Accuracy of Homing Devices? Pro's and Con's of Different Sensors

[Biermann]

After a lot of searching, and much reading, I have found many different opinions and suggestions on the types of sensors/methods used for homing.  I need to outfit a mill and lathe and am looking for the best accuracy and repeatability while maintaining robustness.  Figuring that there will be metal swarf, coolant and oil potentially reaching these sensors.  From what I've read electrical/magnetic switches such as inductive proximity, and hall effect sensors, can shift register due to temperature and electrical interference in the region of a few .001's over the course of a few hours.  On the plus side there are no mechanical part to wear out and it easy to get IP65/67 version's.  On the other hand you have a micro-switch which does not suffer from the previously mentioned climate shifts and electrical interference dilemma, but have mechanical mechanisms that can wear out and fail, that and it's hard to come by waterproof versions cheaply.  

So if you guy's wouldn't mind listing what systems you are using, and/or have tried and scrapped for something better, and what level of accuracy/repeatability you experience.  Is the position shift from some types of sensors only critical if you have to perform manual tool changes over the course of a long cycle, or are using setup plates for repeat parts w/o re-zeroing on the workpiece?  Also, in regards to proximity switches if you use them can you list the model# and the sensing distance.

Which is most accurate and repeatable, and has the least worry of false triggers of the following types of systems:

1) Proximity (inductive)
2) Micro Switch
3) Laser
4) Hall Effect
5) Other...

Thanks.

[Jeff_Birt]

No matter what type of sensor or switch you use it will not be super accurate all by itself. For really accurate homing it is best to use a combination of a home sensor/switch and an index pulse on the axis motor itself. The home sensor/switch and the index pulse are seen at the same time the controller considers this 'home'. Since the index pulse is based on the angle of the axis motor itself it is very accurate.

Having said all of that you have to stop and ask yourself if having a very accurate homing sensor is even needed for your method of operation. A home sensor only sets the zero for the machine coordinates, it lets the machine get a bearing on where each axis is physically when you start up. For most operations you bolt a part/fixture to the mill table and then you zero the machine to some feature of the part/fixture (local coordinates), you can even have multiple fixtures and multiple local coordinate systems set up. There will be a relationship between the mechanical and these local coordinate systems (an offset), but since you will likely always zero to the part or fixture each time the accuracy of the home sensor is not important.