Any recomendations for ball screw retrofits for knee mills?
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  1. #1
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    Default Any recomendations for ball screw retrofits for knee mills?

    I am thinking of replacing the Acme screws on my 20 + year old Enco 10" x 54" knee mill with ball leadscrews. Not doing a CNC conversion or anything like that just want to eliminate the backlash so I can climb mill. I've searched the net and sent inquiries to several places but I would appreciate any feedback from folks who may have done this. How hard is it, who did you use, how long did it take? Thanks for any help.

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    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.

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    Yes, I went thru a lot of the threads here on this topic and several discussed how slippery ball screws could be. If there is no /little backlash how could the table get pulled into the cut? I have manual feed for the Y axis and power feed on the X. If I control the feed won't that prevent puling into the cut? I lock the table and saddle when drilling so no problem there. I'm having a little trouble imagining that a ballscrew with no slop could be worse than an Acme with lots of it. But that's why I ask questions and read previous posts.

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    There is a LOT of parasitic drag or friction in an Acme screw and nut. Almost none in a ballscrew. It isn't a matter of slop of course. And where you're going to run into big trouble is doing things like milling into a corner. You can only control one direction at a time with the feed. If the tool grabs it will easily run away with the table if you're taking a heavy cut.

    The ballscrew effectively has very nearly the same friction to move the table as the table itself with no screw. So imagine the Acme screw with a bunch of backlash - the same trouble comes with the unbraked ballscrew - except that the ballscrew will let the table run the whole length of the part, not just the distance of the backlash like the Acme.

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    My Taiwan knee mill has ballscrews. I've never once thought "boy I sure wish I had acme screws".

    Yeah, the table will turn the screws, but the locks work fine.

    I view a knee mill as a pretty basic disposable machine and would never bother to fix an old machine with worn screws, but if I was buying a new knee mill I would spend a few extra bucks and get ballscrews.

    Makes it a lot easier when you use CNC's 99.9% of the time and you forget all the stuff a manual mill can't do.

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    My Enco is Taiwanese iron, it is pretty heavy and isn't appreciably worn so I am not trying to restore an old clapped out machine expecting a miracle. I am not trying to turn it into a CNC frankenmill, I have a TM1-P that works fine for that. I just want a smoother feeding machine that I can climb mill with (the way I mill on the Haas).

    Right now on the manual mill if I feed in one direction I always lock the other axis. If I feed into a corner I lock one axis before unlocking the other to change feed directions. I lock both axes for drilling. The Y axis would always be under my hand control. The X axis would either be under my hand control or would be fed by a power feed that can't be backdriven manually. Would this be enough to keep the table under control? I mean it's not like I will need to learn new habits.

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    You should be fine as long as you aren't going to do any heavy climb milling cuts.

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    I normally use conventional milling for all roughing passes, I save climb milling for finish cuts. Climb milling seems to work a lot better for finishing cuts on so many materials. Lately we've been getting a lot of work for small lots (1-12 pcs) of parts made from A2, O1, or some grade of SS. A lot of it can be partially or totally done on manual equipment and the small lot sizes make it impractical to write much code. SS for one seems to finish better climb milling, I think it is because it allows cutting into the part where it isn't work hardened. Of course not pounding recuts and stuck-on chips into the finished surface is always a benefit of climb milling no matter what the material.

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    I put ball screws on a bridgeport years and soon sold the mill do to them ,,, when ever you lock a table the table moves ,,, with the acme screws I can just put the lock on so it starts to drag some and still turn the screw and it hold position fine ,,, I seen zero benefit to ball screws and a lot of down side to them ,,

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    This is one of those times I wish I knew somebody that had a ball screw equipped mill to try out.

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    my kondia came with ball screws.
    never noticed the table move, no need to lock it,
    virtually no backlash, conventional or climb milling does not make a difference,i love it.
    recently had a look what just a front bearing assembly for the y axis costs, it was very pricey.
    you had better sit down before looking up the price for the entire conversion.

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    The ones I've seen advertised run between $900 - $1800. I'd be willing to invest $1000 - $1200 if I thought it was an improvement. I just got a $1200 check from Uncle Sam.

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    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

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    Thank you for that info. That may be the difference between the $900 units and the $1800 units.

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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....

