elliptical mirror finally complete

[Jeffrey Makiel]

It looks fabulous! Well done.
-Jeff

[Guy Germaine]

Very nice work. Would you mind elaborating a bit on the jig for the elipses? I've seen commercial versions at Rockler, but I figured it should be easy enough to make my own. Looks like you did!

[Rich Torino]

Real nice piece of work John... I like the detail..

[John Piwaron]

Very nice work. Would you mind elaborating a bit on the jig for the elipses? I've seen commercial versions at Rockler, but I figured it should be easy enough to make my own. Looks like you did!

It is easy. Figuring out the size of the ellipses I needed and the jig to cut them was the first order of business. FWIW, I think I spent about $30 building this. That doesn't include the polycarbonate router base which I already had lying around. Most of that money is for the Rockler T guide channel.

I saw an article in FWW that described what was needed to make a jig, I think it was entitled "cutting big curves" or somesuch.

Between that article and my own layout work, it took a pretty good amount of time. And through all this, the extensive amount of time I took building this was a matter of learing. Learning takes time.

I used Autocad to layout the ellipse and design my jig. Mostly, there's 2 centerpoints it pivots on. Each pivot point is actually a steel dowel pin pressed into a T shaped piece of hard maple about 6 inches long. That T rides in some of that aluminum track you can buy at Rockler. I had a machinist friend cut the ends of 4 pieces so they'd join tightly at the center. If you can't do that, there's a kit available with 4 small lengths that have that same kind of cut on the ends. I chose to make the cuts on longer pieces to avoid an extra 4 joints. I wanted to be sure the pivots would slide smooth and catch free. The base of the jig is 3/4" MDF. In addition to figuring out the sizes and relative positioning of my ellipses, it was critical to determine how the router base would pass over the jig base, determine where to to put the foot under the router base to hold up the unsupported end, and most critical of all, that the end mill I used would itself miss the base while cutting.

It is important to not that my particular jig will not cut every ellipse. If I get a yen to make another of different (narrower) shape, I'll have to figure out a different jig. But if I decide to cut an ellipse similar to this one, I can probaby reuse most of it.

The layout of the ellipse is pretty easy. The distance between the first two points that are always on the jig is 1/2 of the width of the desired ellipse. The distance from the outer pivot to the point of where the cutter is tangent to the ellipse is 1/2 of the long length of the elllipse. I made only 1 base, but two arms since the actual shape of the outer and inner ellipses are different. When I made the template that I used to cut the offset ellipse that would place the mirror retaining piece, I move the arm out by 1". This does NOT make a different shaped ellipse, it offsets the same ellipse by that amount. That fact was very useful.

Also, I found that my jigs repeatability wasn't really all that good, so the jig I cut the first time through was it. No resetting and going back for something I missed or screwed up. How'd I learn that? Well, on the template that I made for that offset ellipse, I tried resetting the jig to make another outside shape. I had planned to simply line up that template with the offset ellipse onto the outer ellipse after using the main template. Well, the second outside ellipse didn't match the first one.

So what I did while I still could was mark all the centerlines onto all templates and onto my work piece. So long as I kept things organized and worked deliberately enough to minimize brain farts, I was o.k. What I'm saying is that I lined up the offset template with the centerlines. And it wasn't super critical anyway since it only had to hold the mirror to the frame.

I used a 1/2" dia solid carbide spiral end mill to do the cutting.

And last, if all that is still confusing, I have an Autocad 2000i drawing of my layout if you want to study it to see what I mean.