Thread Rolling

Evirt Italia provides thread rolling machines for a wide variety of thread rolling processes.

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FUNDAMENTALS OF THREAD ROLLING

Thread Rolling is a highly versatile cold forming process which can be done on any type of ductile metal to harden, strengthen, or change the material’s physical properties in other ways.

Thread Rolling makes use of hardened steel cylindrical dies to shape materials; the dies, imprinted with the thread profile, penetrate the work metal at ultra-high pressures in order to displace and mold it into a mirror-image thread. Unlike traditional cutting and grinding methods, no metal is lost or cut away during the thread rolling process; it is simply reshaped.

The main functionality of Thread Rolling is represented by a component pressed in rotation by two or three rolling dies installed on two or three rolling heads (slides). At least one rolling slide has a feed movement to perform the forming process which simultaneously moves in contact with the workpiece while dies are rotating synchronously, whereas the profile will be formed out.

THREAD ROLLING METHODS

Infeed Rolling with Circular dies (Plunge)

Infeed rolling process uses a timed cycle for increased production. It’s used for general rolling of parts up to the maximum length of dies eventually presenting also shouldered or headed geometries.

Main Characteristics of Infeed Rolling Processes:

Helix angle is ground on the dies surface;
Work piece and dies are kept parallel on the machine;
Work piece rotates between the dies, presenting a little or no axial movement.

Technical Description of Infeed Rolling Processes:

The Infeed thread rolling process with Circular dies is based on one main parameter: the infeed rolling dies present a thread angle equal in magnitude but opposite in direction form that one realized on the workpiece.

The Infeed thread rolling consists of moving some (two or three) rotating cylindrical dies radially toward the center of the rolling workpiece resulting in a synchronal rotational movement of part and dies.

A minimum amount of axial movement between the dies and the workpiece occurs during the rolling cycle. Even if this movement does not affect thread quality, it may restrict the ability to produce a full thread as close as possible to the workpiece’s shoulder and will reduce the amount of full thread that can be produced. The movement can be drastically reduced by designing the rolling dies with an effective lead.

The effective lead angle varies slightly during die penetration so that some workpiece axial movement occurs. The amount of movement is usually insignificant when rolling standard threads but it can be considerable when rolling blunt or very deep thread forms, or those with large lead angles.

When two or three dies are used, they must be matched so that the helical path produced by one die is a continuation of the one produced by the other die or dies. Conversely, there will be steps in the product thread. Dies are matched by rotating one or more dies in relation to the others, or by moving one or more dies axially, to produce a continuous helix on the workpiece.

The dies size for infeed rolling is calculated from the pitch diameter of the die which must be a multiple of the pitch diameter of the finished workpiece. For single-lead threads, the number of threads’ starts is equal to the ratio of die-to-work pitch diameters. For rolling multiple-lead threads, the number of die threads’ starts equals the number of threads’ starts on the workpiece multiplied by the ratio of die-to-work pitch diameters.

Three-die machines generally cannot use dies larger than about five times the work diameter, because larger dies will contact each other before reaching full thread depth in the work. Slightly larger dies can be used for rolling multiple-lead threads.

The biggest Advantage of infeed rolling process is the Speed of Process – there is no faster process to manufacture threads especially when dealing with big sizes.

Through-feed rolling with Groove Dies

Through-feed rolling process is used to roll threads that exceed the maximum width of the dies as well as for continuous rolling of long threaded bars.

Main Characteristics of Through-feed Rolling Processes:

It typically uses annular ring type dies presenting the thread form ground on the die in straight rings with no helix.
Helix angle is performed on components by tilting the machine’s spindles to the same helix as the work piece.
Multiple start threads (within range of machine) can be rolled by increasing helix angle tilt on the machine.
Speed-up dies are used for high-speed rolling.

Technical Description of Through-feed Rolling Processes:

In through-feed rolling the workpiece moves axially through the dies.

Through-feed dies have 3 different areas: Lead-in, Calibration area & Lead-out.

Through-feed dies are designed without any lead angle (angle=0°) so that the part can feed. Their profile is made with annular grooves thus, the axes of the dies are set at an angle equal to the required lead angle of the product thread. Moreover, the dies’ lead angle has the same hand as the workpiece thread – right-hand lead dies produce right-hand threads.

The dies are made with a starling taper, so that the thread is formed progressively as the blank feeds through the dies. Also due to this characteristic, the rolling pressure is released gradually without marking the workpiece.

When through-feed roll threading the full length of a blank, the dies are held in the closed position by a hydraulic or mechanical system. The same rolling process can also be used to roll threads on only a portion of the blank, either by feeding through the piece until the desired length has been threaded and then opening the dies, or by inserting the workpiece between the open dies to the correct position and then closing the dies so that the piece feeds out of the dies.

Some basic machines models which are designed specifically for through-feed rolling of the entire workpiece’s length are not even equipped with any kind of die advance-retract mechanisms that permit partial-length rolling.

Cylindrical through-feed rolling process can be done with two-die (Planar Series) and three-die (Radial Series) end-feeding heads.

The rate of feed per revolution is equal to the lead of the thread being rolled.

Dies for through-feed rolling are usually relieved at both ends. Moreover, through-feed dies rolling process for Acme threads, worm screws or other threads often have a modified, pointed thread shape at the starting end for an efficient penetration into the blank.

Compared to infeed dies of similar diameter for a specific thread, the annular through-feed dies have no thread starts.

Typical examples of parts made by through-feed rolling are commercially threaded rod, high-strength studs, headless set screws, threaded mounting tubes for electrical fillings, pole line hardware, recirculating ball screws, and jackscrews of all types.

Threads are through-feed rolled to partial length on parts such as compressor studs, large-diameter cap screws, clamp and jackscrews, finned heat-exchanger tubing, and reinforcing rods.

Through Feed Rolling with Lead Corrected Dies

Lead corrected Dies are different from Groove Dies indeed they have a lead included in profile. Additionally, these dies are swiveled. The sum of lead angle of dies and swivel angle = lead angle of workpiece.

Technical Description of Through-feed Lead Corrected Rolling Processes:

In through-feed Lead Corrected rolling, the workpiece moves axially through the dies. Through-feed Lead Corrected dies are designed with a lead angle generally different from the one of the pieces, so that the part can feed. The dies are made with a starling taper, so that the thread is formed progressively as the blank feeds through the dies. Also due to this characteristic, the rolling pressure is released gradually without marking the workpiece.

Some basic machines models are designed specifically for through-feed rolling of the entire workpiece are not equipped with die advance-retract mechanisms that permit partial-length rolling.

Cylindrical through-feed rolling process can be done with two-die (Planar Series) and three-die (Radial Series) end-feeding heads.

The number of threads starts, which, together with the diameter of the die, determines the lead angle, is different lor through-feed rolling on cylindrical machines than for infeed rolling a similar size thread.

Compared to infeed dies of similar diameter for a specific thread, the through-feed dies may have more or fewer starts for parallel-axis rolling.

With the through feed rolling using lead corrected Rolling Dies even smaller machines with medium Rolling force can be used for big threads.