Friday, April 26, 2013

Dialing it In

I successfully used the new rocker routing fixture to cut the slots in the walnut chair that I am finishing up.

I suppose that a quick word is in order about my reasons for going to the trouble of building this tool. While I enjoy doing many things with hands tools, because they give me lots of control, freedom and results that machines can't match, when it comes to rockers, I have a different priority.
Designing and building rockers is a process full of variables. To rock successfully, rockers must be oriented to each other and the chair correctly. Any variation in the process of cutting the slots and fitting the rockers can make it difficult to refine the design. Also, the references used to locate the slots can greatly affect the consistency from chair to chair.
What this fixture does, is create repeatable and consistent rocker slots based on references that allow me to focus on the other variables that go into making a rocker.  For me, nailing down the relationship between the seat and the rockers is the next step towards a deeper understanding and freedom in rocking chair design.

I am going to show some photos here, and hopefully you can see that while it has some adjustable parts, the fixture is simple and does a simple job. The photo below shows one the primary advantages of the fixture, which is that the two platforms are automatically parallel to each other which insures that the slots are as well.
Here are the slots already routed. The straight edge lies dead flat against the inside of the slot on both legs.

 Another variable that the fixture addresses is the different splay of the legs. You can see this in the photo below if you look closely.
The slot on the leg in the foreground is slightly tilted off the axis of the leg. You can especially notice this if you compare the half circle shaped material remaining at the bottom of the leg. The slot in the leg in the background does the same thing, but in the opposite direction. The reason for this is that the slots must be oriented to split the difference between the splay of the front and rear legs.
Most importantly, the slot is in the center of each leg at the deepest point, for strength. The jig does this automatically. I'll explain how it does this later, but for now, I just want to point out the variables.
The process follows these steps.
First, I measure the splay of the front and rear legs, in this case 13 degrees for the front and 19 for the rear, which gives me an average of 16 degrees. I pivot the central panel to 16 degrees, push the chair up against the jig and position the two platforms. Then I route the slots.
Next, I swing the pivoting panel the opposite direction (16 degrees again), with the platforms still fixed, reposition the chair, and route the other legs.
Using the fixture was a breeze and so many of the troublesome layout and fitting issues that I've always encountered were either eliminated or greatly simplified.

Above, you can see how I measure the slot angles to transfer to my rocker pattern. If all is right, there is no fiddling around or fitting and I can repeat the results on the next chair. I will be shooting video of the process, but I think that introducing the variable over a few posts might make it all more understandable.


Bern said...

Well done and thank you Pete. Such a beautifully simple jig that cuts right through the complexities of allignment. I couldn't imagine creating slots any other way having been lucky enough to have been one of your Antipodean guinea pigs.
You should get down here more often. A lot of good ideas in a stubby of Coopers Green.

Anonymous said...

Aloha Peter from Oahu,Hi. , looking at your allinment jig seems Very straught forward,one big thumbs up from me !!! I'm on the disabled list now, but have hopes to return to maybe part-time some day. I made Koa mission and Hawaiian style rocker in eight styles that used a 1.5 koa thick rocker and a simpler tenon forming 'pin'. Most of all of my chairs (about 5 aweek) were shipped with the rocker 'off' to be mounted when the new owner recieved the crated rocker. This save over 75.00 shipping and we passed the savings on to the buyer. My ninth design for a rocker I think becomes do-a-bule with your jig. It is a rocker built for two, but facing each other and also side by side. I have been holding back refining the design for 12 years. Thanks for inspiring me to finish my rocker for two. The 'rug cutter' rocker ( Boston,ect.) has nor sold as well when made in Koa so I shall continue with the Hawaiian style to finish this new one. Koa seems to be king tho we sometimes use Mango, what wood do you work with the most? Mahalo, Mark on Oahu !