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Thread cutting vs Thread rolling

04/03/2009 6:55 PM

Does Thread Rolling method generate less stress and make the material less brittle than Thread Cutting does?

It seems thread cut studs tend to be more brittle.

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#1

Re: Thread cutting vs Thread rolling

04/03/2009 9:36 PM

Thread rolling makes stronger threads because flow of material reinforces the shape, the cold work increases ts and yield ( and especially ys/ts ratio, and usually the surface finsh is smoother so less chance of nucleating stress fracture. Plus there is more material. My guess is what you describe as more brittle in cut threads is related to the sharp nature at the root which can initiate stress failure. I'm pretty sure a nice discussion of this can be found at http://www.keystonethreaded.com/why.htm

Come on back if you have additional questions. milo

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#2
In reply to #1

Re: Thread cutting vs Thread rolling

04/04/2009 9:18 AM

Additionally the residual compressive stress on the threads are another thing which helps it in attaining better fatigue life.

If the thread is rolled properly then infact you are likely to have a better root radius and hence again improved fatigue life.

Though our specifications call for rolled thread, we have faced some fatigue failures earlier and and on analysis it was seen that those which failed were cut and not rolled (usual malpractices one comes across once in a while).

As Milo said in a rolled thread there are are multiple points from which the crack can nucleate - root, flank etc. This is avoided by the rolling.

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#3

Re: Thread cutting vs Thread rolling

04/04/2009 11:42 PM

The two methods of forming external threads can provide equivalent service in many applications. The ultimate holding power under steady load is still determined by the shear area of the thread root. However, properly ROLLED threads exhibit far superior life expectancy in applications where loadings vary cyclically--as in cylinder head fasteners, connecting rod bolts, and similar highly stressed cyclical loadings.

PROPERLY cut threads are extremely durable in most applications. However, if the cutting tool and lubricants used do not produce a smooth flank and root, fatigue life is reduced significantly in highly stressed cyclical loadings. Proper radius of the root is very important for long life in such cyclical loadings.

Rolled threads are not a substitute for proper sizing and material selection however. Heat treating after rolling is very important for attaining high strength, long cyclical loading life, and smooth torque vs clamping force. Improper rolling techniques will not produce a strong thread.

'Brittleness' is not a characteristic caused by differences between cutting and rolling. It is easy to mis-analyze a thread failure and attribute the failure to an incorrect cause. Professional analysis of the failed part may be warranted if the cost of failure is high or life safety is involved.

Using an improper thread lubricant, or exposing the threaded connection to corrosive environmental factors can cause 'brittle fracture' appearance that is really caused by inter-granular corrosion.

Higher strength fasteners (such as Grade 8) are, by nature, more brittle than lower strength metals. Failure may occur with little visually apparent permanent deformation (stretching) at the failure plane.

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#4
In reply to #3

Re: Thread cutting vs Thread rolling

04/05/2009 1:22 AM

It's just my opinion but, if a high load requiring the ability to withstand extreme tempurature swings and shifts in directional forces ect. were the required design needs, i am sure alot of factors would have to be considered and cost would be greater to produce threads to meet the those requirements. A precise steel and alloy mixture along with the exact rockwell hardness would be the first consideration. As far as how to acheive the strongest treads, i am sure that cnc machining with specialized carbide incerts changed at predetermined intervals would be staged in multiple passes over the threaded area. Small chip loads on the tooling with a roughing cut and then a finishing cut would be the choice i would make. I would leave about .003-.005 thread pitch over size to be followed by a self tracking 3 roller pop open head fed by a lead screw. Little heat would be produced and quality control samples would be measured before the thread roller to keep an eye on the life of the pre cutting incerts. The steel alloy mixture of choice would be heat treated after the machining process to meet the precise strength needed. Minimal removal of material in stages is the key to the machining process. A light thread rolling is just to burnish the surfaces of the threads. Of course this is an example of a costly way af making a specifc thread for an extreme need. Most above standard applications have no need for such a process however the cost of just the steel before any machining of such a thread would be a special request item also. The tolerances of even the best steel vary greatly and most machining companies that get these contracts have their own foundrys to control the quality of the steel itself. Now we are even getting into Quality control checking parts with xray equipment and high magnification measuring. It can get extreme in every reguard depending on the need. I guess the key word for type of thread forming method that makes the best quality thread is Combination.

