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Threads: How They are Made and Which to Use

Posted May 21, 2015 12:41 PM by gbb1277
Pathfinder Tags: bolts Fasteners Threads

Threaded fasteners are common is all aspects of industry. The most important part of the threaded fastener is the threads. Threads can be created in a multitude of ways with material removal (cutting), material deformation (rolling), and material addition (3D printing). The first two are the most widely used, while the third is still limited in application. However as more work is done with 3D printing that allows for stronger materials and smoother finishes, the additive manufacturing method could become a viable option in the future.

Thread rolling

Rolled threads are the most common type as they are the easiest to mass produce and cheapest in material. The threads are formed by pressing a die into the metal blank, thereby deforming and displacing the metal to form the thread peaks and valleys. This is done to a blank that is pitch diameter and not full diameter, so there is a savings in material weight and cost. Rolled threads tend to be stronger and more durable than cut threads because they are cold formed and therefore work hardened. Since the threads are pressed, bolt material is restricted to ductile materials only.

Thread cutting

Cut threads are not as popular because there is a significant increase in cost and time to manufacture. However cut threads are made from a full-diameter blank, making the shear strength of the bolt itself higher for the same thread size. Thread cutting is required when working with a brittle material such as cast iron.

How do I know and which to choose

Identifying rolled vs. cut threads is typically very easy. If the thread diameter is larger than the blank diameter then it will almost always be a rolled thread. Choosing between the two depends on the application. If you need to use a material that is strong and brittle then cut threads is the superior option. If shear strength is more important than thread strength then cut threads are the way to go. When cost is the most important selection parameter, rolled-thread fasteners are usually chosen.

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#1

Re: Threads: How They are Made and Which to Use

05/21/2015 2:35 PM

I don't really care how these were made, and they should never have been used.

[edit] Oh, wait ... Threads... Fasteners! Never mind. I should have read the thread.

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#2

Re: Threads: How They are Made and Which to Use

05/22/2015 12:17 AM

I hate to be this way, but this blog is fluffy.

I opened it hoping to learn something.

Even if I had never seen a threaded fastener before, I'd not know more after reading this. Except that there are two ways to form an outside thread. 3D printing is a non-starter here. I suppose there MAY be a way to roll an inside thread, I don't know.

Sorry gbb1277, no offense to you but I fail to see the point. It's an interesting curiosity, how threads are formed, but does it really matter in the end?

If I'm an engineer spec'ing a thread, I'm more interested in tensile strength and materials of construction/compatibility rather than how the thread was formed. Purchasing will select the least expensive item that meets my requirements.

Rolling threads is far more production friendly, but when I buy a threaded fastener that is far from the most important factor in selection.

If I walk in to a hardware store and ask for 1/4-20 "bolt" it won't matter to me how the threads are formed.

Sorry to be be so negative.

(Disclaimer, I'm on <legal>Vicodin at the moment)

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#3
In reply to #2

Re: Threads: How They are Made and Which to Use

05/22/2015 6:46 AM

I agree about the fluffiness. It certainly does not explain how a thread can be formed by rolling. I come from Derbyshire, UK, where one of the local heroes was Sir Josiah Whitworth, one of the finest of the Victorian engineers. He did not actually invent the screw-cutting lathe, but his designs brought a previously unheard-of degree of precision into fine machinery, and one of his notable achievements was the first national standardisation of screw threads. Up till now I believed that the most precise screws were still manufactured by cutting, so it was a surprise to discover that precise threads could be formed by rolling.

Unfortunately the illustration of the rolling process here confuses rather than illuminates. How can the linear slots of the two dies produce a spiral thread?

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#5
In reply to #3

Re: Threads: How They are Made and Which to Use

05/22/2015 7:30 AM

Maybe the photo here helps. ↑I had a '58 Triumph motorcycle with Whitworth threaded fasteners. Some threaded items were hard to find in the U.S.

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#10
In reply to #5

Re: Threads: How They are Made and Which to Use

05/22/2015 10:58 AM

Thank you. As I suspected, the diagram in my link was wrongly drawn.

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#4

Re: Threads: How They are Made and Which to Use

05/22/2015 7:00 AM

I also disagree with this evaluation of threaded fasteners:

"The most important part of the threaded fastener is the threads."

I think there are more important things such as:

Grade [tensile strength]

Length

Diameter

Head type

And even color

Then I might care how the threads were formed.

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#6

Re: Threads: How They are Made and Which to Use

05/22/2015 9:34 AM

Sorry everyone. I am new to this blogging thing. I will think it through a bit more next time. I appreciate the feedback and will delve deeper next time to make it worth reading.

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#7
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Re: Threads: How They are Made and Which to Use

05/22/2015 10:18 AM

Please don't be offended. It's just that we are a tough audience. Many of us are old geezers who have been in engineering many many years. About 40 for me.

There's nothing wrong with your blog. It's just pretty basic stuff.

Welcome to the forum.

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#9
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Re: Threads: How They are Made and Which to Use

05/22/2015 10:48 AM

Gbb: I am not the advanced mech these other gents are, and I did learn something. I also have a question: Does the cold-rolling thread production produce enough heat within the blank to lessen the quantity of work expended in producing the threads, or does it simply take place so fast, the metal does not have time to "get hot" before it is already being shot out of the die roller?

