View Full Version : value of home switches

03-04-2012, 01:58 PM
I'm a newcomer to CNC. I've converted a standard bench mill with all the recommended bits and pieces - limit switches wired in series for each of x,y and z with the lower z limit adjustable to cutter length so there's no chance of it meeting the table. In the absence of any 'bench mark' CNC knowledge the mill is currently set up as per the Mach3 manual with relatively slow feed rates and I've run a couple of the included wizards in aluminium to test everything - especially me!!!

So far, so good... My questions relate to the use of home settings. I've read the manual and posts, here and elsewhere but I'm having trouble understanding the real value of home switches and where the home reference 0,0,0 points apply in the software. After years of setting up conventional mills with the 3 DROs zeroed at a designated datum point, I've approached CNC on the same basis. I've tweeked the G Code to establish where the 0,0,0 datum is, jogged the mill to the work piece, zeroed XY and Z, moved all three axes away and engaged warp drive... The Estop is on a flying lead and the limit switches protect the machine and so far its done what was expected.

However I'm on a steep learning curve and easily confused with this new terminology. I'd appreciate some logical (and uncomplicated) explanation as to where I'm missing the point with home switches/settings - what are they used for? I know the size of the table and how large my working area is. Logically the only time limit switches become vital is when human error overrides software. Having said that it would be relatively straight forward to redesignate the limit switches as home switches as presumably they would fulfill the same function of not overunning the machine but I need some clarification of the shortcomings of this plan.

03-04-2012, 03:34 PM
I can only relate to the way "I" work.
My machines use a fixed point probing for tool length offset (in G53 coordinates), I always use G54 coordinates for my work piece, and on each setup, I designate a location 0,0 in G59 coordinates to do tool changes and this location varies depending on my setup.
My routine is to home the machine, designate the location for the tool changes as desired and locate my workpiece datum. Then I save these various offsets in case of say a power failure, a crash cause I did something stupid, etc. This way, I can start over with confidence that things will be located in a proper positions.
My setup program also gives me a chance while homing etc to make sure oil is distributed properly etc and just generally check things out before doing the actual work.

03-04-2012, 11:14 PM
Mach3 can use limit switches as home switches. For a Mill I don't know the full use possible for a Router the uses is pretty good as it let's you set up multiple areas for cutting and have the home switches to reset the system for changing where you are cutting or resetting after a snag in a program or user error.

Outside of that I couldn't tell you.


04-04-2012, 12:01 AM
I once clamped a home switch on the Y-axis of my CNC router and didn't find it particularly useful so when it fell off I just left it. So long as the machine does not loose position and you're careful not to command it to go outside the working area then there's no reason for the home switches to be pressed. The latter is easy to avoid as it's easy to check if the job fits in the available space by checking the extents in mach3. If the machine stalls then you should find out why and make sure it never happens again, not rely on home switches to reset regularly. I left the dials on my mill and always zeroed them before each use so if something did go wrong I could quickly reset 'manually'.
But none of that really answers your question...essentially they compensate for user error most of the time. If you have an automatic tool-changer however they're vital so that the position of the tool rack is known (relative to the home position).

Having said that the next machine will have somewhat elaborate home switches - inductive sensor and IR sensor shining through hole in pulley to get greater resolution than switching off a part that moves linearly, resulting in the ability to reset (theoretically) within 1/8th of a step.