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#5

Re: Thread cutting vs Thread rolling

04/05/2009 5:23 AM

To my mind, thread rolling must be best, especially where heavy loads are being used,, but I have found that expensive 1 meter stainless steel rods (8mm thread) that I bought were inconsistent in diameter and I re-cut them myself to improve consistency. (for a CNC machine that I am building!)....(no heavy loads)

It was a long slow process to re-cut them all.......

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#6

Re: Thread cutting vs Thread rolling

04/05/2009 2:00 PM

Cut or rolled...it's the material that used , not the way the thread is applied.

Even a heat treated bolt will break if you are rough with it.

Usually a rolled thread is stronger because no material was taken away. Usually on smaller diameters will a thread be rolled.

Also...American made bolts and studs are stronger. My suggestion is to talk to your supplier and tell them what your application will be, and to recommend the appropriate material. maybe the size of stud is too small. That's another issue to look at.

After 40+ years as a toolmaker I've seen about everything when it comes to fasteners. Have a great week.

Thanks

Ronnie

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#7

Re: Thread cutting vs Thread rolling

04/05/2009 7:51 PM

Rolling is superior. A cold forge process that aligns the grain structure. Rolls leave a superior finish which minimizes stress risers. Can work harden material if its chemistry allows.

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#8

Re: Thread cutting vs Thread rolling

04/06/2009 10:00 AM

Something to consider in specific circumstances of abrasive or corrosive wear or failure in higher carbon steels, Deep Cryogenic -300 F. treating: 1) converts retained austenite to martensite in hardened steels; 2) relieves residual stresses; 3) precipitates fine eta-carbides in appropriate steels; and 4) refines the crystalline structure of the metal. These metallurgical changes result in a stronger, but not harder, and a less abrasive material with increased distortion resistance furthering the improvements gained by heat treating. dave@300below.com

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#9
In reply to #8

Re: Thread cutting vs Thread rolling

04/06/2009 10:13 AM

Hi Davek1.

After precipitating out new martensite from retained austenite, that new martensite is untempered, right? (brittle!) So after the cryo treatment, once more into the temper furnace, right?

I have seen electronmicrographs of the "eta carbides" but I have to say that from a volume fraction/ frequency severity, I remain unconvinced that 'eta carbides' actually do anything as opposed to being merely correlated with the treatment you have described.

When you say "refine the crystalline structure of the metal," what exactly do you mean? Grain size modification?

Great to see you on here.

milo

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#10
In reply to #9

Re: Thread cutting vs Thread rolling

04/06/2009 10:51 AM

Done properly, after the cryogenic treating, the material is heated up to +300F three times which tempers the martensite. As you alluded to, if it isn't done right, the benefit is not fully gained. Thank you! dkimmel@300below.com

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#12
In reply to #10

Re: Thread cutting vs Thread rolling

04/06/2009 11:21 AM

Thank you all for contributing the helpful answers

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#13
In reply to #10

Re: Thread cutting vs Thread rolling

04/06/2009 11:25 AM

Thanks!

If you were in AZ you could just leave it in the car a couple of afternoons to get that 300degrees .

300 is prettttttyyyy low.

Appreciate the comeback.

milo

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#11

Re: Thread cutting vs Thread rolling

04/06/2009 11:19 AM

I have a further question that interests me.

I use 8mm stainless threaded rod, which I then cut to a consistent size myself. Its a shitty job and takes hours because the die(s) heats up so much. To recut 1 meter takes about 2 hours, even working with power tools, with lots of cooling periods in between!!

