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.

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Related essay:

Without Metric Threads We’re Screwed

Yes! We have no metric drill bits