    Bill
    A good theory, what happens when you override the "spring" on accs and decs or a hard cut?
    Acme screws do not backfeed , ball screws so very happy to do so.
    A cnc hangs onto the handle with a force back and forth as needed, a manual machine does not.
    Ballscrews nice on a manual mill but keeps those locks down snug.
    Most manual machine users not so happy with the changeover.
    Smaller lash so not having to deal with 1/4 turn on direction change but... new and other .
    CNC and manual use is just not the same.

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    one of my mills has a ball screw on y. it never moved. the mill is 4000 lbs and max climb cut i ever took is probably 3 mm.

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    When I converted my Rockwell mill to CNC, I installed Rockford ballscrews and nuts. I was able to specify how much of the ends I wanted annealed so that I could turn them down to suit the bearings and mechanical components. I can't speak for the backfeed issues with ballscrews but installing the ball bearing nuit assemblies can be a finicky process. Probably be a lot easier to insatll new acme screws for a manual mill and would get you 90% of what ballscrews would do for you. And you could invest the money saved in a halfway decent DRO.

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    Not a good idea for a manual machine, fit new acme,s and go on...Phil

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    Quote Originally Posted by theperfessor View Post
    My Enco is Taiwanese iron, it is pretty heavy and isn't appreciably worn so I am not trying to restore an old clapped out machine expecting a miracle. I am not trying to turn it into a CNC frankenmill, I have a TM1-P that works fine for that. I just want a smoother feeding machine that I can climb mill with (the way I mill on the Haas).

    Right now on the manual mill if I feed in one direction I always lock the other axis. If I feed into a corner I lock one axis before unlocking the other to change feed directions. I lock both axes for drilling. The Y axis would always be under my hand control. The X axis would either be under my hand control or would be fed by a power feed that can't be backdriven manually. Would this be enough to keep the table under control? I mean it's not like I will need to learn new habits.
    My 1987 Taiwan Webb 4VH 10 X 50 mill (3,500#) came to me with ball screws and servo motors but no computer. It had been a factory CNC modification that had quit working, The machine was very sound and I refurbished it with manual controls but left the ball screws in place. The X axis has about 7 degree marks of backlash total (3.5 each way) and the Y axis has 0 backlash. The machine had been used so that's where the X backlash developed, I'm sure. On the good side, even after 7 years of constant use and lots of projects, the backlash hasn't changed a bit.

    I'm not the resident expert on ball screws but i suspect that even new ones might have some small amount of backlash. That's just a guess, though. .

    In any case, I've never been sorry that the machine has ball screws. The Webb has dual table locks on the X Axis and I always keep some drag on them or lock them down totally when milling. The milling process has yet to drive the table even when climb milling. I'm quite pleased with them all in all.

    The bigger issue in converting an older machine to ball screws would be the overall cost. I think it would be easy to exceed the value of the machine when doing such a conversion since everything has to be changed.

    I listened to all of the negative talk about ball screws on a manual machine when I first got it and considered converting it to standard acme screws. With all of the parts from Webb, it would have come close to $1,500. which is about what I paid for the machine when I got it! LOL

    Good luck on whatever you decide.

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    Quote Originally Posted by Newman109 View Post
    My 1987 Taiwan Webb 4VH 10 X 50 mill (3,500#) came to me with ball screws and servo motors but no computer. It had been a factory CNC modification that had quit working, The machine was very sound and I refurbished it with manual controls but left the ball screws in place. The X axis has about 7 degree marks of backlash total (3.5 each way) and the Y axis has 0 backlash. The machine had been used so that's where the X backlash developed, I'm sure. On the good side, even after 7 years of constant use and lots of projects, the backlash hasn't changed a bit.

    I'm not the resident expert on ball screws but i suspect that even new ones might have some small amount of backlash. That's just a guess, though. .

    In any case, I've never been sorry that the machine has ball screws. The Webb has dual table locks on the X Axis and I always keep some drag on them or lock them down totally when milling. The milling process has yet to drive the table even when climb milling. I'm quite pleased with them all in all.

    The bigger issue in converting an older machine to ball screws would be the overall cost. I think it would be easy to exceed the value of the machine when doing such a conversion since everything has to be changed.

    I listened to all of the negative talk about ball screws on a manual machine when I first got it and considered converting it to standard acme screws. With all of the parts from Webb, it would have come close to $1,500. which is about what I paid for the machine when I got it! LOL

    Good luck on whatever you decide.
    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.


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