Re: Encoder index and homing

Yes...I'd say that you were pretty well on point. On my CNC router
and CNC plasma rigs, I define 0,0,0 wherever I want on the table, and
reset the control software's DROs to reflect this.

While it may not make as much sense on a mill, a Homing routine on a
router does. Typically 'Home' is the center of the bit the bottom
left corner of my spoilboard. This makes it easy to tell the machine
to go Home, when I have aligned a sheet of plywood to the X and Y of
the spoilbaord. I know that I am perfectly aligned with mechanical
0,0 and there is no fiddling around to center the bit on the corner.

Generally for all of the work that I have done, I do not put the Z-
axis in the mix. I just make sure that it is well above the material,
and then zero it after everything is fixtured.

A repeatable Home position in X & Y would be helpful on a mill just
as an accurate reference point in the event that you were running
expensive material & had a crash, lost steps or the power went out
etc. You of course would need to know what the offset is from Home to
your part's defined 0,0...which is something that I do religiously
because glitches DO happen.

-Brady

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

>
> 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