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[machinist] NC vs NF (threads)

From: Lawrence London <lfljvenaura@gmail.com>
To: machinist@lists.ibiblio.org
Subject: [machinist] NC vs NF (threads)
Date: Sun, 15 Nov 2015 21:33:05 -0500

http://www.practicalmachinist.com/vb/general-archive/nc-vs-nf-96455/

C9
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This one's probably been discussed before, but I did a search and didn't find a specific comparison.

I hang out on a few hot rod oriented boards and I'm seeing a few comments about weak threads in the NF category and the recommendation to use NC in most applications.

My understanding was that NF was the stronger of the two due to more threads engaged.
Even if the depth of thread is shallower.

Perhaps it has to do with the choice of metals used.
I make a lot of hot rod oriented stuff, simple engine parts - both internal and external - and in most cases the choice of material is 6061 T6 aluminum.
A lot of the threading is done under power in the lathe and NF is the thread of choice - 1/4" & up fwiw.
The lathe is reversed 2-3 times - depending on thread size and depth - and chips are cleaned out.
I do start the smaller taps in the lathe and finish them by hand.

The biggest reason NF is chosen is because it's easier on the lathe or mill/drill.

I understand as well that full thread strength is generated by cutting threads to a depth at least equal to the diameter of the bolt to be used.
IE: 1/2" bolt gets 1/2" worth of threads.

NC is used - in my case - for threaded weld bungs that get welded into the chassis as well as for decorative items like escutcheon plates.
These finish off the holes for the brake pedal - and helps hold the carpet down - as well as for the throttle rod to enter the cockpit area.
To my eye, a much nicer look than a sewn carpet binding around the hole and the aluminum escutcheon plate doesn't wear away.

I note as well that NF is used in the engine internals in several areas utilizing smaller bolts - 3/8" rod bolts for example - and NC is used for main cap bolts or studs.

Externally, most engine threads seem to use NC.

Is the choice between NF and NC simply a manufacturing convenience or a strength consideration for rod bolts and oil pumps?

I see that most torque values listed for NF vs NC have NF getting a bit less torque.
I understand as well that this lesser torque with NF generates the same clamping force as the higher torque of the NC threads when similar sized bolts/studs are used.

Color me confused....

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03-04-2005, 02:40 PM #2

randyc
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IE: 1/2" bolt gets 1/2" worth of threads

"Machinery's Handbook" has a good treatise on thread stresses. You may calculate the optimum length of thread engagement for most thread configurations and materials.

A thread engagement equal to 2.5 X thread O.D. is a rule of thumb that is often used (steel fasteners in aluminum). (The reasoning is that, with this length of engagement, the male fastener will usually break before stripping the female threads.)

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03-04-2005, 02:49 PM #3

Forrest Addy
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I don't blame you for being confused. You'll never find technical expertise at the lower end of the automotive food chain where ignorance and BS trumps hard-won expertise. Most of the confident-sounding talkers are just that: they're sheep copying what the big guys do and who convincingly parrot what was parroted to them by people who don't know what they're talking about thus perpetuating confusionand ignorance.

People who tout the superiority of NF Vs NC threads without qualifying their remarks with knowledgeable commentary on materials or fastener class or who cannot quantify their statements using numbers applied to engineering analysis don't know what they are talking about. You don't have to contradict them; many are otherwise great guys; kind, funny, and helpful. Just don't be pursuaded by them without separately confirming their convictions with competent authority.

NF and NC threads are roughly equal in strength with the NF thread being slightly stronger because of its larger root area. How much stronger depends on the materials, the class of fastener, the lenght of engagement, etc. They both have their advantages and which to use in an application is a technical decision fraught with considerations.

Automotive engineers are among the best there is. They get an enormous amount of function and value for the buck. Some study of their products will reveal a pattern of usage that if intelligently applied to new work will lead to strong reliable bolted connecitons.

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03-04-2005, 03:11 PM #4

Bert
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C9,
I was always taught that a good rule of thumb when selecting an NC or NF, that you use a NC in soft material and NF in higher strenght material.

Don't know exactly where "soft and higher strenght" cross paths, but in the automotive world; aluminum would always get NC, and heat treated steel would get NF.

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03-04-2005, 03:20 PM #5

JRouche
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I use ARP fasteners throughout my car. They appear to have the technical skills needed to produce and recommend proper fasteners. You could contact them, I have and I found them to be more than helpful. JRouche

http://www.arp-bolts.com/pages/contact.html

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03-04-2005, 04:04 PM #6

rklopp
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The often-violated rule of thumb is NF only on hard materials: nuts and steel on steel; NC on everything else: cast iron, aluminum, zamak. Fine threads on aluminum often give trouble if the screw is removed often. Many times you will find studs threaded coarse on one end into cast iron or aluminum and fine on the other for a nut. This practice is for fractional sizes, not machine screws. Machine screws are rarely used in the NF sizes except 10-32, which is now the de facto standard.

