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  • Leadscrew nuts

    Rustybolt asked me where I purchased my leadscrew stock from and I advised him to hold off on buying nuts to go with it. The reason is that I have been playing around with acme tapping. I can and have turned left hand inside acme thread in the past but for what I wanted to do a tap would be a lot easier and would be nice to have in the tool drawer.

    I bought 10 tpi .5" left hand acme rod for the leadscrews on my mill (I can't at this time justify ball screws). I also bought brass nuts with them to make the order up to the $50 minimum. However, I am experimenting with different materials in places on this project, especially polymers.

    The leadscrews are rolled 4140 and are about half hard. I made an acme tap from a short length of it. It's a triple flute tap and I ground it mainly by hand. I used an angle grinder fastened to the tool post on the SB9 to make the main flutes but after that it was all relieved and sharpened by using a 4" CBN wheel running horizontally in a drill press at 3000 rpm. It works very well.

    One of the materials I am experimenting with for nuts is acetal copolymer. This is similar to Delrin which is acetal homopolymer. The acetal I am using is branded Acetron. It differs from Delrin in that is has slightly lower properties but more importantly does not suffer from reduced density or porosity in the core of an extrusion like Delrin does.

    Acetal is truly nice to machine but has the same drawbacks that most polymers have when drilling and tapping. It tends to grab as it heats and also likes to pull in the drill bit like brass does.

    Acetal is being used a lot now for leadscrew nuts in many applications. It's self lubricating and can be run dry but does work better with oil. It is completely impervious to petroleum oils and solvents as well as any of the common coolants and cutting fluids used in machining. It is frequently used for gears in small to medium power transmissions up to 3/4 horsepower running in an oil bath.

    The tap has a very long lead in to help with the fact that it is removing so much material. I haven't heat treated it although I may. I have used it to cut acme threads in nylatron, acetal and bronze and it works well. It takes a lot of power to cut the threads. For the bronze nut I used the lathe and cut the threads in back gear.

    For the acetron I used the lathe to start the tap straight and then removed it from the lathe and continued by hand in a vise. You can't power the tap all the way through in acetal in one go. Acetal has a fairly low melting point and the tapping (and drilling) generates a lot of heat when done under power. I tried it and it ripped out the threads. Besides, trying to run the tap through under power all at once makes a sound like strangling a bobcat. I should have recorded it to play outside on speakers on halloween.




    The bronze nut is cut in 660 bearing bronze. I cut it at 90% of full thread and it made it through. It turned out pretty well. You have to run the tap through a number of times to help with the fit since this is an exact size tap. Putting the work in the fridge for a while and running the tap through while it is cold will give that slight extra clearance that is needed to prevent binding.

    Last edited by Evan; 06-28-2006, 02:17 AM.
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  • #2
    Hi Evan,

    Very nice

    A question: You mention that the tap is "exact size", and you had to accommodate for that when tapping the nut. I would think it was actually slightly undersize, since the acme rod from which it was made should have been on the minus tolerance side to start with. Y/N/?
    Leigh
    The entire content of this post is copyright by, and is the sole property of, the author. No assignment
    of title nor right of publication shall ensue from presentation of this material on any computer site.

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    • #3
      They are very close to exact size. The proper grade of acme thread for leadscrews is called a "centralizing" grade and uses the designation C with a number that indicates the accuracy spec. The ones I bought are 3C. They have a spec limit on the OD of +0.0/ -0.0011

      They are also very tight on the thread depth and the pitch error.

      Standard general purpose acme in a .5" is class G and can be as much as .005 undersize .

      [edit]
      More to the point, it is exact size since it is made from the same stock as the leadscrew that runs in it.
      Last edited by Evan; 06-28-2006, 04:02 AM.
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      • #4
        Oops, looks like my server is down. We have been having some power glitches tonight and I suspect the power may be out in town.

