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  1. #1
    Join Date
    Mar 2012
    Posts
    378

    Z axis to ball screw or not to ball screw

    I was wondering what others thought about wheather or not to convert to a ballscrew on the knee of a Bridgeport. As far as I know, and I am very likely missing something, the big advantage to the ball screw is that you can get zero backlash and you can lock the axis with just locking the motor. Neither one of these advantages are really show stoppers on the knee. I assume that the reduced friction will allow the use of a smaller motor, but not by that much unless you also do a gas spring or counterweight. If you use a good sized spring or counterweight then the zero backlash becomes more important, but still not sure it is necessary. Easiest is just to use the acme screw and bigger motor.

    Thoughts, opinions?

  2. #2
    Join Date
    Jan 2006
    Posts
    2985
    The knee is zero backlash unless you are pulling up with more force than the weight of the knee, saddle, table and vice combined. Doubtful. The Z axis is effectively locked for this same reason. Ballscrews are back-driveable, that means that if you push on the nut, the screw will spin. This happens because they have low friction. If a ballscrew was used, the weight of the knee would let it fall freely (or in a controlled manner if you have a gas spring in there). You would need a brake on the motor and additional logic to insure that the brake was always on when the motor wasn't so that the table didn't fall.

    Reason enough to stick with the acme screw??

    These issues are why lots of guys end up driving the quill.

    Matt

  3. #3
    Join Date
    Mar 2012
    Posts
    378
    I was thinking of just using a hundred or two pounds of gas springs and keeping the acme screw. I guess you could do a motor driven lock on the Z axis pretty easy if you wanted to or just manually lock the knee if you are doing some really heavy cutting that is generating a lot of up force. I can only remember one time when I cut heavy enough to lift an unlocked knee and I probably should have been using a roughing cutter at the time which would not have been trying to lift the table near as bad.

  4. #4
    Join Date
    May 2008
    Posts
    132
    I converted my 66 J head to a Ball screw for the Z. Installed a 1.5" dia 5 TPI ground screw I found on ebay. The knee lead screw on my CNC conversion is driven directly through a 3:1 timing belt and stepper. With the old acme screw I could do about 10 IPM now I can do 75 IPM. I had some thoughts of back drive but it wasn't an issue even without the stepper attached. I do have a pair of 200 pound gas springs to take off some of the load.

    Craig

  5. #5
    Join Date
    Mar 2012
    Posts
    378
    Tigerpilot, your build was one of the ones I looked at. Well documented. I like your design and am seriously considering mounting my motor and direct driving the lead screw like you did. That is also where I got the idea for using the gas springs. I think that is a much better alternative than driving from the crank handle, but I am not sure yet if I want to go with the extra work of removing the knee. Certainly if I use a ball screw I will do it that way, just still not sure yet.

    Looking at the size motor you used I am estimating that you have about 500 in oz of torque when running up to about 500 RPM so I am wondering what was limiting your speed. Plugging in the numbers even with only a 70% efficiency on the screw shows that you would have 1,374 pounds of force if you drove it at a 1:1 ratio. I am wondering if this might be one of those cases where the speed reduction for more torque is actually working against you because you have to drive the motor faster and thus loose torque.

    Can you elaborate more on what your previous setup was and if the ballscrew, gas springs, or another change is what got you the increased speed.

  6. #6
    Join Date
    Mar 2012
    Posts
    378
    Now that I made the previous post I found another calculator on screw efficiency. My 70% on the acme was pulled out of the air thinking that the acme would probably be more efficient than that. Doing some research it seems that it could actually be much lower. Found good calculators here:

    Calculators and Charts

  7. #7
    Join Date
    May 2008
    Posts
    132
    There was quiete a difference between the old acme nut and the new ball screw. I suspect it was mainly just nut friction that limited the speed. Its hard to know -- I would just set the acceleration very low then keep increasing the velocity till it falulted then backed off and went back to adjust the acceleration. With steppers the big elephant is the mass you have to accelerate -- once you stall its over, not like a servo where you can set the tracking error limits.

    With the ball screw I found I could easly do 75 ipm though I didn't really play with it that much. I am sure it would do 100 ipm but I would need to change the kernel rate to up the pulse rate to try this out. I could very well be over the curve and running into reduced torque due to excessive RPM. But again it's the initial acceleration that kills you on the knee. The knee, saddle, table and all the accessory stuff net out to over 700 lbs. I have stalled it a couple of times on rapids but only when it was very cold. Since I don't cut with the knee it doesn't really matter.

    Craig

  8. #8
    Join Date
    Mar 2012
    Posts
    378
    I plan on using the 1125 in oz peak (410 continuous) servos from Automation Technologies. I don't have the ball screws ordered yet and I want to make all my motor mounts, etc. before I take the machine apart so I ordered one motor and driver today to do some testing. I will try rigging up the one motor temporarily off the handle so I can experiment and see what ratio pulleys I want to use and if there is a need to use ball screws or gas springs or not. I suspect that at a minimum I will want a few hundred pounds of gas springs anyway. I suspect that I should easily be able to do 50 IPM like this and I can't imagine that would not be fast enough, but we will see.

    I'll put up a build post once I start experimenting. Let the chips fly!

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