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Machining small valve spindles

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  • #16
    QUOTE=JCHannum;841089I have worked on a few of these things as well. What you are calling a plug valve is more properly termed a rotary plug valve. It, like a ball valve, is not used in throttling applications. Its primary application is on-off control of flow.
    Application or not , a plug valve and a globe are not remotely similar'

    When it comes to throttling applications, globe style valves are used. These are loosely referred to as sliding stem valves as the valve stem operates in an up-down motion. There are three basic types: disc, which are used for on-off control and offer coarse control of flow; plug, which offer positive shutoff yet still provide finer control of flow than the disc and needle, which offer very fine control of flow but are not good for positive shutoff conditions.
    So I guess you never heard of a gate valve?

    The valve in question is a plug style and in the small size, a floating plug is not needed. The tapered plug, machined concentrically with the stem, will seat in the tapered seat of the valve body if reasonable care is taken in its construction. These are model valves for low pressure applications and are used primarily for flow control to control the speed of the model. There is usually a primary shut off valve on the supply line so there is no real concern that the valve be capable of going to full shutoff condition.

    My question was how would the concentricity be assured. The OP is being advised to change set ups and all manner of things that may affect the concentricity. I was curious about how to deal with it without complicating the model valve. I am well aquainted with valves and valve types. I merely asked how it would be dealt with. I did not need a bunch of BS from people who do not know one type from another.

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    • #17
      Of course I have heard of gate valves. I have also heard of ball valves, pinch valves, butterfly valves, diaphragm valves, you name it. They are not what is being discussed here, so it would be pointless to mention them.

      Machining a model valve of this type is no different than machining the full sized version. The machining of the body, bonnet and stem requires care be taken to ensure the areas that are critical to alignment are handled appropriately. To this end, if those areas are machined in one setup each, adequate precision will be attained. I do not see where the OP has been given instructions that would lead to a wildly inaccurate product. With any project of this type, it is important to understand the requirements of the final product and take appropriate care to ensure the desired outcome is achieved.

      My procedure would be to turn the stem in a collet or chuck from oversized material. I would turn with the stem toward the tailstock and the plug at the headstock end. The bonnet, I would also turn with the lower end toward the tailstock. I would drill and thread for the stem, turn the OD and single point the threads for attachment to the body.

      The body is a bit more of a project due to its being a casting and multiple setups required. I start by cleaning up the casting with a file and squaring the ends and "cheeks". I then drill and tap the inlet and outlet. It takes a bit of fixturing I will not go into, but I now set the body up in the milling vise and drill for the seat per the instructions included with the castings. The body is drilled and tapped to accept the bonnet at this point.

      I am not some BS'er as you imply. My career was in the chemical and food processing industries in engineering and maintenance. As far as the project in question, since I have actually made several of these items, I am fully aware of the requirements of the job. PM Research provides the castings in kit form. The valve body castings are provided in a tree of four bodies, the kit also includes handwheel castings, SS stock for the stem and three pieces of brass hex for the bonnet, packing nut and handwheel nut.

      This photo is of one valve I made several years ago with a lubricator attached as well as a raw body casting with handwheel casting and barstock as supplied by PM Research. The valve is 1/4" model pipe thread and is 11/16" long. The tee fitting is also from PM Research castings and was machined in a similar manner.

      Jim H.

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