In view of the fact that the deviations from sphericity of the formed 

 sector were less than 1.0% of the specified radius, no real incentive existed to 

 modify at that time the existing curvature of the mold by remachining it to an 

 exact 33-inch radius. Besides, it was predicted (and subsequently verified by 

 actual measurements) that the deviations from specified sphericity during the 

 thermoforming process are minor compared to the deviations resulting from 

 machining of the pentagons. However, the generated data on deviations in 

 sphericity proved conclusively that specifying the nominal radius of curvature 

 of the mold 33.187 instead of 33.000 inches produced thermoformed sectors 

 with radii of curvature larger rather than smaller than the specified 33.000 

 inches. Thus, it is recommended that before additional 66-inch acrylic plastic 

 spheres are fabricated, the curvature of the female mold be remachined 

 accurately to the specified radius of curvature for the sphere. 



Machining. The transformation of the spherical sectors into spherical 

 pentagon modules was performed in two steps. The first step consisted of band- 

 sawing the spherical sector into an oversized spherical pentagon (Figure 31). 

 The second step, reducing the oversized spherical pentagon to the dimensions 

 calculated for the spherical pentagon module, was performed on a vertical mill. 

 For machining of model spherical pentagons, a manually operated mill was 

 utilized (Figure 32), while for the machining of full-scale pentagons, a magnetic- 

 tape-controlled mill was chosen (Figure 33). To facilitate the holding of the 

 33-inch-radius spherical sector in the bandsaw and in the vertical mill, a vacuum 

 chuck (Figure 34) was designed and built by the shop personnel at the Pacific 

 Missile Range. This chuck not only held the spherical sector in place but also 

 served as a pneumatically operated indexing head for accurately locating the 

 five straight beveled edges of the pentagon. 



The machining of the large pentagons was performed with a 2-inch- 

 diameter, helical-type milling cutter rotating at 3,800 rpm and fed at 10 in./min 

 (Figure 33). A detergent and water cutting fluid was used to keep the cutter 

 and the work piece cool so that serious stress concentrations would not be 

 introduced into the spherical pentagon. The selected cutter, rotational speed, 

 feed rate, and coolant resulted in a 63-rms finish for the beveled edges of the 

 spherical pentagon. 



Regardless of whether the pentagons were machined manually, as was 

 the case with the model scale ones, or by preprogramed tape, as with the large 

 pentagons, each pentagon had to be dimensionally checked before acceptance. 

 The dimensional check consisted of measuring the linear distance between 

 opposite tips of the pentagon with a specially built caliper having a 46-inch 

 throat (Figure 35). Only if all the measurements between the opposite tips 

 of the pentagon were within the specified dimensional range was the pentagon 

 considered completed. To reduce the influence of temperature on the mea- 

 surements, all readings were taken in the 65 to 75°F range. Although in the 



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