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Thread: CONFUSED... Where to home too... Help! where should my home switches be.

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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    So just to help out this newbie...

    Homing a machine is really on necessary if you are using offsets and fixtures correct? there is no other reason to do it?



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by truckeic View Post
    So just to help out this newbie...

    Homing a machine is really on necessary if you are using offsets and fixtures correct? there is no other reason to do it?
    About half the members here are going to disagree with this so I'll put my flame suit on, but...........

    For the purposes of this discussion I define required homing accuracy as +/- 0.0005 inch or better. This only applies to mills and lathes, there is some logic to using homing limits on a router because extreme accuracy is not normally required.

    The only reason to home a mill is if you have a tool changer and need to move the head and/or table to a specific location to effect a tool change. Other than that, the zero should be set to the work or fixture on the table and everything should be relative to that 0,0,0 position.

    Limit switches are almost required for overtravel protection, but are normally not accurate enough for absolutely locating the machine coordinates. There are limit switches available that do provide micron repeatability, but normally only found on higher end machines because they are expensive. A proper homing routine uses the index pulse on the servo encoder for final positioning. If your machine does not use servos, the homing can not be accurate.

    My knee mill does not have any homing provision, not needed. My Haas does home, it has a tool changer. My lathe also homes. Both the Haas and the lathe use the servo index pulses for final home positioning.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    The only reason to home a mill is if you have a tool changer and need to move the head and/or table to a specific location to effect a tool change.
    The above statement is just flat out wrong. There are many reasons to have a home position and you don't have to have switches to have a "home" position either . A home position can simply be located by witness marks so that you can get back to about that same spot within a reasonable accuracy.

    - Many controllers have software limits that allow definition of the work envelope to prevent reaching the end of travel. If properly set up and barring a failure the controller won't even allow the machine to be moved to its hard limit. Depending on the machine I would consider this a safety feature since some machines have axis drives that can break stuff. If you don't have any switches, depending on the controller you may be able to still use software limits, but to use soft limits you have to have a home position.

    - Not using a home position means that machine coordinates are essentially useless. Things like G28 and G30 locations are based on machine coodinate systems. G28 and G30 are commonly used by CAM programs for manual (not automatic) tool changes, but the definition requires using a home position.

    - G53 moves (Move in machine coordinates) would be arbitrary if you don't use a home position

    Other than that, the zero should be set to the work or fixture on the table and everything should be relative to that 0,0,0 position.
    Everything? Yep if you only use the part zero everything will be relative to part zero. Lets say moving the Z all the way up is 10 inches above one part, but the next part is 2" shorter. Now Z all the way up is 12 inches above the part because you are referencing the machine from the part. On a machine using a home position, Z all the way up is just G53Z0. Nothing to remember or change. Same goes for X and Y coordinates. Now if I'm changing tools manually I want my machine at the clearest part of my table which is always at the same machine coordinates, but pretty much always at different coordinates relative to the part. To use the machine reference, you have to have a home position.

    Limit switches are almost required for overtravel protection, but are normally not accurate enough for absolutely locating the machine coordinates. There are limit switches available that do provide micron repeatability, but normally only found on higher end machines because they are expensive. A proper homing routine uses the index pulse on the servo encoder for final positioning. If your machine does not use servos, the homing can not be accurate.
    Totally wrong. Homing can be as accurate as the switches and what is an appropriate level of homing accuracy is something each person has to decide. Just because you want sub-micron accuracy doesn't mean the next person needs it. Homing for use of things like G28, soft limits, etc, doesn't even have to be all that accurate. +/- 0.05" accuracy would be just fine for this kind of stuff.

    Granted the most accurate homing will be using an index pulse on the rotary encoder on the stepper/servo, but saying without this that "homing can not be accurate" is just BS. I have inductive proximity switches that I have tested the repeatability to less than 0.0005" (micron repeatability) and they cost me $30 for a pack of 10. Now if I'm making parts that have a +/- 0.005" tolerance, then explain why homing within 0.0005" is not acceptable. I would bet on 0.0005" homing accuracy on my machine but it is probably a little better than that.



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by 109jb View Post
    The above statement is just flat out wrong. There are many reasons to have a home position and you don't have to have switches to have a "home" position either . A home position can simply be located by witness marks so that you can get back to about that same spot within a reasonable accuracy.

    - Many controllers have software limits that allow definition of the work envelope to prevent reaching the end of travel. If properly set up and barring a failure the controller won't even allow the machine to be moved to its hard limit. Depending on the machine I would consider this a safety feature since some machines have axis drives that can break stuff. If you don't have any switches, depending on the controller you may be able to still use software limits, but to use soft limits you have to have a home position.

    - Not using a home position means that machine coordinates are essentially useless. Things like G28 and G30 locations are based on machine coodinate systems. G28 and G30 are commonly used by CAM programs for manual (not automatic) tool changes, but the definition requires using a home position.

