Several conclusions can be made from that comparison. First, the 

 deviations from specified sphericity are larger after machining operations 

 than prior to them by a factor of 2 to 4. Second, even with the considerable 

 increases in spherical deviations, they are less than 1% of specified radius. 



Similar measurements (Appendix C) were taken on model pentagons 

 (Figure 38). The only difference here was that the measurements did not 

 reflect any deviations from sphericity due to annealing process, as the model 

 pentagons were not annealed until they were assembled and bonded into 

 spheres. 



Bonding. Initially the assembly of both the models and the 66-inch 

 spheres was planned to be by the solvent-cement technique. The arguments 

 for this bonding technique were that (1) the tensile strength of solvent-bonded 

 joints can be made to equal that of the parent material by proper curing 

 schedule, (2) all the operational parameters of this bonding technique are 

 well known, and (3) the joint, if properly made, is completely colorless and 

 transparent. 



Although the arguments cited in support of this technique are valid, 

 the results from bonding the first model capsule were quite disappointing. 

 Either the joints were full of air cavities (Figure 39) and surfaces unwetted 

 by the solvent because of insufficient clearance between the individual pen- 

 tagon for penetration of solvent, or the joints were completely wetted by 

 the solvent, but because of too large a separation between pentagons, the 

 solvent upon drying left a very weak joint. 



The reasons for this unsatisfactory performance of solvent-cement 

 technique were many. The major ones were ( 1 ) all the pentagons had to be 

 bonded simultaneously so that mismatches between individual pentagons 

 could be distributed over the whole sphere, (2) edges of pentagons were not 

 presoaked in solvent prior to assembly because of physical impossibility of 

 presoaking the five edges of all pentagons simultaneously prior to assembly, 

 (3) joint clearances and clamping forces varied from point to point depending 

 on the dimensional deviation from nominal pentagon dimensions, and (4) it 

 was nearly impossible to apply a calibrated clamping restraint on the whole 

 sphere during bonding. 



In order to improve the quality of the joints, an attempt was made to 

 control the clearances in the joints so that complete penetration of the joints 

 by solvent would take place. After an exploratory investigation into this pro- 

 blem, it was found that a 0.005-inch clearance was required to create the 

 capillary force that would draw the solvent into all the joint spaces. 

 Unfortunately, the joint with a 0.005-inch clearance maintained by spacers 

 had nearly zero strength as the evaporation of the solvent from the wide 

 joint resulted in very inferior joint bond strength. The decrease of the joint 

 clearance to 0.001 inch by placement of 0.001-inch-thick spacers resulted in 

 higher joint bond strength, but did not permit the solvent to wet all the joint 

 surfaces. 



64 



