IT WAS MORE WORK THAN I THOUGHT, BUT IT WAS FUN


I was somewhat at loose ends. I was between work assignments as the programs I had been working on had wound down. I was thinking I had to start getting serious about finding a project to work on. I do not know how it came about, but I was asked to go talk to Gerry Hardesty* about helping him with the design of components of the new ESG MiniSins system. This was the system that was to become the replacement for the  Autonetics N16, then in service as the navigation system for the USS Los Angeles class submarines. The new navigation system used a new version of the ESG. This design had two rotor assemblies mounted on a common block. (This commonality proved to have its own unique set of problems. I remember talking to Gerry about this. He believed he had no choice in choosing this approach as the stable platform had to fit the existing N16 system gimbals. He had the problem of cramming 10 pounds into the proverbial 5 pound bag. Not enough space for two separate ESG assemblies.) The new system made use of some other parts of the N16, but it was an entirely new design of the stable platform. Gerry informed me that a magnetic shield that covered the ESG assembly was proving to be troublesome to manufacture. He asked me look into the problem and determine what corrective actions could be found. I obtained the drawing for the part and spent some time trying to understand the problem. I was confused by the drawing. I remember visiting the vendor shop that had a contract to produce the shield and being greeted with less than full enthusiasm. It turned out they were as confused by the drawing as I was and were tired of  meetings that resolved nothing. (I learned from the contractor that they were experiencing little difficulty fabricating the part, but they found it nearly impossible get the part though the final inspection process. This was a surprise to me as I had assumed the major problems would be associated with the deep drawing operations.)

The shield was made from a metal alloy commonly used for magnetic shields (mu-metal ?). The part was made by several successive deep drawing operations,  followed by a complex final heat treatment which that was intended to maximize the shielding properties. The drawing had an explicit warning note to not bend or work the shield after heat treatment. Any such working or bending of the shield would degrade the shield’s effectiveness. Since the drawing was not clear as to the final shape of the shield, shields not meeting the dimensional requirements of the drawing were rejected and could not be reworked because any rework of the rejected shield meant a redo of the heat treatment. ( An alternative way to resolve the problem would be to require that each shield be subjected to a final magnetic field test that demonstrated that each shield attenuated the ambient magnetic  field to an acceptable level. When I made this suggestion, the reaction was a vocal and resolute “no way”.)

 After spending some time going over the drawing and listening to the contractors shop people, I came to realize  that we did not have shield fabrication problem; We had a drawing problem. To correct the problem it would be necessary to completely redo the drawing.

I had worked as a machinist and I had seen many drawings in doing that work. I had been frustrated by many drawings that were downright confusing to me and the others that I consulted. The shield drawing was just another example of the downside of drawings being made without a consistent methodology as regards drawing standards. It seemed to me as though each draftsman and designer had his or her way of doing things. I do not remember the circumstances, but I became aware of what was the current ANSI/ASME standard for mechanical drawing dimensions. I remember obtaining a copy and studying it. I had two reactions: “thank god” and “I don’t understand this at all”. I struggled with the concepts of dimensioning presented in the standard, but I began to see how the new standard could used as the basis  for a revised shield drawing.

A magnetic shield has but one purpose, to reduce the deleterious effects of ambient magnetic fields on a sensitive component. In this case, the sensitive component was the ESG assembly. The shield in question was deep drawn from a single sheet of  nickel alloy. The shield was then subjected to a complex heat treatment that was the final step in the manufacturing process. The shield was handled carefully following the heat treatment to avoid degradation of the shield’s effectiveness. The assembly requirements for the shield were minimal. The shield had to slide into place easily and the screw holes had to line up without any pushing or pulling on the shield. A space could be defined in which the shield could take any reasonable form. Precise dimensioning of the entire shield was not necessary in order to ensure the shield would fit at the next assembly. My task was to define that space using dimensions consistent with the dimension standard. That proved to be a difficult task. I had to be sure I understood each dimension that I contemplated using; I did not want to create a new set of problems. I marked up a shield drawing using red pencil and, after a period of review and reflection, I took the marked up drawing to Gerry. It looked like someone had bled all over it. Gerry went from skeptic to a reluctant semi – believer as I went over my reasoning with him. He did give me his tentative approval which meant I could take the marked up drawing to the contractor and repeat my pitch. I went to the vendor’s shop and presented my ideas about basing the shield dimensions on the requirements of the dimensional standard. They realized they were no longer being required to demonstrate that the shields met dimensional requirements that had only a vague relationship to the actual use of the shield. They only had to demonstrate that the shield surfaces were within a defined space and the screw holes lined up. Once that realization sunk in they began to think about inspection fixtures that would greatly simplify the inspection of the shield. The shift in the tenor of the meeting was a good sign and I began to relax a little bit.

I continued my involvement with the Minisins project by carrying out tests of the effectiveness of shields made according to the revised drawing. I was able to demonstrate that the shields were effective in reducing the ambient magnetic field near the ESG. I seem to also remember demonstrating the effectiveness of the final heat treatment. The heat treatment process was complicated and time consuming but necessary.

The new dimension standard went into great detail as to the rationale for a scheme with which to define a  frame of reference for any part. A lack of a clear definition of the reference frame was a common problem found on drawings I was familiar with. I also realized that it was possible to completely eliminate the horror represented by a dimension that required a measurement to an imaginary point, such as the center of a hole. For me it was like someone had opened a window in a smelly room and let the fresh air in. My only regret was that I did not have this dimensional standard at my disposal during the ten years that I was the Responsible Engineer for the G9 gyro.

  • Gerry and his wife were lost in the North Atlantic ocean while they were sailing to Ireland. He was a brilliant engineer and his wife Carol was the former owner of a book shop. He and Carol lived on his boat exclusively after he retired. The name of his boat was “HAPPY ENDING”.

One response to this post.

  1. Posted by Anonymous on December 10, 2017 at 3:25 pm

    A bit over my head, but interesting none the less. Ironic the name of the boat.

    Reply

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