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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > Limit/homing switch questions
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  1. #1
    Join Date
    Mar 2017
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    98

    Limit/homing switch questions

    Hi all I am trying to work out which electrical bits I'm going to need for my cabinet, and have hit the problem of limit/homing switches.

    Questions... Is the accuracy of the limit switches really that important? I have never used a cnc machine before, but I would guess that you would set your 0,0,0 point using indicators off your vice or part you're going to mill before starting an operation.

    Do you want them to be wired to the motion controller or do you even perhaps want a switch that shuts off all power to the mill if it's ever hit? Something that activates an e-stop, like through a relay- hardwired. Maybe both?

    How do you work out the travel required on the switch? Would this have something to do with your table speeds? Or is this irrelevant as the only time you should encounter the switch is when you're homing and this is done at a controlled speed.

    Is there a difference between limit and homing switches? I see the term used interchangeably sometimes.

    Is there a sweet spot for price/performance? I see Tormach use Tend TM1308-1

    Do you want to use a high voltage limit switch? I would think that 24V would be safer, but I don't know if there are disadvantages.

    Any good examples of limit switches or wiring on a mill that was done really well would be helpful, especially if it uses high voltage servos (which I'm using).

    Thanks.

  2. #2
    Join Date
    Dec 2013
    Posts
    5717

    Re: Limit/homing switch questions

    Quote Originally Posted by j3dprints View Post
    Hi all I am trying to work out which electrical bits I'm going to need for my cabinet, and have hit the problem of limit/homing switches.

    Questions... Is the accuracy of the limit switches really that important? I have never used a cnc machine before, but I would guess that you would set your 0,0,0 point using indicators off your vice or part you're going to mill before starting an operation.
    There are a couple schools of thought about homing switches. Some people think that there should be a machine absolute 0,0,0 located relative to home switches, then the work or fixtures are an offset from that point. My opinion is like yours, machine zero and work zero should be the same point. The machine could be set to go to a convenient ''parking position'' once the job is complete. The one exception to this is when using an automatic tool changer, in that case the machine needs to be located from a fixed position to change the tool. I don't have homing switches on my mill.

    Do you want them to be wired to the motion controller or do you even perhaps want a switch that shuts off all power to the mill if it's ever hit? Something that activates an e-stop, like through a relay- hardwired. Maybe both?
    Normally the limit switches are wired to the motion controller. If it's doing its job correctly, it should stop any further axis movement in that direction and the software would take any pre-programmed action needed at that point. Normally the machine would stop and a message be presented to the user that a limit switch was tripped.

    How do you work out the travel required on the switch? Would this have something to do with your table speeds? Or is this irrelevant as the only time you should encounter the switch is when you're homing and this is done at a controlled speed.
    Normally the limit switches are set to trip near the mechanical limits of travel. Depending on the machine, this distance could be as little as a couple of mm. I think my lathe cams are set to about 5mm

    Is there a difference between limit and homing switches? I see the term used interchangeably sometimes.
    Physically? Normally not unless extreme accuracy is required. There are high precision limit switches available for around US$100. Now to really confuse the issue, the axis travel limit switch could also be used as the home switch. It depends on the software you are using. If the software overrides the travel limit function during a homing cycle then the same switch can be used. If you are using a single switch on an axis, then there needs to be a software or hardware override, or other provision to allow the axis to move off of the limit under operator control, normally done at very slow speed.

    Is there a sweet spot for price/performance? I see Tormach use Tend TM1308-1
    I have successfully used $2.00 snap switches as machine limits and home switches with 0.01 mm repeatability. It does require a bit of fiddling to make that happen. I normally use a bit better switches. A ''slow acting'' limit switch is much more repeatable than a ''snap acting'' switch. If coolant is involved, then an IP65 or IP67 rated switch should be used.

    Do you want to use a high voltage limit switch? I would think that 24V would be safer, but I don't know if there are disadvantages.
    I normally run the limit switches at 5VDC or 24VDC depending on the application. Wire the switches Normally Closed so they open when tripped. That way a wire break has the same effect as a tripped limit, and is noise immune. I think the limits on all of my machine are wired for 5VDC.

    Any good examples of limit switches or wiring on a mill that was done really well would be helpful, especially if it uses high voltage servos (which I'm using).

    Thanks.
    The servo voltage does not matter, the travel limit is not going to be wired into any high voltage motor circuit. The travel limit could be wired directly to the enable input on the axis servo, but this is normally not required. Some servo drives have travel limit inputs. The controller should be capable of handling the tripped condition.

    Normally one switch per axis is all that's required. The switch is mounted to a fixed location relative to the axis and is operated by a cams attached to either end of the moving axis.

    Here is a picture of the X limit switch on my mill. There are one of these cams on either end of the table


    And a picture of the Y axis limit, again there is a cam at each end of the travel.



    Here is a pic of a limit on a dispensing machine I built out of a router frame. There are one of these cams at each end of travel.



    And another view of a cam on the same machine. About a 15° ramp.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
    Join Date
    Jun 2013
    Posts
    1041

    Re: Limit/homing switch questions

    I am of the opposite opinion. I think homing is a very important part of a machine.
    It allows the machine to prevent itself from being driven into a switch in the first place.
    It allows you to turn off the machine for the night. After you home the next morning you can hit start and be right back making parts. The machine references itself so no need for you to find a zero again.
    If you use a tool changer it's a must. If you use more then one offset in a program there is no longer a argument to have part zero and machine zero match. Your other offsets from part zero would be in relation to part zero. Why not then just let everything be in relation to machine zero.
    I do this for a hobby as many here do. I have also programmed and operated machines in various cnc fields for almost 20 years. All of those machines rely on a homing routine except a few machines with absolute encoders. They always knew where they were at from machine zero even after turning off and restarting.
    If I had no limit switches at all then referencing to part zero as machine zero would be alright. Otherwise I don't see a compelling argument to leave out homing.

