We all need limits...
This is a continuation of a thread that was started here.
As I am sure many of you know, limit switches detect the position of moving parts when they are near the extreme ends of their linear positions. This prevents collisions of things like ballnuts at the ends of the ball screws, and are also used as points of reference for when the software resets the table to go to the "home" position. Some systems use mechanical switches but in my case I chose to use optical interupter switches made by Fairchild. The advantages of optointerupters like the H21LOB are that they create TTL logic and are fairly accurate.
After some testing I came up with this circuit diagram ( jpg pdf ) for the interupters. In this circuit you'll note that in order to daisy chain the optointerupters together it was useful to solder a resistor directly to the interupter. The H21LOB wiring was bundled up with lots of heat shrink tubing (also shown here). The circuit diagram also shows the pinouts for each of DIN connectors which eventually will plug into the back of the electronics enclosure.
Although its very difficult to see in the picture, my cnc table came with a nice mounting bracket (note red arrow) which had some old school interupt switches. I pulled off the bracket, removed the switches, and strung in my interupter circuit into the mount (view here and here). Hot glue was used to make all the wiring that was crammed into the rail stay in place.
The pictures look kind of crappy but once the rails were bolted into place everything looked really good. There are two rails, one for each axis, and each rail has three switches: limit+, limit- and home. The table comes equiped with small pieces of sheet metal that project down onto the rails. When the table slides towards the ends of its total possible travel (around 18 inches) the sheet metal tabs travel between the opto-interupters. The state of the opto-interupters changes, the universal stepper controller picks up the change, and my software throws a complaint.
owen