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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:
Below is a picture of my setup.
The second picture shows a close up of the threading insert.
Next I center drilled and then drilled a hole 1/2 way through the fitting.
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.
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.
- 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
The second picture shows a close up of the threading insert.
2. Close up of threading tool
Next I center drilled and then drilled a hole 1/2 way through the fitting.
3. Drilling center hole
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
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
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