Thread Size Variety

[Gabe]

Simple question: why are there different thread sizes for same diameter screws, and why are certain material screws seen in one size more commonly than other sizes? We have 6-32, 8-32, 10-32, and 1/4-20 stainless. But in black alloys screws we find the thread size commonly in 8-24 and 10-24? And why does 1/4-28 even exist? A detailed technical explanation would be very helpful.

[AdamHeard]

from wikipedia

Unified Thread Standard

The United States has its own system, usually called the Unified Thread Standard, which is also extensively used in Canada and in most other countries around the world. At least 85% of the world's fasteners are dimensioned to Unified thread dimensions, and the biggest selection of fastener sizes and materials are found supplied in this standard (Source: World Fastener Review, Industrial Press, 2006). A version of this standard, called SAE for the Society of Automotive Engineers, was used in the American automobile industry. The SAE is still associated with inch-based fasteners by the public, even though the U.S. auto industry (and other heavy industries relying on SAE) have gradually converted to ISO preferred series fasteners for some assemblies from the 1970s onward, because global parts sourcing and product marketing favor international standardization. However, all automobiles sold in around the world contain both metric (engine assemblies) and Imperial fasteners (for example, lug nuts, oxygen sensors, internal electrical assemblies, body fasteners, lamps, steering, brake and suspension parts).

Machine screws are described as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32, 10-32, 10-24, etc. up to size 16. The first number can be translated to a diameter, the second is the number of threads per inch. There is a coarse thread and a fine thread for each size, the fine thread being preferred in thin materials or when its slightly greater strength is desired.

The numbering system follows a roughly logarithmic series where an increase in each screw number size approximately doubles the tensile strength of the screw and is given by Dia = (#N X .013") + .060" Using this formula a #5 screw has a major diameter of .125" (1/8"), a #10 screw has a diameter of .190" (or 3/16" in practical terms), etc. The formula applies for screw thread numbers #0 and higher, but does NOT apply to smaller Unified miniature screw thread series. Typically screws smaller than size #0 are supplied in the Unified Miniature Series. The formula for number sizes smaller than size #0 is given by Dia = .060" - (#zerosize X .013). So a #00 screw is .047" dia, #000 is .034" dia, etc.

The number series of machine screws once included odd numbers (7, 9, etc.) and extended up to #16 or more. Standardization efforts in the late 19th and the early part of the 20th century reduced the range of sizes considerably. Now, it is less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 thru #12.

Sizes 1/4" diameter and larger are designated as 1/4"-20, 1/4"-28, etc. the first number giving the diameter in inches and the second number being threads per inch. Most thread sizes are available in UNC or UC (Unified Coarse Thread, example 1/4"-20) or UNF (example 1/4"-28 UNF or UF).

[MrForbes]

because all jobs don't have the same requirements.

In general, fine threads allow for more clamping force. Coarse threads work better for soft materials. You'll see coarse threads used for screws that thread into castings, and fine threads used for screws that thread into nuts or into stronger materials such as steel.

For some applications such as aerospace and aircraft, weight is very important so each fastener must provide the most strenght possible for the weight. In other applications ease of machining and assembly might be more important, so a coarse thread would be more economical.

On a robot, it would be nice to standardize everything so you only have a few different sizes of fasteners to deal with (for the sake of your sanity, if nothing else). I tried to get 1726 to do this with the 2007 robot, which meant I had to stop by the hardware store just about every day during build season!

[AdamHeard]

Originally Posted by squirrel

because all jobs don't have the same requirements.

In general, fine threads allow for more clamping force. Coarse threads work better for soft materials. You'll see coarse threads used for screws that thread into castings, and fine threads used for screws that thread into nuts or into stronger materials such as steel.

For some applications such as aerospace and aircraft, weight is very important so each fastener must provide the most strenght possible for the weight. In other applications ease of machining and assembly might be more important, so a coarse thread would be more economical.

On a robot, it would be nice to standardize everything so you only have a few different sizes of fasteners to deal with (for the sake of your sanity, if nothing else). I tried to get 1726 to do this with the 2007 robot, which meant I had to stop by the hardware store just about every day during build season!

From what I have seen has a 2 year intern in the assembly portion of the aerospace sector of Northrop Grumman, I can confirm some of these statements.

NG tries to use fine threads whenever possible. If the part is a soft material such as a composite or aluminum, they will put a threaded steel insert in that the fastener threads into as a method of getting them rather than coarse.

[Jeremiah Johnson]

I'll agree with what they last people said, no arguments there.

On a completely unrelated note:

I work in the hardware section of K&K True Value and I can pinpoint what thread size any screw is by looking at it, that includes standard, metric, or if it's lamp or pipe threads. Maybe someday I'll upload a picture of the hardware department that I work in... there's probably over 2,500 bins of different nuts, bolts, washers, and other miscellanious hardware items.

[artdutra04]

In addition to being used in softer materials, coarse threads are also used when you want to quickly (dis)assemble something, as you have less threads per inch to rotate your nut/bolt. Fine threads are used in higher strength applications, as well as when you are going through something thin.

As for some screws being offered in some sizes and not in others, it depends on material strength and application. Using stainless for a fine threaded screw makes sense, because many of the applications where you would a fine threaded screw also need/benefit from stainless steel hardware.

[Jeremiah Johnson]

Originally Posted by artdutra04

In addition to being used in softer materials, coarse threads are also used when you want to quickly (dis)assemble something, as you have less threads per inch to rotate your nut/bolt. Fine threads are used in higher strength applications, as well as when you are going through something thin.

As for some screws being offered in some sizes and not in others, it depends on material strength and application. Using stainless for a fine threaded screw makes sense, because many of the applications where you would a fine threaded screw also need/benefit from stainless steel hardware.

Most of the stainless steel that I've worked with and/or sold is mostly coarse threaded. Take the screw size 10, for example. It comes in two "standard" sizes: 10-24 and 10-32. Both are used as much as the other, but 10-24 (coarse thread) are the most widely used in stainless steel. Don't get me wrong, they make all sizes in stainless steel, it's just more difficult to find some of them.

Chrome on the other hand... well, let's not go there, it's just pretty stainless steel.

[Qbranch]

Another reason for fine threads, aside from the high clamping force, is their ability to be used for fine adjusts on mechanesms. For instance, on a machine a buddy and I made at work to load boards from a casette onto a conveyor, we used a simple threaded hole and a fine threaded socket head bolt with 3B threads so theyre sticky (grab the bolt a little) to finely adjust the pitch at which the gripper and board intersect.

Also, fine threads are often more and more useful as screws get smaller since they increase the mechanical advantage of the screw. Basically, the finer the thread is (ignoring friction) the less input torque on the bolt head it takes to get the same compression on the material. So on a small screw (like the machinemaker's favorite, the 5-40) the shaft of the screw is so thin that very little torque can be put on it. So, the high thread count makes up for it.

On the standard sizes... I think its just manufacurers of certain devices like pneumatics and such had to standardize on something, and then machine designers had to use these components, so over the years the standardization has filtered down through industry so now we all know the famous 1/4-20s, 8-32s, 10-24s , etc.

just my 2¢.

-q

[Jeremiah Johnson]

On another note: Anyone ever try counting 300+ 0-80 stainless steel screws, washers, and nuts? Normally, I would have used a magnet when doing inventory... but as we all know, stainless steel is not attracted to magnets. Wow... what a fun day.

Sorry, I'm passionate about my nuts (and bolts), I just got off work.

[Dick Linn]

"...why are there different thread sizes for same diameter screws...?"

Simple answer? Because we have engineers