Know this isn't exactly "general electronics" but I couldn't find a more appropriate section to ask this in.
What is the difference between screws such as M3 M5 and the ones labelled #6 32 #8 32 etc,,
They seem to have different heads but that is about it.
I just read that M3 and M5 the number stands for the size whereas the #6 #8 stands for their size and then 32 is the number of threads per inch or something in that respect.
Are they used for different tasks or why is there a difference between the two?
Their are about half a dozen types of small screws. They differ in thread pitch and metric and imperial.
For electrical work the BA type were popular mainly the 4BA and the 6BA, they are imperial and are what I grew up with. The 4BA are larger than the 6BA, the bigger the number the smaller the screw. Then the U.K went metric so now I use the M3 which needs a 3mm clearance hole and is roughly the same size as a 4BA. Then for a smaller screw the M2 is a bit smaller than a 6BA. With these the smaller the number the smaller the screw.
Their are other threads like the BSW, BSF and UNC but these are mainly larger screws.
Their is no specific use except that some situations have standard screws. For example rack mounting equipment uses UNC threads.
Anything with an "M" in front is metric. The number following is the nominal size = basically the size of the hole you have to drill for the screw to fit through the hole. Metric screws have a defined thread pitch for each size. Usually 1mm per thread, obviously smaller mm/thread for smaller screws.
The "numbered" screws are SAE (american) sizes. #6 is bigger than #4. These sizes relate to the wire size used to make the screw. That's useless information for you. You have to look up a chart to find out what size hole to drill. Above a certain size = 1/4 inch = the size is the size of the hole.
The SAE system of nuts and bolts usually has both "coarse" and "fine" thread in each size. You can get a 1/4" bolts with either 20 threads per inch (TPI) or 28TPI. You might hear a bolt called the quarter-twenty which is 1/4" diameter and 20 threads per inch. This is the standard bolt used for camera tripods. But the coarse threads make the bolt weaker because they cut deeper into the bolt. A fine pitch 1/4-28 bolt is stronger and that's what you use building airplanes.
So even the small screws like #4 still need to specify their TPI. A 4-40 has 40 threads per inch. That's a really common bolt for small electronics stuff. It's about the same as an M3 but obviously you can't screw a metric bolt into an SAE nut.
Some metric bolts do have fine and coarse variants. The water pump bolts on a Renault are 7mm fine. They are really difficult to find outside of France.
sethwilliam:
Know this isn't exactly "general electronics" but I couldn't find a more appropriate section to ask this in.
What is the difference between screws such as M3 M5 and the ones labelled #6 32 #8 32 etc,,
They seem to have different heads but that is about it.
I just read that M3 and M5 the number stands for the size whereas the #6 #8 stands for their size and then 32 is the number of threads per inch or something in that respect.
Are they used for different tasks or why is there a difference between the two?
Hi,
They are from different 'systems'. The metric and SAE systems.
Amazingly, there is one thread that i know of in SAE where there is a metric nut that will fit the bolt. The SAE size is 10-32, but i'd have to look up the metric nut that will fit that.
This isnt a perfect fit so a deep hole with the metric threads will not allow 10-32 bolt to screw all the way down, but because the threads are so close to being the same that a short body of the metric nut will actually fit on a 10-32 screw.
This is the only metric/SAE partial coincidence i know of but there may be others.
Be aware that it is not the best idea to do this, but it works to some degree.
Most threads and diameters are so different the nuts from one system wont fit on a bolt from anther system.
If you look up metric sizes and you look up SAE sizes you'll see the two systems and how they are different.
There's also the hardness of the metal used. These vary from soft to very very hard, and there are sometimes different markings on the hardened hardware to indicate the hardness. It's bad to use the wrong hardness on some things, it should match the original.
Thanks guys, that really helps clear things up! I believe I have a better understanding of the two different systems now.
Really appreciate you guys taking the time to help me, thanks!
And here's a more sensible all-metric chart
