X2 Limit switches

[michaelf]

Hi all, not sure if this is the place to be asking this question, but anyway here go’s. I have just completed (well almost) converting my X2 to CNC, I thought I may be able to get away without using any limit switches by just setting soft limits in Mach3, however it looks like the safest way to go is to use limit switches. My question is, do I need two switches on each axis or can I get away with just having one?
Thanks Michael

[LeeWay]

Personally, I think if you are using steppers, you don't need any limits. Perhaps one at the top of Z. This is all I use on mine. It isn't harmful to the motors to hit the dead stops at the ends of travel. That is provided you do have hard stops.

I think they are needed on a servo machine to keep from tearing something up. Also very useful on a router that would maintain a home position and it would likely be the same home for all parts cut.

On my mill, I have so many jigs, tooling plates and vise setups, that any home position is really useless.

That said, lots of guys do use limits. Check out Hoss's X2 Freak. Good explanations and placement as well as how he built covers for his.

[wildwestpat]

Hi Michael

Just to add to Lee's good advice if you will need to zero refference each axis. It is practical on a small machine to use a single physical switch for homing and end of travel limits. This is easily done by mounting blocks on the table to operate a single switch. That said zeroing can be achieved manually and there are a variety of methods depending on how the machine movements are related to the code used to move the cutter.

There are several types of switch - micro switch - optical vane - magnetic - inductive - capacitative. Each has its own strengths and weaknesses. My personal prefference is for the inductive or capacitive type as these are imune to coolant and swarf damage. Currently a good source of these is Ebay from China (type in Inductive Proximity Switch). I am currently paying £6 GBP which includes postage and here is the ebay link

http://cgi.ebay.co.uk/Inductive-Prox...item48380a3903

Hope this helps clarify the choices.

Pat

[michaelf]

Thanks for the reply Lee; I wasn’t sure if hitting a hard stop would do any damage to the steppers so it’s handy to know that it won’t. It will certainly simplify things by not having any limit switches; I guess the X2 is small enough to keep an eye on where everything is.
Michael

[michaelf]

Thanks for that Pat; I had a look at the inductive switches on eBay and they look interesting. How many would I need, would just one on each axis at the home position be enough, or would I need two on each axis? How would you hook them up, they say they need a 6 – 36 DCV power supply, would you have a simple circuit diagram?
Michael

[wildwestpat]

Hi Michae

With a little bit of mechanical design it is possible to provide two solid metal projections that will operate an inductive proximity sensor on each axis. This is the simplest option as Mach3 will let you programme the pins and ports to take XY&Z limit switches on individual inputs to the LPT port. Using separate inputs to Mach3's LPT avoids the problems associated with ganging electronic switches which can not be added in series connection like microswitches. Mach is configured to home on the appropriate limit switch and this is done one at a time by the software.

There are two types of out put from electronic sensors i.e. NPN or PNP . Which type you use will depend on the way you want to trigger the switch - moving into proximity of the sensing edge or moving out of range for the operated condition.

The operating range is the maximum gap between the sensing edge and the switch face the larger ranges available are intended for package manipulation applications. The smaller ranges (gaps) are better suited to CNC axis apps. The repeatability of the sensing action of these switches is good but the speed of sensing limits their use for rotation sensing - ok for several hundred RPM but look at the data sheet.

There are other mechanical configurations available and the source I have indicated is the best I have found to date at arround £6 GBP per switch.

Hope this helps

Regards

Pat

PS Mach3 can be configured for separate home switches or to home at the limit of travel switch see the Mach3 manual for further detail.

[michaelf]

Thanks Pat; I'll need to do some more research.
Michael

[wildwestpat]

Hi Michael

I have just checked and the ones I have to hand are ok at +5 volts. I will try and sort you out the connections for youe box.Are you using a breakout / buffer board between the Mach3 and the driver electronics? This is a good idea if the board provides opto isolation as the currents used to drive the motors can generate spikes that if you are unlucky can see off your LPT port on your PC. I use boards from CNC4PC despite having to pay import duty as I am in the UK. i have sourced some opto isolator chips and will eventually make my own breakout boards if I ever find the time - some hope!!!!

Suggest at this point in your build you list the inputs you need for Mach3 control. The Mach3 manual will guide you. Many people use desktop PC's with extra LPT cards plugged in to give one or two more lots of inputs. There are other alternatives but the LPT cards are less than £20 on ebay for a desktop PC. Ley me know you actual pick for interfacing and I will try and help.

