How can soft limits work without referencing home at table corner?


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    Member QuinnSjoblom's Avatar
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    Default How can soft limits work without referencing home at table corner?

    I've been talking with a guy that I'm about to order my electronics from and he's trying to explain to me that soft limits can be used without ever needing to reference home at the corner of the table. How can this work? He is suggesting to not worry about using any switches at all and just home the machine on the corner of my workpiece in the middle of the table and my soft limits will protect me. I understand switches aren't needed for soft limits but wouldn't I at least need to jog over to the corner of the table and reference home there so mach3 has a reference of where my soft limits are? Let's say for example my X travel is 1000mm long, so I set soft limits to 0,1000 for x. If I just home on the corner of my work piece somewhere in the middle of the table, won't mach3 think my soft limit at 1000 is way off the table? How can soft limits work if mach3 doesn't actually know where the physical corner point is on my table?

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    Default Re: How can soft limits work without referencing home at table corner?

    You are correct. You need to reference the machine in order for softlimits to know where the limits actually are.

    Gerry

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    Default Re: How can soft limits work without referencing home at table corner?

    The home does not need to be in the corner though. It just needs to be consistent. Lets say your home of x is just in the middle, then the limits would be -500 +500. And it would work. But you have to home the machine on the same position for them to work.



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    Default Re: How can soft limits work without referencing home at table corner?

    OK thanks guys. That's what I thought. Maybe im just confused by his terminology. Maybe when he says "home" on the part, he just means zero the work offset on the part. Although he's still leaving out the part of actually "homing" somewhere consistent on the table each time to give limits a reference before zeroing on the part.

    I do like the idea of having my actual home in the center of my table. It will be much closer to where I'll be doing work so I won't ever need to move all the way over to the corner. 99% of my work is less than a tenth the size of my table so I can setup soft limits a couple inches away from the end of my travel and ill still have plenty of room to work. when I reference home in the middle I don't need to be precise about it. I'll just put a couple reference marks to jog over to for homing.



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    Default Re: How can soft limits work without referencing home at table corner?

    My opinion is that it is moe convenient for home to be in at the right top corner. The reason is uniformity. So all the moving range for each axis will be negative value in g53. Also it is important to have the home switches very repeatable. This will allow you to have fixed positions on the table that are valid after you ref home the machine. For example you have an mdf piece as fixture for some parts on the table ant is has location pins so you can position your material. If you do not have repeatable home location the location of the fixture with the location pins is lost if the machine loses its exact home position after. power down.



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    Quote Originally Posted by jimskeet View Post
    My opinion is that it is moe convenient for home to be in at the right top corner. The reason is uniformity. So all the moving range for each axis will be negative value in g53. Also it is important to have the home switches very repeatable. This will allow you to have fixed positions on the table that are valid after you ref home the machine. For example you have an mdf piece as fixture for some parts on the table ant is has location pins so you can position your material. If you do not have repeatable home location the location of the fixture with the location pins is lost if the machine loses its exact home position after. power down.
    Yes I agree with that if I was doing repeated operations and had jigs setup for doing parts more than once, but that's not the case for me. This isn't a work machine. I'll be doing mostly one off parts out of carbon fiber and aluminum so won't really have the need to setup a jig that i can accurately repeat on. I'll just be setting up a piece of stock and finding edges on it to zero.

    On that subject though, let's say I did end up doing a production run of some aluminum parts some day, are home switches accurate enough to place me over a part for a second operation after flipping a part? Obviously don't need perfect placement if I'm cutting all sides of a peice of stock, but if I want to flip the part and run the other side, will an offset from home switches give me accuracy to within a thou or so? If not then I would still need to find edges anyway even for doing multiples of a second operation where accuracy of the pick up on the part is really important.



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    Quote Originally Posted by QuinnSjoblom View Post
    Yes I agree with that if I was doing repeated operations and had jigs setup for doing parts more than once, but that's not the case for me. This isn't a work machine. I'll be doing mostly one off parts out of carbon fiber and aluminum so won't really have the need to setup a jig that i can accurately repeat on. I'll just be setting up a piece of stock and finding edges on it to zero.

    On that subject though, let's say I did end up doing a production run of some aluminum parts some day, are home switches accurate enough to place me over a part for a second operation after flipping a part? Obviously don't need perfect placement if I'm cutting all sides of a peice of stock, but if I want to flip the part and run the other side, will an offset from home switches give me accuracy to within a thou or so? If not then I would still need to find edges anyway even for doing multiples of a second operation where accuracy of the pick up on the part is really important.
    Yes. I use optical home switches and they are very accurate. I have locating holes for exmple at x-645 y-220 and after ref home I go to that location and the spindle is always in the center of the hole. If I lower my Z the endmill goes right into the hole.
    I use those pins for the 2nd operarion on a part that I am flipping. So I have my XY zero set once and have the coordinates stored. As long as the mdf board is not removed from the table I keep running the parts without any need to re zero the X and Y. I just ref the machine and the offsets are exactly at the same place.

