Re: [CAD_CAM_EDM_DRO] New question about limit and home switches

John,

Here is how I view the home and limit switch functions.

On my system, I have a limit switch at the lower limit position for the X
and Y locations and the high limit for the Z.

When I home my system it sets some values internally in the controller so
that it knows where the limits are. In the controller you may have values
that tell the system the size of the table. Anytime that your router goes
beyond the low or high values, you want the system to do a soft stop and
tell you that you it has hit a limit. You do not want your router to hit a
hard stop or run off the end of your rails.

The way that you described setting a new home position is a normal
procedure. On my system, I set a G92 command to set a new home. The system
(controller) still knows where it is in relation to the high and low limits.

Limits are very important in relation to safety. One of the things that
your instructor should stress is safety. CNC machines can hurt you if you
do not respect them. Use all the safety procedure that you have and enjoy
your cnc router or mill.

Bob Campbell

----- Original Message -----
From: "johnhe" <johnhe@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, October 11, 2001 5:51 PM
Subject: Re: [CAD_CAM_EDM_DRO] New question about limit and home switches


> > If I understand correctly, you are asking what happens if you drive
> > over a home switch while running a program, subsequent to homing
> > the machine. The answer is nothing - the home switch only gets
> > looked at during homing. Running over a limit switch on the other
> > hand will shut the machine down.
>
> I'm not sure I understand the need for them in CNC. On the mill I used in
> school we have a cursor keypad on the control to position the spindle
> somewhere over where we want to consider home. Then you lower it until
it's
> pressing against the bit of plassy you want to cut into a shape and press
> the okay key, setting this position as the home. Start the commands going
> out, lots of most definitly cool whirling noises later you have a shape. I
> can see the need for them if say, you were using a power feed so you don't
> wack the table into the end of the travel but that's a limit switch isn't
> it. Why do you need home switches if you can tell the mill where you want
it
> to home to when you start it? Would be it for things like, if the power
was
> turned off and you left the work clamped in exactly the same place you
could
> turn the mill on the next day, ask it to find home again and it'd set it's
> home to exactly the same place as it was at the day before? Because if you
> set it only as a position in the computers memory it's taking it as a
> position such and such increments away from where the motors have the
table,
> which won't be remembered the next day. Or is it possible save the table's
> position, motor increments from home, in the control so you can turn it
off
> and then ask it to use a set home position in relation to the saved table
> position? I always thought the idea of CNC was that the machine knew where
> it's own motors were going in relation to table size etc while it's
running,
> so that's how I've come to the perhaps completely mad ideas above.
>
> John H.
>
>
>
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