Rocker box for the
12.5-inch
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The rocker box is fairly straightforward and similar to what is described in the Kriege and Berry book. It is made from 1/2" baltic birch except for the sides carrying the altitude bearings which are 3/4"-inch thick. As can be seen two 1" squares of 1/8"-thick teflon are nailed inside to prevent wood-on-wood scuffing. This works well. The altitude bearings are 1/16"-thick 3/4"-wide 1.5"-long bondable teflon (Small Parts U-VTE-60) glued on with Liquid Nails. The dimensions of the box are 19"W x 19 .25"D (o.d.) and 17.5"W x 18.25"D (i.d.). The extra 3/4" is for bearing storage (see Packing up the Scope). The heights of the sides for the altitude bearings are 11.125" max and 9" at the bottom of the curve (o.d). The height of the front is 9.25" and the back is 3" (o.d.). The inside of the box (and the mirror box) is spray painted with Ultra Flat by Krylon (the best). |
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The bottom of the rocker box. The ground board (1/2"-thick baltic
birch, 17.25" in diameter) has three 1.25"-square pieces of 1/8"-thick teflon
nailed on. The size of the teflon for both bearings was calculated
as described in the Kriege and Berry book and,
judging by how well the completed scope works, I think they are right. The
bottom of the box as well as the altitude bearings are covered with Ebony
Star formica which you can buy pre-cut in a strip 3/4" wide and a square
18" on a side from Crazy Ed
Optical. Ed is a nice guy and you might want to call him to discuss
any size you need. The Ebony Star is glued on with Liquid Nails.
The square of 1/4" baltic birch in the lower left is used when shipping as described in Packing up the Scope. |
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The pivit bolt is a stainless steel (it gets wet; no rust) 3/8" carriage
bolt, cut to the right length. The nut is an all metal lock nut (difficult
to find; The Nutty Company)
that tightens before it goes all the way on. To avoid a problem that
can occur if the bolt simply goes through a hole in the wood (the bearing
can get sloppy after the wood begins to wear), I put in a 3/8" tee-nut in
the ground board and rocker box and then used a 3/8" drill to drill
out the threads. This worked great. The tee-nuts from Woodworker's
Supply (call 800-645-9292 for a great catalog that has all kinds of stuff)
were much sturdier than the ones from my local hardware store.
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The mirror box, with altitude bearings attached, rides on the
rocker box. On the rocker box, just below the altitude bearing, you
can see the (2) brass threaded inserts that are used to bolt (#10-32) the
aluminum rectangles that join the top and bottom of the rocker box (for
shipping). Similarly there are threaded inserts on the right and left
edges of this side of the box used to bolt the aluminum angle (see
Packing up the Scope)
The altitude bearings are of 3/4" medium density fiber board as recommended by Ray Cash. It is strong and won't warp, but it is brittle, so avoid sharp blows.
The method of cutting circles
recommended by Berry and Kriege worked really well for the altitude
bearings, the arcs in the rocker box, the upper tube assembly rings, and
the baffle and dust cover. |