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  • Acetal Ball Screw nuts

    New thread as the old one went off tilt a bit but prompted initially by Darrin's thread over on PM a while ago and Evans regurgitation of it I resurrected my trials on it.

    First a bit of history to see why I went this way.

    I get called on the modify ballscrews for CNC machines, just machining the ends. The ball-screw is dispatched in long lengths with a ball nut fitted and these are cut with a slitting disk to length and machined, often the waste length can be up to 600 - 700 mm long, they don't bother because the ball-screw is relatively cheap, it's the nut that costs the money. So I have a series of cut off lengths that are basically worthless.

    First try about a year ago was to grind 3 flutes in a small section and tap it, the end result wasn't very good so it went on the back burner, then Darrins remark prompted a go at doing it which worked but I wasn't happy with the result.

    Evan did roughly the same as Darrin but posted his results better laid out for people to follow.

    Anyway after a few trials and tribulations I came up with this.



    A one piece ball nut in some crappy plastic that was on hand, probably nylon or nylon 66 with no splits in it and a fully formed thread inside.

    Can't get a shot inside as the whit nylon fuzzes the picture up, runs nice on the ball screw and my next step is to do a square nut and replace the Y axis standard nut and block on a X1L machine I have here for testing.
    The X1L already has CNC on the Y but using standard parts, it will be interesting to see how it behaves.

    BTW, none of this is new, Moore international have made round thread screws and plastic nuts for quite a few years, called Rondo Thread.

    http://www.moore-international.com/l...nd_thread.html

    On their web site they go on to say:-

    Rondo round thread has been developed as a true alternative to Acme / trapezoidal threads. They deliver outstanding efficiency and quiet running due to their round thread profile. In terms of performance Rondo has almost twice the efficiency against standard acme, trapezoidal threads.

    More on this method later when I get some process shots.

    .
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.




  • #2
    Originally posted by John
    First try about a year ago was to grind 3 flutes in a small section and tap it, the end result wasn't very good so it went on the back burner, then Darrins remark prompted a go at doing it which worked but I wasn't happy with the result.

    Evan did roughly the same as Darrin but posted his results better laid out for people to follow.
    Actually John, I never even saw that thread or I would have posted in it.

    http://bbs.homeshopmachinist.net/showthread.php?t=30977


    Originally posted by darryl
    Some time ago I made a 3 axis gantry router and used a leadscrew that came from a garage door opener to run the motions. This stuff was 1/2 inch diameter, 3 tpi, and 3 start threads. I molded some nuts around this rod using of all things- coffee table epoxy. I figured I would have some problems removing the nuts from the shaft, which I had treated with some kind of release compound, and I wasn't dissapointed. Once I had a nut removed, I ran a tap through it (made from the same leadscrew of course) and that cleared it out just enough to run easily. Two of these nuts carried the weight of the gantry with the router on it without problem, and they hadn't developed any play by the time I dismantled the machine. I didn't use the machine as much as I thought I would, and it was too big to keep around taking up space. I used the pieces for other things, one of which (using one of these nuts) was to push and pull a machine of about 100 lbs across a pair of steel rails, sliding on uhmw pads. That got a fair amount of use, and there is still no play in the nut. (I just checked that- the mechanism is sitting right beside me, waiting for its next application).

    I've also played with uhmw to make nuts from. It doesn't machine out well enough with a tap to make a satisfactory part, not IMO anyway, but if I heated and molded it around the leadscrew it made a nice thread. I didn't get it to fill the groove though, and abandoned the idea.
    Originally posted by John
    BTW, none of this is new, Moore international have made round thread screws and plastic nuts for quite a few years, called Rondo Thread.
    What is new is that I developed a method to make home shop acetal nuts that even you can follow.

    added: What I thought up and what Darryl didn't and what you took from my idea is to heat the screw and use the hot metal to form the threads.
    Last edited by Evan; 09-25-2010, 08:08 AM.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      I'm just wondering, why make nuts like that out of plastic at all? Isn't it just easier to grind a HSS threading tool and make them out of a much-stronger metal?

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      • #4
        Look at the ball screw in the picture. How will you make a tool that is an accurate fit to that profile? It is much easier to use the ball screw itself as a hot forming tap. Further, plastics are superior to a metal part because they are self lubricating and do not tend to wear in use.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          If you knew the size of the balls in the original screw, could you wind up a spring* using rod of the same diameter as the balls, twist it onto the screw and then use this to hot form your nut? Bore out the minor diameter of the nut so it clears the major diameter of the screw. Drill two holes** and fill with balls. More thought required on the recirculating channel...

          * probably on a slightly smaller dia. former than the minor dia. of the screw, and maybe to a slightly slower pitch so that it's tight when wound onto the screw stock.

          ** determining the linear and angular position of these holes relative to the axis of the shaft may be a challenge. Perhaps they should be tangential to the screw's helix?

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          • #6
            Nice job John! I would imagine these are much lower friction than the acetal acme nut?

