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  1. #31
    Its always best to refer to the electrical machinery regulations for the latest interpertation of good practice re estops and safety circuity.

    I would paste the link but Im not quite In control of this tablet yet :)


    Just google It


    QUOTE=birchy;39536]I would much prefer to kill the power and re-zero rather than rely on software. At the end of the day, it's a safety circuit. I'm not going to add spindle speed control at this stage, however I may add it in the future if I need to replace the motor and/or upgrade to an inverter drive. Latest drawing is attached. Please let me know if you see any mistakes or can suggest improvements!


    Attachment 7653[/QUOTE]

  2. #32
    Yes, the regulations are 2006/42/EC. I know they make reference to machinery working within its limits but that is for lifting equipment where uncontrolled movement could be hazardous to the operator.

  3. #33
    Quote Originally Posted by birchy View Post
    My reasoning is that if a drive goes to the end of its travel and activates a limit switch, we don't want it to kill everything and have to carry out a manual reset. It's not an emergency or a safety issue. Even if the drive keeps going, the physical stops will prevent it going any further. It might be an inconvenience to the operator but not a safety hazard...unless the machine is badly designed mechanically. So my idea of a limit switch is that it should stop the drive and inform the software but NOT kill the whole machine. Would you not agree?
    No don't agree at all. Course it's a safety issue it's just crashed for some reason how do you know what that reason may be.? To have the system restart with out a manual reset is plan dangerous.!! . . You wouldn't have the machine restart upon releasing E-stop so why would you a Safety limit switch.?

    You'll be hard pushed to find any commercial CNC machine that doesn't throw the machine into a reset condition from a limit trip.!

    Relaying on Physical stops is bad practice and unhealthy to any machine and relaying on drive over current protection is just plain asking for trouble.!


    End of the day it's your machine and components so do it how you feel best.! . . . Me I'll stick with being safe and the slight inconvenience of a reset thank you very much.!

  4. #34
    It seems that we have different opinions of what a LIMIT switch and a SAFETY switch are. Obviously safety switches should be dual circuit and fail-safe. A limit switch/sensor is usually a single switch that tells the PLC or machine computer its current position. So I guess the safest way is to have LIMIT switches near the end of travel and SAFETY switches just before the physical stops. I think we're both on the same wavelength, but not using the same terminology!

  5. #35
    Quote Originally Posted by birchy View Post
    It seems that we have different opinions of what a LIMIT switch and a SAFETY switch are. Obviously safety switches should be dual circuit and fail-safe. A limit switch/sensor is usually a single switch that tells the PLC or machine computer its current position. So I guess the safest way is to have LIMIT switches near the end of travel and SAFETY switches just before the physical stops. I think we're both on the same wavelength, but not using the same terminology!
    I may be talking tosh but this was a question that I asked when I first started thinking/designing my first machine. I got confused about home switches and limit switches and safety switches and I ended up having several physical work areas on my machine, depending on the size of the piece/part I was making, so if my machine sailed out of the physical working area I had defined, there was a good reason for it and it activated an E-stop shutting the machine down via the software stopping the program in its tracks, these physical working areas were movable/definable the in software, and a manual restart would be required whatever the condition, there were also a switches at the physical ends of all the axis, that was more brutal and just shut the power of completely. I even went and purchased a multiple input/output card (pokeys55) to achieve this. However as JAZZ has said all unexpected movement of an axis would imply a safety condition exists and should shut down the machine until it can be fully investigated..Rick
    Always bear in mind that your own resolution to succeed is more important than any other - Abe Lincoln

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