200-inch telescope being welded electrically

The mounting structure for the 200-inch telescope being built on Mt. Palomar, in Southern California, USA, will consist of electric-arc-welded units. The mounting for the telescope will consist of 2 parts: a tube, and a cradle or yoke which will carry the tube. The tube, which will serve as the supporting structure for the principal optical elements, will rotate about 2 ball-bearing trunnions supported in a horseshoes-shaped yoke. The decision to use arc welding so extensively in the construction of the tube and mounting is considered an outstanding example of the engineer's confidence in the welding art. Most of the material being used for the tube and mounting is quite ordinary mild carbon steel, except for very few pieces made of special alloys that have low expansivity. All the materials being welded are of standard commercial grades - an important practical consideration. Arc welding only is being employed, and heavily coated electrodes of the mineral-coating or slag-producing type are being used. Overhead welding has been avoided. The electrodes are of such nature that they are equally satisfactory with either dc or ac welding ap paratus. Both are being used throughout; ac welding was found to be especially useful for welding into comers. All welding on the tube parts and to date on the "horse shoe" has been manual welding. All the parts are being carefully annealed to relieve internal stresses. As an example of the accu racy with which the parts were held to size throughout welding and annealing, when the prime-focus cage was placed on the boring mill to face the lower end where it bolts to the top ring of the tube, the departure from roundness was not greater than 1/16 inch.