Most of the set screws used on older PHI optics are the Bristol set screw type. The main advantage to this drive system is that almost all of the turning force is applied at right angles to the fastener spline face, which reduces the possibility of stripping the fastener. For this reason Bristol set screw are often used in softer, non-ferrous metals. Compared to an Allen drive, Bristol set screws are less likely to strip for the same amount of torque. Bristol set screws come with 4 flutes on the smaller sizes and 6 flutes on larger sizes.
When working on optics such as ion sources, analyzers, specimen stages and so on it is not uncommon to strip the set screws when removing them. A trick that can help prevent striping is to use a small amount of isopropanol as a lubricant before removing the set screws.
Some sources for replacement Bristol set screws are –
Fasteners & Metal Products Corporation
Note that these set screws are not cleaned for UHV and so you will need to run them through a cleaning process before using them for UHV optics applications.
When using Bristol set screws you need to use Bristol spline drivers. A source for Bristol spline drivers is –
Bristol Wrench Company, Inc 4 Hershey Drive, Ansonia, CT 06401
Note that Bristol Wrench no longer sells individual spline wrenches, only in sets of 10. Good news is that you can get individual Bristol spline wrenches from McMaster Carr –
I find that for these small screws it is good to sputter them with gold after a glow discharge final cleaning. A basic table top sputter coater intended for preparation of SEM samples can usually do both jobs without breaking vacuum so you do not risk trapping any organic films under the gold.
The gold deposited this way will not be uniform or pretty but it seems to be adequate to reduce seizing and make future disassembly easier.