CONVERTING METRIC & INFERIAL THREAD PITCHES

[mklotz]

Lots of folks seem confused about the process of converting from one system to another. This note provides an easy-to-remember rule that can be used for both conversions - inferial to metric or vice versa.

M = metric pitch measured in mm/th(read)
E = inferial pitch measured in tpi [th(read)/inch]

25.4 (mm/in) / M (mm/th) = (25.4 / M) (mm/in * th/mm) = 25.4 / M (th/in) = 25.4 / M tpi

25.4 (mm/in) / E (th/in) = (25.4 / E) (mm/in * in/th) = 25.4 / E (mm/th)

i.e., whatever you've got, E or M, divide it into 25.4 to get the other

[JoeVanGeaux]

" inferial "

Ha! I'm going to use that forevermore!

[mklotz]

" inferial "

Ha! I'm going to use that forevermore!

And you get to decide what the "inf-" part was derived from...

inferior
infuriating
infernal
infantile
...

[wizard69]

Hi Marc

While I use the metric system daily at work , I can’t really call the Imperial thread system inferior. Now old fashion maybe, but the reality is the clean ratios means manual machining is a snap.

Great post by the way

[mklotz]

Hi Marc

While I use the metric system daily at work , I can’t really call the Imperial thread system inferior. Now old fashion maybe, but the reality is the clean ratios means manual machining is a snap.

Great post by the way

First off, my name is MarV, not MarC.

The inferial system, in addition to having a bizarre collection of units with arbitrary relations, suffers from some extremely bad nomenclature problems.
The most important information about a drill is the size hole it makes. Naming drills with arbitrary letters and numbers hides their true size. The fractional nomenclature makes it difficult to decide what the next largest/smaller size is unless you're adept at mental fraction arithmetic. In the areas where the metric measurement system is used, drills are labeled by their diameter, the size hole they make.

Thread labeling is equally bizarre. You need to be able to mentally compute a formula to obtain the diameter of a numbered thread of have a table to hand. Then, because the labeling is not open-ended you end up with labels like 000-120. The formula still works but you have to know how to convert 000 to a negative number. Tap drill computations are equally complex and need not be. In metric areas threads are labeled directly with their diameter and pitch and tap drill computations are simple subtraction.

You just can't beat a system that's been thought out and designed to be easy to use. The inferial "system" (really a hodge-podge collection) is its own best argument for why it should be discarded.