Re: [CAD_CAM_EDM_DRO] Orientation was Re: Repeatability of Home switches

Caudlet,
Take a look at the photo switch that I posted pictures of, in the
Marv's Folder today.

Marv Frankel
Los Angeles

----- Original Message -----
From: "caudlet" <info@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, April 04, 2003 3:27 PM
Subject: [CAD_CAM_EDM_DRO] Orientation was Re: Repeatability of Home
switches


> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "mrgamber" <mgamber@c...>
> wrote:
> >
> > What is the recommended orientation of the switch?
> >
> > Should it be in the path of the axis and depress the lever or
> button?
> > or is it better to let it "wipe" the trigger? Wiping seems safer
> but
> > less accurate.
>
> All of my limit and home switches are the lever (wiper) type
> microswitches with rollers on the lever. The are mounted on slotted
> brackets that allow for some adjustment. The are setup so that a
> plate on the axis with rounded corners contacts the roller as it goes
> buy. If it goes too far nothing happens to the switch. I have
> hardwired limit switches past each home so that in the event of a
> runaway, it drops out the contactor feeding DC to the motors. I find
> that the home (reference in MACH1) is repeatable to .001 with these
> switches. The issue is not the step accuracy of your machine but
> rather the overall error from backlash, leadscrew non-linearity,
> etc. If you have a machine that can consistantly hold .002 after
> moving all over the place then you have a really tight machine. Why
> would you want a zero reference that was ten times more accurate than
> your machine?
> The smallest increment you can move (as long as it is less than
> desired accuracy) has little to do with overall accuracy and
> repeatability. You could make the argument that each incremental
> move would create error if your step resolution was not tiny but the
> error tends to cancel out in multiple moves. On small machines or
> precision lathes and mills they may need to hold closer tolerances
> than .001 but that requires the mechanicals be really rigid and
> ballscrews with zero backlash are in order.
>
> Non-contact switches can be made to hold a closer tolerance. One
> precision machine I just finished disassembling uses an optical
> sensor with a shutter plate that has a tiny slit. As it passes the
> sensor the slot creates a very accurate reference. They must have
> logic that first senses the edge of the plate (dark) followed by the
> slot (light) since it is light most of the time. There are proximity
> sensors, Halleffect sensors, modulated light sensors and tons of
> mechanical switch types.
>
>
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