screw threads and what are they nc or fine thread

[Brett Hurt]

So I know what they are BUT! What do you use them for and what applications. In touque and holding, screwing,, down ect, and what thread holds the most touque. nc or fine thread. Lets say 1/4 bolt 1.5000 long holding two pices of steel together. This is general questions , but you guys are smarter than iam about this.

[BCRider]

It sounds counter intuitive but the fine threads actually are stronger than the coarse threads. And the fine threads are less likely to vibrate loose.

So why don't we use more fine threads in things? Because to work as well as they should the fine threads need to be made to a tighter tolerance. And when pennies add up to dollars coarse threads win out. We can be more sloppy with coarse threads and they still hold "well enough".

[J Tiers]

One also uses coarse threads in material that is soft and easily bunged up. Fine threads in aluminum, for example, are normally a bad idea just on the damage and jamming basis. Coarse threads are preferred.

Fine threads have a lower "wedge angle" and can get a higher clamping force with the same torque, just as a thin wedge is more easily pounded in than a steep one. The trade off is having to go more turns.

[Erich]

BC is right, A fine thread bolt is stronger in tension than a coarse bolt.
If you are threading the bolt into a material that is softer and weaker than the bolt, the coarse threads may increase the pull out strength in that material.
IE bolts threaded into plastic or soft aluminum.

[38_Cal]

Fine threads are preferred when you are drilling and tapping a blind hole and can't go more than about 1 screw diameter deep...think scope mounts or sights on a rifle barrel where #6-48 or 8-40 threads are normally used.

[BCRider]

Excellent points on the coarse threads in softer materials. I didn't consider that. Hence why wood screws are even more coarse than metal screws.

Having said that it's interesting to read about the tensile strength of machine threads in wood. Sadly I can't find anything out on the interwebz at the moment. Wrong search words apparently. But there was an article in Fine Woodworking magzine many decades ago about using machine screws in different woods. And trust me. 3/4 of an inch of threading of a 1/4 inch bolt into hardwood will hold just as well as a regular nut. Namely the bolt will snap before the thread tears out.

[thaiguzzi]

I have never understood Harley Davidson's preference for NC threads over NF threads, especially as they are known, as, er, a vibration motorcycle par excellence.
I'm trying to remember if i've ever seen an NF bolt on one....

[rzbill]

I have never understood Harley Davidson's preference for NC threads over NF threads, especially as they are known, as, er, a vibration motorcycle par excellence.
I'm trying to remember if i've ever seen an NF bolt on one....

Are they simply following the rules noted above about threading into softer metals??

Well, they are known for being heavy so maybe many parts remain steel vs lighter metal and the threads could be fine. Dunno.

[Tungsten dipper]

Pitch diameter or class of fit also plays a part in the strength of the thread.

[Mcgyver]

The other consideration is environment. The fasteners are not all used by white coated types in brightly lite tiled floor rooms putting together microscopes. Its easier to cross thread fine threads than coarse and they are more easily damaged; the same sized bruise on a NC is a bigger deal on a NF. When assemblying some big thing with 500 1" bolts they are soon going at it by rote without a huge level of care and using big air hammers; I can see how NF would lead more F'ups.

NF isn't that much stronger to give up the advantages of a more robust profile unless its clearly a matter of getting the max strength in small area. If you are the designer picking the fastener, you're thinking the worst of those potentially working with, it not the best. From my observations it would be rare to find fine threads on most industrial machinery.

[Bill Houghton]

I have never understood Harley Davidson's preference for NC threads over NF threads, especially as they are known, as, er, a vibration motorcycle par excellence.
I'm trying to remember if i've ever seen an NF bolt on one....

And some of the older Harleys used pitches that are no longer standard. I once sourced a cap screw for a Stanley plane (Stanley also being big on non-standard thread pitches) from a local mechanic who specialized in old Harleys.

[Tony907]

I worked on and flew on helicopters for about 12 years or so. The CH-47 and UH-60 to be exact. Almost everything on those helicopters if NF. As all could imagine, helicopters are nothing but vibration and tons of stuff that, if it comes apart, you die. Very rarely did I ever find a loose fastener during an inspection. So when I'm building something and loose is not an option, I reach for NF with a nylock nut. Of course, in the event that I would need to disassemble it frequently, I would use a castellated nut and cotter pin or lockwire. It also helps that I still have lots of hardware from the scrap bin

[MattiJ]

I worked on and flew on helicopters for about 12 years or so. The CH-47 and UH-60 to be exact. Almost everything on those helicopters if NF. As all could imagine, helicopters are nothing but vibration and tons of stuff that, if it comes apart, you die. Very rarely did I ever find a loose fastener during an inspection. So when I'm building something and loose is not an option, I reach for NF with a nylock nut. Of course, in the event that I would need to disassemble it frequently, I would use a castellated nut and cotter pin or lockwire. It also helps that I still have lots of hardware from the scrap bin

Flying contraptions are also very weight-sensitive applications, ideal case fine threaded bolt joint is stronger/lighter than coarse thread.

[Brett Hurt]

thanks for the info it help me a lot nc on the soft stuff

[thaiguzzi]

Are they simply following the rules noted above about threading into softer metals??

Well, they are known for being heavy so maybe many parts remain steel vs lighter metal and the threads could be fine. Dunno.

No i'm not talking about cap heads screwed into alloy casings.
I'm talking about run of the mill fasteners (nuts & bolts) on the cycle parts. Unique to HD i think. Certainly the Brits were either BSF or CEI or UNF on similar fastening recquirements.