I could tell you horror stories about attempting to cut inside threads in some (unknown grade) of 1/8" thick steel plate, and not realizing I didn't even have all the proper tools for that job. Needed a tap center/loader on my drill press. Went Rambo on it, and invariably I nearly ruined the parts by not keeping the die normal to the metal. (I told you guys I am a cave man when it comes to my home shop).

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#12
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Re: Threads: How They are Made and Which to Use

05/22/2015 12:27 PM

Hi James: Cold working or rolling is done at a fast pace. I am sure there is some heat build up within the blank as the threads are being formed. I wouldnt think it would be enough to change the work needed to roll the threads. If it were to add that much heat the grains in the metal that where just dislocated would begin to relax and decrease the strength of the threads

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#8

Re: Threads: How They are Made and Which to Use

05/22/2015 10:33 AM

Where does this puff piece say which thread to use? It doesn't. It tries to palm off which fabrication technique a fabricator must use if they have no control over the original raw stock material. What lame mass production fastener fabricator has no control over the stock material? Also, since when is three considered a multitude. Three is the second smallest primary integer. This is far away from a large number. Words have meanings. If you don't understand what they mean, don't use them.

I was expecting a discussion of the merits or drawbacks of the wide variety of different types of threads available for machine bolts, sheet metal screws, wood screws, self tapping, vented, self locking, zero back lash and maybe even a type completely new to me.

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#11
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Re: Threads: How They are Made and Which to Use

05/22/2015 12:12 PM

Red Fred: Even though I agree with you, that I would have liked to have learned more about various types of thread, beyond how they are formed, I did learn something.

Your reply was harsh enough that my banana came out of its peel with a bruise on it! WTG....at least most of the banana was still edible.

I thought 2 was the second smallest primary integer. Not that I have a pit bull in this ring.

Anyone have any great tool tips, or technical videos of good thread cutting set-up and practice on a lathe, or using drill press to line up a pipe thread tap?

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#13
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Re: Threads: How They are Made and Which to Use

05/22/2015 12:32 PM

My apologies to everyone and especially gbb for being so harsh. I'm having a very bad day. A friend is burying his infant son today. The opportunity to vent on this horrific day got the better of me.

0 and 1 are not prime numbers. The first prime number is 2.

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Re: Threads: How They are Made and Which to Use

05/22/2015 1:02 PM

Sorry to hear of your friend's loss.

I thought that didn't sound like you, but we both can be "stern" at times.

gbb1277 is new here. He'll get the hang of it.

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Re: Threads: How They are Made and Which to Use

05/22/2015 1:46 PM

And my apologies to you RedFred, and my condolences to that family and to you, as this is about the worst thing imaginable that can happen in any family. One of my teachers in school lost two children within a week once, first one was stillborn, other passed away the next couple of days from a rattlesnake bite (while staying with others in the same town where the mom was in labor at hospital). They never were the same after that, but there is healing down the road.

About primes: A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. A natural number greater than 1 that is not a prime number is called a composite number (Wikipedia link you provided, an excerpt). If you define it that way, I would have to say yes. I am not sure why the reason is there to say "greater than 1", since other than that 1 would most certainly be a prime number. (As usual, this is my own bent view of mathematics at its worst, I suppose).

God Bless us all, every one, and God Bless the USA.

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#16

Re: Threads: How They are Made and Which to Use

05/23/2015 5:22 PM

I recall visiting the Cloisters in NY. They had an impressive display of medieval armor. The screws were hand cut. Later of course early firearms had hand made screws. Some fine craftsmen in those times. I doubt we could do it again today.

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#17
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Re: Threads: How They are Made and Which to Use

05/26/2015 3:21 PM

I happened upon a tv show about the making of muskets. They had a master craftsman making the entire thing which included the screws. Each screw had to be used ONLY in the place it was intended! No swapping them as each was different. An interesting art to make that gun...he had to know how to do molds, castings, machining, wood work, welding, forge work, metallurgy, chemistry, etc., a true engineer!

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#18
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Re: Threads: How They are Made and Which to Use

05/26/2015 4:14 PM

All I can say is wow to that! Look what a wrong way we have come? Play on words.

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#19

Re: Threads: How They are Made and Which to Use

05/27/2015 1:34 AM

OK kinda off the path here but has anyone here ever had to cut buttress threads?

UGH!!!! what a nightmare those were....but i have to admit they take a beating, hold incredible torsional loads and are easily parted even after overloading (ask an oil rig roughneck).

Actually came up with an idea years ago about repairing tie bars on die cast machines after breaking the head off of it using a buttress thread...it held up admirably...the boss was pleased and we got through the run after a 36 hour repair.

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#20

Re: Threads: How They are Made and Which to Use

05/28/2015 3:00 PM

gbb1277

OK, so the BLOG is a bit FLUFFY. I like Fluffy!

The BLOG may have been a bit simplistic but someone that has zero experience in producing threads, this would be a good basic informational piece!

Being a Machinist for so many years, it was less of a question of how to produce the threaded portion of our part because it was going to be cut on the exterior and then chased by a die to get the proper fit and for an internal hole, 99% of the time it would be tapped. Now, a lot of internal holes are thread milled in CNC's or in a lathe with live tooling.

If we had 1000 parts to produce with a particular thread, we would look at the part as a whole and then figure out HOW can we put the thread on this part the cheapest way and have the proper tolerances! Sometimes it was cheaper to send it out to a shop with a screw machine or a thread roller.

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