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Making Acetal leadscrew nuts the easy way

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  • Originally posted by Evan View Post
    The boiling point of water isn't quite hot enough. That is still below the glass transition temp. It must be at least 120C and preferably a bit higher. Below that temp the molecules are being sheared apart and that weakens the entire matrix. Above that temp the molecules are free to move without damage. Both Delrin and acetal are crystalline polymers although they don't look anything like a crystal.
    Oil?
    Helder Ferreira
    Setubal, Portugal

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    • Originally posted by Noitoen View Post
      Oil?

      Good God man it's the 21st Century and not the time to be pouring boiling oil off the ramparts.
      .

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



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      • Originally posted by John Stevenson View Post
        I have made quite a few Delrin nuts with a variation of the theme.

        My method is slightly different in that the two previous methods you had to split the nut and force it round the screw.
        I made mine with the bore of the nut the same as the OD but tight, in other words a pre-stripped nut. Cut the middle man out.
        That's an interesting approach, basically semi-injection molding. I'm wondering if you've had occasion to check the generated nut threads for fill relative to the screw? The exit leak path for air vacating the screw threads as the acetal fills in should be the yet unfilled upper threads. And the thread fill should occur from bottom to top given the cup taper, so ideally air pocket entrapment shouldn't occur. If the ram was sized to seat snug in the cup to minimize molding flash between both, an air hole in the center of the ram/screw would provide a positive escape for the displaced air.

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        • Boiling water was mentioned 130؛C for oil is just a little over the 100؛C of the water
          Helder Ferreira
          Setubal, Portugal

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          • That should work just fine if you don't mind playing with hot oil. A cooking oil such as peanut oil should work well. No toxicity to worry about and widely available. Peanut oil has a high working temp. If not that then palm oil. Also easily removed with washing detergent.

            It isn't a bad idea really. Nice even temp control and if not married then the kitchen stove should work just fine.

            130C should be an appropriate temp and at that temp there shouldn't be much if any molding flash. That is still below the injection molding melting temp. There is a temp difference between Delrin and acetal so that is something to look up and pay attention to. I don't off hand recall the difference but it isn't a big difference. 120C transition temp is correct for Delrin, that I am sure of.
            Last edited by Evan; 10-24-2016, 02:31 PM.
            Free software for calculating bolt circles and similar: Click Here

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            • Hotter water? Pressure cooker... standard cooker at 1 bar is 121C. 1.5 or so bar should do it for 130C (if you can find one that will go that high).
              Last edited by lakeside53; 10-24-2016, 09:32 PM.

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              • Even more entertaining: Use oil in the pressure cooker

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                • Originally posted by elf View Post
                  Even more entertaining: Use oil in the pressure cooker
                  That is KFC's chicken frying method
                  Helder Ferreira
                  Setubal, Portugal

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                  • Originally posted by Noitoen View Post
                    That is KFC's chicken frying method
                    I don't care for KFC, but broasted chicken is great!

                    BTW: Using hot oil is a great idea... I have one of those induction hot plates which should work really well.

                    Robert

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                    • Originally posted by Noitoen View Post
                      That is KFC's chicken frying method
                      We've got a KFC in town I wonder if I could take my parts in and ask if they'd "just warm them up" for me.
                      .
                      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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                      • Originally posted by TGTool View Post
                        We've got a KFC in town I wonder if I could take my parts in and ask if they'd "just warm them up" for me.
                        If they'd do it the parts would probably taste better than their chicken. Popeye's for me. Wish they had a buffet like KFC.
                        “I know lots of people who are educated far beyond their intelligence”

                        Lewis Grizzard

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                        • The point has probably been raised, but above a certain (and fairly low) temperature, acetal decomposes into toxic gas. I'd expect a deep fryer would result in explosive decomposition.

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                          • Originally posted by Evan View Post
                            That should work just fine if you don't mind playing with hot oil.
                            One concern I'd have here would be oil inclusions during the molding process. Not so much with the two blank "clamshell" approach but with the tapered cup/ram operation. In that case I'd expect some amount of turbulent flow such that considerable surface mixing would occur. Here the oil may act as an unintended parting agent against the molten acetal as it mixes against itself.

                            At the other extreme would be oil/moly/etc.. filled composites. But in that case the distribution is uniform to the extent large inclusions of the fill are statistically unlikely to be a structural issue.

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                            • You may be right. That is what experimentation is all about.
                              Free software for calculating bolt circles and similar: Click Here

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                              • power controller for hot tap

                                So, going all the way back to posts #129 and #131 back in 2011, Paul Diamond describes using a diesel engine "glow plug" inserted into a piece of hollowed lead screw to create a "hot tap" for form tapping.

                                He described a series of problem from the glow plug overheating, due to no way of controlling power.

                                The idea doesn't seem to have generated any interest, and I am curious as to why that is. The solution to controlling power seems straightforward enough. Glow Plugs are simple resistive elements. A PWM (Pulse Width Modulation) circuit using a 555 timer to switch a MOSFET at high frequency can be used to control the duty cycle (via a potentiometer). The glow plug could also be replaced by a custom heating element made with Nichrome (resistive heating element) wire. Another method entirely is to use a halogen lamp as the heating element. The ones with an external 11.5V AC transformer. These lamps are dimmable using an off the shelf light dimmer, allowing you to control power output. Not sure how small their diameters go though.

                                I can see arguments where form tapping the thread leads to more errors than press forming, but the concept of using an internally heated threaded element can be employed for both.

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