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I was wondering if the possibility is available to separate the
limit switches and the home switch. I seriously believe that there
is a serious safety concern when the limit/home function is the same
contacts.

My experience in equipment is that the limit switches are part of
the E-STOP ladder and the home position is separate and that is to
prevent damage to the equipment. I would think as hobbiests we need
the protection this would give us. If necessary I think the
activation of LPT2 is an issue to consider.

ART, Thanks for the great product.

--- In mach1mach2cnc@..., "capteod" <mfgguru@c...> wrote:

I was wondering if the possibility is available to separate the
limit switches and the home switch. I seriously believe that

there

is a serious safety concern when the limit/home function is the

same

contacts.

My experience in equipment is that the limit switches are part of
the E-STOP ladder and the home position is separate and that is to
prevent damage to the equipment. I would think as hobbiests we

need

the protection this would give us. If necessary I think the
activation of LPT2 is an issue to consider.

<snip>

I would welcome your thoughts on or off list how we could improve
section 4.6 and appendix 4 of the Mach2MillManual. (Rev A1.8 in
Files and download from www.artofcnc.ca) I have found it hard to get
balance of risks and features here.

Thanks, John Prentice

I have found the same quandry myself. I my experience, the limit
switches should be there to prevent damage to the machine, and in
effect initiate an E-Stop condition. It realy dosen't matter which
switch is actavated, it should prevent movement of the machine and
spin down the motor. The Home switches should be there to position
the unit for initial setup and zeroing of DRO's and should be
positioned inside the envelope of safe machine travel.

Also had a few thoughts about the physical qualties of the
switches ...The limit switches need only to be mechanical in nature
and need only a gross resolution. For the home switches, I've been
playing around with the inexpensive laser diodes (laser pointers)
that can be bought almost anywhere. What I have come up with is the
laser diode powered by a 5v DC "wallwart" transformer and a 220ohm
resistor to limit current and a small NPN transistor on the output of
a photoreceptor. A switch is guaged "accurate" mainly by its
switching speed - so this switch will opperate in the billionth of a
second range and should prove to be very accurate indeed.

As a physical layout, this scheme would require an e-stop,x+,X-,Y+,y-
,Z+ limit switches wired to 1 input. The z+,X+,X-,Y+,Y- home switches
would require their own seperate inputs for a total of 6.

Thoughts?



--- In mach1mach2cnc@..., "capteod" <mfgguru@c...> wrote:

I was wondering if the possibility is available to separate the
limit switches and the home switch. I seriously believe that there
is a serious safety concern when the limit/home function is the

same

contacts.

My experience in equipment is that the limit switches are part of
the E-STOP ladder and the home position is separate and that is to
prevent damage to the equipment. I would think as hobbiests we

need

the protection this would give us. If necessary I think the
activation of LPT2 is an issue to consider.

ART, Thanks for the great product.

Gentlemen:

I agree that the limit switches are there to save a crash and should
do two things:

Instantly stop the motors on all axis
Issue an e-stop signal back to the software.

In the best interest of safety the switches should be all wired in
series in a normally closed configuration so that activation of any
switch in the ladder breaks the string and removes power from the
motors. By using a normally closed configuration any wire break would
cause the unit to stop as well. In planning my controller power
supply I made provisions for a totally mechanical limit switch
configuration. The limit switchs are wired in series to ground. The
ground goes to the neg side of a 12VDC control relay. All switches
have to be open (contacts shorted) for the relay to activate. Any
disturbance in the string will cause the relay to fall out and its
contacts to open. Included in the limit string is a front
panel "STOP" switch and a table maounted "PANIC" switch. All of the
DC runs to the motor modules go through a large contactor with a
115VAC coil. For the motors to have power both the front panel RUN
switch AND the control relay contacts have to be activated. Tripping
a limit will cause the control relay to drop out and shut down the DC
to all of the motors. A separate set of contacts on the control
relay will issue an e-stop back to the software. No additional
parallel port pins required.

In my design the Main contactor supplying the DC is wired in
a "latching" configuration so that once it drops out it requires an
activation of a "RUN" momentary button on the front panel to
reactivate. Once a fault or limit has caused the contactor to open
the DC path it requires a manual restart to prevent intermittent
limit conditions from restarting the machine.

There you have it. In building a safe machine it should be designed
so that a failed component results in a no-go condition. In other
words it takes every component in the string to be working properly
before the machine will produce motion.

I like software. I like computers. I don't trust the safety of my
machine or myself to either.

Tom C

--- In mach1mach2cnc@..., "caudlet" <thom@t...> wrote:

Gentlemen:

I agree that the limit switches are there to save a crash and

should

do two things:

Instantly stop the motors on all axis
Issue an e-stop signal back to the software.

<snip>

Many thanks for original point and everyone's contributions to this.
In light of them I have decided to strengthen the advice in the
Mach2Mill manual to use a totally hardware EStop, limits strategy.
Look out for this in the next release which documents the new F-key
screen switching and XML profiles.

Best wishes

John Prentice

--- In mach1mach2cnc@..., "japrenticeuk" <john@c...>
wrote:

--- In mach1mach2cnc@..., "caudlet" <thom@t...> wrote:

Gentlemen:

I agree that the limit switches are there to save a crash and

should

do two things:

Instantly stop the motors on all axis
Issue an e-stop signal back to the software.

<snip>

Many thanks for original point and everyone's contributions to

this.

In light of them I have decided to strengthen the advice in the
Mach2Mill manual to use a totally hardware EStop, limits strategy.
Look out for this in the next release which documents the new F-key
screen switching and XML profiles.

John, If it would help I think I have an electronic copy of a
schematic for a strictly electromechanical (relays and switches) of
providing a failsafe limit. Let me know if you want it for inclusion.

TOM C

Best wishes

John Prentice