PDA

View Full Version : when to use NC or NF threads



pasofino
09-01-2004, 01:57 PM
When designing or building a project and you have the option, when should you use NC threads and when should you use NF. And why

SGW
09-01-2004, 02:07 PM
Here's some of the things I think about, which may or may not be relevant or sensible. http://bbs.homeshopmachinist.net//smile.gif

Coarse threads tend to be a better choice in soft materials, like plastics.

Generally speaking, if you're making a scale model, fine threads turn out to be more "scale" than coarse threads.

If you're threading into thin work, you'll probably want to use fine threads so more threads will engage.

Since the core diameter of a fine-thread bolt is larger, it's slightly stronger.

Coarse threads are probably easier to put nuts on and thread into holes, than fine threads.

gglines
09-01-2004, 02:11 PM
Not a simple answer to the question. Much depends on the device design and the materials used. "Nuts, Bolts, Fasteners and Plumbing Handbook" by Carrol Smith is a good book to check out.

There are a few rules of thumb such as having at least 3 threads in contact, use coarse threads if you are going to fasten and unfasten the joint a lot, coarse threads in soft materials, fine threads may be stronger because you have more root material, etc.

George

kap pullen
09-01-2004, 02:12 PM
I use nf threads because;
1) larger root in the bolt (stronger)
2) less stress tapping (Less tap breakage)

Many high production outfits (auto makers) use nc for non critical work because of shorter cycle time tapping, and assembling.

Many assemblies use self tapping bolts to eliminate a tapping operation now.

I think the bolts on your car wheels are probably national fine thread because of the stress.

My view + $1.00 will get you a whopper junior.

I don't make plastic stuff.

Kap

[This message has been edited by kap pullen (edited 09-01-2004).]

spope14
09-01-2004, 03:43 PM
Coarse threads where more holding strength is needed, and torque applied, such as transmission mounts, machine fastening, tool mounting. More surface area between OD and ID threads surfaces, and less apt to 'strip out" when torqued and applied to stresses of normal every day use.

Fine threads for adjustments, fine fastening, and thin work. Three threads in contact as noted is the general rule.

Fine thread bolts themselves are stronger as a single structure, but the actual "holding power" between items may be less due to less thread depth in the tapped hole, and on the shaft. Stripping of the hole, stretching of the thread. This is "tensile" application, shear may be different.

This is under higher torques, as noted. For lower torque locking, such as hand power, the fine thread "locks up" better upon contact. I use the mag light as an example later on....

Other uses for fine threads, in low torque applications that may experience repeated 'cycles' of loading and unloading, thus the thread may loosen up. less apt for the bolt to come out of the hole as fast.

FINE threads are also used in "hollow mounting" situations. An example of this may be the battery end of the maglight flashlight, where the thread is of fine quality because it allows for more "opening" at the root diameter of the thread (the 75% depth allows for a greater ID, thus more hollow area allowed). The flashlight body is thus a bit smaller in external diameter (OD) than it might be otherwise with a coarse thread, which has greater depth. The battery opening leads to "less wall". In this application as well, from experience, the thread allows for better water tightness, though not complete without the gasket. The fine thread on a mag light 'smallest version and 2 cell D version, if this were coarse thread, the contact wih the O ring or the locking gasket would be of less turns, an may not "hand lock" as well as a fine thread, that allows for almost a full turn and 1/2 of "lock" at the O-ring contact point or on the small version, the gasket. If this loosens up, you have to loosen it almost a full turn before the gasket / O-ring seal is broken free for full water penetration, and the end also tends to stay put better. The coarse version may have about 1/2 to 3/4 turn only, thus when it may be released, water infiltrates faster, and it never really seals up quite right without a plier.

back to Coarse, you would not attach a transmission with fine thread and 80# torque using a 1/2 bolt (coarse 13, .047 thred depth theor., fine 20, .030 thread depth theor.) for the tapped hole would strip or stretch. Same with aloris tool holders which use a 5/16 - 18 instead of a 5/16 -24.

