Re: Stupid Home SW location vs Limit SW location question..

Jon,

Good point about thermal expansion. Probably not a significant
issue on home-built machines because the components usually aren't
accurate enough for a little thermal expansion to make a noticeable
difference.

It's a bit simpler to have the "home" position switch located in
close proximity to the axis limit switch. On my mills the home
sensor and limit switch are built into the same housing. A single
armored cable handles both switches. Another point, it's much
simpler to initiate the homing procedure, you don't have to give any
thought to whether you're on the wrong side of the home switch.

Until today I had never seen a commercial, production mill with
center of table home position. A Tree CNC mill at auction appeared
to have center of travel arrows which are to be lined up prior to
intiating the homing sequence. The machine had a warning sticker
indicating programs should not be run unless it had been homed,
that's very sloppy IMO, a control should not allow a program to run
prior to homing.

Doug

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> doug98105 wrote:
>
> >IMO, it makes no sense to put them anyplace except at the exteme
> >ends of travel. One of my commercial machines "homes" at the NE
> >corner, the other at the SW corner.
> >
> >
> One of the reasons for having home switches is to repeatably
position
> the machine
> in the same place, relative to a fixture located for some time, or
> permanently, on
> the machine. On larger machines, thermal expansion can be a
significant
> problem. If you locate your home switches far from the center of
the
> machine's
> travel, then whatever errors develop due to thermal expansion
between the
> home position and the part will cause the part to be cut with all
positions
> shifted by some amount. In a worst case, a very small part would
be totally
> unaffected by thermal errors itself, but its alignment with the
fixture
> would
> be off. If your machine is in a well-regulated environment, that
won't
> be a problem.
> But, in a basement or garage, there can be large temperature
swings.
>
> If you put the home switch in the middle of travel, you halve the
thermal
> errors, overall, but reduce the shifting of the fixture relative
to the home
> position by how close the fixture is to the home position.
>
> On some routers, etc. that have a fence built into 2 adjacent
edges of
> the table,
> then the home position should probably be more toward the locating
corner
> these provide. But, on a milling machine, where work is often
located in
> a vise near the center of the table, then a central home would be
best.
>
> Jon