Thread Rolling and Blank Diameter

[BarryS]

When rolling threads, I understand you reduce the body diameter to a few thou over the pitch diameter before the rolling operation. At least that is what Machinery's says. But most commercial fasteners seem to have rolled threads and a full major diameter on the shank. So do you reduce the diameter in just the area to receive threads? Or do some types of rolling equipment accept full major diameter blanks and squeeze hard enough to make the part grow some length?

Barry

 

[L Vanice]

Looking closely at modern cap screws, I usually can see that the threaded section was originally a smaller diameter than the unthreaded section of the shank. I can see a bit of a shoulder right at the end of the last thread. The shank diameter is apparently cold headed along with the screw head.

Deck and drywall screws usually have the unthreaded part visibly smaller in diameter than the thread o. d.

Larry

 

[J-Head]

L Vanice is correct. Wire is fed into a cold header where the headed end is formed using multiple dies, and the material flows into the unthreaded shank region. The blank is then sheared off, ejected, and transfered to a thread rolling machine. Smaller screws are typically rolled on a reciprocating flat die machine, bolts between 1/4 and 3/8 are typically made on a planetary die machine, and larger diameter bolts are made on either opposing two die or three die machines.
In addition to using a blank diameter equal to approxamently the pitch diameter, the depth of the rolling dies must be set such that there is less than 100% flow into the die form. As the material flow approaches 100% wear on the dies and the forces exerted on the machine become tremendous. Many aerospace thread rolling applications call for high percentages of die fill in order to maximize full thread form, and are therefore very difficult.

 

[BarryS]

Thank you both for the help. I am considering a RFQ for a part I now single point on a manual lathe and am trying to better understand what I am asking for.

I have also noticed that common studs have a reduced shank for the full length. Which makes sense as there would be no need to do any prep except straighten the wire.

 

[PsiMan]

We roll alot of threads at my work, Mostly 1/2-20 on 1/2 rod. The before thread is turned down to .465 dia. After the roll it's .495, then we adjust the roll till its almost touching the 1/2 original diameter.

 

[surplusjohn]

interesting point for filing under general knowledge. Cold headed bolts have to have taper shanks in order for them to be ejected from the header dies. that taper can be a puzzler if you don't know to look for it.

 

[Iceman Motorsports]

We make 100's of millions of cold formed thread rolled fasteners and parts of all shapes and sizes. Usually hold diameter within .0005" to a size that we have found works for the particular thread on that particular part. Material and part geometry, thin wall with thru hole, etc as well as threading equipment, three die, flat die, planetary all play into it. Usually run very little taper. Experience determines the final size.

 

[russ61]

Try contacting central thread roll.They have a free catalog that has all the info you need.What we have learned is that for every .001 the blank dia. goes up the thread will go up .003.Usually the blank dia.is equal to the pitch dia.

Just finished modifying a machine to roll threads on al. blanks .750 long,threading around 5 parts per second.Pretty cool to watch them come out.
Russ

 

[Mold Mak]

We roll alot of threads at my work, Mostly 1/2-20 on 1/2 rod. The before thread is turned down to .465 dia. After the roll it's .495, then we adjust the roll till its almost touching the 1/2 original diameter.

Once we have reduced the blank diameter to .465, how or where do we begin to adjust rolling head? do we use a "sample thread" or blank diameter?

 

[FredC]

You pulled a very old thread and I do not think I have seen PsiMan here in the last 10 years. I you are adjusting a 3 roll head I would assume the distance between the rolls would be a little smaller than your minor diameter. The reason is I think your die head would spring a little. How much I have no idea, so I might try setting the rolls to touch a pin gauge of the correct minor diameter then make a few parts till I get the finished pitch diameter and major diameter in tolerance.
Some awfully smart people hang around here and they might give you a better answer.

 

[atex57]

Once we have reduced the blank diameter to .465, how or where do we begin to adjust rolling head? do we use a "sample thread" or blank diameter?

I use a sample to get close, open the head adjustment, insert a sample, adjust down to touch, and lock it there. You will still have to adjust it down from there. Sometimes many trials.

Are you using thread mikes, thread wires, sample nut to check fit?

Ed.