Regards

Pat

[michaelf]

Hi Pat; I’m not sure if I have a breakout / buffer board between the Mach3 and driver electronics, I certainly haven’t added anything other than the box supplied by Keling that uses the Gecko G540, http://www.kelinginc.net/ControlSystem.html (it’s the black box at the bottom of the page).
It does have a serial port for connecting homing switches, and I assume it would have some DCV in that port as well. You’ll have to forgive my ignorance, this is still a steep learning curve for me.
Michael

[wildwestpat]

Hi Michael

Nice control box. It is fully opto isolated and based on the Geckodrive G540.

The 9 pin connector is not an RS232 port but a special provided by Keiling to make it simple for the end user to connect to their machine. The wiring for this box to the external drives and switches is given by Keiling here http://www.kelinginc.net/G540WD.pdf and should have been supplied with the box. I like to save the documents to the PC used for each machine so that they are to hand when needed! (Assuming that the PC is OK!)

The + 10 volts on this connector is part of the speed control (VFD) circuitry and is expecting to connect to the speed set variable resistor in the spindle motor control. Best to leave this for the intended purpose as there may be insulation / isolation problems - not too certain on this point as I have no experience of these integrated systems but would expect it to be isolated as there are mains supply voltages on the spindle drive circuit board.

The G540 is expecting to see normally closed switch contacts. Four separate switch contacts are provided - one for each of the four axis which this box can drive. The limit / home switched lines being connected through the G540 to the LPT 25 pin connection to your PC's parallel port.

Thus you have several options for connecting switches and each has its merits as well as possible disadvantages depending on what you are happy to construct.

1. Use micro switches - no adaptation other than proofing the switches against mechanical damage and ingress of swarf and coolant. This is the G540 expected solution and is the basis for the detail in the Mach3 manual on limit switches. Many pics and details on the web including this site.

2. Use electronic switches there are several types categorised by how they sense the proximity of the table stop. These are (a.) optical - prone to dust and expect flood coolant problems but otherwise good - (b.) magnetic - prone to fouling with magnetic swarf so not good for a machine used for cutting steel but ok with most other things and ok with flood coolant. - (c.) capacitive reasonably good with coolant and fouling - (d.) inductive similar to c but in my experience better tolerance to coolant flooding and general fouling.

If you want to use the electronic switches you have to convert them so that thaey look like a closed / open contact. This means you have two options.

1. Use a reed relay with it's coil operated by the sensor black wire. For the NPN type the other end of the coil goes to the brown wire and supply positive. For the PNP type the coil is between supply negative and the black wire. Use a wall wart 9 to 12 volt DC supply (just make sure it is DC) as a simple way of getting the requiured voltage for the sensors. You could derive the sensor supply from inside the Keiling box but I don't know how confident you are with the electrics or what warrenty you have on the box.

2. Use an external wall wart as in 1. above. Add a 1K ohm resistor in series with a diode (1N4001 series will do) positive end of the diode goes to the negative side of the wall-wart. The negative side to the Keiling box common zero pin. The sensors are wired with their brown leads to the + 9 to 12 volts of the wall wart and their blue leads to the minus along with the bar end of the diode. The inputs to the Keiling box are the black wires. The diode biases the NPN outputs to around zero volts by off setting the on condition voltage across the npn output transistor.

My personal preference would be to use the read relays and mount them from a new terminal strip/s inside the Keiling box along with a small 9 volt supply. The reed relays can be had for £1 to £2 here in the UK from any hobby electronics outlet.

Hope this helps

Pat

[michaelf]

Hi Pat,
Thank you for a very informative reply. Unfortunately I didn’t receive that information with the controller box, I did get some spare screws rattling around inside, I guess they put them in there so you had to pull the lid off to have a look, it was then that you found everything was lose and needed to be tightened up, there again everything probable shook lose when they dropped the unit before packing, this caused a large dent and missing paint.
Anyway enough of that, at least it worked. I will probably go the simplest route at this stage and use limit switches, however I still like the look of your inductive switches, I may have a serious look at them after I become more familiar with what I have.
Once again thank you Pat for your patience and spending the time explaining things so well.
Michael

[wildwestpat]

Hi Michael

Here is a simple cover for a pair of microswitches designed to accomodate a small amount of over travel. The switches are operated by small blocks attached to the moving part and the hole in the lower part is for the cable.

I suggest you get the machine up and running because with stepper motors on the screws you don't have a compelling need for limit switches other than for homing. Just make sure you are not loosing steps. See the Hoss video on how to calibrate the travel for Mach3 on his X2 mill. If you are missing steps and eventually hit the buffers the motors make a horid noise as they try and cog to the next step but the potential for damage is the inertia of the machine hitting the buffers which for most builds would be OK.

Good luck and fine machining

Pat