    - G53 moves (Move in machine coordinates) would be arbitrary if you don't use a home position



    Everything? Yep if you only use the part zero everything will be relative to part zero. Lets say moving the Z all the way up is 10 inches above one part, but the next part is 2" shorter. Now Z all the way up is 12 inches above the part because you are referencing the machine from the part. On a machine using a home position, Z all the way up is just G53Z0. Nothing to remember or change. Same goes for X and Y coordinates. Now if I'm changing tools manually I want my machine at the clearest part of my table which is always at the same machine coordinates, but pretty much always at different coordinates relative to the part. To use the machine reference, you have to have a home position.



    Totally wrong. Homing can be as accurate as the switches and what is an appropriate level of homing accuracy is something each person has to decide. Just because you want sub-micron accuracy doesn't mean the next person needs it. Homing for use of things like G28, soft limits, etc, doesn't even have to be all that accurate. +/- 0.05" accuracy would be just fine for this kind of stuff.

    Granted the most accurate homing will be using an index pulse on the rotary encoder on the stepper/servo, but saying without this that "homing can not be accurate" is just BS. I have inductive proximity switches that I have tested the repeatability to less than 0.0005" (micron repeatability) and they cost me $30 for a pack of 10. Now if I'm making parts that have a +/- 0.005" tolerance, then explain why homing within 0.0005" is not acceptable. I would bet on 0.0005" homing accuracy on my machine but it is probably a little better than that.
    Still learning here but I don't understand all the G28 and G30. If you feel like explaining them would love to learn. I am using Soft limits s I guess what you are saying is homing the machine everttime will make sure these soft limits are still "true". That does make sense.
    I have just installed simple switches on the mill I am not sure how accurate the mill is as it is a Harbor Freight mill with 1605 C7 Ballscrews added.

    Thanks all for the info!



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    G28 and G30 are used as "predefined position". Invoking them moves the machine to the location that you the user has set for them. For example, on my Machine invoking G28 sends the machine to my tool change position which is Z all the way up,Table full left and back toward the column. So putting a G28 command always moves the machine to that same spot. That position is remembered by the controller even after a re-boot, BUT it requires using a home position. As said before that home position can be set manually, or you can home to switches. To set a position, all you have to do is jog to where you want it to be and send the G28.1 (or G30.1) command and where the machine is when that command is sent is stored as the location to go to when you invoke a G28 command.

    G53 is a coordinate system just like G54-G59 except that G53 is non-modal meaning that every time you want to move using the G53 coordinate system you have to put a g53 on the line. G54 for example is modal, so a G54 is remembered until changed by another coordinate system. The G53 coordinate system is considered to be the non-changing machine coodinate system. The G54-g59 are work coordinates and they change from job to job but G53 remains the same.



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by 109jb View Post
    G28 and G30 are used as "predefined position". Invoking them moves the machine to the location that you the user has set for them. For example, on my Machine invoking G28 sends the machine to my tool change position which is Z all the way up,Table full left and back toward the column. So putting a G28 command always moves the machine to that same spot. That position is remembered by the controller even after a re-boot, BUT it requires using a home position. As said before that home position can be set manually, or you can home to switches. To set a position, all you have to do is jog to where you want it to be and send the G28.1 (or G30.1) command and where the machine is when that command is sent is stored as the location to go to when you invoke a G28 command.

    G53 is a coordinate system just like G54-G59 except that G53 is non-modal meaning that every time you want to move using the G53 coordinate system you have to put a g53 on the line. G54 for example is modal, so a G54 is remembered until changed by another coordinate system. The G53 coordinate system is considered to be the non-changing machine coodinate system. The G54-g59 are work coordinates and they change from job to job but G53 remains the same.
    A G53 can't be used if you don't use or have a homing system as it relies on the machine coordinates to move to

    G53 temporarily cancels work offset when used

    G53 is an Absolute move

    G28 is an Incremental move

    So both are quite different how the move also

    Attached Thumbnails Attached Thumbnails CONFUSED... Where to home too... Help! where should my home switches be.-g53-2-png  
    Mactec54


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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by truckeic View Post
    Still learning here but I don't understand all the G28 and G30. If you feel like explaining them would love to learn. I am using Soft limits s I guess what you are saying is homing the machine everttime will make sure these soft limits are still "true". That does make sense.
    I have just installed simple switches on the mill I am not sure how accurate the mill is as it is a Harbor Freight mill with 1605 C7 Ballscrews added.

