Any recomendations for ball screw retrofits for knee mills? - Page 2
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  1. #21
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    Yes exactly Tyrone. That's what I was talking about in my very first response in this thread... Don't drag the table locks! And fully clamped!? Good god. Poor machine. Ways must look like a banana.

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    Indeed. As someone else said - fitting a 3 axis DRO would give your much more bang for your buck than swapping over the Leadscrews.

    Regards Tyrone.

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    Quote Originally Posted by Tyrone Shoelaces View Post
    Indeed. As someone else said - fitting a 3 axis DRO would give your much more bang for your buck than swapping over the Leadscrews.

    Regards Tyrone.
    For sure.

    But if you have a full turn of backlash like my BP you need both.

  4. #24
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    Quote Originally Posted by Tyrone Shoelaces View Post
    I'm not sure power feeding any axis with the manual axis locks on would be recommended by the manufacturer. Repairing the " fire-ups " caused by operators forgetting the manual locks were on and power feeding the table or saddle provided me with quite a lot of work back in the day.
    Cast iron to cast iron ways " fire-up " very easily without regular lubrication.

    Regards Tyrone.

    It could happen, but I've not had the problem. I wouldn't use the locks if I were power feeding so it's just a matter of being careful. It's a matter of situational awareness.

    OTOH, the 10X50 table is very heavy so that helps.

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    Quote Originally Posted by eKretz View Post
    Yes exactly Tyrone. That's what I was talking about in my very first response in this thread... Don't drag the table locks! And fully clamped!? Good god. Poor machine. Ways must look like a banana.
    As I mentioned to Tyrone, I don't use the locks when moving the table. Banana indeed.

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    Quote Originally Posted by ChipSplitter View Post
    For sure.

    But if you have a full turn of backlash like my BP you need both.
    I once had an operator of a " Richards " PRT 5 come to see me. That's a 5" spindle Hor bore probably weighing about 35 tons all up, saddle, table and top table maybe 7 tons plus.

    He was complaining about the saddle/table long feed fluctuating. When I got it all apart the long travel brass nut, about 4" bore and 15" long, had no thread in it at all. When I say no thread, I mean no thread. It was bolted and dowelled to the underside of the saddle with two wings that were part of the nut. Both dowels had fallen out plus most of the bolts over the years and been swept up in the swarf. The remaining couple of bolts were allowing the nut to cant from side to side slightly and the contact of the leadscrew on the corners of the nut was all that was enabling the saddle to move under power.

    If I hadn't have seen it with my own eyes I wouldn't have believed it.

    Regards Tyrone.

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    Quote Originally Posted by Tyrone Shoelaces View Post
    I once had an operator of a " Richards " PRT 5 come to see me. That's a 5" spindle Hor bore probably weighing about 35 tons all up, saddle, table and top table maybe 7 tons plus.

    He was complaining about the saddle/table long feed fluctuating. When I got it all apart the long travel brass nut, about 4" bore and 15" long, had no thread in it at all. When I say no thread, I mean no thread. It was bolted and dowelled to the underside of the saddle with two wings that were part of the nut. Both dowels had fallen out plus most of the bolts over the years and been swept up in the swarf. The remaining couple of bolts were allowing the nut to cant from side to side slightly and the contact of the leadscrew on the corners of the nut was all that was enabling the saddle to move under power.

    If I hadn't have seen it with my own eyes I wouldn't have believed it.

    Regards Tyrone.
    I'm having a hard time of it myself.......

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    Quote Originally Posted by eKretz View Post
    I only ever ran one machine with a ballscrew retrofit and it wasn't a turret mill. However, my number one recommendation would be to make sure you add a braking device to the screw, ​rather than create drag with the table lock screw like everyone seems to do.
    Good advice but I always run a very light pressure on the lock even if a Acme screw.
    Exception being one way loaded as in roughing with a face mill with power feed. Here I cut in on direction only, up, rapid back, down and again.
    Everything has backlash somewhere if you are picky.
    Screws are big stiff springs that wind up under load. Linear scales no matter if in sub-micron are mounted on the table outside not the cut on the work-piece.

    Advice good and maybe a simple spring loaded brass pin set screw but that makes the handle hard to turn when you want to twist it fast so trade-offs.
    The lock you can pop off and just touch very light or more as needed. It is basically a gib adjustment.

    I don't think anyone back drives Acme screws, the problem being the lash and jumping that when climb milling.
    Oh poop, big chip. Part out of vise, tool broken.

    I do love precision ballscrews as the best thing since sliced bread and will drop $4000 on a 8 inch travel one in a heartbeat where needed but not so much on a manual B-port type mill.
    Bob

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    I was reading this mostly for my own education as I do not have anything with ball screws. But a thought came to me and I am going to throw it out, for better or worse.

    Acme has backlash. Ball screws do not but they can be back feed by the table and cutting forces. CNC machines have stepper or servo motors that prevent any stray motions with the ball screws.

