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  • #16
    Originally posted by Evan
    Acetal withstands shock loads very well.
    One thing that worries me about acetal is its temperature range: 32آ° to 180آ° F

    What happens if it gets too cold?
    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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    • #17
      The mill is in my basement shop. If it gets to freezing down there I will have much more important things to worry about.
      Free software for calculating bolt circles and similar: Click Here

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      • #18
        Hmm. I'm not sure where you found those numbers Leigh. The glass transition temp for acetal is -50 to -60 and the minimum service temp is -40 according to here:

        http://www.omnexus.com/tc/pom/properties.aspx
        Free software for calculating bolt circles and similar: Click Here

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        • #19
          I think we need a tips book 3, just made up of Evans contributions. Excellent work Evan!

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          • #20
            Yep, Nice and shiny.

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            • #21
              Evan is correct in that a piece of acme thread can be used as a tap.
              This is a piece of Myford leadscew from an old lathe.



              The worn part has been tapered off and fluted to make a long tap.
              It's only normal soft mild steel and I sent it into the hardeners to have 10 thou of case applied.
              I have made a few nuts with this with no problem and since I have had the gear hobber it's been pressed into service making 40, 60, 72 and 90 tooth worm wheels for Gert to sell on Ebay.




              It has little or no relief but cuts well, it just needs a lick with the dremmel every so often.
              So far this tap has got to have cut over 200 wormwheels and still has plenty of life in it yet.

              .
              .

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



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              • #22
                I picked up some small diameter precision ss acme from mcmaster recently and got the nuts with it. They were black plastic, felt more like Ultem than Delrin but the fit was pretty loose. A good fit can be had by splitting a plastic nut lengthwise and spring loading one half, securing the other. The leadscrew is always digging into the plastic under pressure and backlash is close to zero. The spring tension will affect how well the backlash behaves under actual load. In another precision (but quite small physically) application, we have a standard v thread squeezed between two flat plates of delrin. This works well also and I've considered something similar for a lathe cross slide, but on a larger scale.

                Regarding "EMCO/Maier stuff made of acetal ..."

                About a year ago I bought some of the white acetal, tumbler reverse gears for my small Maximat from Blue Ridge. They cost dearly but were original Emco, sealed parts. Long term , only clones may be available.

                The ones on the Maximat 7 are the same as the 8.6 and "look" the same as the V10 sized machine. By the time you get to the Maximat 11, they are currently stocked replacement parts and are quite a bit larger.

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                • #23
                  If you folks could clear up an area of confusion and misinformation I'd appreciate it.

                  For some obscure reason I seem to be under the impresssion that recirculating ball screws are -not- desirable for manual operation.

                  Is that correct?
                  Len

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                  • #24
                    Recirculating ball screws have so little friction that they don't stay where you put them if operated manually. If on CNC then the holding force of the drive keeps them in place when not moving.
                    Last edited by Evan; 06-28-2006, 09:29 PM.
                    Free software for calculating bolt circles and similar: Click Here

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                    • #25
                      Originally posted by Evan
                      Hmm. I'm not sure where you found those numbers Leigh. The glass transition temp for acetal is -50 to -60 and the minimum service temp is -40 according to here:

                      http://www.omnexus.com/tc/pom/properties.aspx
                      From page 3379 of the McMaster-Carr catalog for stock material (rod, sheet, etc.):

                      Acetal Copolymer
                      • Color: Opaque white or black
                      (unless noted)
                      • Temp. Range: 32آ° to 180آ° F
                      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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                      • #26
                        Originally posted by Evan
                        Recirculating ball screws have so little friction that they don't stay where you put them if operated manually. If on CNC then the holding force of the drive keeps them in place when not moving.


