Home Products Forums Ask the Engineer Tap for gaseous fuel mix

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    We are making several prototype gaseous engine fuel mixture controllers of plastic material. It is necessary to move a loose fitting rectangular plastic plug in and out of a plastic hole to create a partial obstruction to the flow of the gas.

    It seems reasonable to tap through the plastic block for your 1/4-20 stainless acme lead screw material and drive the rod with a stepper motor.

    The lead screw needs to couple to a 5mm shaft. There are hardly any forces, fit is totally non critical, but it should not rattle around. Are there suitable couplers or will we have to grind the stub end? We are not married to the 5 mm shaft, there are others

    What do you suggest for a suitable tap? Any suggestions for nice tapping plastic plug material would be appreciated. The prototypes will only be in service until some ideas are proven, so long term suitability of the plastic is not an issue.

    We considered using your plastic nuts for the extending-retracting plug, but cannot figure out how to keep it from spining in the hole. Mill flats on the sides?


    For the material we recommend acetal plastic (Delrin or equiv.). The internal threads should be ½” maximum length.

    We don’t make couplings. Most users couple directly to the stepper motor output shaft with commercially available, helical cut aluminum body couplings with set screws which work well for limited loads. I would use ¼” output shafting on the stepper and ¼” bore couplings and you won’t have to do any machining.

    Any method to prevent rotation will work, mill flat(s), a keyway, pin(s), etc…


    Linear space is limited, but I could use a larger shaft. How does this sound? We use your plastic nuts for the plugs, mill them flat on the sides and maybe all four sides. Put a piece of 3/8 lead screw in a lathe collet, drill and ream it to the size of the stepper shaft. Glue or set screw it solid. There would be threads almost all the way to the stepper body. It is not important that the attachment is perfectly coaxial. It can wobble around in the hole a bit and still do the job.

    How much torque will it take to turn the shaft in the nut?


    Normally steppers have no thrust capability so direct coupling is rarely done. You can try it and see what happens.

    Most installations isolate the thrust with separately mounted thrust or angular contact bearings and then couple with a flexible coupling to allow for some misalignment.


    I realize what you say is true, but for what we are doing and for the length of time it will be in service,  it will work.  It is mostly for software development so the logic and algorithms can be proven and demonstrated on a little test engine.  Big engies already have big expensive valves

    After it homes, softly, on each startup it will know where it is and not go to the ends of travel during operation.  Pushing the plastic nut in and out of the natural gas fuel stream will load nothing.  It is more of an obstruction than a

    Does the stainless lead material drill nicely?


    The material is 304ss. It machines readily with carbide and at slow speed with HSS tooling.

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