A Hole in The Screw Head

By The Metric Maven

Thern is a friend who is a mechanical engineer, and has taught me much of what I know about  the mechanical arts. I spent a considerable time trying to learn how to use a common CAD program for drawing, but managed to get nowhere fast. I had purchased a book, but to no avail. Thern looked at my struggles and said “They’re aren’t any good books for this, but I’ll bet I can have you drawing in an hour.”  He was correct, and it opened an entirely new world for me. Thern had me draw all the component parts and then, most importantly, had me fit them together to make certain they  did not interfere. He showed me how to allow for clearances and other important information. When all the the parts for the design arrived, and they all fit together perfectly, it was sublime.

I became more and more confident about creating my own mechanical drawings for my electrical designs. Unfortunately confidence does not beget knowledge. I took the drawings over to Thern, who was actually favorably impressed by what I had done. But he noted one detail. I had holes in the design through which fasteners were to pass. They were dimensioned in decimal US Anarchy Units, as was the entire design. I do not recall the hole size, other than it seemed like a “nice round number.” Thern pointed out that if I left the nice round number on the drawing, the machinists would take me at my word and either machine the hole exactly, or obtain a custom drill bit and drill the holes. One should always look up a standard sized drill bit to avoid the extra costs. Thern cited a “standard” sized USAU drill bit, which I then called out on the drawing. Thern then told me “don’t feel bad, you have no idea how many mechanical draftsmen/engineers don’t bother to call out sizes that fit a standard drill bit.”

It turns out that in US Anarchy Units, standard drill bits are a bit difficult to pin down. What I mean is that one has to admit that a great thing about USAU is that there are so many “standards” from which one can choose.

The first standard is Fractional-inch Drill Bits. According to Wikipedia:

ANSI B94.11M-1979 sets size standards for jobber length straight shank twist drill bits from 1/64 inch through 1 inch in 1/64 inch increments. For morse taper shank drill bits, the standard continues in 1/64 inch increments up to 1¾ inch, then 1/32 inch increments up to 2¼ inch, 1/16 inch increments up to 3 inches, 1/8 inch increments up to 3¼ inches, and a single 1/4 inch increment to 3½ inches.

One disadvantage of this scheme of sizing is that the size increment between drill bits is very large for the smaller sizes, 100% for the first step. The implication is that number gauge drill bits have to be used to bridge the gaps. (emphasis mine)

Another disadvantage is the convention in labeling the bits. Rather than an integral number of 64ths of an inch, drill bit sizes are written down as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is always written as 1 7/32 inch. This can lead to confusion and mistakes unless great care is taken.

The decimal equivalents of these fractional drill bits are numbers that I suggest no one other than an idiot savant could master and retrieve for placement on a drawing. The middle paragraph even indicates there is a nudge-nudge wink-wink assumption that people know the sizes are not very complete for small sizes, and so they need to use drill bits from another standard to fill in the gaps!

The other “standard” is US number and letter gauge drill bit sizes. Again according to Wikipedia:

Number gauge is routinely used from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Number gauge is actually defined at least down to size 97, but these smaller sizes are rarely encountered….

The numbers and letters correspond in no logical manner to the diameter of the hole to be drilled. In fact, they do not even have a linear or understandable mathematical relationship. As all the sizes are generally less than an inch, I guess I don’t know why they are not called out in barleycorns.

It is my understanding that drill bits are often used to make holes through which fasteners such as machine screws are to pass. One would expect that if an actual system existed, that there would be a correlation between the fastener designation and drill bits. There is not. For instance the size of machine screws is 0, 1, 2, 3, 4 to 10. These are all gauge numbers, which means they are meaningless numbers. A number 4 machine screw has a 0.1112 inch diameter, a number 4 drill bit has a 0.209 inch diameter.

Oh, I forgot to mention that when the diameter of a machine screw is 1/4 inch or larger, it is no longer designated with an integer “gauge number,” but is then in fractions of an inch. So above 1/4 inch we have  5/16, 3/8, 7/16, 1/2, 5/8, 3/4 and more fractions. Of course modern CAD programs are in decimals, so one has to do some computations to figure out what drill size would be the closest to a desired fastener size. I’ve often wondered how much time is wasted, and mistakes made in the US because we need to constantly use fraction to decimal equivalent charts. My father has them all over his shop.

When a fastener diameter is the same as a drilled hole diameter, this is often called a “friction fit.” There is no clearance. If there is only one fastener and hole, this is not a problem, but when there are multiple holes, their relation to one another has to be very close, if all the screws are to pass through without binding. In the case of US Anarchy Units it takes some time to figure out what clearances are possible using our “standard” drill bits and fasteners.

When I finally made the decision that my Engineering Practice would use all metric, the simplicity of metric fasteners and drill bits came as a shock. My first metric-only design used M6 machine screws. M6 is Metric six millimeter diameter. I’ll bet you can guess what M2, M3, M4, and so on all mean. So to get a friction fit with an M6 machine screw, I would need to use a six millimeter diameter drill bit, which is standard. If I need a little clearance?—use a 6.5 mm drill bit.

I bought some M6 machine screws at a business who has sold only metric fasteners and hardware for years. I asked the man behind the counter “so in your experience if I drill a 6 mm hole this M6 machine screw will pass through?”

He looked at me like I was Forrest Gump and said “they’re both six millimeter. Why wouldn’t it fit through?”

