[Shop-talk] Tachometers

[Randall]

> Check e bay for a machinist's tachometer. It is a little 
> device that looks
> like a large key. 

Thanks, Gerald.  I am familiar with those, but using one is somewhat
problematic for me as I don't carry a watch (nor a wall clock in the shop
area).  It also makes me nervous to have my hand near a spinning shaft while
I have to look away at something else.  And I like the idea of being able to
measure surface speed directly, rather than having to do the computation.

Instead, I picked up one of the digital contact types at HF this morning.
In fact, I just came in from measuring all 16 lathe speeds (but haven't
tried to check the numbers for consistency yet).  It seems to work well
enough.  The readings are not rock-solid, but I think that is probably
because the lathe and it's single phase induction motor don't actually hold
a constant speed very well.  The reading got more stable when I laid my hand
on the chuck (to provide a bit of load) and the worst case waver was less
than 1% of the reading.

-- Randall