    Last edited by jimskeet; 01-24-2018 at 11:58 AM.


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    Default Re: How can soft limits work without referencing home at table corner?

    My opinion is that it is moe convenient for home to be in at the right top corner
    Fortunately, that's just your opinion, because it makes no sense to me...

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: How can soft limits work without referencing home at table corner?

    What makes no sense? Home location to be on the right top corner? I think this is the location most machines (like haas etc) have as home position. Z home has to be on the top of the travel. so the g28 command will be safe because if you have Z home low and not on the top of travel a g28 command at the end of a program could crash the machine if the part was taller the the home position. So any Z movement is negative in machine coordinates. So I see it convenient to have the X and Y home locations at the max value of their travel so any X and Y movement is negative in the machine coordinate too.



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    Quote Originally Posted by jimskeet View Post
    Yes. I use optical home switches and they are very accurate. I have locating holes for exmple at x-645 y-220 and after ref home I go to that location and the spindle is always in the center of the hole. If I lower my Z the endmill goes right into the hole.
    I use those pins for the 2nd operarion on a part that I am flipping. So I have my XY zero set once and have the coordinates stored. As long as the mdf board is not removed from the table I keep running the parts without any need to re zero the X and Y. I just ref the machine and the offsets are exactly at the same place.
    Have you measured the repeatability of your optical switches with a dial indicator? An endmill dropping into an existing hole in mdf doesn't really mean much in terms of the accuracy I would want for flipping an aluminum part, placing it in another fixture and finishing an outside profile that it started in the first operation. I'm sure you could be a few thou off and an endmill would still drop into an existing hole in mdf. In my case a few thou step in a surface of an aluminum part would definitely be noticeable. I still doubt I'll ever be using this machine for production with multiple parts, I'm more just curious how repeatable an optical switch can actually be.



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    Default Re: How can soft limits work without referencing home at table corner?

    I will check it with a dial indicator and report back the accuracy level. The holes are drilled with an 8mm flat endmill and are a tight fit on my 8mm steel pins. So the fact that the spindle drops right into hole after ref home shows me that the switches are very accurate. I will report back with dial result though.



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    Default Re: How can soft limits work without referencing home at table corner?

    I tested the repeatability of the home switches today and the worst result I got was 0.02mm off once. Most times was 0.01mm or lower. Here is a video of the test
    https://www.dropbox.com/s/sh80emy7ra...0Test.mp4?dl=0

    I had the machine homed before starting the video. Then type in mdi x-3.
    After the movement I again click Ref home for the machine to ref the axis again and then type again x-3. in the mdi.



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    Quote Originally Posted by jimskeet View Post
    I tested the repeatability of the home switches today and the worst result I got was 0.02mm off once. Most times was 0.01mm or lower. Here is a video of the test
    https://www.dropbox.com/s/sh80emy7ra...0Test.mp4?dl=0

    I had the machine homed before starting the video. Then type in mdi x-3.
    After the movement I again click Ref home for the machine to ref the axis again and then type again x-3. in the mdi.
    That's pretty good. Under a thou at the most. Sounds like optical switches are pretty good.



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    Default Re: How can soft limits work without referencing home at table corner?

    Quote Originally Posted by QuinnSjoblom View Post
    That's pretty good. Under a thou at the most. Sounds like optical switches are pretty good.
    Yeah and that 0.01mm maybe is not cause of the switches themselves. One correction though.
    I wrongly said that my switches are optical. They are proximity switches.
    Like the one on this photo. https://4.imimg.com/data4/BP/WY/MY-2...ch-500x500.jpg
    So they signal when they come close to something metal.



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    Default Re: How can soft limits work without referencing home at table corner?

    I just converted my mill and before I had the switches installed I just moved each axis to where I was planning on putting the home switches. Z all the way up, X Table to the right, Y Table back. I would then click on the buttons to home each axis. I'm using LinuxCNC. Then I would touch-off on the back left corner of the stock in the vise for the X & Y, Then touch off the tool to the top of the stock for the Z. Not good enough for flipping the part but I am just playing at this point. Now that I have the home switches installed it is easier to home the machine. I no longer have to jog to the home position and then press the home buttons for each axis. Now I just press the Home All button and the homing happens automatically. I also have physical limit switches in addition to the soft limits.

    John



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    Default Re: How can soft limits work without referencing home at table corner?

    The home switches pay off when you want to have an accurate home location. Like when you machine a part and for some reason power goes down or a fault happens and you maybe have lost some steps. If you cannot have the same home location this part is lost. With proper homing switches you can just reset, ref home again and continue the operation from where it stopped.
    Also for having fixtures on the table that you can still use without needed to indicate them after powering down the machine.



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How can soft limits work without referencing home at table corner?

How can soft limits work without referencing home at table corner?