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.