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.

John - Technically, this is not a "home" position because you're changing it
for each part. "Home" generally refers to a set location of something
(perhaps the milling table) in the "universe" (the mill). It does not
change. That's why you use a set of home switches to determine where it is.

Now, why would you want a "home?" Here are a couple of reasons. Consider an
automated machine where a parts holder brings a part into position. Your
fixture locates this part to some point with respect to the "universe." You
would want to start your milling operation with the tool adjusted to some
point with respect to this same universe, so the initial tool location would
always be set to the same point relative to the part. The "home" switches
allow you to do this.

Another reason may be that you've detected that your motors have lost some
steps during the operation. You might be able to correct for this by
bringing the motors to the "home" position and then moving to the correct
position. I am aware of one industrial control system where the steppers
were run at such a high speed that they normally dropped steps. After each
fifty parts, the motors automatically homed themselves to get back to a good
position.

-- Jerry



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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