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sheepherder · 05-02-2014, 03:06 PM

I was asked WTF that scribbling above is...

It's part of my simple math to decide which size tap to use for the Nambu barrel thread...The left half compares 1/2", 9/16", and 5/8" thread diameters with the Nambu receiver to decide on the best compromise tap to use...(I'll thread the barrel to match the tapped receiver)...

The right side compares different ways to determine Luger barrel thread crush factor...Some say 'extractor notch width', some say 1/16 (or 1/8) revolution of the barrel, and "the book" says .003" (which can be measured with a feeler gauge)...I compared them all, given that the Luger uses 20tpi and 1/8 revolution is un-lapped, while 1/16 rev is 'lapped in' by being tightened & loosened at least three (preferably more) times...

The yellow post-it shows common NEF (National Extra Fine) thread pitches and the corresponding dimension between threads...

I frequently do my calculations on the backs of envelopes and when I can see ahead a step or two I abandon the envelope and go directly to the lathe/mill...Leaving the written account unfinished...

Below is my final comparison of the three tap sizes. Because the Luger barrel thread is ~.708" dia and the Nambu bolt is ~.704" dia, I decided not to use the OEM Luger barrel thread...It would not have left any boltface in the receiver. In a perfect world, I would want equal wall thickness for the barrel stub and receiver front. Or in this case, barrel stub and bolt recess diameter. That would give me 35/64" wall thickness for each. There's no corresponding commonly available tap for that, so I chose the closest tap giving any extra material to the boltface. I want to keep as much of the OEM boltface as possible. It simplifies things considerably (headspace, breech clearance, locking block engagement, etc). That tap is 9/16" NEF [24tpi].

Some of this math crap gets tossed out, some gets saved...Some gets left around for months and when I come across it, I ask myself...WTF is this???

05-02-2014, 03:06 PM	#20
sheepherder Lifer Lifetime Forum Patron Join Date: Feb 2009 Location: ...on the 'ol Erie Canal... Posts: 8,181 Thanks: 1,398 Thanked 4,438 Times in 2,327 Posts	Crib Sheet I was asked *WTF* that scribbling above is... It's part of my simple math to decide which size tap to use for the Nambu barrel thread...The left half compares 1/2", 9/16", and 5/8" thread diameters with the Nambu receiver to decide on the best compromise tap to use...(I'll thread the barrel to match the tapped receiver)... The right side compares different ways to determine Luger barrel thread *crush factor*...Some say 'extractor notch width', some say 1/16 (or 1/8) revolution of the barrel, and "the book" says .003" (which can be measured with a feeler gauge)...I compared them all, given that the Luger uses 20tpi and 1/8 revolution is un-lapped, while 1/16 rev is 'lapped in' by being tightened & loosened at least three (preferably more) times... The yellow post-it shows common NEF (National Extra Fine) thread pitches and the corresponding dimension between threads... I frequently do my calculations on the backs of envelopes and when I can see ahead a step or two I abandon the envelope and go directly to the lathe/mill...Leaving the written account unfinished... Below is my final comparison of the three tap sizes. Because the Luger barrel thread is ~.708" dia and the Nambu bolt is ~.704" dia, I decided not to use the OEM Luger barrel thread...It would not have left any boltface in the receiver. In a perfect world, I would want equal wall thickness for the barrel stub and receiver front. Or in this case, barrel stub and bolt recess diameter. That would give me 35/64" wall thickness for each. There's no corresponding commonly available tap for that, so I chose the closest tap giving any extra material to the boltface. I want to keep as much of the OEM boltface as possible. It simplifies things considerably (headspace, breech clearance, locking block engagement, etc). That tap is 9/16" NEF [24tpi]. Some of this math crap gets tossed out, some gets saved...Some gets left around for months and when I come across it, I ask myself...WTF is this??? Attached Thumbnails __________________ I like my coffee the way I like my women... ...Cold and bitter...