. .
  1. #1
    Dear Members: I am new here, so if this is not the correct place or manner to pose this question, please forgive me and show me the right way
    to proceed...

    I am contemplating, for many years, converting my RF31 clone to CNC... yes, I know: round column - ugh... but this is what I have and
    she-who-must-be-obeyed will not let me buy a better mill...

    Initially, I intend to convert only the Z axis, since this is the one that gives me the most trouble, with the awful backlash in
    the rack-and-pinion mechanism.

    But, all the conversions I have seen lately simply adapt a stepper to this inherently flawed implementation of the Z movement.

    Finally this is my question: has anybody seem a Z-axis conversion on one of these machines that replaces the rack-and-pinion with
    a ballscrew/nut? I believe it would have to be a rotating nut design, and I remember seeing, many years ago, a beautiful conversion
    in which the guy placed a rotating nut inside the head of the machine, but I could not find the post nowadays...

    Has anybody seems this post or any other, or has a suggestion about converting the Z drive on a RF31 to ballscrew/nut?

    Thank you so much for your attention.

    Arquibaldo

  2. #2
    Dear Members:

    Unfortunately, nobody could help me with a RF-31 Mill-Drill Z axis conversion from
    the factory rack&pinion to a rotating ballnut, so I decided to apply my (extremely)
    limited mechanical design habilities to this problem!

    I will post here my progress(?), hoping that someone will take the time to evaluate
    what I propose and offer me improvement tips (or, more likely, tell me this will
    never work...)

    My requirements are:

    1. Extremely low cost.

    2. Minimal modifications to the mill (hopefully only a few threaded holes
    on the head casting) so that, if this project fails, I will still have my old
    mill (flawed, but better than nothing).

    3. If at all possible, retain the ability to drive the quill manually, using
    the factory handles, with only a simple setup to choose between manual and
    stepper driven quill operation.

    4. Under stepper power, capable of precisely moving the quill for light-duty
    hobby use only, mainly machining aluminum or mild steel.

    5. The stepper will initially be manually operated, with a precision multi-turn
    potentiometer, or I may even attach encoders to the current handles
    for the quill operation - computer and digital electronics are (were?) my area.

    I started by reviewing everything I could find online about rotating ballnuts.
    I found many interesting designs, particularly on the thread
    "Rotating Ballnut - design ideas" by Jonathan, on this site,
    but most were sophisticated, expensive mechanisms aiming at high speeds on very
    long ballscrews. This is the oposite of my needs: very low speed with a very
    modest travel: about 150mm only.

    I found only one design that I thought I could adapt to my needs:

    https://www.youtube.com/watch?v=iuAQnf6vSAA

    So, I started modelling my rotating ballnut!

    I attach pictures of my initial attempt.

    It uses a single 5208 double angular contact ball bearing (downsized from
    the original 5212). These bearings are used in the automotive industry
    and are very cheap (if chinese...): about 10 US dollars...

    I also opted for a SFU1605 ballscrew&nut (which I already have) and a
    timing belt & pulleys system with a HTD5M 15mm wide profile: 50/20 teeth
    pulleysm and a 360mm belt, yielding a center distance of 89.4mm.
    This kit can be bought for about 15 US dollars, plus shipping.

    So, here are my initial worries:

    - Is a 16mm ballcrew rigid enough in this application? I hope it won't
    flex much over a length of only 150mm... I would prefer a 20 or even
    25mm ballscrew, but these are more expensive and take more space,
    requiring a larger bearing and the whole mechanism might not fit
    in the limited space available.
    Actually, a 20mm screw could fit in this design with minimal changes,
    so do you think this upgrade ie worth the cost: 32 dollars (plus
    shipping) for a SFU2005 ballnut&300mm ballscrew or 55 dollars
    (plus shipping) for a DFU2005 double ballnut&300mm ballscrew?

    - Is a HTD5M 15mm belt adequate here? This choice is not critical:
    if this belt proves inadequate, it can be upgraded with minimal
    impact on the rest of the system.

    Note that this transmission has a 1-to-2.5 reduction ratio and with
    a 5mm pitch screw we will have very fine increments per step:
    0.005mm per pulse at 400 pulses per revolution, and a tremendous
    power gain, so that I think a NEMA23 stepper will be more than
    adequate.

    Also, this mill has a spring return mechanism which can be adjusted
    to just balance the weight of the quill.

    Then, there is the problem of where to place this mechanism on
    the mill and how to atach it to the quill.

    Originally, I intended to mount the system inside the head casting,
    but it proved very difficult to mount: there simply is not enough
    space.

    I think I found a suitable spot on the left side of the head - there
    are some obstacles, but they can be overcome.

    Attaching the system to the quill is the remaining, difficult problem.
    The ideal solution would be to have two screws, one on each side of
    the head, working synchronously thus balancing the forces on the quill.
    But this would be very difficult to implement and very expensive.

    So, I am hoping that my unbalanced solution works:

    There will be a very strong (25mm thick) plate attaching
    the quill to the ballscrew. Note that, in this machine, the quill is
    very beefy (75mm in diameter) and I think it will support the forces
    involved.

    Note also that this is a preliminary design and many details and refinments
    are missing, in particular the means to attach the mechanism to the
    left side of the mill head.

    Before cutting metal, I intend to build a mockup with a cheap, easy
    to machine material (MDF or PEAD) to test the concept. Also, I must
    decide which parts will be aluminum and which must be steel... what
    do you think?

    Here I stand... I hope at least one member has the patience to read all
    this and offer some criticism and suggestions.

    Regardless, I will keep posting my progress, if any.

