Rocker Blade Jig

 

If you can't change it, you got to stand it

Ennis Delmar, BrokeBack Mountain, Annie Proulx

So I come from a casework background and as such I try to use that knowledge and those tools to simplify or at least speed up certain aspects of chair making. 

A good example is how I make my rocker blades. Years ago I made this jig to cut some large circles out of plywood for some project and I altered the dimensions to work with cutting the rocker blades. 

The base for the jig is this 12 by 52 inch piece of ¾ plywood.  Its fastened to the bandsaw table with a couple bolts drilled thru the table. 

The hole that the rocker pin goes in is 46 inches from the blade. As you have probably guessed the bottom of the blade is a section of a circle which seems to work fine and 46 does give a nice steady rocking motion.  

 

I built this one with looks like mostly pocket screws but anything will work provided the bottom stays flat and free from obstructions.  

Below you can see the top side. Its just a big T shaped piece that measures 52 on both axis.  The pin hole is 46 away from the blade and a couple toggle clamps hold it firmly in place. I figure out the angle of the front legs where they meet the floor (which is the same angle as the stretcher) and set the bandsaw table to that angle.  Then clamp it in place and cut it. The typical size of the blank is ½ x 6 x 42 which seems to work well. Also please note that there is a left and right. 

Further note the blade pattern I use is from Pete Galberts rocker plan found in FWW. I use the same pattern on all my chairs the only difference is on smaller chairs I shorten the length a little.

Why did it take so long

 There’s value in work you enjoy, or that serves a need. There’s no value in work for its own sake

Elizabeth Bear, Ancestral Nights

(if you like Sci-fi check out Elizabeth Bear)

One thing thats always bugged me was turning the bottom 6⁰ taper smooth after the end (and thus the center) has been cut off. It's near impossible to find the absolute dead center again and so the piece spins off center which is doesn't help at all. This is especially problematic with arm posts as they are double ended and they are visible...very visible.  I use a tenoner from Elia Bizzari which makes a great 6⁰ tenon but it does leave a rough spot where it ends that must be turned away. 

So tonite after 15 years and maybe hundreds of arm posts and legs it dawned on me to just put the tapered end in the open #2 Morse taper on the drive end and stick the other end (which still has a center) on the live center and there ya go. Bob's your uncle. A perfectly centered piece that can be trued up and looks great. On one hand I'm glad I thought of it on the other I'm kinda wondering why it took 15 years to see this. 

Fast Growth is Stronger (sometimes)

The Devil can quote scripture, after all. And monsters can say “please” and “thank you” same as any mother’s son.

Elizabeth Bear, Karen Memory

I noticed this piece of red oak and thought it would be a good opportunity to explain why fast growing wood is, in some cases, stronger than slow growing wood. You can see below the holes (more of a matrix really) that is seen in ring porous woods during early wood growth stage. This is during the spring of the year when the sap runs and the tree is growing quickly.  The size of this growth, ie the width, is pretty consistent year over year. Next as the starches in the roots run out the tree transitions to so called late wood this is the growth that is put on during the summer and early fall.  

Below you can clearly see years of good late wood growth and years of poor late wood growth.  The difference in strength thus becomes the ratio of early growth to late wood. In the lower part of the picture you can see the ratio is about 50 50 vs the upper part where it looks to me to be about 4 or 5 to 1. So the wood in the top of the picture is much stronger than the wood in the bottom. 

A couple points. Early wood and late wood are not to be confused with juvenile (sometimes called reaction) wood which is something else entirely.  

Second this does not apply to diffuse porous woods (like maple) which are stronger the slower they grow. 

So when you are looking at ash logs or oak for chairs be sure to look for faster growing trees. 

(There is no red oak where I live (Northern Maine) so this log came from central New Brunswick and if you zoom the below picture you can clearly see times of fast growth and for some reason two 5 to 8 year periods of very slow growth).