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Manually machining 316 SS bulk head fittings on lathe

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  • Manually machining 316 SS bulk head fittings on lathe

    I am building a 570 square foot addition to the back of my garage. My friend Ray helped me to install and spackle the drywall. He needed two custom 316 stainless steel bulk head fittings for his boat, so I machined them for him (It was the least that I could do). The exterior thread is a M22 x 2.5. My lathe has an English lead screw so I couldn't open the split nut, without loosing alignment so I chose to:
    • Use a left hand insert
    • Install the threading tool on the back of the lathe
    • Run the lathe backwards starting in the undercut
    • Thread from the chuck outwards towards the tailstock.

    Below is a picture of my setup.
    1. Blank turned ready for threading
    Click image for larger version

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    The second picture shows a close up of the threading insert.
    2. Close up of threading tool
    Click image for larger version

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    Next I center drilled and then drilled a hole 1/2 way through the fitting.
    3. Drilling center hole
    Click image for larger version

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    I needed a 1/4"-18 NPT thread. My lathe doesn't have a taper attachment so I used a tap. Tapping stainless steel can be difficult and can break the tap if it isn't done correctly. I used the .422" tap drill, which is recommended for a reamed hole. (as opposed to the .438" hole, which is recommended when the hole isn't reamed.) I used the compound on the lathe to "ream" the hole on a 1:16 or 3/4" per foot taper on the inside diameter of the hole, because I didn't feel like purchasing a reamer. The reamed hole will minimize the metal that the tap must remove and reduce the torque on the tap.
    4. Reaming (Boring) hole with compound

    Click image for larger version

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    I also used a tap that had a special coating, which was designed for stainless steel. I used tap magic tapping fluid that was for general use including stainless steel. I chucked up the tap and tapped the hole about 3/4 of the way under power. I did the last two threads by hand so that I could get 4 threads of engagement when the fitting was hand tightened.
    5. Tapping center hole

    Click image for larger version

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    Miller Thunderbolt
    Smith Oxyacetylene Torch
    Miller Dynasty 200DX
    Lincoln SP-250 MIG Welder
    Lincoln LE 31 MP
    Lincoln 210 MP
    Clausing/Coldchester 15" Lathe
    16" DuAll Saw
    15" Drill Press
    7" x 9" Swivel Head Horizontal Band Saw
    20 Ton Arbor Press
    Bridgeport

  • #2
    The tapping process was easy -- it was no different than tapping mild steel or aluminum.
    6. Close up of tapped hole

    Click image for larger version

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    Next I used a parting tool to remove the partially machined fitting from the lathe.
    7. Parting the fitting off

    Click image for larger version

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    The last picture shows the fittings all done.
    8. Fittings all done

    Click image for larger version

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    Miller Thunderbolt
    Smith Oxyacetylene Torch
    Miller Dynasty 200DX
    Lincoln SP-250 MIG Welder
    Lincoln LE 31 MP
    Lincoln 210 MP
    Clausing/Coldchester 15" Lathe
    16" DuAll Saw
    15" Drill Press
    7" x 9" Swivel Head Horizontal Band Saw
    20 Ton Arbor Press
    Bridgeport

    Comment


    • #3
      Good pictures along the journey, as usual, Don.

      Comment


      • #4
        Nice pictures, great reading, excellent workmanship. I'm again very impressed.

        Comment


        • #5
          Awesome job. Love the progression of pictures.

          Comment


          • #6
            Nice work, thanks for the clear explanations and pics
            Richard

            Comment


            • #7
              Originally posted by ryanjones2150 View Post
              Good pictures along the journey, as usual, Don.
              Originally posted by Noel View Post
              Nice pictures, great reading, excellent workmanship. I'm again very impressed.

              Originally posted by Oldgrandad View Post
              Awesome job. Love the progression of pictures.
              Originally posted by Ltbadd View Post
              Nice work, thanks for the clear explanations and pics
              Thanks, for all of your kind comments.
              -Don
              Last edited by Don52; 06-15-2019, 02:02 PM.
              Miller Thunderbolt
              Smith Oxyacetylene Torch
              Miller Dynasty 200DX
              Lincoln SP-250 MIG Welder
              Lincoln LE 31 MP
              Lincoln 210 MP
              Clausing/Coldchester 15" Lathe
              16" DuAll Saw
              15" Drill Press
              7" x 9" Swivel Head Horizontal Band Saw
              20 Ton Arbor Press
              Bridgeport

              Comment

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