Without Metric Threads We’re Screwed

By The Metric Maven

I only had a casual acquaintance with screws during much of my working career. Where I worked there were clear plastic organizers with machine screws inside, and label designations outside like #4-40, #6-32 and #1/4-20. I had little idea what the nomenclature meant. One day I decided it was time for me to cure my ignorance. I discovered rectifying this lack of knowledge would be much more of a herculean effort than I expected.

I asked a number of people what the designation #4-40 meant, as I had seen a large number of these machine screws used on projects. The answer was almost uniform, and went something like this: “The 40 means forty threads per inch” I would then ask what the #4 designated. This was generally met with a blank stare and speculative guessing would commence.

Finally I met an engineer who knew the answer off the top of his head. He told me that below 1/4 of an inch the number was in gauge, above that the designation was in fractions of an inch. I was gobsmacked, and could not make any logical sense of this. When I looked up a dictionary definition of gauge, I discovered that it had the alternative spelling of gage. The definition that seems to fit this situation is: “A standard dimension, quantity, or capacity.” What was the dimension of the gauge size below 1/4 inch? After some research I found this answer:

“In Unified threads (measured in inches) there are numbered diameters #0 through #10, with 0 the smallest and 10 the largest. (Diameters #12 and #14 may also be found, but are usually on older equipment and needed for repairs or restorations.  #14 is close to, but not exactly the same as, 1/4-inch.) The major diameter in Unified threads = 0.060″ + 0.013″ x (numbered diameter). So #2 has a major diameter of 0.086″. The odd numbers exist, but the even numbers are in far more common use.”

WHEW!!! This is how we designate screws in the US?! So you take 60 thousandths of an inch and add the gauge number times 13 thousandths of an inch to obtain the size in inches from the gauge number!!!? This is what we viscerally protect as a rational method of articulating screws in the US? But the given diameter formula only is valid for the gauge numbered screws. Above 1/4 inch, the values are directly in inches—well as directly as a fraction may be called “direct.” One generally has to compute a decimal value from the fraction to actually perform computations. It is clear that even this demarcation at 1/4 inch is an apparent ad hoc attempt to merge the two separate nomenclatures, given that #14 is close to 1/4″.

We see that  #14 would be 0.242 inches and #1/4 is 0.250 inches. One can see that this is only a 0.008″ or 0.2 mm (200 um) difference between screw diameters. It is not difficult to imagine many situations where this size similarity caused mistakes. The answer offered to solve this problem appears to be “just stop using #14.”

Recently I met a former machinist who smiled when I told him “I only use metric in my designs.” His countenance indicated he really didn’t see that changing to metric was anything other than a frivolous, fringe issue. I looked at him and asked a simple question: “what size drill bit do you  choose to allow a #4-40 screw to just pass through a sheet of material?–to produce a friction fit.” Suddenly his face became blank with uncertainty. “What I usually do is get a pair of calipers and measure the diameter in inches and then choose a drill bit.” I then asked what type of drill bit designation he then used to obtain the correct drill bit size based on his measurement.

There is more than one option. A fractional set of drill bits does not match the decimal inch value he measured with a calipers. He would need to first go to a decimal equivalent chart and choose the closest fractional drill bit size. A  #4 screw has a diameter of 0.112″ according the the formula. The closest fractional size is 0.125″ which is a 1/8 inch drill bit. That’s as close as it gets! There are also US number and letter size drill bits, but I’ve  tortured myself long enough explaining this Rube-Goldberg method of just trying to figure out what size drill bit to choose, for a given screw, when using imperial measurement. I didn’t inquire, but I suppose the former machinist is content to measure all the candidate drill bits with his calipers until he finds one acceptable. How is it we are content to have size designations on screws and drill bits that have no correlation?—and reject any change or reform?

Let’s now explain how screw threads are designated in metric. The closest size screw in metric to a #4-40 machine screw is M3. What does M3 mean? Well, the M means metric and the 3 means three millimeters in diameter. My metric drill index has a three millimeter diameter drill bit, which if I then drill a three millimeter hole, allows the three millimeter M3 screw to just pass through the plastic, metal or whatever sheet material I drill. Yes I have done this, it works. It’s that simple! If you want some clearance choose a 3.1 mm drill bit or 3.2 mm. They’re both standard sizes.

When Australia switched to the metric system in the late 1970s, their manufacturers quickly discovered they could get rid of a large amount of screws and bolts they stocked. One car manufacturer was able to reduce their inventory of fasteners by 80% which saved considerable money. The small business community in this country, who use commonly available imperial fasteners, don’t realize that without metric threads they’re screwed.

Related essays:

Don’t Get Engaged With Gage

A Hole in The Screw Head

Yes! We have no metric drill bits.

2 thoughts on “Without Metric Threads We’re Screwed

  1. You have only scratched the surface of screw loopiness. The curious world of Whitworth and British steam pipe threads await. These unique threads bear no relationship to each and you need separate thread guages to reads them.
    With metric you will get a bolt that is M10 x 1.25, so a 10mm diameter bolt with 1.25mm between threads. If you need a coarser thread it would be M10 X 1.5.

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