Saving machine settings between sessions

[EddyCurrent]

These are the ones I used and they work fine so far 2pcs DC 10-30V 30mA NPN NO 3-wire 4mm Inductive Proximity Sensor Switch SN04-N | eBay

Because the Home switches are not safety related it's okay to use normally open type, I used NPN because they act like a switch to short the bob input to ground and I like 3 wire type so that the supply voltage can be 24v dc.

[Neale]

Thanks, Eddy - if they are repeatable to "a couple of tenths" as per the video clip, that's good enough for me!

[EddyCurrent]

Use a good 'target' with a square edge like the one here (second pic.), http://www.mycncuk.com/forums/gantry...html#post56428
it's a piece of 6mm thick steel on an aluminium bracket, make sure no other nearby metal will influence it.
Sensing distance is 4mm so you want to set it in the middle with a 2mm gap.

[JAZZCNC]

What kind of performance would you expect from the kind of proximity switch that seem to be very popular for routers? Any recommendations? Accurate homing is part of the "save session state" process, I guess.

Yes accurate homing as saved my arse many times along with "Run From Here".

Most of the cheap prox switches on Ebay either those Eddy suggested or the round bullet type with threaded body work well and are very repeatable. I mostly use the Round type has they are easier to mount and reach or fit into more places better.

[JAZZCNC]

Ok well Neale you got me wondering about repeatabilty of those switches and as it happens i'm just setting up a machine so lashed together a test for you of the Ebay switches I use.
Think these where the ones.? http://www.ebay.co.uk/itm/10pcs-New-...item3f29967ebb

Anyway thought a Video was in order so here you go hope it helps.!!

[Neale]

Interesting demo, Jazz! Better than +-0.01mm repeatability, which is better than I would have expected. You look as if you are bringing the ballscrew towards the sensor from the side, and maybe it would be even better if you came at it end-on? But I guess we're starting to talk numbers which are down in the region of ballscrew and nut clearance, etc, and as you say in the video, it's certainly good enough for a router! I'll be looking to use something like this, now I've seen what it can do in practice.

[JAZZCNC]

Interesting demo, Jazz! Better than +-0.01mm repeatability, which is better than I would have expected. You look as if you are bringing the ballscrew towards the sensor from the side, and maybe it would be even better if you came at it end-on?

Well possibly but it's not a good idea because you can smash into them so it's always better to slide by.! . . . But yes although I knew they where more than good enough for a router I was quite surprised and before the Video I'd done loads of tests on each axis just to be sure and they where all about the same so consistant repeatabilty across the differant switches.!!

If you look at the video around 4.02 mark you'll see another switch that is the traveling limit switch for X axis and at 4.08 you'll see the washer that is the target.!
This way I can adjust the limit distance by moving the target and keep the wires/switches to a minimum.

[Neale]

I'd assumed that you needed to approach end-on to get repeatability, but knowing that you can take a safer approach and still get good results is useful info.

[JAZZCNC]

I'd assumed that you needed to approach end-on to get repeatability, but knowing that you can take a safer approach and still get good results is useful info.

Again it's down to how you approach or more the point the speed you approach. If you fly upto the switch then yes repeatabilty will drop but as you could see at 1mtr min it still did a good job with the side approach. How much better it would be end on I'd have to test to find out but that will have to wait for another time as this machines due to leave me soon.

[m_c]

Sliding past is usually more accurate.

The difference is, as you approach from the side, you have a solid egde that disrupts the inductive field more rapidly at close range, than if you approach end on where you have a solid lump that slowly disrupts the inductive field as it approaches from a distance.

However, if you want accuracy, I personally use optical slot sensors, as they're far less affected by temperature, and have a much smaller switching point, however they need to be reasonably well protected from contamination.
To give some theoretical figures, I just checked, and the last slot sensors I used are officially repeatable to 0.03mm over thier entire operating temperature, whereas having just checked the inductive sensor datasheet for the ones on my current lathe, and they list a +/-10% tolerance over their full operating temperature range, with switching distances of 8mm+/-10%, which gives a much wider switching point of 1.6mm +/-10%!
Off course, in practice repeatability is usually very good.

My current lathe uses inductive sensors, as they only provide a rough position which is then refined using the servo encoder index pulses to give highly repeatable homing (0.6 microns if my memory and calculations are correct!)

If you want to waste 15minutes, check out thise thread - Accuracy of Homing Devices? Pro's and Con's of Different Sensors
It has a couple links to other tests, and towards the end I helped Dickeybird setup accurate homing using a microswitch combined with a slotted disc and slot sensor.