Why Use Fractions?

Well, that is the way it used to be done. The only definite advantages of the modern standardized measurement systems are for easier communication between designer and builder and for parts interchangeability. You can get a pretty astonishing degree of precision with relatively simple tools. If you are building your own, start with a water level or a plumb bob and those get you to a straight edge and square. 3 flat pieces of material with some abrasives or cutting tools and you can make a plane. Add a compass and some dividers and you are in business. The next huge leap is when you originate a screw.
 
I wonder how many remember the teacher saying, class we are about to learn a new math 'easier way'. The decimal system. Proud to say of my straight "Fs" in math, but I never forgot that. A ruler, I have to count the tick marks with a pencil. I can look at decimals and instantly know where I am. Ya all know I am doing the machining on a 1923 Ford which is all in fractions. And I had to google how to convert decimals
into fractions.? bad huh.. I do remember calculators in high school but you had to move them with a fork lift. So I'm machining thrust
shims recessed in the crank and I find out my new mill dros convert to fractions. Bad habit, not reading instructions. To me math is the
same as english. Why is there a to, too, two, 2 and II??? same as 5s and 6s and 1/2s ? And then Metrics. Thats a flat No in this shop.
 
Mr.Kirtley states it perfectly when he distinguishes accuracy from precision. It's worth remembering that the building of machinery like cannons, clocks, or steam engines and the machine tools that built them predates any standardized system of measurement. They had a straight edge, a compass, and a rigorous knowledge of Euclidean geometry; and that enabled them to do very sophisticated and accurate building.

Start with just those simple tools to design in terms of pure, and perfectly accurate, geometry without numbers of any kind getting in the way and you will find it very natural to divide circles and then lines into thirds and fourths.

Today workers need to be trained to use precision tools, which have evolved to divide into tenths; but the most precise work will still be done by those educated to understand the principles behind the mechanisms we build, which are based on fractional division.
 
Thanks to everyone who replied. I learned some history. As for the "ruler trick", I already knew that one although I don't believe I've ever used it.

However, no one really answered my question, perhaps because I did not ask it well.

What I was pointing out was that Elmer's drawings are made so that someone can duplicate the device. Most of the operations will be done on a lathe or on a mill. The dials of these machines have decimal markings, not fractional. So why doesn't Elmer use decimals rather than fractions? Was his equipment marked in fractions? Did he make the drawings and then convert to decimals when he machined a part? If so, why the extra effort? It may be easy to add fractions, but it is even easier to add decimal numbers, and you don't have to convert.

If you need a part that's "about 7/64" (= 0.109375), why not just specify 0.109 or 0.110?

So what am I missing?
 
Mainly like all things it comes down to the cost to manufacture something. When a machinist looks at a drawing and sees dimensions in fractions they can pretty much assume that the tolerance is at least + or - 1/64" unless specified. A trained machinist can layout fractional hole locations, drill them with a drill press and hold +- 1/64". If its a part size it can be sawed and sanded to size. This is much cheaper that setting it up in a mill or lathe to be machined. Sometimes in a production shop they may have a machinist layout the part and then have a grunt do the drilling, sawing or sanding.

I can remember long ago I was applying for a job to do grunt work the person doing the interview tested me on fractions and decimals. They pointed to a spot on a ruler and asked what fraction it was. Then I was asked what the decimal equivalents were of several common fractions. Do that to a 16 year old today and see how many would get the job without using a calculator or Google.
 
CPCaballero said:
Thanks to everyone who replied. I learned some history. As for the "ruler trick", I already knew that one although I don't believe I've ever used it.

However, no one really answered my question, perhaps because I did not ask it well.

So what am I missing?
Click to expand...

Precision is expensive. Either time or money.

There are measurements that are appropriate to the tools. You don't specify a bandsaw cut to a thousandth. It is not accurate enough. You also don't buy stock to that precision either (usually -- you can buy precision ground stock but it is a premium price and available in limited sizes.) Another example is drill bits. They don't make an accurate hole. They are "close enough." If you need and exact fit, you use other methods (boring and reaming).

You make the decision based on what you are doing and how things fit together. Some measurements are critical, some are not. I was making some tool holders for my QCTP. The size of the block was important to about a 0.25 in. The size of the slot for the holder was important to about 0.05. The size of the dovetail was critical to the 0.001 to get a good fit.

The way you specify dimensions reflects the precision required. If you specify something as being 1/4" implies a level of precision which is different than when you specify 0.25 in or 0.250 in or 0.25000 in. Some are specified as tolerances. Some of these ways to specify measurements are just customary. That is part of the meat that you get in machinist training.

For amateurs like me, I just look it up or make a best guess.
 
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