Thursday, February 9, 2012

Sightlines Revisited

I recently had a "eureka" moment, or perhaps a "duh" moment working out some sightlines for the two walnut armchairs that I am making. ***please see the edit at the end of the post***

I've shot a short video explaining the details.

Hopefully the video clarifies the process enough, but if not, here is the brief synopsis. The sightline is based on a relationship between the amount of rake and splay that you want. If the rake and splay are equal, the sightline will always be 45 degrees. By using a straight edge with marks based on degrees from a single point, the incremental increase of distance is accounted for and we can use this scale for direct layout. If you are dying to know why this works, take a look at this drawing of the graphical process. (and perhaps visit this posting and it's second part for a refresher)

You'll notice that all of the views of the chair come together to make a small triangle. Well, by using the special rule based on degrees, we skip all the steps and simply make the triangle directly on the pattern. What enchanted me about this was the ease of finding the resultant angle by measuring the hypotenuse. Perhaps I'm just a total nerd, but this one made my day!

*** One of the comments that I got questioned whether the Bevel Boss scale was created by measuring from a single point for all of the angles. This method presupposes that it is, and it isn't! That's the bad news, the good news is that the angles below 30 degrees do converge nicely and since theses are the numbers usually used in chairs, the scale should work fine. .***


Greg Pennington said...

Genius, what more can I say. And I thought I couldn't love the bevel boss any more.

Andrew Adams said...


Being a bit geeky myself, I enjoy learning about shortcuts like this, but I do have a concern.

Bevel boards like the Bevel Boss can be constructed in at least a couple of different ways. One runs all the lines from a common point, which is what you want for your sightlines trick. Another just sets a common displacement interval along one edge and then marks angles from each interval.

I can't tell from marketing pictures which the Bevel Boss uses. So have you checked that the Bevel Boss lines all converge on one point?

Peter Galbert said...

great catch!!!! You are correct that the Bevel Boss does not have all the angles originate at one point. I gave it a cursory check when I was playing with this, but obviously made the assumption that it was a proper rule from the accurate readings that I checked against the trig charts.
Luckily, the margin of error below 30 degrees or so is small enough that it still works out well, but to be truly accurate, we need a better scale,
thanks very much for the comment!

Anonymous said...

you need some children,you have wwwaayyyy too much time to think about this stuff. If you had diapers and skating practice and swimming, you'd be as bleary-eyed and stupid as the rest of us, lol.

Anonymous said...

I was following your new measuring method for drilling legs and then you stated "it was 14.5". What is 14.5? degrees? Inches? Please explain.

Bigchief said...

My understanding is that a starting point was marking a desired location (target) of the leg's foot with unknown rake and splay as well as the drilling points in the seat of the common leg pairs. This target, I'm assuming, would be something visually pleasing with thought given to the relation of perpindicular line of the seat above. Finding the rake,splay and resultant angles using your angle square or boss method made perfect sense to me except that length of the leg was not taken into account so while all leg lengths would pass through the target point not all would end up there.A child's chair vs a tall barstool with the same rake, splay and resultant drilling angle would not end up at the same target footprint at the floor. What am I missing or misinterpreting? Is there a way to make this work by calculating length of the leg into the equation so that it will end up on a target point at the floor level regardless of leg length? Thanks, Greg