Sunday, September 13, 2009

Three steps backward but four foward!

Every year I make a ceremonial bowl. As a result when I discovered rotary tables I became enamored. Not only could I make gears but I could finish bowls. Apart from that, they are beautiful machines.

I looked at rotary tables, used a big one on a Bridgeport, and realized the only practical size was an 8" table which can hold a piece that is 5" - 6". They weigh 70 to 80 pounds which is about what I can lift onto the mill. After lusting for about a year I got finally nabbed a nice used table. Perfect - except most of my bowls are larger than 8" and I needed at least a 12" table.

A solution is to extend the size of the table by finding a big disk of plate steel and fastening it to the table. The only problem is that I did not have the steel and, depending on the thickness, it too could weigh another 70 pounds. Because my acquisition of a rotary table was aligned with the Universe (apologies if this is too California) I soon came across a posting by someone who had a similar problem and solved it with an extender. This is four pieces of rectangular steel, each with a protrusion that fits exactly into one of the slots of the table. It has some countersunk holes so it can be fastened to the table, and sticks out a few inches so a large work piece can be held down.

The progression is bowl -> rotary table -> project. The project consisted of four pieces of steel, exactly the same size, with a protrusion that just fits the slots in the table, and countersunk holes right down the middle to hold it down with T nuts. I also drilled an tapped two additional holes so I could screw in pieces from a clamping kit. I can not tell you how many mistakes I made. At least three major mistakes and they were so embarrassing that is all I am going to say about them. Fortunately, I found a way of fixing all of them and making it all work.

The first photo shows the extender in action. There are three extenders (hidden) that are used to hold the various clamps and the mill is cutting a nice arc in 1/4" plate. The work piece itself is about 18" long.

The next photo shows how one of the extension pieces fits in the groove. There are two countersunk holes for bolts that fit the T nuts. The last photo shows this piece turned on its side and one T nut is shown. The extensions convert the 8" table into a 12" table.