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  • Chinese ballscrews?

    Got a project I would like to do where I need a small ball leadscrew and was wondering about the smaller 1204 metric units coming in from China.Anybody tried these?Opinions?Backlash?

    https://www.banggood.com/250mm-SFU12...l?rmmds=search

    Last ones I used were from Taiwan,but much more expensive.
    I just need one more tool,just one!

  • #2
    Initially thought of something along the lines of the "Malay boot".

    Maybe that IS pretty close, if the accuracy difference tracks the price difference, and you use it in a CNC application.........
    1601

    Keep eye on ball.
    Hashim Khan

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    • #3
      Tormach uses Chinese ballscrews and I think they are pretty good.

      Chinese ballscrews are like Chinese bearings - usually pretty good initially - just don't last as long so your maintenance costs are higher.
      www.thecogwheel.net

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      • #4
        A friend of mine used some of these (not exact ones) and had trouble
        with them whipping. I guess it depends on the overall length.
        olf20 / Bob

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        • #5
          You may want to do some more shopping: https://www.aliexpress.com/w/wholesa...ll+screw+250mm

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          • #6
            Originally posted by olf20 View Post
            A friend of mine used some of these (not exact ones) and had trouble
            with them whipping. I guess it depends on the overall length.
            olf20 / Bob
            Whipping comes from having not bearings supports on both ends and/or the length to diameter ratio being too small and/or rotating them too fast. If the thing needs to turn at fast speed, is has to have supports on both ends and the diameter thick enough for it to not bend from the rotation.
            Amount of experience is in direct proportion to the value of broken equipment.

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            • #7
              His was on a small cnc router. According to their specs they
              should have work. The ball screw were not perfectly round.
              Worked good at slower speeds.
              olf20 / Bob

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              • #8
                Originally posted by Jaakko Fagerlund View Post
                Whipping comes from having not bearings supports on both ends and/or the length to diameter ratio being too small and/or rotating them too fast. If the thing needs to turn at fast speed, is has to have supports on both ends and the diameter thick enough for it to not bend from the rotation.
                Depends on what you mean by "whipping".

                They would have to spin very fast indeed tor eally "whip" from rotation. But on a router, if the axis motor is strong, and the cutting force is high, a thin screw might deflect sideways due to the forces. Especially if it is not made very well, and the balls do not all contact the screw at once. If one "side" of the screw contacts more than the other, that could put a bending force on it that can start the problem.

                Bearings at both ends help a good deal, but may not be enough to fix it when you have a long, thin, imperfect screw, and fairly high cutting forces while the nut is at the far end, so a lot of the screw is unsupported. It's just like a long thin column, and can be unstable in the same way.

                Basically a bad design, or a good design pushed past its limits.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  Originally posted by J Tiers View Post
                  Depends on what you mean by "whipping".

                  They would have to spin very fast indeed tor eally "whip" from rotation. But on a router, if the axis motor is strong, and the cutting force is high, a thin screw might deflect sideways due to the forces. Especially if it is not made very well, and the balls do not all contact the screw at once. If one "side" of the screw contacts more than the other, that could put a bending force on it that can start the problem.

                  Bearings at both ends help a good deal, but may not be enough to fix it when you have a long, thin, imperfect screw, and fairly high cutting forces while the nut is at the far end, so a lot of the screw is unsupported. It's just like a long thin column, and can be unstable in the same way.

                  Basically a bad design, or a good design pushed past its limits.
                  I meant just that, the screw starts whipping around from the rotational forces. Long unsupported section bends from its own weight already and is thus out of center of axis of rotation and whips once turned.

                  The problem source was not descriped any further by olf20, so hard to say what was wrong as there was just a mention of "being in spec", whatever that means? Made of steel?
                  Amount of experience is in direct proportion to the value of broken equipment.

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                  • #10
                    I don't remember how long they were, maybe 36 overall.
                    They were supported on both ends. He had purchased them
                    so he could get more speed on his router. They are ok, but
                    for the price they have limits.
                    olf20 / Bob

                    Comment


                    • #11
                      Originally posted by Jaakko Fagerlund View Post
                      I meant just that, the screw starts whipping around from the rotational forces. Long unsupported section bends from its own weight already and is thus out of center of axis of rotation and whips once turned.

                      The problem source was not descriped any further by olf20, so hard to say what was wrong as there was just a mention of "being in spec", whatever that means? Made of steel?
                      It would seem that at the sort of speeds normally used, the screw would need to be already bent before it would really whip enough to make a difference in a wood router. What sort of RPM are we discussing? Normal cutting travel, or some sort of fast "rapid" as it returns to 0-0 or the like?

                      Spec = "made of steel".... LOL Could be about it....
                      1601

                      Keep eye on ball.
                      Hashim Khan

                      Comment


                      • #12
                        Originally posted by J Tiers View Post
                        It would seem that at the sort of speeds normally used, the screw would need to be already bent before it would really whip enough to make a difference in a wood router. What sort of RPM are we discussing? Normal cutting travel, or some sort of fast "rapid" as it returns to 0-0 or the like?

                        Spec = "made of steel".... LOL Could be about it....
                        Actually, ballscrews 'whipping" is a well known and documented behavior. The term is "critical speed" It is not uncommon at all to run up against the critical speed limits in DIY designs at speeds not all that fast. The main factors determining critical speed are the screw diameter, length, and how it is supported by its bearings.

                        Here is one description: http://www.beaver-online.com/uploade...%20Formula.pdf Most ballscrew makers have info on calculating critical speed in their technical docs. As a example, bridgeport boss cnc's use type A mounting for the Y axis because the screw is short, and type C mounting for the X axis which is much longer, the screw diameters are the same.

                        Keeping the screw rpm down is one of the main reasons cnc routers commonly use pretty steep pitch ballscrews. They want fast speeds but also keep screw diameter to a reasonable size, all to avoid hitting the critical speed.
                        Last edited by Sparky_NY; 05-01-2017, 02:45 PM.

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                        • #13
                          Originally posted by Sparky_NY View Post
                          Actually, ballscrews 'whipping" is a well known and documented behavior. The term is "critical speed" .....
                          Keeping the screw rpm down is one of the main reasons cnc routers commonly use pretty steep pitch ballscrews. They want fast speeds but also keep screw diameter to a reasonable size, all to avoid hitting the critical speed.
                          AAAAAAAAAAANNNNNND that would be why I asked about the actual rpm.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            Originally posted by J Tiers View Post
                            AAAAAAAAAAANNNNNND that would be why I asked about the actual rpm.
                            It often happens at the speeds "normally used" in wood routers, IF approached without knowledge or checking the critical speed. The rpm can be surprisingly low with the diameter and length of screws very commonly used.
                            The RPM alone won't tell anything without the mounting method and screw diameter.

                            Lets say the RPM was 866, what would that tell you? Is that below the critical rpm? You can guess, or calculate it and not have any surprises, its pretty simple.

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                            • #15
                              Originally posted by Sparky_NY View Post
                              It often happens at the speeds "normally used" in wood routers, IF approached without knowledge or checking the critical speed. The rpm can be surprisingly low with the diameter and length of screws very commonly used.
                              The RPM alone won't tell anything without the mounting method and screw diameter.

                              Lets say the RPM was 866, what would that tell you? Is that below the critical rpm? You can guess, or calculate it and not have any surprises, its pretty simple.
                              It would tell me "fast enough to be considered". If you said 60 RPM, I''d be thinking it was not a problem unless crazy skinny. or bent. Just wanted a range. I have absolutely no idea how fast they TYPICALLY DO turn in actual use in actual units, which is why I asked.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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