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somewhat more crazy idea - DIY lever

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  • somewhat more crazy idea - DIY lever

    I have an old Starbucks Barista (actually I have two) that I rescued sometime in the dim dark past when my other half was a Starbucks Barista.

    9 bars is 130psi. You need to move about 60-80mls (4-5 cubic inches) of water in total for a double shot, so if you had a 1sq inch cylinder, you would need to move it 4-5 inches in height, and apply 130lbs of force. With a standard 30lb tamping force you need about 4.3:1 lever action, or to move the lever 18-23 inches.

    So if I bored out a cylinder of the right diameter, tapped a hose fitting into it, and put that where the pump was, could I convert one of these old machines to a lever machine? I would make a little open feed so I could feed in boiled water beforehand.

    (the other option is to put this between the boiler and the group but I don't know how easy that is to disassemble).

    I could also put some heating resistors into the cylinder so that it could maintain temperature (would be too low power to boil water at any decent speed).

  • #2
    I'm confused. Is tamping anything to do with the pressure needed to push water through coffee? To me tamping is the pressure needed to compact the grind enough to slow the hot water long enough to allow prop[er extraction of all the components of the bean.

    Also, and please forgive a newbie for what might be a dumb question, is 9 bar the pressure needed? I thought it was less than that for good espresso extraction.

    Even with that aside, the average group filter is a lot larger than 1 inch2 - at somewhere closer to 2 across the area is 6.28 inch2 so wouldn't htat change considerably the pressure applied and the travel for the lever? 130psi sounds like a hell of a force to be applying to coffee... Do it in a plunger and you'd be picking shards out of people across the room.


    • #3
      the only reason I mentioned tamping force is that it's a force most people should be able to apply evenly on the lever across the pull.

      espresso is traditionally brewed at 9 bars. that's equivalent to 130psi.

      pressure in a closed cylinder is (modulo flow effects) the same everywhere - so if you put 130lbs into a 1sq in cylinder, you get 9 bars of pressure out the far end even though the fluid has expanded to a 4sq inch area. you just need to move it 4 times as far so that energy is conserved. I could make it a 52mm cylinder and have it move less and more lever arm on the lever, but that would be both more brass to machine, and also more hoop stress on the brass (hoop stress is related to the diameter of the cylinder).

      and yeah, you can't do it in a plunger - this is why espresso groups are metal, and why an aeropress is not espresso.


      • #4
        OK... just checking to see if I am as confused as I thought I might be when I read it first... Do you hang out with Gym Junkies? Most people I know couldn't do 130psi without hanging their body off the lever...


        • #5
          Lever machines don't brew at a static 9 bar. 9 bar was arrived at as a standard when pump machines became available. Most probably because it best approximated the taste profile one gets from a lever machine. From what I've read maximum pressure of a lever is anywhere up to 12 bar and this quickly reduces in a linear fashion as the shot progresses to finish a log way below 9 bar (I've seen a scace log but cant remember the specifics)


          • #6
            remember - pressure is not the same as force - you can easily do 130psi if you're willing to have a tiny cylinder.

            (130psi is only 9kg of force if it's across 1sq cm).

            the problem with 1 sq cm is that you need to move it half a metre to get 35mls into a cup - so I'm presuming that 15kg (typical tamping force) is what your average barista can handle - or moving it about 30cm to get a single lungo or double ristretto. that's what I'm aiming for.

            I'm looking at a disassembly diagram of the machine I have and I can either take the boiler off completely, or put the pressure through the boiler. I can PID the boiler and then just pressurise the inflow liquid (like a standard pump machine does), or I can take the boiler off and maybe have a kettle pourover inlet. the problem with using the boiler as is is that there isn't a solenoid - it's a pressure valve on the exit to the boiler, so low pressure preinfusion can't happen.


            • #7
              It's funny how when you break it down, you really see how 'violent' the process actually is and you see how vital getting every variable correct is! Anyways, sounds like a very cool project Hildy! Keep us posted


              • #8
                does anyone know where I can get food safe aluminium braze? or do I need to do this in brass?

                I think the only portion I will use will be the actual group head and surround, and I will make it a true gravity fed manual lever with preinfusion governed by how long you keep the lever arm up for.


                • #9
                  Re: somewhat more crazy idea - DIY lever

                  I think brass is used because it is dense and easy to cast. I could be wrong but I think aluminium has a high coefficient of thermal expansion (i.e. not dimensionally stable under variations in temperature).

                  Aluminium may also present corrosion issues.


                  • #10
                    Originally posted by Hildy View Post
                    ...does anyone know where I can get food safe aluminium braze?.

                    "aluminium braze" ?? .. i dont think such a material exists ?
                    There are many aluminium alloys, usually chosen for their various strenght / weight ratios.
                    "Food safe" ..? .. i would not worry about food safe with aluminium, ..lots of pans and coffee m/c boilers are made from it already.
                    Try "O Brians Metals" in Wetheril Park.
                    Have you seen this..
                    website, EspressoStrietman
                    Click image for larger version

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                    • #11
                      I haven't seen the espressostrietman - but 1600 euros is a lot of money!

                      yes, I'm happy enough using something like anodised 6061/2011, and even to make it out of a bit of solid billet if need be (eg bored out of 50mm bar) - but if I wanted to join two pieces together, I'd normally use a brazing rod like AlumAloy. Given that these brazing rods all have "secret ingredients" I'd like to make sure that was food safe before I started putting it in the cylinder.

                      aluminium is just easier to machine but harder to solder, and much cheaper than brass. I can buy a solid block the size of a la pavoni group for about $15 and bore out a 30mm hole in the middle for the piston.


                      • #12
                        Originally posted by Hildy View Post
                        does anyone know where I can get food safe aluminium braze? or do I need to do this in brass?

                        I think the only portion I will use will be the actual group head and surround, and I will make it a true gravity fed manual lever with preinfusion governed by how long you keep the lever arm up for.
                        Silicon Bronze braze. Works a treat on Aluminium Need a TIG or torch. If you are fiddling with Aluminium though - you can't weld any old piece of ally... So brazing is a good choice and it is very strong. I don't have the strength figures to hand, but I think it is better than the base metal. You can use Si Bronze to braze steel and stainless to good effect too...



                        • #13
                          For reference, factoring in the approximate density and specific heat capacity, aluminium has about 77% of the heat capacity of brass (per unit volume). It may not be significant for temperature stability anyway, but you could probably compensate by having a proportionately larger group head.


                          • #14
                            There are many ways to make a cylinder / piston assy, but for a test unit i might be tempted to consider using a section of copper tube ( thick wall), or even "re-purpose" some brass industrial valve body ? Easy to work with and assemble.
                            With the copper pipe approach you could easily construct a concentric assy with a hot water "jacket" for temp stability, much the same as a commercial lever uses !
                            Just thinking aloud !
                            Last edited by blend52; 8 March 2013, 12:41 PM.


                            • #15
                              Re: somewhat more crazy idea - DIY lever

                              Machining a lump of something would probably be easier than sourcing copper pipe in the required size. Probably cheaper also.