View Full Version : machine stops when homing

26-03-2016, 07:27 PM
My machine has developed a problem today during homing. Z and Y home fine, but the long twin motor X axis doesn't home properly.

I press the home button and both X motors move together until it hits the micro switch and clicks. But it then just stops right at that point. Note that it does not e-stop, and you can still jog the machine further onto, or right back off the switch.

It should back off again until the switch clicks off and then stop there instead, then zero the machine and put the offsets in the DRO, but it does none of that.

This only happened today and I can't see why it would start doing this. It was annoying because I was carrying on a job and needed to regain position on the part. In the end I had to step jog off the switch manually (in 0.05mm increments) until the switch clicked off, then I typed the offset position into the X DRO so it would know where the 0,0 of the job was. Luckily I always write the tool offset values down.

It could be noise and I will start by changing the switch and playing with the debounce when I'm next in the workshop, but if it is noise surely it should e-stop? But it just stops. It's also not the longest cable, in fact it is the shortest one (Y and Z home cables are much longer).

Any ideas?

26-03-2016, 08:35 PM
Don't think it will be noise and I wouldn't mess with de-bounce has it will just add a delay to the homing process and make it less accurate.

More than likely you have Dodgy switch or lose wire connection, possibly bit of crap in the switch if Micro type.?

Did you try restarting PC and trying again.? Could have possibly been a Mach issue i've seen it do daft stuff like this before and be fine after restart.!

26-03-2016, 09:10 PM
Hi Dean,

I only had an hour today so didn't get to try out many options including PC re-boot. The switch did have bits of swarf around it so could be that. It's been on that machine for a while so maybe it's starting to play up.

I'll see if it is OK next time I fire up the PC. I'll also try a new switch and give the terminal connectors a bit of a squeeze before sliding them onto the switch again to get a tight electrical joint.


26-03-2016, 10:20 PM
OK managed to have a quick check in the workshop.

Fired up PC and still same problem.

Unplugged the microswitch and used a multimeter across the terminals whilst pressing the lever. The multimeter beeped but as the switch clicked across the beep was a bit warbly just before it stopped as if the switch contacts were not that clean.

I fitted another microswitch and squashed the terminal connectors a bit on the cables to make sure the joints were tight. Tried homing again but the same problem.

Tried debounce 5000 (x40us) and it homed properly although there was the expected delay. Then tried 1000, 500, 100, 50, and finally 2 (x40us) and they all homed properly with reducing delay. I kept the 2 (x40us) value and homed 10 times in a row and each time was perfect. I'll leave it like that for now.

I think the machine knows it is building it's successor and is chucking a few things at me along the way!

27-03-2016, 02:39 PM
are you using any screen set? some times i have same problem and but only hapens on Gary screenset 2010. but you can check the homing configs on setup homming and limits, if you hava a negative and positive limits that's is a possible problem.

27-03-2016, 03:38 PM
I am using screenset 2010 as well. Next time I'm in the workshop I'll have a look at the config settings.

27-03-2016, 04:09 PM
Although the homing is working now I decided to have another look at getting the proximity probes to work with my BOB. It is a basic DB25 so won't take the 10-30V direct output from the prox sensor:


Last time I looked at this I tried all sorts of things including resistors arranged as voltage dividers, current limiters, opto isolator boards etc. but nothing worked. I've had one of these relay boards for a while so thought I'd try that:


24V prox connected on one side, and green&white relay contact output cables connect to pins 11 and GND on the BOB.


It worked fine but I know relays are not recommended for homing (or probes) as there is a switching delay of a few milliseconds. But then I wondered if this was a constant delay, and if so, it could still be used?

So I did lots of trials using a DTI and prox set up on the X axis (aimed at a big 12mm bolt head) and it homed back to the same point on the DTI every time without fail.


I jogged away, tried it, repeatedly homed without jogging, jogged to random places then home. Same spot every time. I did 10 minutes of various combinations but each time right back on zero on the DTI. This sounds like even though there is a delay with the relay it is always the same and therefore could be used for homing?

I even tried jogging 0.01mm manually around the switch point and it always tripped on and off at the same point on the DTI. It also showed just how small the hysteris region is on these sensors, and how repeatable they are compared to microswitches.

Perhaps over time the relay switch time could drift, but again unless you have a jig reference point on the board (which I don't), or you are cutting the same part over many weeks/months with lots of homing then that doesn't matter does it?

Thoughts? . . .

Clive S
27-03-2016, 04:24 PM
I used to use proximity sensors on 24V straight into the pin on the cheapo BOBs There is a 10K R inside a lot of them so it should only put about 2.4mA into the BOB you can check this by putting your meter on mA range and connecting between the black wire sensor and ground. and see what current it draws. (at your own risk of course)

27-03-2016, 04:53 PM
Ok cheers Clive. I'm away from the machine for a while now so will have a go at that next time.

27-03-2016, 05:06 PM
I used to use proximity sensors on 24V straight into the pin on the cheapo BOBs There is a 10K R inside a lot of them so it should only put about 2.4mA into the BOB you can check this by putting your meter on mA range and connecting between the black wire sensor and ground. and see what current it draws. (at your own risk of course)

If you use an external 10k connected to the sensor output, and measure from the other end of the resistor you won't blow the sensor up and you can take that into account in your calculations. I use them straight into the BoB with a 12v supply to the sensor with no problems.
I have just set up a lj12a3-4-z/bx on the breadboard.
Using a 30v supply the output shows 30v and drops to zero when triggered. BUT the current through a 10k load is 6mA, and without the 10k it is still 6mA. So there must be current limitation.

Then I tried it at 15v and it is the same but 3mA current.

27-03-2016, 05:32 PM
in 2010 screenset on tab "Limits" active "soft limits"

and put off " Automatic override" and " Manual override"

and if is the same problem i have somtimes its ok now.