Today is a very exciting day because we get to talk about one of my very favorite topics. thread pitch (while some people use the term screw pitch, it’s just not proper terminology). All good fastener geeks have something they’re really into. Yes, I am a proud fastener geek. I’ve been interested in thread pitches since I was a kid. For some odd reason the symmetry of the concept greatly appealed to me. Right about now you’re thinking, “why do I care about you being a fastener geek?” A very valid point. Except I actually taught you something already.
I used the word symmetry and it’s important because fastener threads are almost all symmetrical. The word, like most words, have several meanings of varying nuance. The one we want is from the Oxford English Dictionary, “the quality of being made up of exactly similar parts facing each other or around an axis.” See the last part “around an axis”? Yep, that’s fasteners. (For some good books on the topic, see the coda to this blog post.]
When I was a child, in the Pleistocene Era, I started noticing threads on fasteners. I noticed they had different angles and directions. They were closer or further apart. I didn’t have words for them yet, but I found it interesting in some sort of abstract fashion.
Symmetrical threads are when both sides of the thread are angled to the same degree (parts that take nuts and most screws). Asymmetrical threads are when one side of the thread is at a different angel than the other side. An asymmetric thread form allows the thread to have low friction symmetrical forms in one direction, but at but higher friction and inferior load bearing in the opposite direction. If you don’t plan to ever remove the screw, this isn’t necessarily a big concern. Particle Board screws have asymmetrical threads for just this reason (and others too.)
THREADS THAT TAKE NUTS
“The thread pitch is the distance between corresponding points on adjacent threads. Measurements must be taken parallel to the thread axis.” (Tip of the hat to Science Direct for that quote) Please note that TPI (thread per inch) is not the same as thread pitch.
For fractional (US or Imperial) parts we use TPI and for metric parts we use thread pitch. TPI is easy. Measure 1” of the fastener and count the threads, and if the bolt is large enough to see the threads easily, you’ll have the TPI.
Metric fasteners are specified with a thread pitch instead of a thread count (TPI). The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example, a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm. It’s related to threads per inch, obviously.
If you have an existing description for your bolt, the smaller (closer to zero) of the two numbers is the finer of the two.
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Take a 1/2” diameter bolt as an example: 13 is coarse thread (never write course which is something you race on) and 20 is fine thread. Because fine threads are closer together there are more per inch. The smaller (closer to zero) of the two numbers is the coarser of the two and 13 is a smaller number than 20. This trick works for ACME, UNx, NPT, BSW, BSF, UNS, UNEF, and many more threads.
- When it comes to metric fasteners, it’s the same thing but reversed because we’re talking pitch not TPI. If you take a 6mm bolt, coarse thread is 1.00 and fine thread is 0.75 – the smaller (closer to zero) of the two numbers is the finer of the two and 0.75 is a smaller number than 1.0.
Now, keep in mind there is more than one thread pitch standard, so we’re only dealing with coarse and fine in the actual article. But there are dozens of thread pitches that vary in angle of degrees; number of threads per inch; whether the thread is rounded, squared, or trapezoidal; and so much more. See the spiffy chart I made just for you? It shows just a few of the thread pitches you might see for some common diameters (sizes) of bolts. This chart is for parts that accept a nut.
Let’s talk conventions in writing thread pitches. For US and British fasteners (Imperial), it’s easy. You always write the number of threads per inch after the diameter separating them with a hyphen. So, a ½” bolt is written as ½-13 (coarse) or ½-20 (fine) or ½-10 (Acme) or ½-13 (NPT) or ½-12 (BSW) or ½-16 (BSF) and so on. A typical part call-out would appear thus:
½-13 x 4” Hex Machine Bolt Grade 307-A Zinc/Cr3 plated
½-13UNC x 4” Hex Machine Bolt Grade 307-A Zinc/Cr3 plated
½-13NC x 4” Hex Machine Bolt Grade 307-A Zinc/Cr3 plated
I stick to the first, as the second two I believe are overly redundant.
I guess we should talk about part call-outs while we’re on the subject. The order is always Diameter x Thread x Length, Head / Drive / Description / Grade / Plating / Add-Ons (if applicable). I’ll write an article on this separately but make sure that the information for your bolt is included in the correct order. That is not every possible permutation, but it’ll keep you honest in the meantime.
For Metric fasteners, it’s a little more obscure. You write the pitch after the diameter separating them with a hyphen or a lowercase “x”. Maybe. Here are the most common ways to indicate metric threads. The differences are between the 6 and the 25; the rest is the same.
