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View Full Version : Rolled threads vs Screwcut threads.



David Powell
06-09-2009, 10:13 AM
As a favour I have just made 2 new threaded rods for a puller. The originals apparently were made from 9/16" stock and the threads seem to be rolled threads, nominally 5/8" but no where could i find more than ,615" dia. Mine were made from 5/8" 1018 which happened to be at hand, they threaded well,with a good looking thread apparently well fitting at a thou over nominal thread depth and an od , after a quick run over with a file,of ,623 ". I used new commercial nuts and needed to run a good tap through them before they would run really sweetly down the length ( 10") of thread, My question here is, are my threads likely to be as strong as the originals? I know nothing of the practicalities of thread rolling but have been told that rolled threads are stronger than screwcut ones. Certainly all modern fasteners seem to have rolled threads. regards David Powell.

Ken_Shea
06-09-2009, 10:29 AM
You are correct, rolled are stronger, supposedly up to 25% or so, consequently, using the same material, your cut threads will not be as strong as the original rolled threads. Still does not mean that they will not be suitable, that would depend on the application I guess. There are other benefits for rolled threads besides strength.

Do a search, lots of useful and much more complete information that I can share.

Ken

rklopp
06-09-2009, 10:56 AM
Rolled threads are stronger, especially in fatigue loading. I would not expect fatigue to be an issue for a puller - after all, how long would it take to do a million pulls?

The reasons rolled threads are stronger are that the material is work-hardened by the process, raising the local yield strength. Also, the surface finish is typically better for rolled than cut threads, reducing the number of stress-raisers. Also, thread rolling deforms the underlying microstructure to follow the contour of the hills and valleys of the thread's longitudinal section. This means there are fewer surface-breaking defects to initiate cracks.

From a manufacturing perspective, thread rolling is also fast compared to thread cutting, and there are no chip handling issues. The price for this convenience is higher tooling cost.

ahidley
06-09-2009, 11:01 AM
To the thread gurus.... I know that thread forming taps are readily available but why arnt thread forming dies available?

J. R. Williams
06-09-2009, 11:38 AM
ahidley

Thread rolls are available to produce rolled threads on round material. Typically they use three rolls in a holder to form the thread. Bring money..

JRW

Rich Carlstedt
06-09-2009, 11:41 AM
Search for "Landis" heads

Rich

lazlo
06-09-2009, 11:59 AM
I think most threads are rolled nowadays because it's vastly cheaper to mass produce screws on a roll former than a screw machine. If you watch the "How Its Made" video of the bolt factory in Canada, those are mass produced hot rolled, where I presume you don't get the work hardening effect...


Rolled threads are stronger, especially in fatigue loading.

How much does that really matter though? A bolt reaches maximum strength at about 1 1/2 times its major diameter. So say the rolled thread form is 25% stronger, as per Ken. So you could max-out the bolt strength at about 1.1 times the bolt diameter, instead of 1 1/2. Is that really a major issue?

Seems like David just needs to keep to the thread length = 1 1/2 bolt diameter rule, and he's all set.

http://www.youtube.com/v/7ORomNNCSUQ&hl=en&fs=1&rel=0

PeteM
06-09-2009, 12:23 PM
The original likely used a stronger and heat-treated material. I used 4140 pre-hard to replace a puller screw --- so far it's been OK with moderate use.

tattoomike68
06-09-2009, 04:53 PM
Iv seen screw machine thread rolling and it was super fast, like "SNAP" and its done the part might olny do 1.5 revolutions and its over.

the type I seen running was like a scissor type knurling tool with a timing chain to time the 2 rolls and an air cylender that powered a wedge to put the bite down to an adjustable degree, If you ever seen one in action you would like it.

I have even seen end endworking treadroll that mounts in a tailstock, its was just as fast but cost more and was on the fancy side of the tooling spectrum.

Even the worlds fastest CNC single point thread cutting is slow compared to thread rolling, not even close. There again there is no reason a thread roll cant be mounted in a cnc.

Last time I worked on those machines the toolholders were $3,000-$5,000 + and thats not including the rolling dies, those were $1,000-$2,000 per thread pitch. the upside was you could tread 30,000 parts and nothing took much wear unless you rolled a part with a broken off carbide insert into it.

If I had a job in the 10,000 to millions of threaded parts I would roll them.

knedvecki
06-09-2009, 10:40 PM
Search for Acme-Fette Rolling heads. Forms a UNJ type thread with rounded root and crest. Material is cut the pitch diameter. Used in aircraft.

rklopp
06-10-2009, 12:00 AM
lazlo,
You've got your factors of 1.5 mixed up. I think you mean that a threaded connection reaches the strength of the bolt once at least 1.5 time the diameter worth of thread engagement is achieved. That's a rough rule of thumb. For a given material for a female thread, a stronger bolt will require more thread engagement and a weaker bolt less engagement, before the bolt breaks instead of the thread stripping. There really is a difference between strong bolts and weak bolts, whether that strength difference is due to thread rolling or heat treating, etc.

lazlo
06-10-2009, 12:12 AM
I think you mean that a threaded connection reaches the strength of the bolt once at least 1.5 time the diameter worth of thread engagement is achieved.

Exactly. So if the threads on a roll threaded bolt are 25% stronger, it will reach it's maximum strength in 25% less thread engagement. So if you're really worried about it, just make the threads 25% longer, or use a material that's 25% stronger than 1018 (4140 PH has 50% higher yield strength, for example).

oldtiffie
06-10-2009, 08:57 AM
Good video Lazlo - many thanks.

The other half/side of the equation of course is the "nut" or tapped hole and its material and its state.

Sometimes the rolled bolt or what-ever is screwed and torqued into aluminium or cast-iron (motor vehicle parts - heads, manifolds, transmissions etc.) and other times passes straight through and has a nut (and washer) on the other end (pipe-work flanges, structural building frames, machines, machine tools etc.) where the torque can be applied either to the head of the bolt or the nut - or both.

The rod that is rolled to form the thread is about the pitch diameter so that as the roller die is pushed into the rod, the displaced material will (partially at least) fill the void between the pitch diameter and the outside diameter of the finished bolt/screw in the rolling dies.

Mcgyver
06-10-2009, 09:34 AM
To the thread gurus.... I know that thread forming taps are readily available but why arnt thread forming dies available?

I've a thread rolling die kicking around i kept as a curiosity. its a rectangular hunk of steel about 1.5"x .75 x 4. There are finely ground groves, angled at the thread's helix angle running its length. the grooves progress from tiny serrated groves to full depth smooth over their length. A very complex and nicely done bit of grinding - I'll try to remember to get a pic up

looks expensive and the machine they'd be loaded in would be specialized and able to impart some force on the blanks.