I know there is more than one type of stainless, which one I have I have no idea....so my question may not be answerable....but:-

Is Stainless steel usually harder (generally speaking) than the normal mild steel threaded rod that one can buy in DIY shops?

If yes, I will have to change what I use eventually. My idea was to reduce the possibility of rust!!!!

Is there a grade of steel rod that is has good wearing characteristics, but can easily be re-cut?

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#14
In reply to #11

Re: Thread cutting vs Thread rolling

04/06/2009 1:43 PM

Andy

I know that drilling stainless requires cobalt tipped drill bits... otherwise it takes forever. I wonder if they might make similar taps and dies for stainless? I am not a professional at this so I don't know. Most of the machining I do is in aluminum.

Bill

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#17
In reply to #14

Re: Thread cutting vs Thread rolling

04/06/2009 3:22 PM

I think now that stainless is harder to work, your comments fit in with my thoughts.

I have not found any dies specially for stainless. I will have a further look around...next time I will use mild steel.....

Thanks for your very welcome comments..

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#18
In reply to #17

Re: Thread cutting vs Thread rolling

04/06/2009 3:41 PM

Stainless will work harden and they're "sticky". You'll need some coating on the die to prevent material sticking to the die. Rolling will be easier.

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#21
In reply to #17

Re: Thread cutting vs Thread rolling

04/07/2009 12:00 AM

At our work place some worker has come out with a novel idea check it up whether it helps.

use Curd (yes you heard it correctly curd made from milk) as a cutting fluid/ paste for cutting Stainless steel. It really improved the cutting speed, reduced the chattering and improved the surface finish.

Try out. It especially worked well with the work hardening steel.

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#22
In reply to #21

Re: Thread cutting vs Thread rolling

04/07/2009 12:11 PM

Will do, its certainly worth a try. Thanx to you both.....

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#15
In reply to #11

Re: Thread cutting vs Thread rolling

04/06/2009 2:01 PM

303 is FREE CUTTING. .15% min sulfur

416 was the original free cutting stainless. .15% min sulfur.

304 is acceptable machining, Carpeneter makes a project 70 304 with process controls that make it midway between 303 and regular 304 .

316 is a b*tch to cut using "home equipment."

Schmolz+Bickenbach has a series of UGIMA stainlesses that have optimized process control and some Metallurigical magic that make them the best choice for high production applications using coated carbides.

The problem is getting the chip to break on stainless compared to mild steel.

milo

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#19
In reply to #15

Re: Thread cutting vs Thread rolling

04/06/2009 3:52 PM

I think I just found out with your help, which stainless I have....

Many, many thanks.

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#20
In reply to #11

Re: Thread cutting vs Thread rolling

04/06/2009 10:54 PM

Yes Stainless is more work than mild steel. However there are different grades of stainless steel as you may already know.

304 stainless seems to machine easier than 303 for example.

But ...if I may ask. Why do you need to start with an 8mm threaded rod, and then make it something else ? I would think that every possible threaded rod imaginable can be purchased online.

If you resort to grunt work, I'd suggest making your life easier by going with a steel, and not stainless and use grease or oil to prevent rust. Surface treatments to the steel can also prevent rust, if grease or oil is not desired.

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#16

Re: Thread cutting vs Thread rolling

04/06/2009 2:37 PM

I think Jerry NH hit the nail on the head. Rolling is not actualy a "machining" process, it's a forging process whereas thread cutting is a "machining" process. The difference, thread cutting cuts across grain in the steel whereas rolling forms the grain without actually cutting/damaging it. It is a difference similar to taking a round wooden rod and steaming it to bend it into an arc verses taking a piece of wood and cutting out the shape. The same applies for steel, but on a microscopic level. Forging creates a stronger product in all materials (steel , aluminum, etc.). Heat treating helps, but the damage is already done and cannot be reversed short of heating the steel to a molten state and allowing it to cool and re-establish the grain pattern. Both machining and forging have their place, but just as the processes are different, so are the outcomes concerning the strength of the product.

