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| A Microscope Repair. |
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| Pete W.:
Hi there, all, Here is one of my first projects of 2015. Several years ago, a friend gave me a Cooke, Troughton & Simms polarising/petrological microscope. It was in need of a repair to the substage cluster, which should look like this: At the very top, immediately beneath the stage, the condenser and its iris diaphragm sits a filter holder and beneath that the polariser (the component with the 0 to 360 degree graduations). Next down is one of the two lugs that support the entire swing-out sub-stage cluster, followed by another filter holder. The lowest swing-out component is a supplementary condenser lens beneath which is a second support lug. The threaded portion of the spindle next to the major diameter of the tapered section screws into the hole in the upper mounting lug and the lower end of the plain section is secured in the lower mounting lug with a tiny grub-screw. The polariser unit is retained by a nut that fits the other threaded section of the spindle. The assembly is interspersed with several plain and one spring washers. Oh, and I forgot to say that the upper filter holder, swinging on the plain section of the spindle, is retained by a collar and grub-screw. However, some previous owner (not he who gave the microscope to me!) had modified (butchered!!!) it. I presume that the polariser unit had suffered damage to its polarising material and so had been removed, complete with the upper filter holder. In the process, the spindle on which all the swing-out components pivot was sawn in two. When the microscope came to me, the empty polariser unit and upper filter holder came separately, complete with half the spindle! I had put off doing this repair for far too long despite having acquired a complete polariser unit via that well-known on-line auction site but I've decided to make it one of my first workshop projects for 2015. The first step was to strip out the part of the spindle that was still mounted on the microscope and to have a measuring session. The two sections of the spindle, arranged to simulate a new spindle, look like this: To give an idea of the size of this part, the threaded sections are ¼" by 40 tpi ME thread (i.e. Whitworth form) and the overall length of the entire spindle will be roughly 1½ inches. The polariser unit swings on the tapered section of the spindle. I could measure the length and maximum diameter of that tapered section easily but could only get an approximate measurement of the minor diameter. I plan to set over the top-slide of my lathe and machine a dummy spindle with just the tapered section then check the taper in the polariser with engineers' blue. Then adjust the set-over until I get a good enough fit and machine the tapered section of the replacement spindle at that setting. The original part was probably drawn with dimensions referenced to some particular point on the spindle - I have no way to retrieve the original manufacturers' dimensions. So I just have to measure each feature of the spindle sections, compare them with measurements of the mounting lugs and proceed as best I can. If the first attempt doesn't work, I'll try again - I just need to avoid resetting the top-slide setting until I'm sure I don't need it again!! I'm going to machine the new spindle from EN1A material. Watch this space!! :mmr: :mmr: :mmr: |
| awemawson:
Hi Pete, glad to see you active in the workshop :thumbup: Being a lazy sod, I'd just face up those two parts, bore them, and either hold them together with a stud and loctite, or a pin and silver solder. If much material has been removed between them then a suitably shaped pin would make up for it I would think. |
| vtsteam:
Looking forward to this... :coffee: :beer: |
| Jonny:
Good one Pete, if a jobs worth doing, do it it right or not at all. Have both studs to be cock on inline, most likely. If so machine in one from bigger dia bar to avoid flex, I do it all the time. |
| BaronJ:
Hi Pete, One way of getting a very close idea of the diameter of the small end of that taper would be to cut a hole in some thin card and press the small end into it. I would use my drill press to get enough pressure without doing any damage to the part. Then measure the diameter of the depression in the card. |
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