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03-04-2005, 04:18 PM #7

mrainey
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Here's a good discussion to complement this one:

http://www.eng-tips.com/viewthread.cfm?qid=72123

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03-04-2005, 04:34 PM #8

JimK
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I have always been under the impression that the NF (at one time referred to as S.A.E.) thread series was used in applications where the material was strong and there was considerable vibration present.

Of course automotive work comes right to mind.

The NF threads have a less oblique helix angle, so they tend to wedge tighter when they are brought up to torque. You can see why they would stand vibration better.

It has been my experience that the NF series wors best with nut and bolt construction. Both parts of the fastening are usually strong grades of steel.

There was a time when it was not reccomended to use cap screws in cast iron at all.

The order of the day was to tap an interferance thread of the coarse series in the casting and set a steel stud into it.

Subsequent assembly and removal of the fastened part consisted of tightening or loostening a steel nut on the outward end of the steel stud.

This was the standard practice for internal combustion engine cylinder head assembly for many years, It is still used on some expensive engines.

One other trick for holding things together in severe service conditions is to turn the body of the bolt or stud to the minor diameter of the thread.

A short length of body size is left near either end of the bolt or stud for alignment.

When the bolt is brought up tight, the stretch happens along the length of the turned section. Otherwise all the bolt or stud stretch would occur right at the minor diameter of the exposed thread sections only.

This is a strong but elastic fastening that holds it's tightness under sress reversals and under the stress of thermal expansion of the fastened parts.

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03-04-2005, 04:59 PM #9

metlmunchr
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Echoing what Forrest said about usage patterns in the automotive industry, if you look closely at what's used in engines you'll notice that there's NEVER a fine thread hole in cast iron or cast aluminum. A carburetor mounting stud is a good example. Most all of them used fine thread nuts to retain the carb, but the end of the stud which threads into the manifold is always coarse. I'd imagine one of the major reasons for selecting fine threads in many driveline parts is the increased resistance to loosening due to vibration of a fine thread due to its smaller helix angle. As soon as you move away from the driveline per se, most things are attached with coarse thread fasteners. In a production assembly environment its much less likely for a worker to crossthread and gall a coarse thread nut or bolt than a fine one. Everything now is such a mish-mash of English and metric within a car that one doesn't really notice the specifics of the fasteners, but if you look back 30 or so years you find the auto industry using grade 6 (4 marks) and grade 7 (5 marks) bolts. This is something you basically never see anywhere else and further backs up Forrest's assertion of how the auto industry has to actually engineer each and every part instead of just slapping on a grade 5 or if that ain't good enough then automatically going to a grade 8. When all is said and done, carmakers use fine threads only when they're deemed an engineering necessity. Rod bolts would be max strength for size and vibration resistance. Leaf spring u-bolts would be fine for vibration resistance and to keep the undersize of the rod necessary for thread rolling to a minimum since u-bolt threads are typically rolled from stock that is not reduced in the thread area prior to rolling. Wheel studs would be fine for vibration loosening resistance. Other instances of fine threads would be selected for similar specific reasons, and most everything else you'll find is coarse.

I make some motorcycle related parts from 6061, and I would caution against using fine threads on it unless absolutely necessary. Moisture can cause some "white rust" between aluminum and steel fasteners, and if the part later needs to be disassembled then this corrosion can cause the internal threads to rip out along with the bolt whereas the same fastener will come out okay with a coarse thread. I run a couple parts that assemble to each other via a 1.25-20 thread, and I find I have to watch the pitch diameter very closely in chasing the external thread. There's just a few thousandths difference between a properly fitting fine thread and one that will gall upon assembly when the materials are soft. You have more latitude in this respect with coarse threads. Your statement about backing the tap out and clearing chips when power tapping makes me assume you're using hand taps. For power tapping you should use spiral point taps on thru holes and helical flute taps on blind holes. This would speed up the process and it keeps you from shaving the threads on re-entry and ending up with loose fitting threads. You might also look at thread forming taps, as they produce better finishes in aluminum and the rolled thread is stronger to boot as the grain flow in the thread matches the thread profile instead of being cut across the grain. I like to use them in aluminum wherever possible, and unless there's something grossly wrong somewhere, its almost impossible to break a decent sized thread forming tap. Overall, its just a lot simpler to produce a good coarse thread on a consistent basis in 6061 than a fine thread, regardless of the method used.

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03-04-2005, 07:01 PM #10

jkilroy
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Also consider that the automotive world has another reason to use coarse threads whereever it can, speed. If the guy or gal on the assembly line can get that fastener in 2 seconds faster, and you can add that up and N nubmer of stations thats time saved. As we all know, time is money.

They also must consider the average joe at an average dealership behind the impact when servicing these things. Will they clean the mating surfaces before torquing the bolt? Will they cross thread it? The automotive world would only use a fine thread if they really felt they had to.

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03-04-2005, 08:06 PM #11

Dave A
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jkilroy makes a good point (as have many others). Properly cleaned threads, lubricant and/or anti-seize on both threads makes a huge difference in proper assembly and the ability to take it apart multiple times. Too many people use a torque wrench that is not checked or calibrated on unlubricated threads and there is no way the fastener is properly tightened.