    Ben

    Sent from my HTC6525LVW using Tapatalk

  4. #4
    Join Date
    Nov 2012
    Posts
    1267

    Re: Limit/homing switch questions

    ^^^ Good points, exactly what I wanted to reply but was too lazy.

    I just want to add that many machines do not rely on homing switches for accuracy. The homing routine first seeks the switch for "ballpark" homing, and then uses the encoder's index pulse for "fine" homing.

  5. #5
    Join Date
    Feb 2018
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    92

    Re: Limit/homing switch questions

    The way I've planned to do my setup is with both. A cheap mechanical switch will trigger my servo controllers to shut down before smashing into the end of travel, but an optical switch will home it in the 0,0,0 position. I can setup my Acorn motion controller so the optical switch will be interrupted in the -x/-y/-z to 0 positions, then open from there through +x/+y/+z. This way, the controller always knows which direction to go in order to find 0,0,0.

  6. #6
    Join Date
    Jan 2005
    Posts
    1943

    Re: Limit/homing switch questions

    Homing switches are used to tell the machine where machine home is. Machine home is different than part zero. It is important especially if you have a tool changer that relies on an accurate home position for tool changes. It is also important because G28 and G30 are offsets from machine zero, and have nothing to do with part zero. For example, if the user has not defined G28 or G30 positions, they are default to X0 Y0 Z0 in machine coordinates, which will raise the Z axis all the way up and move X and Y to the machine zero, usually table toward the operator and all the way left. If you don't use a machine home position you have to define G28 and G30 every time you set up your machine, or not use them. They are very useful, so for me I have to have a machine home position. Also, if you intend to use sofware limits in addition to the hardware switches, then you need an accurate machine home position so the controller knows how far it can move in each axis. Software limits are nice because the controller can interrogate the g-code program and determine if any moves are outside machine limits before running the program. Finally, if you have accurate home switches and have an occasion where you can't finish a part in one setting, you can safely shut the machine down and when you come back, just re-home and resume.

    Homing switches and limit switches on industrial machines are separate things. The limit switches on an industrial machine are wired to a contactor that will cut all power to the spindle and drive motors. On hobby machines, the limits are commonly wired to the controller and the controller is responsible for performing these functions. The problem with this is if the problem is due to a controller error in the first place you are relying on the errant controller to do the right thing. Normally though the problem is a programming error, or user problem and the controller does a good enough job.

    On hobby machines you can combine the homing and limit switches. When homing they work as a homing switch and afterward they revert to limits. This is useful if you are limited in the amount of inputs your controller setup can accept, and has less switches to set up. I have my machine set up with a single switch on each axis which performs as a combined home and limit switch. Each switch only acts as a limit in only one direction of travel and I use software limits for the opposite direction of travel. I then have the software limits set about 0.1" from where the switch actually activates. This way prevents false triggers when using combined home/limit.

    In regard to the accuracy of limit switches, they don't have to be too terribly accurate and are usually setup a short distance from the actual hard limit where something mechanical prevents further movement. Home switches on the other hand should be as accurate as you need for your machine. I use inductive proximity switches and have tested their accuracy. The results were impressive with repeatability within a few ten-thousandths of an inch (0.0001"). That is plenty accurate enough for my purposes.

    Finally, You can still use most of the features of a machine home position without having actual home switches. On my first machine, which used low power steppers, I simply ran the axes into their hard stops slowly until the open loop stepper stalled, and then backed off 0.1" and "homing" the machine. On that machine, since no switches were installed the controller I use (LinuxCNC) knew to set the machine coordinates to zero when I did this. This allowed me to use software limits on that machine and to still use g28/g30, and G53 commands. It wasn't accurate enough to be able to shut off and then resume, but was very close and retained many of the things that machine coordinates give you.

  7. #7
    Join Date
    Nov 2009
    Posts
    4415

    Re: Limit/homing switch questions

    Quote Originally Posted by hackish View Post
    The way I've planned to do my setup is with both. A cheap mechanical switch will trigger my servo controllers to shut down before smashing into the end of travel, but an optical switch will home it in the 0,0,0 position. I can setup my Acorn motion controller so the optical switch will be interrupted in the -x/-y/-z to 0 positions, then open from there through +x/+y/+z. This way, the controller always knows which direction to go in order to find 0,0,0.
    Wouldnt the encoders on your servos tell you where you are? My machine has no limit switches but the encoders know where the machine is. I set my machine in a visually verifiable location, then press home all axis. It then seeks a home position based on the encoder. If there is a difference which at time there are, like .00015. I accept the change and move on. I am running FlashCut
    A lazy man does it twice.

  8. #8
    Join Date
    Feb 2018
    Posts
    92

    Re: Limit/homing switch questions

    Quote Originally Posted by Fastest1 View Post
    Wouldnt the encoders on your servos tell you where you are? My machine has no limit switches but the encoders know where the machine is. I set my machine in a visually verifiable location, then press home all axis. It then seeks a home position based on the encoder. If there is a difference which at time there are, like .00015. I accept the change and move on. I am running FlashCut
    The encoders on my servo motors count 2500 pulses per revolution, but when the machine is powered up, it has no way to know if it was properly parked on the last shutdown, or even moved. If I had a linear encoder hooked up, then homing would not be necessary. The way it is setup, once powered, the servo controllers will fault if it ever fails to seek the requested location, so, homing is a cold start operation.

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