look.

I'm no Sketchup expert, but here's how I did it anyway. Its fun and easy. My explanation will assume you have some experience drawing in Sketchup.

First, make sure you're working in parallel projection.  Draw a vertical plumb line at each leg location using the tape measure tool.

Now draw the legs as simple lines. They are full length and start at the top face of the table's top surface.

Now switch to front view so everything looks like a vector drawing. Select two legs from the left or right end (you'll need to orbit a bit to select them both) then rotate 10 degrees (or whatever) for the splay. I'm not rotating at the top of the table, but my pivot point is where the leg meets the underside of the batten. This keeps the legs centered on the batten.

Here's what it looks like when you orbit. Legs are splayed only.

Now switch to left or right view and rake the legs to 13 degrees the same way.

You can make copies of the table and play around with different leg positions, rake and splay until you like what you see.

When happy, draw a line from the end of the leg over to the vertical guideline. Don't worry about the angle of this line, it doesn't matter as long as you connect it with the plumb line.

Now draw a line straight up the guideline and stop it past the top. The triangle will close and create a face. This is your sightline.

I learned from Pete Galbert that the sightline is visible when you walk around a chair (or table) and the leg in question appears to be dead plumb, that is, you can't see any rake or splay.

Orbit the table ("walk around it"), and you'll see your angled leg line up perfectly with the vertical plumb line once you're viewing directly along the sightline. Like this:

The sightline is the plane in which you tilt your drill at a certain angle to create rake and splay. In other words, keep your drill dead plumb in one plane (along the sightline) then tilt it to the correct angle (the resultant) and your rake and splay will happen automatically. It's super simple once you've tried it.

Here's how to find the resultant angle.

Select the triangle, copy and move away from the table so its easier to measure.

Use the protractor tool to measure the angle. That's it. That's the angle you tilt your drill to get 10 degree splay and 13 degree rake.

Finally, I select a leg, and move it up just enough (it should only take a smidge) that the end of the leg is visible at the top, then use the dimension tool to find the drilling location from the ends of the top. Notice that it's not at the same location as the plumb line. That's because the leg is centered on the batten below, not at the top. This only applies when drilling through the top and batten at the same time. If you're drilling through the batten only at an angle, then I would just drill through the bottom on the center of the batten.

For further info, see Chairmaker's Notebook, Appendix A: Creating and Using Sightlines.

Also, Pete's video.