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Needle Valve Magic

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  • Needle Valve Magic

    As most of you will know, I am a semi retired design engineer who builds model steam and gasoline engines. I have to publicly admit that while I am a fair hand at most things technical, I am a miserable failure at making needle valves for miniature carburetors. My lathe it too big to start with. Its a 10" x 18" lathe, and the chuck won't even close up small enough to hold a #2-56 screw (which most model carb needle valves seem to have as their main diameter.) I have to hold a smaller chuck (reclaimed off a dead electric drill) in my lathe chuck to even hold the screw. My 3 jaw chuck has about .003 runout in it, which I can live with, but it doesn't help. I sharpen my own HSS lathe bits, but still, trying to cut a 30 degree included taper on something that was only .085" diameter to start with has always eluded me. I have filed them. I have used fine emery paper on my flat file to dress them after filing. I have tried various Dremel type tools to grind them. I have held the screw in my electric drill and spun it against a revolving grindstone. All of this "Sort of" works. Then there are the people who advise that I should drill a .040" hole full length of a #2-56 screw and solder a darning needle into the hole. HAH!!! That trick is somewhat akin to trying to shove a dew worm up a wildcats---Oh never mind. It was only recently brought to my attention that THERE IS A BETTER WAY!!! Instead of trying to put a fine point on an a threaded screw which then screws INTO something else, what if the something else had an external thread and the needle valve actually did incorporate a darning needle soldered into a LARGER DIAMETER part which has an internal thread.--I know, kinda hard to visualize, but a picture is worth a thousand words. This is not my original idea. It is someone elses. I have created the attached 3D model, and what you see is a "section view" thru the carburetor, which clearly shows what I am blathering on about. I'm going to build this---Right Now. It seems like such a wonderful solution. I will post pictures and describe what I am doing as I progress. This one single "How to" could advance the art/craft of building succesful small i.c. engines at least a thousand years!! If it works the way it is supposed to, I will brag about it to you. If its a miserable flop (Like some things I have posted about) I will tell you.---Stay tuned!!!
    Brian Rupnow

  • #2
    Some model airplane gas engines have needle valves like that


    • #3
      For doing small accurate work there is nothing like a set of collets. For what you like to do there is no excuse for not having them.
      Last edited by topct; 03-14-2013, 05:59 PM.


      • #4
        No venturi effect necessary? Helpful?



        • #5
          Guido---The reduction in diameter in the red part (air intake) provides enough venturi effect to lift the fuel 1 to 2" from the tank. I know it works because what I have shown in the solid model is what I am currently running . The only difference is the needle. the red "handle" at the top of the needle and the pale green "spray bar" the needle point goes into. I have just finished the new spray bar. It looks exactly like the old spray bar I am currently running, except the threaded end is 1/4" longer, the thread on the outside is longer by 1/4" as well. and the internal #2-56 threads are not there. The part in the picture in this post is what you see in the solid model. that humongous needle you see beside it is actually .048" diameter x 2 3/4" long. It will be cut off like the grey needle in the solid model when I get the next part (the red "handle for the needle) finished.
          Brian Rupnow


          • #6
            Gosh, isn't that sort of like cheating?


            • #7
              I know eff all about making ME engines but...

              I have a ME engines book by Edgar which he states that the carb venturi should be as local as possible to the carb jet with as little restriction as possible afterward.

              Your design will lead to surface "skin effect" that leads to poor atomisation and wetted walls , if I read the book correctly.



              • #8
                And here we have the second part of the magic needle trick. Now according to the book "The Shop Wisdom of Philip Duclos" here's what happens next. I shove the needle down into the spray bar untill it won't go any farther, which should mean it is totally shutting off any fuel that wnats to come up the spray bar from the fuel line (which is hooked to the other end of the spraybar when all is together.) Then I screw the knurled part onto the spray bar and screw it down on the thread until it bottoms out. (The knurled part has an 8-32 internal thread to match the 8-32 external thread on the spraybar.) Then I back the knurled part off one full turn, which on an 8/32 thread is 1/32". Then------Drumroll please----I solder the needle to the knurled part at the top where the needle exits, and grind off any of the needle that is left sticking out. The split in the knurled part is there so that the body of the knurled part can be "squeezed in" a bit with a pair of pliers. This prevents vibration from the running engine from unscrewing things and changing the setting. Sure is a nifty trick if it works.
                Last edited by brian Rupnow; 03-14-2013, 09:52 PM.
                Brian Rupnow


                • #9
                  For what it is worth Brian, you will probably curse your available thread choices, (UNF,) before you are finished. I suggest that you call Tracy Tools in the UK. As an example, they have ME taps and dies from 1/8X40 to 1/2X 40. Since you are working in brass, carbon steel will be fine. This way you will get valve control without tiny bits to machine. The fact that these are some strange, to us, thread shape does not really matter.
                  I know that when I get into some small models with packing glands, etc., I will order some of these. Our available thread choices do not "scale" worth a damn for smaller work. In fact, I believe that the BA sytstem was designed primarily for model work.
                  Duffy, Gatineau, Quebec


                  • #10
                    Its only cheating if you get caught!!
                    Brian Rupnow


                    • #11
                      It's clearly NOT cheating........

                      Here is a commercial .049 with that design


                      Keep eye on ball.
                      Hashim Khan


                      • #12
                        A couple things.

                        Why can't you hold over size material the size of the gripping surface in your 3 jaw and then machine the hollow stem (including tapping or running a die over it) before cutting or parting off? Would it really matter f the grip is 0.003" off center?

                        BTW, x 2 for threading parts this small with taps & dies. If you're really possessed with accuracy and can spare some material, you could counterbore at teh tap OD for ashrt length before threding and conversely turn a lead in just over the tread miinor diameter to align the die. These lead ins could be cut off after threading.

                        Just my 0.02 TL. YMMV.
                        Design to 0.0001", measure to 1/32", cut with an axe, grind to fit


                        • #13
                          Several years ago I built myself a propane torch head, which is fed through a flexible line from a 5 lb bottle. The head is totally home-made, including the gas flow adjustment valve. I figured to use a very fine thread for the adjustment, and as it turned out, the guide rollers in a vcr have a very fine thread. I ended up modifying one of them (these were stainless) and it works fine. I don't recall what I did for the needle, but there is certainly not a lot of turning to do to go from flame out to full tilt boogie, even with that fine thread. My design has an O-ring to seal the shaft, and when you make the adjustment and let go of the knob, the O-ring turns the shaft back the slightest amount- but it's enough to alter the size of the flame significantly.

                          In general you want that kind of adjustment to be very fine and have no play in it whatsoever- an 8-32 thread is pretty coarse for that. Depends on how long and shallowly tapered the needle is of course.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                          • #14
                            Mr Rupnow, I think it's ingenious to make use of just how finely made bought sewing needles are. You might be interested to see how they are made:


                            Curiously enough, the video features a plant belonging to the company from which I used to buy my health insurance - the needle factory had a cooperative healthcare scheme open to non-employees...


                            • #15
                              Great video on making needles, very interesting!!