>I need to "see" a steel setscrew (grubscrew) in a fiber wheel as the wheel
>rotates at varying RPM from 1000 RPM to 60000 RPM.  I'm only interested in
>determining instantaneous RPM, not in position data.

I suspect you may find an inductive sensor may be easier to use. These
consist of a permeable core with a coil wound on it. The core may have some
residual magnetism built in to give the necessary magnetic field for the
rotating metal to modify the field, and then an op amp to amplify the signal
up to a level where a comparator can be used.

To use a hall sensor the rotating metal will probably need to be magnetised,
and depending on the sensor used may need amplifying. I am not sure that you
will get enough field from a grub screw like you mention to properly trigger
a hall sensor that gives a digital output unless there are sensitive ones I
do not know of.

At 60k rpm you will may need to have 2 set screws to balance the wheel
properly.

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