Z limit switch problem - Not registering

Is the housing of the switch broken? Was it broken before? What kind of material do you mill? Do you use a vacuum cleaner for immediate sawdust removal?

In the german part of the forum we discussed about statical charges and putting earth connections to all parts of the machine to avoid this. Enough charge can produce little sparks. These are under suspect to destroy the alloy running surface and leading to errors in USB communication so far.

I could also imagine that the discharge can also destroy the input of any electronic circuit connected.

Do you use WinPC? USB board?

Not quite that simple as the circuit is also in series with the X axis switch - so the circuit could already be broken here.

you need to test for continuity change.

multimeter across the XZ lmit pins - press switch - should go from "yes continuity" to ... "no continuity". Switching the switch breaks the signal.

it could be that when you disconnect the wire - the other switch is also breaking the circuit so your breaking an already broken circuit?

however with UCCNC if the switch circuit is broken the machine will not reset - no sure about winPC?

The only sure way to know if its the board - is to see the beep from the multimeter as you press the switch at the terminals.

or get a piece of wire and short / open the switch circuit at the switch terminals.

process of elimination.

Is the housing of the switch broken? Was it broken before? What kind of material do you mill? Do you use a vacuum cleaner for immediate sawdust removal?

In the german part of the forum we discussed about statical charges and putting earth connections to all parts of the machine to avoid this. Enough charge can produce little sparks. These are under suspect to destroy the alloy running surface and leading to errors in USB communication so far.

I could also imagine that the discharge can also destroy the input of any electronic circuit connected.

Do you use WinPC? USB board?

The housing of the switch isn't broken as far I can tell, the housing has snap fits on the side to remove and attach that side of it. Milling MDF and PLA at the moment, Aluminium and foam around a year ago and I do use a vacuum cleaner.

Is this input repairable or replaceable or would I have to replace the whole board if this is the problem?

I use WinPC-NC and the USB board.

Not quite that simple as the circuit is also in series with the X axis switch - so the circuit could already be broken here.

you need to test for continuity change.

multimeter across the XZ lmit pins - press switch - should go from "yes continuity" to ... "no continuity". Switching the switch breaks the signal.

it could be that when you disconnect the wire - the other switch is also breaking the circuit so your breaking an already broken circuit?

however with UCCNC if the switch circuit is broken the machine will not reset - no sure about winPC?

The only sure way to know if its the board - is to see the beep from the multimeter as you press the switch at the terminals.

or get a piece of wire and short / open the switch circuit at the switch terminals.

process of elimination.

Ok, thanks, I will use a multimeter on Monday to check for continuity change.

I'm using WinPC-NC.

I have just started to get the same problem 🙁

Referencing was running very well.

Suddenly the Z axis referencing does not work.

I have WinPC-NC USB and SC 600 (just got it 5 months ago).

When referencing, it starts with the Z Axis but it seems that it does not detect the switch and the motor keeps trying to move the axis up.

I have tested the X Z switches with a multimeter and they work fine.

I have tried two different computers that were running fine before and the same problem appears in both of them.

What was the solution to the problem mentioned by jcc26?

Any help would be really welcome.

If you have checked with a multimeter that the signal actually appears at the input terminals of the control board, then it sounds suspiciously like a hardware failure of the input channel.

In the near future I am taking delivery of an SC420/2 system, so I will have a careful look at the input circuitry on my control board to see if there is any potential weakness that could lead to failure.

Not that it would help you, unfortunately, if you have a genuine hardware fault, but a simple zener diode added accross the input terminals could well protect against damage from ESD or other overvoltage.

Does the X work?

all limits pop pin 12.

so you can hit the Y axis limit as its homing in Z to "trick it".

try this also.... wire in the Y axis switch to the Z/X axis input and then press the Y axis switch as it homes in Z.

If it works in this config its the switches - if it doesn't its the card.

Hi Steve,

Did not get round to thoroughly investigating and hence fixing this problem because of time constraints.

In stead of this, I changed a setting called "Start/endposition" in WinPC-NC in the Misc-Parameters section to "Origin + Toollift" so I could continue to run my programs.

I have just received my SC420/2, and so have been able to check the circuitry associated with the limit switch inputs. All three switches are connected in series, so breaking any one will cause the input to change state.

A 1kohm pullup resistor links the combined input (4th terminal from the right) to the internal 5V logic supply and to the input of a CMOS line receiver (HC244) via an R-C low pass filter. This is fine to prevent noise pickup, but probably not able to protect against significant ESD (electrostatic discharge).

A small zener diode between terminals 1 and 4 (working from the right) would provide significant additional protection. A suitable diode would be a BZX55C4V7 (4.7V zener) - connect the cathode (black end) to terminal 4 and the other end to terminal 1 ( logic 0V ). These are available at very low cost on ebay.

With the switches disconnected, the voltage measured at terminal 4 should be around +5Vdc. If it is significantly lower, then there is almost certainly a hardware failure in that input channel. Other failure modes may not affect this voltage though, so a 5V reading does not guarantee a working channel. Use a decent high resistance multimeter, otherwise results could be misleading.

It is also worthe checking that there is low resistance electrical path between all parts of the machine metalwork. There does not, however, appear to be a connection between the power supply 0V and machine metalwork - not sure of the implications of this yet because of connections via USB or parallel port back to the controlling PC or laptop. Unknown territory here so be cautious!!

I had the same problem. I would check all the limit switches with the z tower removed and everything worked fine as soon as I mounted it it would not work. SoI figured it had to be a short to ground I bent the switch connectors slightly away from the body and it works fine.