M6 – 1.0 x 25mm Hex Cap Screw Full Thread DIN 933 gr 8.8 Zinc Plated
M6 x 1.00 x 25mm Hex Cap Screw Full Thread DIN 933 gr 8.8 Zinc Plated
M6 x 1.00 x 25 DIN 933 gr 8.8 Zinc Plated
[The DIN number specifies much of the description but better safe than sorry]
It really doesn’t matter which one you use, but pick one and stick with it. I typically use the 4th, why? For consistency because some diameters (m5 and smaller, m10 through m16) have some pitches such as 1.25 which are two decimals so this way all the parts have two decimals. And they all look the same. (Keep in mind Europeans use a comma as a decimal delimiter so you may see a comma instead of a period in the pitch).
With either system, many people tend to put the word FINE after the size call-out to remind people it’s a fine thread. (You can also put LEFT-HAND in this position as well for a left-handed part!) A nut must match the bolt, or it won’t fit. Everyone knows that, right? In theory, you’re absolutely correct. In practice you are mostly correct (over 95% of the time) but some metric bolts will accept an imperial nut and vice-versa; the fit isn’t great, thus it can lead to some serious problems.
For example, an 832 nut fits m4 bolt sloppily. Should it work? No. Why does it work? Tolerances. All manufactured parts have tolerances and if one part is near the high side and another is near the low side you can have an inadvertent fit. If you’re paying attention, you’ll notice the fit is sloppy.
I recommend a good pitch gauge, not a cheap one. In this case, cheap and good are mutually exclusive. Starrett makes excellent ones – actually, everything Starrett makes is top-notch and we at Hardware Everywhere would be happy to order anything you need from Starrett. Amazon, on the other hand, has very cheap ones. Let me assure you, a $10 pitch gauge works just as well as you’d expect. There are places to be cheap and this isn’t one of them. One bad measurement will cost you more than you saved on the gauge. (Image from Starrett)
When looking at the Pitch Charts, remember I’ve condensed them down to fit with this article. The main one has many more rows and many more columns. But if you look at this chart, you can see the basics. Note there are some sizes where there are alternate pitches listed, even though this is very basic. The full Imperial chart starts at #0000 (close to 1/64” inch diameter) and goes all the way to 7” diameter and has many other pitches listed. The Metric chart starts at 0.20mm and goes all the way to 300mm (12 inches) diameter. The biggest bolt ever made was in 1967 by the Penrith Engineering Works company in Scotland. They were enormous – 27 feet long (8.3m), 4 feet in diameter and weighed almost 13 tons each – and they made 60 of them. The smallest is 0.3mm made by Star Micronics – a video shows it.
Obviously smaller screws have more threads and that is reflected in the chart. You should stick to the ones marked “Y” in the standard column as those are the parts readily available (at least in coarse thread). The “S” stands for semi-standard. The big chart has parts that are no longer considered standards “N”, but I removed those for this article. Avoid those at all costs.
Why pick one thread over another? Some items (like B7 studs) are only commonly available in UN8 pitch once you pass 1”. A325 and A490 bolts only come UNC. A specification for a bolt may mandate thread pitch and thread length – and if you make a part differently, it no longer meets the specification. (For example, fully threaded bolts aren’t as strong so when people ask for fully-threaded A325 bolts, they are defeating the purpose of the specification. Fully-threaded bolts have better gripping strength, but not tensile [breaking] strength.)
Coarse thread bolts are cheaper and therefore are almost always specified. Coarse thread bolts are readily available, and most sizes of fine thread bolts are not. And lack of availability means noticeably higher prices for fine threads. Coarse threads are not affected by plating as much as fine threads. When you zinc plate a bolt, you’re adding perhaps 2 microns of thickness, but on a fine thread bolt, that may cause thread engagement issues.
But a fine thread bolt is usually recommended when the desire for making fine adjustments is required for an application. On the downside, there are a lot more turns required to thread one in. Fine thread bolts are more susceptible to be damaged by banging, more susceptible to thread galling (abrasive wear), and are also more likely to seize up when installed using a machine or impact driver. Fine threads are better for reducing slippage, and they are slightly stronger as well.
Coarse threads are more durable and have greater resistance to stripping and cross-threading. There is more material between each thread making engagement greater. They are less susceptible to being nicked or damaged, and hence unusable. One vendor of ours says on a ½” diameter, the bolt assembles in 65% of the time of a ½” bolt. The explain “The ½-20 bolt advances one inch in 20 revolutions, while the ½-13 bolt advances one inch in only 13 revolutions.” (Kato Fastening Systems) They put it in a way everyone can understand.
All my examples above used ½-13 bolts, but the same logic and pattern is true and applies to metric threads as well.
This concludes the first half of this post, the rest is coming soon. Look for it!