hammerscale

Amateur Machinists' Tools, Techniques and Materials

Archive for November, 2011

Accuracy versus Precision

Posted by hammerscale on November 30, 2011

The recipe calls for precisely two tablespoons of butter

Implying that deviation from the measure is an issue. It’s assumed that you’re striving for accuracy when you whack away with a knife.

To measure chicken’s temp accurately, place the thermometer in the thickest part of the breast.

Here, the temperature is left unknown, as it’s value is irrelevant, but the method to determine the temp is at issue.

Precision is (almost) always accompanied by a value. All though we may say “this is a precision instrument”, meaning finely made, high quality. A Mercedes Benz is precision engineering at it’s best. In these cases, precision is a adjective, a marketing term, a variance of awesome, incredible, really cool. Almost hyperbole.

We don’t say a car is accurately made. We actually hope that it’s accurately made. Precisely made indicates the fineness of design, repeatability of components, closeness of tolerances, consistency of engineering.

Precise is the value (target)

Accurate is the attempt at the target

Precision= fineness of a standard, or value.

Accuracy= closeness to a standard, or value.

Precision is stated before the fact.

Accuracy is determined after the fact.

Accuracy is judged.

Precision is called for.

Finally, if we recall from the movie, during the competition, Robin Hood shoots his arrow last and splits the other guys arrow. An accurate shot, or a precise shot? Or just lucky?

Robin Hood (The Adventures of)

The Adventures of Robin Hood

In another movie, Ulysses (Kirk Douglas) must shoot an arrow through a bunch of ax head eyelets. A precise shot, or an accurate shot?

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More Chatter

Posted by hammerscale on November 22, 2011

Adding to an earlier post regarding chatter, recently while making a few small parts at the lathe, I inserted a lengthy piece of steel rod (48″) in the headstock.  I figured I was fine as long as the material didn’t whip.  Really severe chatter followed, and at first I was stumped, as the tooling was freshly sharpened and I was running the machine as usual;  then I saw the end of the rod sticking out of the left side of the headstock, wagging up and down like a dog’s tail.  Using my hand as an outboard center rest smoothed out the ripples, but it’s neither a safe nor practical answer.  Cutting the stock down to about a third its length solved the problem.

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Building the Unisaw – Woodcraft Youtube Video

Posted by hammerscale on November 4, 2011

 

Nice video on the still Made in USA Delta Unisaw –

 

 

 

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