# course thread or fine thread ?



## lastpatriot (Aug 30, 2010)

Sorry if I'm posting this in the wrong section, (the message boards are a little confusing to me (old fart (youngster @ 50)))

Here goes, is coarse thread stronger for pull out (tension not shear) than fine thread. (in soft metals such as alum.)

I tend to think that coarse thead (female) is the way to go in soft base materials.
I'm not worried about the screw, I'm wondering about the receiver material.

(I realize if I'm worried about it loosing up use fine thread)


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## steamer (Aug 30, 2010)

Depends.

Aluminum,brass and cast iron I use coarse threads....the thread holds better.

With Soft ( 12L14) steel......probably coarse again..

Anything beyond that....( mild steel, alloy steel, high strength bronzes, ect) what ever seems appropriate for the application.

What is the "receiver" for?

Dave


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## lastpatriot (Aug 31, 2010)

I didn't have anything particular in mind, just for general knowledge thanks


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## Blogwitch (Aug 31, 2010)

You will usually find that most threading systems have two distinct sets of threads. The fine for hard materials, the coarse for soft materials.

In the US I suppose it would be UNF (Unified fine) and UNC (Unified coarse), I think also referred to as ANF & ANC, American National. 60o thread form.

For UK in previous times they used to use BSF (British Standards Fine) and BSW (British Standards Whitworth, coarse). 55o form.

Then for Metric you have the two types, Metric Coarse and Metric Fine. 60o form.

There are many, many other thread forms, but for general purpose systems, these three cover almost everything where the two types of threads are required.

As you said, just for general knowledge.


Bogs


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## gbritnell (Aug 31, 2010)

I never sat down to calculate strength of threads, coarse to fine. I was always under the impression that coarse threads were stronger and fine threads were used in vibration situations where the shallower helix would hold tighter. After you asked it got me thinking, say you take a 1/4-20 thread and calculate the surface area of 6 threads which would be 1.5 times the thread diameter. (maximum holding power). Now make that 1/4-28 thread. In 1.5 times the diameter (.30) you have 8.5 threads. Would the cross sectional area of 6 bigger threads be close to the same as 8.5 threads of the smaller size? I could calculate it out but I don't want to burn up that many brains cells. I only have so many left. Or I guess there is a shear chart out there in cyber land which would give you the exact numbers. I do know that working with aluminum the more times you repeat the loose/tighten cycle it fatigues the aluminum to the point that the threads will start breaking out. 
gbritnell


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## lastpatriot (Aug 31, 2010)

I think the angle comes into play quite a bit here. Let's see if I can describe this, say you section a threaded hole lengthwise down the center,
now zoom in if you will to a single thread say at the bottom middle of your cross section.
You would be looking at a triangle with the point on top, now it seems to me that the wider the base the better (in a shearing per thread situation?) or am I all wet cus it gets cancelled out due to fine having more of them?

All I know is on my old Harley fine thread strips out faster in alum. than coarse (which also stripped out) gotta love the rigid mounted unbalanced motors.


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## hammers-n-nails (Aug 31, 2010)

not being an expert myself i would say that with similar materials fine threads would hold better. my reasoning is that the diameter of a cylinder with the circumferance running through the center of the threads would be greater with fine threads than course. given enough threads so that the core of the bolt is the weakest area rather than the threads themselves the fine threads would definatly be stronger because the minor diameter is bigger=more cross-sectional area=more strength.


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## lastpatriot (Sep 1, 2010)

I agree if your looking for bolt strength, not what the bolt goes into.


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## Peter. (Sep 2, 2010)

I don't know about the softer material being the only reason. On my alloy bike engine, all of the higher-torque bolts (head bolts & mains) have fine threads, and all of the standard torque bolts have standard pitch threads. Standard torque for the fine thread head bolts is 35lbs/ft and mine has high-tensile larger bolts (but same pitch) torqued to 75lbs/ft, some people take them to 100.


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## lastpatriot (Sep 2, 2010)

Heat treated alum. may be a factor, some alum. alloys when heat treated and not welded on after, have comperable strength to steels.


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## Blogwitch (Sep 3, 2010)

I found this,and I think it describes things in an easily understood way, rather than getting too technical.

http://www.eisc.com/support/coarsefine.htm

If you click the BACK button on the top of the page after you have finished reading, there are other areas that are covered in an easy to understand language as well.


Bogs


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## fcheslop (Sep 4, 2010)

Thanks for the link Bogs some useful info.
best wishes Frazer


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## jpeter (Dec 5, 2010)

Its common knowledge (machinerys handbook) about how many threads have to engage to reach max strength. Beyond that more threads don't help much. But, I have to agree, course threads do seem to last longer in soft material.


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