Re: [CAD_CAM_EDM_DRO] Re: Limit switches revisited

yet_another_kent wrote:

>I agree, Paul, although I see a lot of examples where the switch is
>indeed the stop. I was thinking I could use a roller-arm switch
>attached to the ways that is tripped by a projection on the bed as it
>passed by to allow for overtravel. My mill will have limited work
>volume and my steppers will be slow, so I don't envision
>any "hurtling," but mashing the switches just seems like shoddy
>engineering to me.
>
>The same "en passant" technique could be used for detecting home
>position with mechanical switches but I'm uncertain how precise and
>repeatable it would be.
>
>

It depends on what you want to use home position for, really. If you
expect to have semi-permanently mounted work holding jigs that reference
partially machined pieces (ie, you mill 50x the "tops" of something,
then want to mill the "bottoms"), then homing will be critical. The
other place where machine home is important is with certain
toolchangers. You have to know exactly where the head is in relation to
a mechanism that's mounted on the machine, and has nothing to do with
any part you may be cutting. If it's just for determining absolute
machine limits, then it can be off a little, as long as nothing will
break if you go over the limit by that same "little".

I think it's desirable to set up 3 (or four) limits, actually. The
first is a soft limit - if you ask the controller to move past this
limit, it politely refuses. Second is a limit switch that the
controller detects, to tell it that it's made a mistake. The controller
stops motion when it senses this. The third is part of the E-stop
chain, and physically shuts down the machine. The fourth would be
mechanical bumpers :)

Incidentally, it is possible to get very accurate homing if you use
encoders with index. Move until you hit the switch, then back off until
you see the index pulse (or any of a number of other methods). This
gives you homing accuracy to +- one count (step, in the gecko-ized servo
world :) ). In a lot of servo systems, that will translate to 1/10000"
or better resolution (I've got 40,000 counts / inch in my setup).

>There seem to be a vocal minority out there in netland who forgo
>detecting home position and reference to edges on the work piece
>instead. I haven't looked into edgefinding technology.
>
>

"Machine home" and "part reference" are two different things, IMO.
Leaving the part off the table, your controller software still needs to
know what absolute coordinates are off limits. Referencing to a part is
nice, so you can use a vise instead of a pinned workholding jig. Using
a part reference requires that your controller understand coordinate
offsets or that you use relative positioning in your part programs.

- Steve