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Name: Joe
Status: Student
Grade: 9-12
Location: NY
Country: United States
Date: November 2006


Question:
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?



Replies:
Hi Joe,

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!

Regards,

Bob Wilson


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.

Vince Calder


Dear Joe,

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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