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Thread: Importance of ball screw pitch size

  1. #21
    Member ger21's Avatar
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    Default Re: Importance of ball screw pitch size

    "Holding Torque" is the torque when the motors are not spinning.
    What you really need is the torque at max speed.

    I keep meaning to read up more on why metal cutting machines use much larger stepper (or servo) motors
    The simple answer is that's a general assumption. A properly machine will have a motor that provides the required torque to meet the performance goals of the machine, plus a certain amount of headroom so the motor won't stall or lose position. This may be 50% more than the minimum required torque.
    A big router will have bigger motors than a small milling machine. It all depends on the specific machine.

    Gerry

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  2. #22
    Community Moderator Al_The_Man's Avatar
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    Default Re: Importance of ball screw pitch size

    Quote Originally Posted by Goemon View Post
    My choice was largely based on the availability of good deals on quality ball screws. I also figured that the larger diameter wouldn't hurt on a longer axis and the one that carries the most weight.

    .
    Also the dia and pitch of a B.S, is equivalent to a Class 1 lever, with Dia being the Effort arm and the pitch, the load arm.
    So an increase in dia, and/or a decrease in pitch will result in less effort required.
    Al.

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  3. #23
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    Default Re: Importance of ball screw pitch size

    So an increase in dia, and/or a decrease in pitch will result in less effort required.
    But an increase in diameter increases inertia, which can require more effort to get it turning.

    Gerry

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    Default Re: Importance of ball screw pitch size

    Quote Originally Posted by ger21 View Post
    But an increase in diameter increases inertia, which can require more effort to get it turning.
    Do you think I should buy a larger motor for my long axis then (the one that has the larger diameter screw)? I currently have the 381oz "low inductance" motors that many here start with.

    looking at the specs for these motors, they seem to have a decent amount of torque and peak torque for the job but i'm not really sure how much torque is optimal for this purpose. My inclination is to wait and see how well these motors cope before deciding to upgrade or not but.... if I am going to need to invest in new motor mounts and motor couplings plus redesigning the axis to make them fit, it would be easier to do that now if you think I'm likely to need more juice?

    If I can do what I need with the 381oz motors then I would prefer to wait until I am ready to upgrade to a decent servo set-up.



  5. #25
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    Default Re: Importance of ball screw pitch size

    I would get the machine running, and then see if what you have works.

    Gerry

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    JointCAM - CNC Dovetails & Box Joints
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  6. #26
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    Default Re: Importance of ball screw pitch size

    Quote Originally Posted by Goemon View Post
    My choice was largely based on the availability of good deals on quality ball screws. I also figured that the larger diameter wouldn't hurt on a longer axis and the one that carries the most weight.

    I guess I'll have to see if it causes problems for me. If it does then I'll have to replace it with a fiber pitch screw and use the old one on another build.

    You are right that speed is of little concern to me unless it causes a cutting problem. My main goal is to be able to cut a mold cavity in 6061 and getting it right first time. 6061 is not super expensive but I can't afford to be screwing up 1.5" x 8" x 36" slabs on a regular basis.

    It would probably be cheaper to upgrade a stepper motor than to buy a new ball screw though. When I bought my stepper motors I was building a different machine with slightly different cutting goals and a lighter spindle. They were on my list of components to revisit to see if there were opportunities to improve anyway.

    I keep meaning to read up more on why metal cutting machines use much larger stepper (or servo) motors to see if I was missing something. I can't shake the nagging thought that more holding torque would be helpful for cutting metal.
    Good deals are only so if you can use them. You're beginning to amass a collection of parts that could have went to funding the right parts the first time. As Gerry says, at some point you just have to get the thing running; it's the only way you'll learn what works and doesn't for you. And we scold you for it, but we've all made crazy purchases in the past. Realizing it and moving on from it is the key.

    I don't think your mechanicals are going to screw up your molds. Most all screw up are from some form of human error. Your machine can only do what you tell it to do, within its capabilities. And you won't know until the most inopportune of time. You WILL screw up, everyone does at one time or another. And it's not the cost of the aluminum that will kill you, it's the 22 hours of machining time into a job, and something goes wrong at 21:30. Or it's the endmill that galls up, going through the aluminum by friction alone, before stalling your spindle and moving your whole setup out of alignment. Then you hit the e-stop. It's not a question of if it'll happen; it WILL, in one way or another.

    Metalworking machines are usually massive. And in an industrial setting, time is money, so moving heavy things at high speeds, as accurately as possible, is key. Then there is the force needed to plow through material, as well as absorb any shock forces, which can be multiple times the cutting force, and not lose position.

    It is not the torque that pushes the machine through metal. (Although on a router-type machine, you're using the speed of the spindle to get through the part) It is the power that is generated from that torque. Think of the scissor jack for your car. Look at that screw. You (small motor) have a finite amount of torque (your weight times the length of the crank) you can apply to that screw. Easy to crank, but takes a bit of time. Now replace that screw with one of those 20mm ballscrews. You'd have a LOT harder time raising the car, and when it gets up there it would be all but impossible to keep it there. A heavier guy (larger motor) could possibly do it, but he's got to eat more (more electricity). Somewhere in between is a screw that gets the car up in a reasonable amount of time, with about the same effort as the standard screw. We don't always have the luxury of picking the ideal parts when we are up at 3am going through eBay listing after listing. You could however achieve the same mechanical resolution in each axis by means of timing pulleys and belts. Which is probably what I'd recommend in your situation.



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    Default Re: Importance of ball screw pitch size

    Quote Originally Posted by ger21 View Post
    I would get the machine running, and then see if what you have works.
    I think that's fair. If the goal is "to make it the best it could possibly be" then I have no doubt that I could spend forever upgrading components - something I have no desire to do. It's time to finish this...

    I'm not far off now (I think). My Z and X ball screw assemblies arrived today and, for the most part, I am happy with what I bought. The parts all move well and, by chance, they fit my existing motor shaft couplers but there is something I don't understand on the NSK parts for the X axis.

    My (THK) Z axis ball screw comes with two pillow blocks with bearings that fit neatly inside but the (NSK) X axis ball screw seems to have bearings at one end that don't fit inside the pillow block:




    The pillow block is loose and I can't push either of the two bearings into the block or remove the black collar at the end (I removed the set-screws but it seems to be stuck).

    Before I get my heat gun and wd40 out to try and force the bearings into the pillow block, Is this a problem you guys have seen before? Am I right in assuming that those two bearings are meant to be be inside the block?

    I looked all over the pillow block for set screws or equivalent but there were only two screws and a broken plate of some kind which did nothing to make it easier to insert the bearings. This is the broken plate I removed from the sides:




    It is going to be a huge issue if I can't push the block and bearings further up because as it stands, I don't have the 30" of travel I need. I didn't realize that the double nut was over 5" long. If the two bearings go inside the block then I have just about enough travel.

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  8. #28
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    Quote Originally Posted by Goemon View Post
    Thank you. That answers my question.

    With what you said plus the tip from Ger21 on using a gear reduction (if needed), I feel better about the deal on some 20mm pitch ground ball screws that has been made available to me.

    It's all coming together. I kinda wish I could pass some of this info back to myself at the beginning of the process. It seems like the largest potential cost in a diy build is changing your mind on the design and having to buy the same parts twice... I'll literally have enough parts for a second build when the first is finished....
    Hey, did you ever go that route and get different pitch screws? If so, what do you think? I’m also looking at different screws on eBay.



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Importance of ball screw pitch size

Importance of ball screw pitch size