Threading of Soda Bottles
Country: United States
Date: November 2006
I am a high school student at
Brooklyn Technical HighSchool. We are working on drilling
and tapping things, strengths of different stuff and
so on. He has told us that anybody who can identify
the thread pitch and diameter of the common soda
bottle will not have to make the drill guide. That
is a lot of filing. I cannot find this information anywhere
on the net. Anybody know where or what the answer is?
Well, I am not going to give you the answer because this
would defeat the purpose of your teacher's question. But
there is a very easy way get the answer.... just measure the
threads on the bottle! Use a pair of calipers (your school is
bound to have some!) to measure the outer diameter of the
threads, and remembering that the measured major thread
diameter is always a little less than the theoretical "book
value", round the value up to the nearest standard increment.
For example, if the thread major diameter measured 0.99", you
can bet that this is a 1" diameter thread.
To get the pitch, simply use the calipers to measure the
distance from across from one thread to the next. Divide this
into 1 inch, and you have the number of threads per inch. Of
course as the a result of inevitable measuring inaccuracy,
the result will not be an even number, so look up a table of
standard thread sizes and find the closest match.
As an example, suppose you picked up a bolt that measured
0.242" in diameter across the threads, and as close as you
could eyeball it, the pitch between threads was 0.053".
Looking up a table of standard threads, you would see that
0.242" is close to 0.250" (1/4 inch), and a 1/4" bolt is
available in several thread pitches, one of which is 20
threads per inch (known as "1/4-20" bolt). Since 20 threads
per inch can be found by simple arithmetic to be the same as
a pitch of 0.050" (very close to your measured 0.053"), this
"unknown" bolt is obviously a standard 1/4-20 bolt.
Now, I will leave it to you to try to measure the pop bottle
threads! By the way, make the drill guide anyway! This is
invaluable practice on how to properly handle tools!
I am not sure that I completely understand your question -- that's
OK. A suggestion: Rather than examining the bottle cap, why not look
at the matching threads of the glass bottle. These have to match the
cap, otherwise the cap would not seal properly and would leak. Why
not measure the thread pitch and diameter of the glass bottle itself?
You would not have to do a lot of filing, and the pitch and diameter
of the cap should closely match that of the bottle.
I have a great idea! Why do not you find a common soda bottle and
measure the thread pitch and diameter?
If you do not know what a thread pitch or a diameter is, send in
another question and I will explain in detail. If you do know, the
measurement should be easy.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012