Re:

My old Hardinge lathes use a magnet embedded in the big sprocket in the
axis belt reduction. They use a micro for rough home. This switch slows
down the axis and then the control uses the prox to find the magnet.

Ray


From: Jon Elson <jmelson@...>

>Subject: Re: Home Switches
>
>
>
>William Scalione wrote:
>
>> From: William Scalione <scalione@...>
>>
>> Anyone know what the best type of switch to use for a home switch?
>> I am using mechanical "Cherry" microswitches but they don't always
>> actuate at the exact same position, which makes them kind of useless.
>>
>> Limit switches don't seem to matter, a couple thousaths either way, as
>> long as they work but my hone switches need to be right on
>>
>> Proximity sensors? Photoelectric sensors? Other? or just better
>> quality mechanical switches?
>
>The way the home switch works on encoder-equipped machines with
>servo control, is that the rotary encoders have an 'Index' channel,
>usually called the 'Z' channel (while the quadrature channels are A and B)
>which generates a short pulse once per revolution. Linear encoders
>have similar functions. The home switch and the encoder are aligned
>so that the home switch closes just before the encoder passes the Index
>location. The Home switch uniquely identifies one pulse of the index
>signal as the true home, other index pulses at other locations are
>ignored. This way, as long as the home switch can click in and out
>in, say, .1" movements, there will be perfect selection of the correct
>index pulse with a .2" lead screw.
>
>If you are using a stepper system, you could put an optical sensor and
>a disc with one very narrow slot on the leadscrew, and use that to refine
>the home switch signal. That should get you to within a few thousandths.