            It's not clear how to tapped the nut? You said you tried to flute the ballscrew, and that you got unsatisfactory results. Did you use heat like Darryl's method?
            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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            • #7
              Originally posted by Evan
              Look at the ball screw in the picture. How will you make a tool that is an accurate fit to that profile? It is much easier to use the ball screw itself as a hot forming tap. Further, plastics are superior to a metal part because they are self lubricating and do not tend to wear in use.
              I agree that it's a good way to get a complex shape done. Personally I would never have thought to take apart a ball screw, discard the the whole recirculating ball system and essentially make a conventional lead screw out of it. I would have made (or bought) an Acme screw and fabricated a two-piece Oilite bronze nut first. Also self-lubricating and (theoretically) stronger against loads than many plastics.

              What was so wrong with the ball screw in your photo that it made a good candidate for a plastic nut? Is it loose in the middle? I'm NOT questioning your engineering or machining prowess after looking at your website and seeing the level of work you do. I'm just looking for an education because I've always been more inclined to seek conventional solutions.

              EDIT: Went back to see these were "scraps" of new ball screw. I still would have gone Acme or tried my hand at making recirculating ball nuts.
              Last edited by PixMan; 09-25-2010, 11:05 AM.

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              • #8
                John has a bunch of left over ball screw rod from cutting them to size. He didn't discard anything. This is an excellent way to use the remainders for other projects. I know this because he e-mailed me about the project.
                Free software for calculating bolt circles and similar: Click Here

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                • #9
                  It's not clear how to tapped the nut? You said you tried to flute the ballscrew, and that you got unsatisfactory results. Did you use heat like Darryl's method?
                  I don't recall Darryl ever saying he used a heated screw as a tap.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    Originally posted by Evan
                    Actually John, I never even saw that thread or I would have posted in it.

                    http://bbs.homeshopmachinist.net/showthread.php?t=30977
                    No Evan read the top of the post again, Darrins post over on PM not here where he used softened UHMW or whatever.



                    What is new is that I developed a method to make home shop acetal nuts that even you can follow.
                    Correction, What is new is that you developed ONE method to make home shop acetal nuts that anyone can follow.

                    If there is one method it follows that there are other methods, some may be better, some may be worse.

                    My method, which isn't new other than being done in the home shop with limited equipment is to do it in one piece as opposed to your two piece so it could be said that this is an improvement.

                    added: What I thought up and what Darryl didn't and what you took from my idea is to heat the screw and use the hot metal to form the threads.
                    My method doesn't heat the screw at all, the screw is there solely as a former for the thread.
                    .

                    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                    • #11
                      No Evan read the top of the post again, Darrins post over on PM not here where he used softened UHMW or whatever.
                      Are you talking about Darin or Darryl?

                      I very rarely look at PM and haven't posted there or read it in a long time.

                      PM is far too slow to load on my connection. The only time I have posted there at all in the last few years was when this board was down for several days. Other than that I don't look at it at all.
                      Last edited by Evan; 09-25-2010, 11:21 AM.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Sorry Darryl, my mistake and it was back in 2005 when you were quite active on PM so i take it you had read it ?
                        .

                        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                        • #13
                          None of you can pin any of this on me

                          I was wondering after Evan posted the melted method if it would work to soften the plastic by boiling it and the screw together and then forcing the thread in while it was soft.
                          Seems like the screw would be the last to cool down forcing the plastic to set around it.

                          It might work,or I might be all wet with boiling water
                          I just need one more tool,just one!

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                          • #14
                            Originally posted by Evan
                            Further, plastics are superior to a metal part because they are self lubricating and do not tend to wear in use.
                            Furthermore, with the right information you can find modified plastics that have different properties. For example I used to mold a part at *arbec Plastics that had a lubricant compounded into the material.

                            If you know you are going to be using a certain type of coolant because of your process requirements, you can select from modified nylon, acetal or even UHMPE. If you know you are going to be putting a severe load on the machine, Torlon or a reinforced material may be a good choice.

                            There is no such thing as "crappy plastic" the reason plastic fails in service is because people choose materials based on cost, color and "shiny-ness" rather than material properties. If you're careful, and you ask the right questions, you could (and it has been done) design a plastic replacement part far superior to a metal part.


                            ...and guess what? I have some of this information! And if I don't know, I know who to contact. So don't hesitate to ask.
                            This product has been determined by the state of California to cause permanent irreversible death. This statement may or may not be recognized as valid by all states.
                            Heirs of an old war/that's what we've become Inheriting troubles I'm mentally numb
                            Plastic Operators Dot Com

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                            • #15
                              Originally posted by Evan
                              I very rarely look at PM and haven't posted there or read it in a long time.
                              Evan, you were a very active participant on PracticalMachinist in 2005, when Darryl posted the heat-formed acme nut technique.

                              http://www.practicalmachinist.com/vb...archid=2814591
                              Last edited by lazlo; 09-25-2010, 12:26 PM.
                              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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