    What about a compromise? Add the ball screws but also add stepper or servo motors. And have manual, electronic controls for the normal control of the ball screws. Put a little drag on those electronic controls so they can't be accidentally bumped. The control knobs could be located in a handy location at the front.

    And this would allow easy conversion to CNC in the future.

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    I hate to over complicate things but I'm wondering if there might be some type of damping that could be applied to prevent axes movement. Pneumatic or hydraulic bleed off systems come to mind, and magnetic damping is also a possibility. And there is also friction, but instead of dragging the axes locks I wonder if an accessory brake strip of some sort might be used, especially on the long X axis.

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    Well if you can have a table lock there's no reason you can't also have a screw lock. Just use friction like Bob said, and leave a handle just like a table lock. You can set it lightly for friction or lock it down in addition to the table lock when you need holding power while moving another axis.

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    I wonder if a hydraulic cylinder on the table (or hydraulic motor on the screw?) with a simple valve and linkage could be used to limit the speed and prevent pulling the table during the feed stroke but allow for bypassing the bleed off valve for rapid traverse or positioning for drilling.

    It might be possible to test such a system with the current Acme screws, if it would prevent pulling into the cut so I could climb mill I might not (feel the) need to change over to ballscrews.

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    Quote Originally Posted by theperfessor View Post
    I hate to over complicate things but I'm wondering if there might be some type of damping that could be applied to prevent axes movement. Pneumatic or hydraulic bleed off systems come to mind, and magnetic damping is also a possibility. And there is also friction, but instead of dragging the axes locks I wonder if an accessory brake strip of some sort might be used, especially on the long X axis.
    You are long into 'lipstick on a pig' territory.

    Sent from my SM-G930R4 using Tapatalk

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  16. #34
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    I like that phrase! Well said. Sometimes I just like to "blue sky" and think about things pracitical and impractical. Even if 90% of my ideas aren't workable the other 10% make up for it. And often impractical ideas eventually lead to good ideas.

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    Quote Originally Posted by theperfessor View Post
    I wonder if a hydraulic cylinder on the table (or hydraulic motor on the screw?) with a simple valve and linkage could be used to limit the speed and prevent pulling the table during the feed stroke but allow for bypassing the bleed off valve for rapid traverse or positioning for drilling.

    It might be possible to test such a system with the current Acme screws, if it would prevent pulling into the cut so I could climb mill I might not (feel the) need to change over to ballscrews.
    Back in the day one or two makers went down the road of hydraulic drives to power machines. I once worked on a " Marwin " ( they got bought out by K&T to become " Kearney-Trecker-Marwin over here ". The owner also had some weird ideas, this machine was an early CNC horizontal machining centre. Everything on it was hydraulic, spindle drive included.
    It was really powerful. I remember watching it drill a series of 1" holes in the side of an overhead crane chassis. On one of the holes the drill stopped but failed to retract fully and left just the tip of the drill in the hole. The " X " axis set off and the tip of the drill planed a very nice groove all the way down the chassis to the next hole !

    Unfortunately it leaked hydraulic oil like a sieve.

    Regards Tyrone.

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    Quote Originally Posted by 9100 View Post
    Proper ball screws have two nuts with a spring between them to take up almost all slop. My 1977 model Boston Digital CNC mill has about .0005-6" backlash due to large screws with spring loaded nuts. A friend spent a ridiculous effort and money to convert a good BP to ball screws and add CNC. He has .002" backlash. He asked the manufacturer of the screws why. Turns out they are not spring loaded but they just put in various sized balls to make it reasonably tight. He wasted his money on something that sort of works, but not really. Be very careful about what you buy.

    Bill
    Maybe ok ballscrews have dual nuts with a spring. Good ballscrews have dual nuts fastened hard together. This way the nut is a hard coupling in either direction.

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    like bolting a shock absorber to the table?

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    Quote Originally Posted by Tyrone Shoelaces View Post
    Back in the day one or two makers went down the road of hydraulic drives to power machines. I once worked on a " Marwin " ( they got bought out by K&T to become " Kearney-Trecker-Marwin over here ".
    I remember one of those, but never had anything directly to do with it. I remember it because it was by far the loudest machine of its type I have ever experienced. Not because it was cutting but the noise from the hydraulics.

  21. #39
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    Quote Originally Posted by billmac View Post
    I remember one of those, but never had anything directly to do with it. I remember it because it was by far the loudest machine of its type I have ever experienced. Not because it was cutting but the noise from the hydraulics.
    Yes, they were pretty noisy and they drank hydraulic oil.

    Regards Tyrone.

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    One shop I worked in had a pair of Cincinnati Hydrotel vertical mills. Table control was hydraulic. When it was working it was great, but there were some problems in that particular iteration. For one, if any air got into the system, the table would jerk around like crazy. In addition, if anyone moved the handles while the machine was off, when it was turned on it would move! There were plenty of crashed cutters and broken carbide tips due to that particular quirk. And yep, they were loud. And leaked a lot.


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