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Question, Producing an accurate bore in a plastic tube

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  • Question, Producing an accurate bore in a plastic tube

    Many espresso machines use a similar or same 15 bar water pump.
    It is comprised of a 115 volt AC coil with a plastic tube about 1/2" ID through it.
    A well fitted steel piston floats freely in this cylinder with no mechanical connection to anything.
    The magnetic field from the coil causes it to vibrate back and forth in the tube resulting in pumping action.
    Check valves control the flow in and out.

    I tested one that gradually lost pressure to make coffee. Needle on gauge vibrated between 50 - 100 psi and held about 50 when turned off.

    A replacement pump that I know fits is $80 to $120 and ones that are likely the same are around $40.

    How can I make a replacement plastic tube with an accurate bore? Material?

  • #2
    How long is the tube? Can't you make the piston fit the tube?

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    • #3
      Polypropylene might be used for this purpose. Well honed small cutting tools to achieve a suitable finish and tolerance. I would not use glass fiber reinforced plastic types for the tube. The fibers usually are quite abrasive and the vibrating piston would wear out quickly rubbing against this stuff. Nylon types of plastic are also an option.....these are very abrasion proof and durable. The nylon stock is pretty costly. PTFE lining would the bestest one (extremely slippery stuff) in terms of friction, but this one might be hard to pull off from a technical point of view. I fear PTFE is not exactly hard and durable enough to withstand the 15bar pressure, also it likes to creep, so just the lining can be made from that material and the main forces have to be held back by something else surrounding the lining.
      Would it be possible to accomodate an existing size of e.g. aluplex tubing as the pumping chamber? These have a very smooth internal surface (if they have not been kinked or bent). You might also machine a polished "calibrating plug" which can be forced through the aluplex tubing to take out any bumps or bends in the tube and straighten it out.

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      • #4
        Steel is "plastic". Can you be more specific with your definition? What are some of the specs? Is it exposed to heat? For all we have right now, a balloon would work.

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        • #5
          Haven't taken the current one apart but about 4" long 1/2" id 1/8" wall. Just pumps demineralized cold tap water.
          Marked duty cycle is usually 60 sec on 90 sec off or thereabouts.
          Typical use is about twenty seconds for 4 oz of water a few times a day (for me)


          I have a little convection oven and a point and shoot thermometer and plenty of recycled HDPE. I think I could heat a steel pipe form full of HDPE and press a polished calibrated plug through it. Perhaps I could do that to the existing one to press the bore back in shape. I'd have the right material that way.
          Last edited by GKman; 03-25-2019, 01:32 PM.

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          • #6
            I am guessing that the travel of the internal piston is an incredibly short distance, since it just vibrates at 60hz.

            We don't know how long that piston is, and whether it has any sealing features - like o-rings. We also don't know how much slop/clearance it has, both in original condition and now, due to wear.

            Those answers will tell a lot about the required tolerances, and over the distance it must be maintained.

            Hmmpf.... You didn't do your homework before posting. You can google this stuff, just like any of us, and so many of these questions are answered. You may just need o-rings.

            https://www.youtube.com/watch?v=we9IZJ5tQFs

            Good video here, with parts on the table.

            https://www.seattlecoffeegear.com/le...ibratory-pumps

            Some new ones appear to be dirt cheap - $26.

            https://www.amazon.com/Ulka-Pump-Mod.../dp/B00NMNA138

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            • #7
              There are ways of boring something like nylon and then finishing the od afterwards, like Jo Piecznski's video which may be of use to you: https://www.youtube.com/watch?v=hXq_HfwG7dA

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              • #8
                Closer inspection made a liar of me of course. Cheaper ones are as I described. this one from a Jura machine looks like steel piston in plastic for the electromagnet to work on then necked down to about 3/16" dia extending out past the coil so the water pumping piston it can run in a metal cylinder. Cylinder (imbedded in plastic so I can't see.

                Do homework just not as good as yours, thanks to Glug and all the rest.

                The Seattle Coffee video looks just like it.

                I'll try the $26 one. 15 bar is 15 bar and the flow isn't demanding. Might have to adapt the tube fittings but surely I can do that.

                Good coffee (and a few other things) has kept me out of the bars for 39 years now. Thanks for getting me back in my barista hat.
                Last edited by GKman; 03-25-2019, 01:57 PM.

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                • #9
                  Originally posted by GKman View Post
                  Good coffee (and a few other things) has kept me out of the bars for 39 years now. Thanks for getting me back in my barista hat.
                  Sorry for taking this OT, but you will get better coffee if you don't use demineralized water. Search for "espresso water chemistry" if you want to learn more about it.

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                  • #10
                    Originally posted by GKman View Post
                    I'll try the $26 one. 15 bar is 15 bar and the flow isn't demanding. Might have to adapt the tube fittings but surely I can do that.
                    Looking at the sold listings, I see several variations on ebay at similar prices. Maybe one will be a closer match, with no adaptors (though I'd prefer amazon if given the option). Once you get the replacement working, it'll be interesting to know how the old one failed.

                    https://www.ebay.com/sch/i.html?_nkw...ete=1&_fosrp=1

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