On Wed, 15 Jun 2011 13:01:45 -0700, "Bob La Londe"
wrote:


I don't like that, because it relies on two separate indicators, and that
introduces uncertainty as to their mutual adjustment an reading.

Nope. While its nice if they are the same its not necessary for them to be.
You can make a home made one all wonky and have it work perfectly. Just set
your manual zero indicator mark the same for both indicators at the same
spot on the table. They can be within the range of the indicator off from
each other and still work perfectly that way. It took me a while to get a
reflexive feel for relative vs absolute measurement, but once I did life
became a lot easier.

I have a piece of square aluminum bar stock with threaded holes all over it
I use for all kinds of things now. I just bolt an indicator and a stud on
wherever I need them. Work on my lathe, any of the mills whatever.

How do you use your home mader? Do you set them to zero, then turn
the device 180º and move the head until it's half of the difference
from the original reading? Rinse & repeat until it's zero?

Are the SPI ones accurately enough made so that you don't have to turn
the device 180º and try to read the back?