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Expanding and cutting new threads on existing threaded hole

skydiverMN

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#1
I'm a newbie and am hoping to get some guidance on enlarging and re-tapping.

I have a steel reducer bushing that has 3/8" NPT female threads that needs to get to 1/2-28. From what I've read I need a 15/32" drill bit to create the hole for my 1/2 tap. Can I drill directly into the existing 3/8" tapped hole in one pass, or should it be done using slightly bigger bits in each pass? I plan on using a manual drillpress, a cobalt 15/32" bit, and plenty of cutting oil. What would be a good rotational speed of the press? I'd appreciate any help or guidance, thanks!
 

CluelessNewB

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#2
But isn't the hole already too big? 3/8" NPT pipe has an nominal outside diameter of 0.675 and tap drills for 3/8 NPT is 37/64 greater than 1/2"!
 

skydiverMN

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#3
Okay, so I'd get 1/4" instead of 3/8, and I'd just have more to drill out.

But I'm still confused. So, when you're talking about 3/8" NPT it's inside diameter for tapping, but when you're talking about 1/2-28 it's outside diameter?
 

CluelessNewB

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#5
Okay, so I'd get 1/4" instead of 3/8, and I'd just have more to drill out.

But I'm still confused. So, when you're talking about 3/8" NPT it's inside diameter for tapping, but when you're talking about 1/2-28 it's outside diameter?
Historically I believe pipe sizes were originally the inside diameter of the pipe but as wall thickness of pipe have become thinner over time the size doesn't mean very much anymore.

I suspect you may need to go down to 1/8" NPT to have a hole small enough for a 1/2-28. There is a link to a table of pipe sizes below.

http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html

Normal (non-pipe) sizes are more or less the outside diameter of the fastener and make much more sense.
 

Bill C.

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#6
Okay, so I'd get 1/4" instead of 3/8, and I'd just have more to drill out.

But I'm still confused. So, when you're talking about 3/8" NPT it's inside diameter for tapping, but when you're talking about 1/2-28 it's outside diameter?
Pipe threads are measured by the inside diameter of the pipe. My Dad was a maintenance-man and threaded a lot of pipe. External threads like 1/2-13 are always measured by the largest outside diameter.

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