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  1. #1

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    knee mill to cnc converstion

    Hi to all members i don't know if im posting in the correct forum please let me no if i have.
    first of all i have done some research on converting my freejoth knee mill to cnc and i now have some questions i would like to ask if possible

    which drive method would be best for the z axis ,moving knee up and down or driving the quill.

    are rolled ball screws of a quality that can be used or do i need to use expensive ground ball screws.

    is it possible to change the vary speed drives for a fixed drive so i can install a vfd or is there a way of doing this without changing discs.

    it appears that my machine would need 25mm ball screws but what pitch would be best for this as i am thinking a/c servo motors.

    i hope these questions don't seem stupid so someone can put me in the right direction please

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Re: knee mill to cnc converstion

    Quote Originally Posted by mechhamo View Post
    Hi to all members i don't know if im posting in the correct forum please let me no if i have.
    first of all i have done some research on converting my freejoth knee mill to cnc and i now have some questions i would like to ask if possible
    This section is as good as any.

    which drive method would be best for the z axis ,moving knee up and down or driving the quill.
    Driving the quill is the ''standard'' method, but moving the knee has successfully been done. For the quill drive, it can be a ball screw or a gear train. I designed a gear drive for mine, makes it easy to switch between manual and CNC quill, just flip a lever to switch between the two.

    are rolled ball screws of a quality that can be used or do i need to use expensive ground ball screws.
    Ground is better, but rolled will work. My machine has ground screws.

    is it possible to change the vary speed drives for a fixed drive so i can install a vfd or is there a way of doing this without changing discs.
    You can use the VFD with the vari-drive, I ran mine that way for years before I changed over to direct drive.

    it appears that my machine would need 25mm ball screws but what pitch would be best for this as i am thinking a/c servo motors.
    My machine has 2505 ball screws. (or 25 x 0.200'', not sure which)


    i hope these questions don't seem stupid so someone can put me in the right direction please
    Your questions are not stupid. That's how we learn.

    I would design the system so that you can keep full manual capabilities. I use mine as a manual machine as much as I use it as a CNC. It is manual, 2, 3, or 4 axis CNC, with switch over in seconds.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
    Community Moderator Al_The_Man's Avatar
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    Re: knee mill to cnc converstion

    The two XLO's I retro-fitted, I fitted VFD's and removed the vari-speed, but they had two speed G.B.s also.
    If you get rolled B.S. remember to get pre-loaded ball nut, The common rolled variety do not have it and can posses a significant back-lash.
    I did get some HiWin (Taiwan) ground B.S. very hard, a ceramic tool had to be used to turn the end down.
    There was a guy on ebay used to bring them in by the case load.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

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

  4. #4

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    Re: knee mill to cnc converstion

    Quote Originally Posted by Jim Dawson View Post
    This section is as good as any.

    Thanks very much for the responses very appreciated

    Driving the quill is the ''standard'' method, but moving the knee has successfully been done. For the quill drive, it can be a ball screw or a gear train. I designed a gear drive for mine, makes it easy to switch between manual and CNC quill, just flip a lever to switch between the two.

    the other problem i thought i might have with moving the knee is stopping the knee from winding down under its own weight is this a problem or is there a way to apply a brake to stop it or do the servo motors come with a brake in them that would stop this happening ,i thought the easier way was to drive the knee but i can now see there are other issues with that method , so from what you guys have said i am better off driving the quill is this your view.



    Ground is better, but rolled will work. My machine has ground screws.
    if i was to use rolled screws which would be the lowest grade you would go to



    You can use the VFD with the vari-drive, I ran mine that way for years before I changed over to direct drive.

    Thats great news i just thought the software would need to know what the spindle speed is.
    so is there a sensor that can monitor the spindle speed and input that to the vfd or the software to control correct speed.

    also i would like to know if the original lead screw yoke be used with ball screws or do i need a new yoke.

    regards wayne.



    My machine has 2505 ball screws. (or 25 x 0.200'', not sure which)




    Your questions are not stupid. That's how we learn.

    I would design the system so that you can keep full manual capabilities. I use mine as a manual machine as much as I use it as a CNC. It is manual, 2, 3, or 4 axis CNC, with switch over in seconds.
    s

  5. #5

    Re: knee mill to cnc converstion

    You may hear different experiences and suggestions and none are right or wrong. Neither of my two knee mills have handles. The Bridgeport conversion originally did but I removed them to improve the response. The Shizuoka never had them to begin with. If you have a decent MPG (wireless is good!), the MPG handle provides a very similar manual input that you can operate wherever suits you to use it. I often use my Shizuoka in manual mode and it's easy enough to operate, possibly more so than with a mechanical connection. The DRO display is very useful in manual mode for squaring off stock, drilling holes etc

    Both my machines have quills driven by ballscrews. This reduces the backlash over what you will get if you use the existing rack drive which has no means of backlash control. You can preload the backlash with a spring but that assumes your axial forces will not overcome the preload. This may be valid but is a compromise to be aware of. Having said that, a ballscrew drive has its challenges but my final solution was actually fairly neat, robust and compact.