    Arquibaldo

  3. #3
    Quote Originally Posted by Arquibaldo View Post
    Dear Members:

    Unfortunately, nobody could help me with a RF-31 Mill-Drill Z axis conversion from
    the factory rack&pinion to a rotating ballnut, so I decided to apply my (extremely)
    limited mechanical design habilities to this problem!

    I will post here my progress(?), hoping that someone will take the time to evaluate
    what I propose and offer me improvement tips (or, more likely, tell me this will
    never work...)

    My requirements are:

    1. Extremely low cost.

    2. Minimal modifications to the mill (hopefully only a few threaded holes
    on the head casting) so that, if this project fails, I will still have my old
    mill (flawed, but better than nothing).

    3. If at all possible, retain the ability to drive the quill manually, using
    the factory handles, with only a simple setup to choose between manual and
    stepper driven quill operation.

    4. Under stepper power, capable of precisely moving the quill for light-duty
    hobby use only, mainly machining aluminum or mild steel.

    5. The stepper will initially be manually operated, with a precision multi-turn
    potentiometer, or I may even attach encoders to the current handles
    for the quill operation - computer and digital electronics are (were?) my area.

    I started by reviewing everything I could find online about rotating ballnuts.
    I found many interesting designs, particularly on the thread
    "Rotating Ballnut - design ideas" by Jonathan, on this site,
    but most were sophisticated, expensive mechanisms aiming at high speeds on very
    long ballscrews. This is the oposite of my needs: very low speed with a very
    modest travel: about 150mm only.

    I found only one design that I thought I could adapt to my needs:

    https://www.youtube.com/watch?v=iuAQnf6vSAA

    So, I started modelling my rotating ballnut!

    I attach pictures of my initial attempt.

    Actually I couldn't find out how to attach a PNG file to a message.
    Will someone help me? This post is meaningless without the
    figures!!!

    It uses a single 5208 double angular contact ball bearing (downsized from
    the original 5212). These bearings are used in the automotive industry
    and are very cheap (if chinese...): about 10 US dollars...

    I also opted for a SFU1605 ballscrew&nut (which I already have) and a
    timing belt & pulleys system with a HTD5M 15mm wide profile: 50/20 teeth
    pulleysm and a 360mm belt, yielding a center distance of 89.4mm.
    This kit can be bought for about 15 US dollars, plus shipping.

    So, here are my initial worries:

    - Is a 16mm ballcrew rigid enough in this application? I hope it won't
    flex much over a length of only 150mm... I would prefer a 20 or even
    25mm ballscrew, but these are more expensive and take more space,
    requiring a larger bearing and the whole mechanism might not fit
    in the limited space available.
    Actually, a 20mm screw could fit in this design with minimal changes,
    so do you think this upgrade ie worth the cost: 32 dollars (plus
    shipping) for a SFU2005 ballnut&300mm ballscrew or 55 dollars
    (plus shipping) for a DFU2005 double ballnut&300mm ballscrew?

    - Is a HTD5M 15mm belt adequate here? This choice is not critical:
    if this belt proves inadequate, it can be upgraded with minimal
    impact on the rest of the system.

    Note that this transmission has a 1-to-2.5 reduction ratio and with
    a 5mm pitch screw we will have very fine increments per step:
    0.005mm per pulse at 400 pulses per revolution, and a tremendous
    power gain, so that I think a NEMA23 stepper will be more than
    adequate.

    Also, this mill has a spring return mechanism which can be adjusted
    to just balance the weight of the quill.

    Then, there is the problem of where to place this mechanism on
    the mill and how to atach it to the quill.

    Originally, I intended to mount the system inside the head casting,
    but it proved very difficult to mount: there simply is not enough
    space.

    I think I found a suitable spot on the left side of the head - there
    are some obstacles, but they can be overcome.

    Attaching the system to the quill is the remaining, difficult problem.
    The ideal solution would be to have two screws, one on each side of
    the head, working synchronously thus balancing the forces on the quill.
    But this would be very difficult to implement and very expensive.

    So, I am hoping that my unbalanced solution works:

    There will be a very strong (25mm thick) plate attaching
    the quill to the ballscrew. Note that, in this machine, the quill is
    very beefy (75mm in diameter) and I think it will support the forces
    involved.

    Note also that this is a preliminary design and many details and refinments
    are missing, in particular the means to attach the mechanism to the
    left side of the mill head.

    Before cutting metal, I intend to build a mockup with a cheap, easy
    to machine material (MDF or PEAD) to test the concept. Also, I must
    decide which parts will be aluminum and which must be steel... what
    do you think?

    Here I stand... I hope at least one member has the patience to read all
    this and offer some criticism and suggestions.

    Regardless, I will keep posting my progress, if any.

    Arquibaldo
    Last edited by Arquibaldo; 26-07-2022 at 08:35 PM. Reason: add a question about attaching files

  4. #4
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  5. #5
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  6. #6
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  7. #7
    phill05's Avatar
    Lives in Derbyshire  UK, United Kingdom. Last Activity: 13 Hours Ago Has been a member for 9-10 years. Has a total post count of 284. Received thanks 26 times, giving thanks to others 12 times.
    Hi there, I made a very similar rotating ball nut to get a longer travel on my Z axis on a router I needed to machine 350 mm High on a 4th axis machine that was meant to cut up to 120 mm high, I used a 20 x 5 screw with a 3 to 1 reduction and it works great,

    Click image for larger version. 

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ID:	31152

    Phill

  8. #8
    Dear Phill05:

    Congratulations - Beautiful design and implementation - You are an accomplished machinist! Arquibaldo

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