    Thanks all for the info!
    And you don't want to even go there when you are learning or may be never G30 is unneeded for normal work you would most likely be doing so take that out of the equation

    So G28 used to be used a lot, not so much today as most machine can use G53 G0 and G53 is used a lot more, G0 Z0. will send the Z axes to the Zero position G53 will do the same thing but for it to be formatted correctly it is normally used with

    G53G0 or G53G1F200. G1 you need a feed move a G0 will move at the machine set rapid speed

    When used like this you can place it any where you want

    G0Z2.0.
    G53X--- Y---

    or it can be used

    G53G0Z0.
    G53X----Y----

    Mactec54


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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by 109jb View Post
    G28 and G30 are used as "predefined position". Invoking them moves the machine to the location that you the user has set for them. For example, on my Machine invoking G28 sends the machine to my tool change position which is Z all the way up,Table full left and back toward the column. So putting a G28 command always moves the machine to that same spot. That position is remembered by the controller even after a re-boot, BUT it requires using a home position. As said before that home position can be set manually, or you can home to switches. To set a position, all you have to do is jog to where you want it to be and send the G28.1 (or G30.1) command and where the machine is when that command is sent is stored as the location to go to when you invoke a G28 command.

    G53 is a coordinate system just like G54-G59 except that G53 is non-modal meaning that every time you want to move using the G53 coordinate system you have to put a g53 on the line. G54 for example is modal, so a G54 is remembered until changed by another coordinate system. The G53 coordinate system is considered to be the non-changing machine coodinate system. The G54-g59 are work coordinates and they change from job to job but G53 remains the same.
    A G53 can't be used if you don't use or have a homing system as it relies on the machine coordinates to move to

    G53 temporarily cancels work offset when used

    G53 is an Absolute move

    G28 is an Incremental move

    So both are quite different how the move also

    Mactec54


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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    G28 is not an "incremental" move and can be used in G90 absolute mode or in G91 incremental mode. Yes G91 is used most often, because it is easier, but G28 works in absolute mode as well.

    G28 G91 X0 Y0 Z0 moves from where it is now in a straight line to the predefined position. Nothing incremental about that

    G28 G91 X0 Y0 Z1 moves the Z up one unit and then it goes straight to the predefined position

    I disagree that G53 is used more or is in any way better because to use it you have to set the machine coordinates that a G53 is going to. Use a different machine and the coordinates will likely need to be different Whereas with a G28, the coordinates are set at the machine and the program doesn't need to be modified. A CAM program can put a G28 G91 X0 Y0 Z1 but it can't put in a G53 unless it knows the coordinates it is sending to

    To make that same move in G53 you need

    G91 G0 Z1
    G53 G0 X??? Y???? Z???? where the ??? are the coordinates of the G28 position.


    G30 is just another predefined position and works just like G28.



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by mactec54 View Post
    And you don't want to even go there when you are learning or may be never G30 is unneeded for normal work you would most likely be doing so take that out of the equation

    So G28 used to be used a lot, not so much today as most machine can use G53 G0 and G53 is used a lot more, G0 Z0. will send the Z axes to the Zero position G53 will do the same thing but for it to be formatted correctly it is normally used with

    G53G0 or G53G1F200. G1 you need a feed move a G0 will move at the machine set rapid speed

    When used like this you can place it any where you want

    G0Z2.0.
    G53X--- Y---

    or it can be used

    G53G0Z0.
    G53X----Y----
    So using G53 do you have to use z y and X.
    Why not use z X y without the g53

    Sent from my XT1635-01 using Tapatalk



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    Quote Originally Posted by 109jb View Post
    G28 is not an "incremental" move and can be used in G90 absolute mode or in G91 incremental mode. Yes G91 is used most often, because it is easier, but G28 works in absolute mode as well.

    G28 G91 X0 Y0 Z0 moves from where it is now in a straight line to the predefined position. Nothing incremental about that

    G28 G91 X0 Y0 Z1 moves the Z up one unit and then it goes straight to the predefined position

    I disagree that G53 is used more or is in any way better because to use it you have to set the machine coordinates that a G53 is going to. Use a different machine and the coordinates will likely need to be different Whereas with a G28, the coordinates are set at the machine and the program doesn't need to be modified. A CAM program can put a G28 G91 X0 Y0 Z1 but it can't put in a G53 unless it knows the coordinates it is sending to

    To make that same move in G53 you need

    G91 G0 Z1
    G53 G0 X??? Y???? Z???? where the ??? are the coordinates of the G28 position.


    G30 is just another predefined position and works just like G28.
    So predefined means these are set in the control software correct?
    So these would be machine specific

    Sent from my XT1635-01 using Tapatalk



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    Default Re: CONFUSED... Where to home too... Help! where should my home switches be.

    G53 uses the machine reference coordinate system

    So, G53 G0 X0 Y0 Z0 would move NOT move to the part zero position. Whereas G0 X0 Y0 Z0 would move to the part zero position (Provided the current coordinate system s the correct one.)

    - - - Updated - - -

    Quote Originally Posted by truckeic View Post
    So predefined means these are set in the control software correct?
    So these would be machine specific

    Sent from my XT1635-01 using Tapatalk
    For G28, YOU set them in the machine controller.



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CONFUSED... Where to home too... Help! where should my home switches be.

CONFUSED... Where to home too... Help! where should my home switches be.