What size bore for threading 1 1/8x8tpi

bustedanvil · May 26, 2017

Okay so I'm making an adapter for my wood lathe with spindle thread of 1 1/8x8tpi and adapting down to more common 1x8 tpi I have the 1x8tpi part threaded I did on my lathe but I can't find what diameter to bore it out to for the 1 1/8 x 8tpi part. I found 1 1/8x7 is 63/64 and 1 1/8x12 is 1 3/64 but can't find anything for 1 1/8 x8.
Picture just because I'm 17 and a lot of the older guys like to see things I make because I'm young.

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Gobo · May 26, 2017

That info must be available somewhere on the web. Or calculate it.

johnoder · May 26, 2017

Add .125" to the info for 1"-8

cpifer3 · May 26, 2017

https://www.barnhillbolt.com/page/specs/InternalThreadsClass2B100.htm

Bobw · May 26, 2017

Same as with a metric thread. Subtract the pitch.. M8x1 is 8mm-1mm = 7mm hole.

1-1/8 - 8(so 1/8 for the pitch).. so 1.125-.125 = 1" hole..

Another example. 3/8-16... 3/8 minus 1/16 = 5/16..

Fulcircleny · May 26, 2017

Here is another way..

1.299 / by number of threads per inch = "double depth" of the thread (2x the "height") 1.299/8= .1623
then multiply your double depth times the percentage of engagement you want say 75% for example .1623 * .75 = .1217
lastly subtract .1217, from 1.125 1.125-.1217 = 1.003 hole size.

this formula works

good luck,
Chris

EPAIII · May 26, 2017

Bob's answer is correct if you are using a tap and want a 75% thread. However, most holes of this size and for lathe spindle attachment are bored and then single point threaded in the lathe. At least that is how I do it.

My Excel Tap Drill Calculator, which is still a work in progress, handles this easily. For a 1 1/8 - 8TPI, 60 degree thread with 12.5% crest flat and root fill and a 100% thread it gives a bore diameter of 0.9626" or 24.45mm. Since I specified 100% thread, that will also be the minor diameter of this thread.

My calculator also suggests the closest available drill size is 31/32" (0.96875").

Bobw said:
Same as with a metric thread. Subtract the pitch.. M8x1 is 8mm-1mm = 7mm hole.

1-1/8 - 8(so 1/8 for the pitch).. so 1.125-.125 = 1" hole..

Another example. 3/8-16... 3/8 minus 1/16 = 5/16..

Delta Iron Works · May 26, 2017

depends on tapping or single point
Tap is 75% of DD
Single point is 83 1/3%

1.125 x 8 tip
Bore= size-(DD x %)
Bore= 1.125-(DD x .833)
Bore= 1.125-(.135)
Bore to .990

bustedanvil · May 26, 2017

EPAIII said:
Bob's answer is correct if you are using a tap and want a 75% thread. However, most holes of this size and for lathe spindle attachment are bored and then single point threaded in the lathe. At least that is how I do it.

My Excel Tap Drill Calculator, which is still a work in progress, handles this easily. For a 1 1/8 - 8TPI, 60 degree thread with 12.5% crest flat and root fill and a 100% thread it gives a bore diameter of 0.9626" or 24.45mm. Since I specified 100% thread, that will also be the minor diameter of this thread.

My calculator also suggests the closest available drill size is 31/32" (0.96875").

Awesome. Thanks to everyone who has responded to this. Y'all are very helpful. I'm threading it on the lathe because I can't afford to buy a 1 1/8x8 tap and a 1x8 die for a one time use and also I've never cut threads on it before. I plan to become a machinist after I do a few years in the navy

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Illinoyance · May 26, 2017

Your handy little Starrett thread pitch gauge had the double depth of thread shown for most common pitches. Subtract DDT from OD to get minor diameter for 100% depth of thread.

Gobo · May 26, 2017

bustedanvil said:
Awesome. Thanks to everyone who has responded to this. Y'all are very helpful. I'm threading it on the lathe because I can't afford to buy a 1 1/8x8 tap and a 1x8 die for a one time use and also I've never cut threads on it before. I plan to become a machinist after I do a few years in the navy

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Keep it up. You are doing great.

manualmachinist · May 26, 2017

My go to when I just need to come up with a bore size that will work is a formula from the old Henrey Ford Trade school textbook (Shop Theory).

Thread OD -(1.0825/TPI) will get a size that will work. Used it many times for oddball stuff. Always worked for me.

Bill D · May 26, 2017

Not sure how it will affect the bore size but I assume this is some kind of acme or buttress thread not a standard pointed thread form.
Bill D.

bustedanvil · May 27, 2017

here are pictures of it completed. Thanks everyone for your input and advice. It fits good could have cut it off longer to leave more inner threads but live and learn. That stainless sure is hard stuff. I want this to serve as a reminder that wether you're 17 like me or 77 if you want to learn something or do something Do it.

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EPAIII · May 27, 2017

Glad it worked!

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