Just my Humble Opinions and Experience, and some information remembered fom engineering school. gawd, I hope this made sense...

[This message has been edited by spope14 (edited 09-01-2004).]

bspooh
09-01-2004, 04:40 PM
course threads have a tendacy to loosen through vibration..fine threads do a better job where there is vibration.

brent

dvk
09-01-2004, 04:40 PM
spope
a wealth of info. I often had questions about that too, thanks for your examples.

abn
09-02-2004, 07:54 AM
True, that fine threads have a bit lower stripping strength FOR A GIVEN LENGTH...where you can increase engagement length enough to take advantage of the higher tensile strength of a fine thread fastener they actually will provide more tension holding strength. So you would want a fine fastener in critical applications where strength to weight is important.

I suspect that the real reasons for coarse thread popularity are ease of assembly (higher helix angle and cross threading) and less sensitivity to tap drill size tolerance.

The Carrol Smith book is great and everyone should own his collection...I assume purchases go to support his family and that seems a noble cause. But if you don't got the scratch, then here's some good downloadable reference material...the meat begins on page 51, and a nice short and understandable thread pitch discussion begins toward the bottom of page 53 (I've parroted parts of it here so I can sound smart http://bbs.homeshopmachinist.net//biggrin.gif)

http://www.spstech.com/unbrako/docs/91430_SPS-LR.exe



<font face="Verdana, Arial" size="2">Originally posted by spope14:
Coarse threads where more holding strength is needed, and torque applied, such as transmission mounts, machine fastening, tool mounting.&gt;snip

</font>



[This message has been edited by abn (edited 09-02-2004).]

sauer38h
09-02-2004, 08:30 AM
The thread systems originated in the days when engineering meant locomotives - British Standard Whitworth, British Standard Fine, American Standard Fine, etc. The coarse threads were for use in cast iron parts. Steel parts used fine threads. Threaded brass parts such as lens cells for microscopes and etc. tended to use unique thread systems such as the lowenherz thread which gradually disappeared in favor of one or the other of the fine or extrafine series. Extrafine is for use on thin-walled tubing, where the deeper threads of the coarse and fine series would cut through the tube walls.

Nowadays in practice it doesn't much seem to matter which thread series is used. At one time things like cylinder head studs for use with cast iron engine blocks had coarse threads at one end and fine threads at the other, but nobody seems to bother much nowadays. If the strength of your joint is so critical that you have to choose between fine or coarse threads, you don't have enough safety margin and should move up to a larger diameter. When using modern materials, very fine threads are a poor choice for aluminum, as the threads and fasteners tend to weld together under even very light loads, or gall badly if they don't actually weld. Stainless steels in general are happier with coarse threads. Again, the problem is galling of fine threads. Hardened threads are less prone to gall, but most stainless fasteners are made of non-hardening alloys, like 300 series, because threads are easier to cut (or roll) in those.

There are all sorts of odd threads still in use for special applications. Threads for structural applications in aircraft have a thread form with a modified root profile to (theoretically) increase fatigue life. There is another thread form which guarantees an interference fit between the thread and the nut, to help prevent the fasteners loosening under vibration. Neither of the standard coarse or fine threads are much good at resisting vibration, which is why God gave us so many types of lock washers.

andypullen
09-02-2004, 10:25 AM
Kap,

I'll spring for the whoppers.....I like fine threads where possible, too. For the most part, that's what I use in my model work.

Brent,

Any thread that is subject to vibration will loosen. Fine threads are less apt to loosen up as you said. (Look at your lugnuts on your cars) If it's going to be in an evironment that has vibration or temperature extremes, I like to put some kind of locking element in or on the thread. A nylon insert on the nut or imbedded in the male thread will stop the loosening problem. I've done hundreds for submarine parts for the Navy. A lockwasher has been proven to loosen over time.

Andy Pullen