Your applications may not need torquing, but the same principles apply to prevent thread damage in the aluminum part.

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03-04-2005, 08:58 PM #12

jim rozen
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What kind of threads should one tap into aluminum
castings? The *helicoil* kind!

The reason that high strength fasteners have
their shanks turned down to the root diameter
isn't about 'stretch' but rather about stress
concentration.

An excellent discussion of this entire topic
can be found in Carroll Smith's "Nuts, Bolts,
Fasteners and Plumbing."

AKA "Screw to Win."

Jim

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03-06-2005, 02:22 PM #13

C9
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A lot of well thought out information and recommendations here.
I appreciate very much the time it took to type them out.
Thanks.

Forrest's comment, "You'll never find technical expertise at the lower end of the automotive food chain where ignorance and BS trumps hard-won expertise. Most of the confident-sounding talkers are just that: they're sheep copying what the big guys do and who convincingly parrot what was parroted to them by people who don't know what they're talking about thus perpetuating confusion and ignorance." holds true in the hot rod arena.

A lot of stuff that was written in the past is still being echoed today.
Bad part, some of it was wrong then and it's still wrong today.
Amazing how an urban type legend can last so long.

One of the big problems in the hot rod field is that the magazines that were our source of information, expertise and how-to-do-it were written largely by guys who'd been there and done it.
And then learned journalism so they could share it.

Today, the situation is reversed.
Journalism majors are getting hired by the mags and it's taking them a while to get up to speed pertaining to building and running cars.
Some of their writing is . . . call it interesting . . . and some of it is more than interesting.

I do very much as Forrest suggests as far as listening to or replying to some of the instant experts . . . usually guys who've had their cars built - 100% - and allude to having done it all themselves.

Nothing quite as educational as getting in the middle of a particular thing, whether it's building a roadster or machining a part and finding that some of the folks who are expert in a particular field have considerably more expertise than you'd imagined.

I like to think that some of the work I'm doing is precise, but after perusing the board here for a couple of months I see that the level of precision attained by expert machinists - regardless of where their shop is located - is flat amazing.

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03-06-2005, 05:12 PM #14

metlmunchr
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I like to think that some of the work I'm doing is precise, but after perusing the board here for a couple of months I see that the level of precision attained by expert machinists - regardless of where their shop is located - is flat amazing.

C9,
Don't be too impressed. None of us start threads about the stuff we put in the dumpster when it's half finished

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03-06-2005, 05:58 PM #15

egpace
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Did I miss it? When deciding between coarse & fine threads in an application, one should note that fine threads are less likely to break loose back out once in place. Particularly important in rotating aplications.
Ed

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03-06-2005, 10:47 PM #16

Pazuzu71
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I was taught the rule of thumb:
Fine threads for PERMANENT connections.
Coarse threads for frequently ASSEMBLED/DISASSEMBELED connections.

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03-06-2005, 11:06 PM #17

pete913
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I have to agree with the 'coarse threads in cast iron/aluminum, and fine threads in steel on steel. One note about torque specs and torquing bolts. Torque specs are determined using bolt stretch on a properly lubricated thread. This means oiled, NOT doused with anti seize compound. Anti seize will reduce the friction a lot and give unreliable readings and excessive bolt stretch. How do I know this? I tried it once on some grade 8 tractor headbolts that were torqued to 260 ft lbs, and I stretched them all into uselessness long before I reached the torque spec.

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03-06-2005, 11:20 PM #18

EPAIII
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Another perspective on this:

In theory, with the same OD and the same length of thread engagement, the shear strength of the coarse and fine threads should be the same. The larger minor diameter of the fine thread would, again in theory, make the bolt stronger.

But, the allowances and tolerances involved are another important factor. If the same allowances and tolerances are applied to coarse and fine threads of the same OD then the fine thread will be weakened a lot more than the coarse. Consider 1/4-20 and 1/4-28. If there is a 0.005" allowance and a tolerance of +/- 0.002" on both the external and internal threads, then the average loss in contact will be 0.005" and the maximum will be 0.009". Half of these numbers on each side.

The thread engagement on 1/4-20 is 0.065" and on 1/4-28 is 0.0464" The 0.005" allowance will be 7.7% of the 1/4-20's thread engagement but 10.8% of the 1/4-28's. Of course, if the tolerances add up in the wrong direction, those numbers could be twice as big in this simple example. Many threads have a larger allowance and looser tolerance.

If you doubt my last statement, I have an extreme example. A strain relief in a professional light for our studio. It has a 1/2-32 thread. Well, that's the nominal size anyway. In reality, the plastic strain relief is so far undersize and the brass nut is so big that I found that there is only abour 3 or 4 thousanths of thread engagement - distributed over both sides so on each side only 1.5 to 2 thousanths. Predictably, they fail. Just barely good enough to allow assembly. I don't know the actual specs they used, but likely something like 0.015" allowance and +/- 15% or 25% tolerance. Only solution is complete replacement.

Paul A.

[machinist] NC vs NF (threads), Lawrence London, 11/15/2015