Re:Home Switches

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Torsten" <torsten@g...>
wrote:

> I do not believe backlash has anything to do with this.
> As long as you aproach the switch from the same direction
> every time backlash will not contribute to the repeatability
> of the trigger action.
> What you could do to test this is mount a dial indicator in
> the spindle, home the axis then move a fixed amount off touching
> with the dial indicator against a fixed object mounted to the
> table.
> Zero the indicator there and repeat homing several times
> noting the reading on the dial the varietions you get will
> indicate the repeatability of the switch.
> These Switches come in several grades what you want is a
> precision microswitch.
> Good Luck
>

Detecting home position accurately is actually a complicated process.
To return an accurate position every time usually involves more than
the switch itself. Normally the machine would head home at a faster
rate to save time stopping the instant the limit switch is activated
then reverses moving off the limit switch at a slower rate to find
the instant the switch contact returns. On industrial machines at
this point the axis continues to move until the encoder home position
is detected and machine zero is set, but since home built machines
don't require that extent of accuracy the axis continues to move a
given number of steps to clear the limit switch and then the machine
zero is set.
Optical sensors do yield better repeatability than the mechanical
switches mostly due to the effect of temperature on the mechanical
spring components of the switch where the effects are much smaller on
electronic componets.

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dave Fisher" <dave@d...>
> wrote:
> > On fitting home switches to our router table, I notice that each
> time
> > I home the table - the X and Y readings differ greatly, some

times

> upto + -
> > 0.5 mm. Seldom never going to zero. Slowing the home traverse

speed

> seems to
> > make little difference.
> >
> > I would have expected there to be some differnces caused by
> backlash etc but
> > not as much as I am getting.
> >
> > I have used good quality lever type micro switches, but wonder if

I

> am
> > suffering from switch bounce or just the mechanical properties of

a

> micro
> > switch. Would I be better using a slotted opto coupler to

eliminate

> > mechanical tolerences or any other method ?
> >
> > The table is being driven with Rutex Amps and The Desk CNC

sofware

> and
> > controller.
> >
> > Any advice would be appreciated
> >
> > Rgds to all
> >
> > Dave Fisher