Newbie How does threading work on CNC lathe


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Thread: How does threading work on CNC lathe

  1. #1
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    Default How does threading work on CNC lathe

    Hello,

    As you can tell I'm a bit of a newb but I have a question on how threads are turned using a CNC lathe (2-axis only).

    I get that the tool is fed at the right ratio to the spindle speed to give you the right pitch. But if the thread is cut in a number of passes then how does the machine calculate when to begin the second pass to make sure the thread cut starts in the same place again for the second cut?

    On a 2-axis lathe, despite there being no control of the C-axis, does the machine still know the spindle position?

    or does the machine simply calculate the relative position by knowing the spindle speed and time between passes?

    I'm sorry for such a newbie question and I'm sure google could have provided the answer if I knew how to phrase the question.

    Thanks

    SJB

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    Default Re: How does threading work on CNC lathe

    Multiple passes just like threading on a manual lathe.

    The spindle needs at least an index pulse trigger that is fed into the controller. This way the controller knows the spindle position once per rev, which is enough to get decent results. A multi-count encoder with index can also be used depending on the controller.



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    Default Re: How does threading work on CNC lathe

    Typically the spindle has an encoder that reads the spindle position and the Z axis travel is "geared"
    in software by the controller to the spindle rotation, This makes it insensitive to spindle speed variations.

    In addition the spindle encoders typically have a once per turn index signal that is used to synchronize the start
    of threading.

    Some simple low cost controller/lathes use a single once per turn spindle position sensor
    but these dont work as well if there are spindle speed variations as they have to estimate the spindle position
    between sensor readings and the estimate (that drives the z axis position) will be in error if the speed changes



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    Default Re: How does threading work on CNC lathe

    Quote Originally Posted by PCW_MESA View Post
    Typically the spindle has an encoder that reads the spindle position and the Z axis travel is "geared"
    in software by the controller to the spindle rotation, This makes it insensitive to spindle speed variations.

    In addition the spindle encoders typically have a once per turn index signal that is used to synchronize the start
    of threading.

    Some simple low cost controller/lathes use a single once per turn spindle position sensor
    but these dont work as well if there are spindle speed variations as they have to estimate the spindle position
    based on a possibly variable velocity
    Thanks PCW_MESA & 109jb for those replies. That makes sense and much appreciated!

    So, if you want to thread only a portion of a bar, is there enough control that the x-axis can get the tool in at the right starting point and out at the end fast enough?



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    Default Re: How does threading work on CNC lathe

    As stated The above explanations apply to systems with a central controller that is capable of electronic gearing, i.e. one master encoder, and one or more slave servo's.
    On systems such as Mach where there is no central servo controller and every drive operates independently, it is done rather more crudely with a once-per-rev signal on the spindle that the Z axis keys off of or sync's to start the threading motion.
    Al..

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Default Re: How does threading work on CNC lathe

    Here is a video example of LinuxCNC doing multiple pass threading using only a single index pulse per rev. Linuxcnc can also use an multi-count encoder, but it still need to have an index pulse to thread.





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How does threading work on CNC lathe

How does threading work on CNC lathe