    The main drawback of a quill drive in my experience is the limited movement. On a Bridgeport clone head, the max movement is 5" or so - but you will lose some of that at each end of travel due to the ballnut yoke and also room for the limit switches to function. You also have to allow for the different tool lengths on a given job as well as the vertical movement you need to do the job itself. All in all, it can be challenging to achieve all of those requirements in one setup. Moving the knee / saddle instead would provide much more flexibility but the combined weight of the knee, saddle, table, vise and workpiece is significant so you would need a counterweight or gas spring to enable you to use a sensible sized servo motor and ballscrew. There are some good examples out there.

  6. #6
    Community Moderator Jim Dawson's Avatar
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    Re: knee mill to cnc converstion

    Quote Originally Posted by mechhamo View Post

    Thanks very much for the responses very appreciated
    You're welcome

    the other problem i thought i might have with moving the knee is stopping the knee from winding down under its own weight is this a problem or is there a way to apply a brake to stop it or do the servo motors come with a brake in them that would stop this happening ,i thought the easier way was to drive the knee but i can now see there are other issues with that method , so from what you guys have said i am better off driving the quill is this your view.
    Servo motors are available with brakes, but not sure if that would be needed. There is a lot of resistance to movement in the knee. As long as the servo motor is powered on this would not be a problem, only when the machine is shut down. Some have counterbalanced the knee with gas springs. I think Bridgeport built some CNC machines like that with an air or hydraulic cylinder counterbalance. You could also actuate the knee locks with an air cylinder. There are also some other ways to do this.

    The quill is limited to 5 inches of travel, but I have not found this to be an issue. As Muzzer says, adding the ball nut/screw assembly to the quill will limit the travel somewhat. That's one of the reasons I went with a gear drive system that uses the original rack & pinion drive. But this does require an anti-backlash system, I used an air cylinder. I have never seen another quill drive like mine, but it works very well, and can be switched between manual and CNC operation by flipping a lever.

    if i was to use rolled screws which would be the lowest grade you would go to
    I would buy the best ones I could find, Hiwin would be my first choice. I would go with double nuts also. You really don't want any backlash in the system. My ballscrews are going out for rebuild pretty quick, and I am going to have them install double nuts.

    great news i just thought the software would need to know what the spindle speed is.
    so is there a sensor that can monitor the spindle speed and input that to the vfd or the software to control correct speed.
    Monitoring the spindle speed on a 2J2 style head is not easy but possible. The only really accessible thing in the head that always turns at spindle speed is the power down feed drive gear. So if you're clever, that is a possibility, you won't be using that with a quill drive anyway.

    You only need to know the spindle speed/position for rigid tapping, other than that you can just manually adjust the speed, and also turn the spindle on/off manually. It just depends on whether you want to fully control the spindle from software. I turn mine on/off (when in CNC mode) from software, but manually adjust the speed. Most modern VFDs have very precise speed control, they drive the motor at the set speed within about 1% or better.


    also i would like to know if the original lead screw yoke be used with ball screws or do i need a new yoke.


    regards wayne.
    I think you would need a new yoke, pretty sure the original bore is too small for a ballnut, and there is not enough meat left to bore them out. You'll need to do some measuring to confirm that. Pretty sure you can buy the full kit for your machine, includes the ball screws and yoke.
    Jim Dawson
    Sandy, Oregon, USA

  7. #7
    Community Moderator Al_The_Man's Avatar
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    Re: knee mill to cnc converstion

    My XLO's had ball screw jacks with brake, to raise/lower the Knee.
    Driven by small motor.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

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

  8. #8

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    Re: knee mill to cnc converstion

    thanks so much for the very helpful information it is so appreciated.

    well guess what i decided to jump right in and at least get the machine converted to ball screws ,i have removed bed screws and yoke ready to measure up , after getting apart i now realise the shaft size is 32mm so im guessing the ball screws are going to be dearer, but i do have another question

    what would be a good price to pay for a 1500x32 ball screw and double nut and a 700x32 ball screw and nut i live in australia so i don't know what i should be paying.

    i will upload some pictures of machine ,shats and yoke for your comments

    kind regards wayneClick image for larger version. 

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  9. #9
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    Re: knee mill to cnc converstion

    The 2 screws will cost about 3-400$.
    The cheap mounts are crap, you need to swap the bearings for AC bearings.

    Plan to make yokes for everything, and upsize yokes by 2x in every dimension for rigidity.

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