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You typically want limit switches to be normally closed in a series loop. That way if a switch fails or you have a wire break the machine will fault.
Have you considered using soft limits in Mach? That might enough to get you going.
bob
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I'm working the the final details of my cnc router and one problem I'm having is with limit switches. I only have a problem cause i'm trying to reduce the price everywhere i possibly can since the final cost of my machine will be about $800 with a cutting area of 15x15x4". It sounds small but it's primarily for aluminum work and will theoretically not flex more than .0002" with a 10lb load in any direction(that's what i calculated for the cutting force for how i'd be using it). back to my question. to be cheap with limit switches i was wondering if anyone has ideas to make limit switches. I was going to connect the machine to one side of the limit input and have a bunch of little pieces of spring steel mounted to something non-conductive and have them all wired in parallel to the other part of the limit input so when the machine makes contact with any part of these springs it will close the circuit and stop the machine. question 1 is would this work or does anyone foresee any problems. question 2 is would it be easier to just have a limit switch(production or diy) be attached to the moving part and have it make contact with the machine on either extent of its travel? question 3 is limit switches seem quite pricey if i need six so i would use the smaller ones which are cheaper but have less travel in the switch so i was wondering how far does the machine usually travel after the switch is activated? i'd assume it may be a matter of a few though since it would hit the brakes hard even with small motors. I don't need anything that looks good or is really elaborate. it just has to stop it consistently so i can squeeze maybe 1 more inch of travel out of the machine. Thanks in advance.
You typically want limit switches to be normally closed in a series loop. That way if a switch fails or you have a wire break the machine will fault.
Have you considered using soft limits in Mach? That might enough to get you going.
bob
what do you mean by soft limits in mach3? I've never heard of it so please explain. I'll make a guess and see if I am right. Do you the the part you're working on zeroed in and then you start this wizard in mach and you bring the machine to all it's limits so it will not go outside those? That would be good for now since i'm on a tight budget. The only problem i could see with that(if i'm right) is that if you lost steps then the machine could go outside the limits even if it thought it was inside them. you're right about the normally closed limit switches. That's the one problem i had with my setup, i wouldn't know if a switch went bad and it'd require a lot of switches being attached in parallel. thank you.
You need to have a method of homing all three axis so that Mach3 knows where it is to cut. The homing can be via the travel limit switches or with a switch set at some other place between the limits defined by the limit switches.
1. Mach3 will operate with software limits after it has been set up correctly. Download the instructions and manual from the Artsoft website. i.e. mechanical switches are nice but not a necessity see 2.
2. Physical end of travel limit switches can be set to protect the axis from mechanical damage. Some machine designs come to no harm if the axis runs at full speed to theend of the axis. You need to think what happens under these crash conditions. In a cost sensitive build you will I asume be using low poer stepper motors. These can stand being stalled. Closed loop servo motors will try and push to overcome the physical end of travel either until something breaks or the motor burns out.
3. A single switch per axis can be used. Best to keep the switch on the fixed parts of the machine as far as practical. Use separate operating vanes or blocks to operate the switch. The home switch can also share the same switch by adding a third operating bane/block.
4. You need to consider the overtravel when the axcis is driven at speed at the point the switch operates. The axis might not stop instantly. .
5. The sort of switch is in part dictated by the environment and the protection given to the switch element / contacts.
6. Magnets and reed switches are cheap. Suffer from magnetic swarf clinging to the permanent magnets.
7. Optical switches are good provided thewre is no dust or other contaminants to obscure the optical path.
8. Microswitches can be expensize if theu need to be water and dust proof.
9. Proximity detectors of the inductive type cost arround $10 if you search on ebay and are available from China in quantity. Western manufactured switches that have a high IP rating can be expensive however.
I would caution against making your own switches as getting reliable operation is tricky but it can be done but why would you? If the switch is going to be used as a switch of last resort then a simple wire link arranged that the axis motion will break the wire is very cheap and as reliable as switches can get. I have used metalised glass as the end stop on very large servos that had enough power to cause real damage if they were driven to the ultimate limits of travel.
Mechanical shear pins or couplings that can slip are also ways of stopping a hard crash of the axis. More expensive is the use of torque limiting clutches. The problem comes with open loop drivers i.e. stepper motor drive in detecting the end has been reached and the motor is just missing steps that the driver thinks have been applied.
Hope this helps you pick a sensible option - Regards - Pat
Something like this? (Check picture)
Would be simple to make & should work..
Hub
http://www.cnczone.com/forums/cnc_wood_router_project_log/125895-my_diy_cnc_cnc2011_%3B.html
a standard microswitch is less than a 1$, hardly expensive for an easy to mount very reliable switch
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