Sunday, October 17, 2010

195/256ths of an Inch

Pictured is my fractional inch vernier caliper in use taking the bore dimension of a hose-thread fitting. I introduced this tool in an earlier post here.

I use fractional inch measure a great deal, but until very recently, I never gave any thought to the possibility that very small fractions of an inch could be useful to actually work with; that was the province of decimal fractions of an inch or millimetres.

It came up that I needed to turn the end of a wooden pan-handle to a diameter that would be an interference/press fit in the bronze fitting in the picture, and that got me looking more closely at the caliper's resolution. What I found is rather interesting. (It is to me, at least.)

The caliper can resolve 128ths of an inch directly, and you can interpolate 256ths quite accurately. If you enlarge the photograph and look closely at the inch cursor, you'll see that it's reading 1/128th of an inch in excess of 3/4s of an inch. (That works out to 97/128ths, but don't bother with such expressions. I titled this post as I did for effect, but in actually working with such tiny fractions, it's much easier to just add them to the nearest larger fraction; hence, "1/128th of an inch in excess of 3/4s of an inch".) Let me digress here slightly with a brief list of some decimal conversions:

1/16th = 0.0625

1/32nd = 0.03125

1/64th = 0.015625

1/128th = 0.0078125

1/256th = 0.00390625

So, 1/256th of an inch is just shy of four thou. For fitting wood into a metal ring, that's press fit territory.

By interpolation, I set the caliper to 1/256th of an inch greater than the metal fitting's bore diameter and locked the slide in place there. And that's what I used to gauge the diameter of the end of the wooden handle as I turned it down. Pictured is the caliper set for that dimension -- 195/256ths of an inch, as in this post's title.

It worked. Once I got the diameter down to where the caliper would just go over it, I had my interference/press fit ready to assemble.

The rest of that story is here. (The outcome turned out not to be entirely successful, but for reasons that don't discount the validity of what Ive laid out here.) I'm looking forward to possibly finding further application for 256ths of an inch, and wood-into-metal press fits.

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