There are a couple schools of thought about homing switches. Some people think that there should be a machine absolute 0,0,0 located relative to home switches, then the work or fixtures are an offset from that point. My opinion is like yours, machine zero and work zero should be the same point. The machine could be set to go to a convenient ''parking position'' once the job is complete. The one exception to this is when using an automatic tool changer, in that case the machine needs to be located from a fixed position to change the tool. I don't have homing switches on my mill.
Normally the limit switches are wired to the motion controller. If it's doing its job correctly, it should stop any further axis movement in that direction and the software would take any pre-programmed action needed at that point. Normally the machine would stop and a message be presented to the user that a limit switch was tripped.Do you want them to be wired to the motion controller or do you even perhaps want a switch that shuts off all power to the mill if it's ever hit? Something that activates an e-stop, like through a relay- hardwired. Maybe both?
Normally the limit switches are set to trip near the mechanical limits of travel. Depending on the machine, this distance could be as little as a couple of mm. I think my lathe cams are set to about 5mmHow do you work out the travel required on the switch? Would this have something to do with your table speeds? Or is this irrelevant as the only time you should encounter the switch is when you're homing and this is done at a controlled speed.
Physically? Normally not unless extreme accuracy is required. There are high precision limit switches available for around US$100. Now to really confuse the issue, the axis travel limit switch could also be used as the home switch. It depends on the software you are using. If the software overrides the travel limit function during a homing cycle then the same switch can be used. If you are using a single switch on an axis, then there needs to be a software or hardware override, or other provision to allow the axis to move off of the limit under operator control, normally done at very slow speed.Is there a difference between limit and homing switches? I see the term used interchangeably sometimes.
I have successfully used $2.00 snap switches as machine limits and home switches with 0.01 mm repeatability. It does require a bit of fiddling to make that happen. I normally use a bit better switches. A ''slow acting'' limit switch is much more repeatable than a ''snap acting'' switch. If coolant is involved, then an IP65 or IP67 rated switch should be used.Is there a sweet spot for price/performance? I see Tormach use Tend TM1308-1
I normally run the limit switches at 5VDC or 24VDC depending on the application. Wire the switches Normally Closed so they open when tripped. That way a wire break has the same effect as a tripped limit, and is noise immune. I think the limits on all of my machine are wired for 5VDC.Do you want to use a high voltage limit switch? I would think that 24V would be safer, but I don't know if there are disadvantages.
The servo voltage does not matter, the travel limit is not going to be wired into any high voltage motor circuit. The travel limit could be wired directly to the enable input on the axis servo, but this is normally not required. Some servo drives have travel limit inputs. The controller should be capable of handling the tripped condition.Any good examples of limit switches or wiring on a mill that was done really well would be helpful, especially if it uses high voltage servos (which I'm using).
Thanks.
Normally one switch per axis is all that's required. The switch is mounted to a fixed location relative to the axis and is operated by a cams attached to either end of the moving axis.
Here is a picture of the X limit switch on my mill. There are one of these cams on either end of the table
And a picture of the Y axis limit, again there is a cam at each end of the travel.
Here is a pic of a limit on a dispensing machine I built out of a router frame. There are one of these cams at each end of travel.
And another view of a cam on the same machine. About a 15° ramp.