Indeed, when I went home and drilled a 6 mm hole, the M6 fastener passed right through—perfectly. I can’t imagine using US Anarchy of fasteners and drill bits now, but many people try to force me. When I call out a drill hole on my metric drawings, I know exactly what values I can use, and what drill bit to call out, and what fastener will work. I can do this all without consulting any anachronistic chart. Anybody that would choose to make drawings in US Ye Olde English Units, must have a hole in their head.

Related essay:

Without Metric Threads We’re Screwed

Yes! We have no metric drill bits

If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page and contribute. Also purchase his books about the metric system:

The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays  that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website,  but the book has them all in one place in print. The book may be purchased from Amazon here.

The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.

The third book is not of direct importance to metric education. It is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.

9 thoughts on “A Hole in The Screw Head

  1. Where I work, there is a mechanical “engineer” and there is me. The mechanical “engineer” always works in Yankee units (or USAU) and I (who they refuse to call engineer) who works in hard metric whenever possible or soft metric because we live in the US. Whenever there is a change to parts and how they interface with each other, the mechanical “engineer” often wonders why my changes are done so fast (usually hours or less), while his part is days or a week long ordeal. Perhaps someday he’ll put down the hammer and feel how good it feels not to hit your self with it.
    Another thing I noticed with metric vs. inch is in the hex head cap screws. An M5 screw is driven with a 4 mm Allen wrench, M6 uses a 5 mm and so on. Whereas a 1/4 inch screw uses a quasi related fraction size that isn’t so intuitive and a #10 is anyone’s guess. Just grab all the wrenches and see what fits.

    • This maybe something you can bring up with your boss or General Manager. Present to him examples of where USC is costing the company money by wasting time and how you using metric is more efficient and productive. If you can show him or her that metrication won’t cost anything yet bring benefits you may be able to metricate your entire operation. If you are a small outfit, metrication may be easier than you think.

      I’m curious though, how does it work out that one produces a “drawing” or design in metric and the other in USC and there isn’t a conflict?

      • Anytime I bring in metric materials for inventory and recommend to the office manager the unit of measure is meter or millimeters, depending on the material. But what she hears me say must be like “I’m going to hold your head under water until you drown.” So, she freaks out (believe me, its quite a scene) and says that will never happen and the material is entered in inventory in inches or feet (depending on her mood). But then, when the material is called out in a BOM, it isn’t necessarily in the same unit. Confusion and accounting errors ensue. Yes, I’ve pointed it out many times before. But only to a deaf, blind and very biased audience. The CEO of the company however, is slowly seeing the benefits of metrication as we sell our products internationally. I’m still waiting for his epiphany.

        Mainly the drawings I give the mechanical “engineer” is for mounting holes (pattern and size) for PC boards and component sizes on the board so he can make sure the board will properly mount and fit in the chassis. I generally know how big a space I have to work with before hand so it makes it easier for the mechanical “engineer.” Any cabling or harness measurements are hard metric also, but I include the inch equivalent for our special worker(s) who have such trouble with integers.

        • How do you get internationally produced hard metric components to fit on inch designed boards? Did you read these two blogs from Metric Maven?



          Here is a similar point made on reddit by a person whose experience parallels what Metric Maven encountered:

          “PCB design is one of those things where metric will unfortunately probably never see total adoption.”

          “Except for US component makers (and even then only for some lines), SMT components are worldwide sized and spaced in metric. This makes it a MASSIVE headache to integrate components on a board where you have mostly worldwide components, and a handful of components that are produced only for US markets.”

          “There are even sizing conflicts! An 0603 in metric sizing is a component 0.6mm by 0.3mm. But in the US (or in US sourced catalogues) an 0603 is 1.6mm by 0.8mm!”


          These MASSIVE HEADACHES usually add so much to the costs that the company will go out of business or relocate to a metric country.

  2. Actually, you left out the best part of metric machine screw – the standard makes sense. A 6mm panhead screw has a head that is 2d – or 12mm

    See http://wiki.xtronics.com/index.php/Screw_Threads
    for my metric thread and machining notes.

    But – a 6mm screw will not necessarily pass through a 6mm hole .You could probably get away with it with thin sheet material. The outer diameter of a threaded 6mm screw is 6mm! So, If you don’t allow clearance, you would have an ‘interference fit’ and would need to be pressed unless you drill with a hand drill and end up with some clearance.

    We also need a bit of clearance to make up for thermal expansion and machining tolerances.

    The ‘clearance holes’ for 6mm are specified as 6.4, 6.6, or 7.0 mm for close, medium, and coarse fit.

    • If you use a 6 mm drill bit to drill the hole, it will actually drill out a slightly larger hole and the screw may fit through.

      Another thing that is overlooked is determining the drill bit needed to drill a hole before threading. There is usually a target percentage one aims for when choosing a drill size. If I want to tap a hole for an M5 bolt with 80 % engagement, I multiply 5 by 0.8 to get 4 mm. That is the hole I need to drill before tapping.

      With unified inch bolts, I would need to look up somewhere the actual diameter of a #10 bolt, then because the diameter will be some long number of decimal places, I will definitely need a calculator. Of course when I get a result, then I have run around looking for a chart to see which of the three series of drill sizes best fits the result. Then I have to check the tool room to see if that drill size is in stock. If it isn’t it is back to the chart for alternatives. This has to be an utter waste of time that those who go through it consider it normal and the money wasted is just ignored and never added to the cost of not going metric.

        • Of course tables and charts like the one you referenced are an absolute necessity when using USC as nothing can be derived in your head. But with metric fasteners I don’t need to waste time looking for a chart to see what I can calculate in my head in a few seconds.

          I can always tell when I’m in a inch based machine shop. Every machine operator has a conversion chart within view. A metric shop is devoid of this handicap.

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