                        Yeah but think of all the advantages, you carfully calculate the material, depth of cut, surface area of cut, when you last adjusted the gibs, the temperature of the machine hence the temp of the vactra, rpm's, tool size and how sharp, how many flutes and spiral %, tool material, weight of X handles and table, (for acceleration purposes) wind drag of X handles, type of coolant, ambient temp. and then of course chatter harmonics adjusted in consideration of all previously mentioned, last but not least --- you take a climb cut and watch your machine perfectly feed itself at the exact rate you predicted,,,, without the use of an autofeed ----------------- priceless.

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                        • #27
                          nice work. Had not thought about the freezer idea to get the needed clearance.
                          John

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                          • #28
                            Looks like Evan's been hanging out at CNCZone

                            One of the materials I am experimenting with for nuts is acetal copolymer. This is similar to Delrin which is acetal homopolymer.
                            Delrin AF is the thermoplastic often used in industrial acme leadscrew applications. It's Delrin embedded with 13% PTFE Fiber. Nylatron GSM (Nylon embedded with molybdenum disulphide) is another common thermoplastic used in low-backlash nuts. I have several sheets and rounds of both that I use for bushings, nuts and low-stress gears.

                            both the acetal and bronze nuts I made have zero backlash as made.
                            You mean low-backlash, right? If the nuts were zero backlash, they would have no thread clearance, and they wouldn't spin

                            That's why your 3C screw stock has a tolerance of +0.0/ -0.0011, and why your tap made from this stock works. If both pieces (screw and nut) had any positive tolerance, the screw and nut combination would bind.

                            If you want even lower backlash, you can find a bunch of posts on CNCZone where they split half of the acetal nut radially in three or four very thin slices, and then wrap the split sections with O-rings. This gives you a poor-man's anti-backlash nut.

                            The industrial anti-backlash nuts typically have two paired nuts held on a captive secondary screw with a very coarse pitch, and an radially wound spring between them, that forces the nuts to unwind apart and take up any backlash. This works better than a spring in compression between the two nuts (which is a lot easier to build), since if you work the table in the direction of the compressed spring (as opposed to the wound spring), you lose some of the backlash reduction in that direction.
                            Last edited by lazlo; 07-01-2006, 12:56 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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                            • #29
                              ou mean low-backlash, right? If the nuts were zero backlash, they would have no thread clearance, and they wouldn't spin
                              Actually, I mean no backlash. The nuts are a zero-zero fit. They spin just fine with only a small amount of friction. As soon as a film of oil is introduced it takes up any slack that may exist and acts as a squeeze film damper. This will also account for future wear, at least for a while. I do have measures incorporated to account for wear. If you look at the acetal nut housing you will see that it is split on one side. This allows for the nut to be split on one side axially and the housing to be compressed by a simple mechanism using a screw.

                              As for the Nylatron I also have a nut made from that that fits the same housing. I plan on experimenting with various alternatives. The nuts are easy to change.
                              Free software for calculating bolt circles and similar: Click Here

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                              • #30
                                Evan, take this is a question not a statement - trying to understand plastics and zero backlash.

                                if it has zero backlash, there must be at least some force acting on both sides of thread, correct? When I've tapped plastic before, the tapped hole ends up feeling slightly tight on the threaded rod or bolt. I've assumed that this is because the tap while its cutting compresses or displaces the plastic slightly as opposed to cutting a perfect profile, hence when the tap is removed is springs back and is tight fitting thread. I can see how this would create the force on both sides of the thread, it ends up being a very light interference fit, but not enough that it won't turn.

                                So if the above is correct, this is why its able to turn with both sides of the thread in contact whereas with metals you wouldn't have this compressibility factor and it if was in contact with both sides of thread at once, it would seize - the compressible nature of the plastic takes the place of clearance??

                                If that's the case, and the plastic is compressible enough that it allows it to turn with zero clearance, wouldn't you then have a problem similar to that of backlash because of this compressibility? ie there would still be an element of movement between the nut and screw when shifted from one direction to the other. In a metal set up its becuase of clearance, with plastic because its not as rigid a material and the transfer of force from one direction to the other changes which side of the the thread is comrpressed?

                                Sorry if this makes no sense, it did when i typed it
                                .

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