Hi,

Quote Originally Posted by ngundtoft View Post

  • If we agree that the purpose of limit switches is safety and prevents the machine from exceeding its working area - why then is it not required to have limit switches in both ends of each of the axes? (ok Z is a little different).
Seeing it from this perspective you'd be right - but my personal opinion is that limit switches are not only unnecessary but even give a deceiving sense of security they cannot provide:


  • If a machine can be damaged by chrashing into its axis limits it is just a crap machine...
    (OK - not talking about professional high speed machining centers where you have tons of material accelerated to very high speeds - but even there a limit switch is just the last line of defense - position feedback will stop those machines way before this could happen)
  • A machinist not noting the workpiece will require machining outside its limits is just a crap machinist... ;-)
  • If the machine looses steps the workpiece will most likely be damaged anyways...
  • And limit switches do not detect step losses - a machine may ruin anything including tool changers or sensors within its work area without the limit switches taking any note of it. Which is also why a machine should never ever run unattended.
  • So basically: if you really need security you need position feedback.



What sometimes makes sense is reference switches to automatically home the machine. But even though they can make life a bit easier in some situations they are rarely necessary:

  • Usually you clamp your workpiece just anywhere on your machine and need to touch it off anyways because your machine has no eyes.
    • In this case knowing machine zero is absolutely pointless - it is just a set of numbers without any meaning for the actual machining process.
    • Only in case of step losses it can safe you a little time to get back to the correct workpiece zero - if your workpiece isn't destroyed anyways which will most likely be the case... And a machine losing steps regularly enough to make you want homing switches is again just crap or badly set up ;-)
    • But even in this case you can usually just touch off again (if the reference edges are still available)...

  • Reference switches are only necessary if you have things like tool changers, tool length sensors or fixed clamping devices on your machine.
  • And: if the switch isn't precise (or a chip gets in between) machine home will also be imprecise accordingly. Many cheap homing switches are much worse than touching off the workpiece. I program CAM Software and I came across quite some people unnecessarily referencing after every toolchange and wonder why there are visible marks on the finished workpiece that wouldn't be there if they just kept X and Y (or at least used proper limit switches with good repeatability instead of the cheap stuff they bought).


Christian