Much that has been discussed is ok for "Hobby" or home type machines.

As someone that retrofits drives and controls fora living I yet to
hear much about the real world issues of safety and "Safety
Standards".

The attached link is a 5 axis digital drive I completed last week and
every possible safety issue has been adressed.

http://s32.photobucket.com/albums/d1...current=m5.jpg

In large gantry machines like this, all axes drive motors have brakes
to ensure a quick stop in an error condition.

Soft over travel limits are set and deal with normal overtravel
condition and extreme overtravel result in a complete drive shutdown.

E Stops are handled by relays designed for this purpose.
http://www.pilz.com.au/products/cont...y/index.en.jsp

Not being critical but none of this would work in any real world
manufacturing plant.

db