        Better, it's back on.
        Last edited by Evan; 06-28-2006, 03:59 AM.
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        • #5
          I dont know the proper name for it but i can find out (delrin AF???) but the guy i work with gave me some to use as plain bearing in my crank and it is wearing better than some of the bronze bushing i used, it is a brown plastic and it has teflon in it, its amazing stuff as where i used it was a high load area and i was very skeptical, now i wish i would have put it in other area's, the only thing i think worth considering is machinery doesnt always run in the best of conditions and where bronze has a harder shell and will push chips out of its way plastic will allow them to imbed in itself resulting in damage and wear to the lead screws...

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          • #6
            Not only do plastics imbed chips, they don't handle shock loads well if I understand (think about the self-feeding that tended to happen while attempting to power tap, wrecking the threads). Assuming this is a lead screw for one of the axis feeds on your ongoing mill project, this makes a tool crash a potentially more critical issue. Of course if one is careful, this will never be a concern.

            I really appreciate you posting this. You just made a nut to fit your lead screw which is probably on order with the best fit nuts that could be purchased...or better. The idea of using some of the acme stock to make a tap was something I had considered when working on a mill vise repair and was going to change thread to something standard. As it turned out, I found a used but nice nut and replacement lead screw before I had all the engineering done in my head. Still, the idea of chilling the work to be cut to provide the needed clearance is not something I had thought of. I considered treating the tap or another piece of rod as sacrificial and lapping the threads using clover compound. Still, the idea of using an abraisive on something like bronze where it might tend to embed and become a permanent part of the nut was not appealing.

            Paul
            Paul Carpenter
            Mapleton, IL

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            • #7
              Acetal withstands shock loads very well. That's one of the reasons it is used in gears in power transmissions. The threads didn't pull out from loading when I attempted to power tap, they melted. [added] Running the tap through under power makes it much too hot to touch.

              Chip embedding isn't that big an issue, especially not for me. I will be mainly machining aluminum but will also be using effective felts on the nuts and perhaps use way covers also. Bronze also makes a good lap and will embed chips. It's arguable that a harder material such as bronze will cause more damage if chips become stuck between it and the screw or ways.

              BTW, both the acetal and bronze nuts I made have zero backlash as made. This will change a bit with wear. They only have a very slight amount of friction on the lead screw and can be easily turned with a pair of fingers.
              Last edited by Evan; 06-28-2006, 12:58 PM.
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              • #8
                Interesting choice Evan. As a plastics man all my life I've been trying to convert people to the benefits of using polymers in place of metals in certain applications.
                I must admit I woudn't have thought of this particular one, but it's an ideal environment. Low speed (relatively - until you CNC it) loading spread over a number of teeth on the bearing length, and its low temperature toughness/ impact strength will no doubt be of benefit in your neck of the woods

                Mind you, the extrusion-generated porosity that you mentioned wouldn't really be an issue as you've bored the centre out, and this is where it would occur most prevelantly.
                Incidentally did you know that Acetal Co-polymers only came about as a way of bypassing the DuPont patent on acetal homopolymers?

                Boomer you're lucky that you were given some Delrin AF. You *don't* want to know how much it costs to buy that stuff. It's loaded with 20% teflon fibres and is probably one of the most expensive materials that DuPont make.

                Pccarpenter, plastics are actually pretty good at taking shock loads, but most of them are incredibly notch sensitive if this is what you mean? Polycarbonate for example can withstand repeated hammer blows and suffer just a dent, but score it first to produce a notch effect and it will shatter under the same treatment.

                Peter

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                • #9
                  Fine work young man, I have used that technique before and it worked very well. If you are going to turn at high RPM's are you going to use any lube? Heavy felt wipers will be fine.

