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  1. #1

    Join Date
    Jun 2019
    Posts
    10

    FNG question on parts/retro

    Hello Gents,
    I have a need for CNC control for lathe parts ~ 2" long x 1.75" D. I currently have a southbend heavy 10 and old pulley head bridgeport along with a multitude of tig/mig welders. I can do 90% of what I want to do on the manual but I turn a lot of titanium/17-4 and form tools don't work so hot on either.

    So on to the questions
    I could do MOST everything I wanted to do by CNC the compound and the cross slide. Obviously no threads with just that but I can thread manually. Is this viable for the occasional use

    If I were to build a SMALL CNC instead, I would want to use collets primarily, preferably something with a big enough spindle bore to pass 1.75". I could use 5C Emergencies and cut everything to size first but then would have 2 operations with a part flip instead. I started looking at builds and linear bearings and came across what is essentially pre-built tables with 2 linear rails and a ball screw built in on eBay/my local automation guy. Are these things viable? Can you just mount one on top of the other and go to town? Here is a cheap example of what I'm referring to. I don't think you can buy the piece parts for that?

    https://www.ebay.com/itm/200MM-Strok....c100667.m2042

    Thanks in advance for the help!

    ETA, tolerance required is probably around +/- 0.003. Not super precise like some. Its more I make a lot of similar shapes that have curves making it difficult to turn on a manual (a ball turner wouldn't help)

  2. #2

    Join Date
    Jun 2019
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    10

    Re: FNG question on parts/retro

    Added one more concept...2 axis table that can just sit on my ways (or my mill for that matter)

  3. #3
    Registered
    Join Date
    Jun 2011
    Posts
    677

    Re: FNG question on parts/retro

    Quite a few have done CNC lathe conversions. Generally the lead screws are replaced with ball screws, so most can't be used manually anymore. Biggest issue with ball screw replacement is that the ball nuts are usually quite a bit larger than acme nuts, so are hard to fit. Most avoid just throwing steppers on the existing screws because the backlash in most of them is too much for CNC.
    I don't think you'd be happy with the surface finish you'd get if you added a CNC stage like that ebay one to your lathe or mill. Nowhere near the rigidity you're used to.

  4. #4

    Join Date
    Jun 2019
    Posts
    10

    Re: FNG question on parts/retro

    Quote Originally Posted by skrubol View Post
    Quite a few have done CNC lathe conversions. Generally the lead screws are replaced with ball screws, so most can't be used manually anymore. Biggest issue with ball screw replacement is that the ball nuts are usually quite a bit larger than acme nuts, so are hard to fit. Most avoid just throwing steppers on the existing screws because the backlash in most of them is too much for CNC.
    I don't think you'd be happy with the surface finish you'd get if you added a CNC stage like that ebay one to your lathe or mill. Nowhere near the rigidity you're used to.
    Well that is disheartening. Big machines = rigidity but I lack the space for a craigslist morei seki (sp) even though there is one less than a mile from my house for 4k. I would hate to convert my southbend entirely since a lot of my work is down and dirty 20 min job that would take longer to program than machine. The nature of the product doesn't let me outsource it unfortunately. Maybe I will have to look into making some sort of tracer instead. I have a taper attachment on my lathe so maybe there are some options there.

  5. #5

    Re: FNG question on parts/retro

    It is POSSIBLE to CNC a manual lathe without losing any manual capability. You use a ball screw on the cross slide but use the existing carriage drive (not the lead screw) to move the carriage. You control the backlash with an air spring, control the accuracy with linear scales on the axes. CNC threading is no problem, just requires an encoder on the spindle. I designed such a system for my 13x40 but never followed through on it because I bought a CNC lathe.
    Jim Dawson
    Sandy, Oregon, USA

  6. #6

    Join Date
    Jun 2019
    Posts
    10

    Re: FNG question on parts/retro

    I think the best solution is going to be just build an appropriately sized CNC lathe. I have all the tools at my disposal to do so. Just a matter of sitting down to design it then build. With as small of a work envelope as I really need I don't think it will get too exorbitant in costs and will be exactly what I want it to be. The trick will be keeping myself limited from trying to go all "live tooling", "tooling turret", micron accuracy on it and realizing what I need it to do is very simple.

