Encoder index and homing

In the last few days there has been discussion about using the index
pulse on an encoder "anded" with a home switch to get ultra precise
home repeatability. At first I was very excited about this solution
to the homing problem and had to take several cold showers. But
after I warmed up and thought about it for awhile, I came to a
different conclusion.

So, as someone that has never actually run a CNC machine I will make
the following observations and ask the group to correct me if I am
wrong…

First, in my mind I see 2 different important positions in a CNC
machine.

1) Home - which I define as finding the table and spindle limits
on power up.
2) Zero – which I define as the zero coordinates on your part or
fixture.

As I see it, home is a rough position to define table limits and zero
needs to be very precise and repeatable so that each part is machined
the same. To further define zero I would say it is a position you
define after the machine homes. You would then find your zero using
something like an edge finder. Or in other words zero is a user
defined location independent of limit switches. And the reason this
would be true is the zero point will change with each setup or every
time you moved the vise or stops.

Home on the other hand is merely a position to find travel limits and
possibly a position to move to, to get clearance to change parts or
tools. Because of this it seems to me that home does not need to be
very repeatable.

The only instance I can think of were precise home repeatability
would be of any concern. Is if you command the machine to home after
you have set your zero and your CNC control software automatically
zeros its position. And since I have never used any CNC software I
do not know if this happens…

As for using the encoder index pulse for zero position, this would
not be feasible. Let's assume the encoder is mounted on a 5TPI
ballscrew. That means the index pulse comes around 1 time per rev,
or 1 time every .200". There is no practical way to sync this
position to your vice or stops.

To sum things up, home is a rough position for table and spindle
limits found with limit switches. And zero is a precise position
defined by the machine operator using the keyboard.

Wally