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                  • #10
                    It is CNC but the loads will still be well within the PV rating by a long way. I expect the leadscrews to turn no more than about 300 rpm in rapid. With oil that will keep the frictional heating to a minimum.
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                    • #11
                      I guess what I was thinking was not that they shear so easily...heck...they are plastic...meaning that they (plastics generally) will deal with some force and then return to shape. However, I have seen things that never actually went far enough to shear, have their shape changed to the point that they had to be replaced...apparently exceeding the elastic limits and reaching the plastic limits On the other hand, the plastic gears that I am familiar with are of dubious composition and failed at a keyway---which is a sharp corner as you mentioned.

                      I would guess that Evan's statement that "acetal withstands shock loads very well" is as a comparison with other plastics and not all materials in general. I sort of presumed that the reason Evan was interested in using an acetal variant here was that he could cut it to a very tight fit and still have it function. If that is the case, a mashed set of threads would not fit the bill. You need something that offers the fit desired with the ability to maintain that over time. What nags at me is the old "If it is such a good idea, why aren't they doing it commercially" question. I just wonder if the wear characteristics are worth the tradeoff. In some ways, the whole tightly fitted plastic nut is a giant fitted wiper that may tend to wipe oil *off* the threads. However, it may be a non-issue in a home shop tool that gets the attention that Evan will give it.

                      One can certainly argue that bronze is also capable of being damaged under crash conditions.

                      Paul
                      Paul Carpenter
                      Mapleton, IL

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                      • #12
                        Acetal is good stuff.....

                        But I think it ages and can become brittle.... Aren't the (no longer available) gears that break on some of the EMCO/Maier stuff made of acetal?

                        I remember seeing some very bitter complaints about them breaking after a number of years, and the fact that they were no longer available. Apparently Emco wasn't supporting them, and suggested that a machine 15 or 20 years was a very old unit...... that they couldn't be expected to support... In fact, I think I saw that in an older copy of THSM mag, in the letters column.

                        Quite a number of plastics age poorly. Typically they become brittle and shatter.

                        Steel and cast iron have their problems, but they do typically last a long time in usable or repairable condition.
                        1601

                        Keep eye on ball.
                        Hashim Khan

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                        • #13
                          What nags at me is the old "If it is such a good idea, why aren't they doing it commercially" question.
                          They are. Acetal nuts and other plastics are replacing ball screws.

                          Here is an example:

                          http://www.machinedesign.com/ASP/str...tedArticle.asp

                          And here:

                          http://news.thomasnet.com/fullstory/14538/920

                          More info here:

                          http://www.findarticles.com/p/articl...10/ai_n8910618
                          Last edited by Evan; 06-28-2006, 02:10 PM.
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                          • #14
                            I have a lot of experience with plastic machine parts, especially gears and sliding bearings. Nearly all of the copiers that I worked on for over two decades had some sort of precision linear motion system in them with just about every configuration you can imagine. There was also every possible system of linear bearings used from recirculating balls to polymer sliding bearings. Also, every possible sort of drive was used from tooth belt to chains, cables and leadscrews.

                            The transmissions often used plastic gears, especially near the end of my time at Xerox when plastic gears, usually either acetal or nylon, were the rule. Plastic gears hold up a lot better in tough environments with contamination and poor lubrication. They are also much quieter than metal as they have some compliance and can be molded to the precise tooth form needed. They will last a long time without lubrication and don't grind themselves to death when filled with dirt. They do sometimes break but that isn't very common in a properly designed drive. Generally the plastic components last at least as long as metal even in tough applications such as chain sprockets running at high speeds and loads.
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                            • #15
                              I've looked at the first one so far....really neat design. An internal torsion spring holds two half nuts under tension. Presumably no adjustment for the life of the part, if you can get the tension to be consistent. That shouldn't be hard really....it doesn't have much rotation to deal with in the life of the half nuts.....no more than the thickness of any thread "land".

                              I think the only trick to that would be to get the spring design to actually apply the torsional force needed to "thread" the two halves together. Part of that trick is just proper assembly.

                              thanks for sharing
                              Paul
                              Paul Carpenter
                              Mapleton, IL

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