    16C collet
    80mm x 140mm Spindle Bearings?
    Servo drive spindle I think?
    Ti and my diameter will require quite low RPM - only 400-800 RPM. 17-4 will be under 3000 RPM still.
    I think ~10" for Z travel and ~10" for X should suffice? I will need, exterior profiling, interior profiling, drilling and cutoff. May be a bit tight for gang tooling?
    ~2-3" max stickout
    1.75" Diam max for bar, maybe more if I use some emergency collets but unlikely
    2 AC bearings on the nose and one radial on the aft end of the spindle.
    I think the HSM 1605 standard ball screws will be sufficient especially if I run proper bearings at the fixed end?
    Steppers should be fine for the ball screws. This isn't going to be a production machine. Direct drive would be nice but not necessary.


    I will have to look into the software/hardware side.

    I have the mill and lathe at home already. We have surface grinders at work. This seems like a bit of a difficult project but not over the top.

  7. #7

    Re: FNG question on parts/retro

    Quote Originally Posted by propeine View Post
    I think the best solution is going to be just build an appropriately sized CNC lathe. I have all the tools at my disposal to do so. Just a matter of sitting down to design it then build. With as small of a work envelope as I really need I don't think it will get too exorbitant in costs and will be exactly what I want it to be.
    Sounds like a good plan. I would buy the spindle assembly/housing, Ebay has a few of them last time I looked. Maybe buying that CNC lathe that you referenced above would be a good starting point, at least it would have a lot of the hardware that you will need. For $4K it might be worth a look, or you might look around for a less expensive one. If I wanted to build a lathe, that's what I would do.

    The trick will be keeping myself limited from trying to go all "live tooling", "tooling turret", micron accuracy on it and realizing what I need it to do is very simple.
    ''Mission creep'' will get you every time
    Jim Dawson
    Sandy, Oregon, USA

  8. #8

    Join Date
    Jun 2019
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    10

    Re: FNG question on parts/retro

    Quote Originally Posted by Jim Dawson View Post
    Sounds like a good plan. I would buy the spindle assembly/housing, Ebay has a few of them last time I looked. Maybe buying that CNC lathe that you referenced above would be a good starting point, at least it would have a lot of the hardware that you will need. For $4K it might be worth a look, or you might look around for a less expensive one. If I wanted to build a lathe, that's what I would do.



    ''Mission creep'' will get you every time
    Well now thats a hell of an idea! Finding the right one may take a bit but with the cost of bearings anyways would probably be a net savings.

  9. #9

    Re: FNG question on parts/retro

    Quote Originally Posted by propeine View Post
    Well now thats a hell of an idea! Finding the right one may take a bit but with the cost of bearings anyways would probably be a net savings.
    Where are you located? I know of a least one locally that you could buy by the pound. There may be another also.
    Jim Dawson
    Sandy, Oregon, USA

  10. #10

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    Jun 2019
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    10

    Re: FNG question on parts/retro

    Quote Originally Posted by Jim Dawson View Post
    Where are you located? I know of a least one locally that you could buy by the pound. There may be another also.
    NW Pennsylvania

  11. #11

    Re: FNG question on parts/retro

    A bit far away to get a machine out here.
    Jim Dawson
    Sandy, Oregon, USA

  12. #12

    Join Date
    Jun 2019
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    10

    Re: FNG question on parts/retro

    Quote Originally Posted by Jim Dawson View Post
    A bit far away to get a machine out here.
    Haha ya just a touch. I'll keep my eyes open here. How about the rest of the list? Does it seem reasonable for what I'm trying to achieve?

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