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#23

Re: Thread cutting vs Thread rolling

04/08/2009 3:57 AM

One of the advantages of rolled threads is that they can give full capacity to the diameter of the bar, thus reducing the amount of material required. This can be useful for aesthetic applications such as cross bracing in a glazed structure where the visual clutter needs to be a minimum.

Tri-Pyramid (http://www.tripyramid.com) use high strength ss bars with cold rolled threads at the ends so that the work-hardening is not lost during the cold forging process.

They also die cut the rolled thread after the initial cold forging. Are all rolled threads also die cut after rolling or can they be used straight from the forge?

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#24
In reply to #23

Re: Thread cutting vs Thread rolling

04/08/2009 7:21 AM

I have very limited personal experience, but I found that the rolled threads were oversize and slightly inconsistent in diameter.....I had to re-cut by hand. Then I had good consistency.

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#25
In reply to #24

Re: Thread cutting vs Thread rolling

04/08/2009 9:37 AM

Tell you something more interesting

Long back (5-6 years) we had some problem with if I remember M36 bolts. These were long bolts and with a about 100-150 mm thread engagement- abnormal i know but that was the design. These were rolled thread. Due to some error in the machine setting the pitch was only fractionally wrong. What has gone wrong don't know- bought out, we don't make, that too from a reputed make.

When you check with gauge everything OK, when you put the component- stops at half way or may be 75%.

After lot of hair scratching (luckily didn't go bald) - we measured pitch over long length and that could show the problem. gauge, normal pitch gauge etc everything OK. Can not blame the inspector. Must be some problem with die .

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#26
In reply to #24

Re: Thread cutting vs Thread rolling

04/09/2009 6:35 AM

Hi Andy Germany

If you have a number of rods to do, it may be worth the effort to anneal them.

Wrap them in alumino-silicate cloth to limit access to air and place in a pottery kiln.

You need to get it soaked at 1045C to ensure all carbides are dissolved.

The bundle should then be quenched in water. (The wrap will probably disintegrate or be otherwise rendered worthless by the quench. It may be feasible to simply tip,them into the water from the wrap) If possible, ensure the rods remain vertical when quench to avoid distorted and bent product.

This will remove the work hardening your rods have acquired during thread rolling (or cutting) giving you a softer material to work with.

Try to do your sizing in one hit as every time you work the material it will work harden some more.

A good lubricant is essential when machining stainless, especially the austentitic grades.

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#27
In reply to #26

Re: Thread cutting vs Thread rolling

04/09/2009 6:44 AM

Hi,

my knowledge is increasing in leaps and bounds with your help. Many thanks...

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#28
In reply to #26

Re: Thread cutting vs Thread rolling

04/09/2009 12:46 PM

Sceptic

Would you have any recommendations for "a good lubricant"? I use something called "A9" when working with Aluminum. I just wonder if there might be special cutting fluids FOR stainless. Of course I realize that brand names might be different between the US, Europe, or Australia.

Bill

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#29
In reply to #28

Re: Thread cutting vs Thread rolling

04/10/2009 7:35 AM

Hi Sciesis2

Would you have any recommendations for "a good lubricant"?

Sorry, not my field.

If in doubt soluble oil should be OK although probably not the best available.

My sole attempt to machine SS using soluble oil was pretty disastrous, but I'm no machinist.

sb's comment on using curd was intriguing. I haven't heard that one before. May be worth a try.

Sorry I can't give you anything useful apart from miscellaneous ramblings (happens as you get older - I started at about 20!)

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#30
In reply to #28

Re: Thread cutting vs Thread rolling

04/10/2009 9:51 AM

MOst home shop s use tap magic. For stainless, the EP additives are crucial. also, here are a couple other ideas.

Most of my clients buy metalworking fluids by the drum or tote...

http://www.tapmagic.com/TMepx.htm

http://www.jacquescoulombe.com/Aervoe1.htm

http://newtecprodukter.devello.se/teknisk/tdmw389_EN.pdf

milo

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