A POWERPOINT PRESENTATION ON THREAD ROLLING

Presented By:- Tklen Haokip (D/15/ME/17) Pritam Sarkar (D/15/ME/18) Shubham Sarkar (D/15/ME/19) Padi Takha (D/15/ME/20) Anirban Das (D/15/ME/21) Taru Yallu (D/15/ME/22) Deepty Pandey (D/15/ME/24)

THREAD ROLLING

THREAD ROLLING PROCESSES

Thread and form rolling is a simple cold forging process confined almost entirely to external threads. It is referred to as a cold forging process because most rolling is done on cold blanks. However, rolling of threads on heated blanks has been beneficial on some applications. Today, thread and form rolling is accepted by many industries as a preferred method of producing uniform, smooth, precise threads of superior physical quantities.

Thread rolling is different from other types of threading processes like cutting, grinding, and chasing. The forming process can be used to produce other special forms, such as knurls. For the best quality threads, the process is performed on precision centerless ground blanks. The blank diameter of a rolled thread is at the pitch diameter, a theoretical point between the major diameter and minor diameter.

Fig: Thread Rolling Process

ADVANTAGES OF ROLLED THREADS :- Rolled threads have improved physical characteristics, greater accuracy and a

high degree of surface finish. They are uniformly produced at high rates of production with no wasting of material. These six major advantages account for the increased use of thread rolling.

PHYSICAL CHARACTERISTICS :- The cold forging that threads receive during the rolling process strengthens them

in tension, shear, and fatigue. TENSILE STRENGTH :- The cold working of the surface increases the tensile strength of the metal

worked, and static tensile test have frequently recorded increases on the order of 10% in the breaking strength of the parts.

SHEAR STRENGTH :- When a thread is rolled the fibers of the material are not severed as they are

in other methods of screw thread production, as shown in Fig. 3, but are re-formed in continuous unbroken lines following the contours of the threads, as in any good forging as shown in Fig. 4. Rolled threads resist the stripping because shear failures must take place across rather than with the grain.

RESISTANCE TO FATIGUE Thread rolling increases the part’s resistance to fatigue failure in several different

ways. Rolling between smooth dies leaves the thread with smooth burnished roots and flanks, free from tears, chatter or cutter marks that can serve as focal points of stress and, therefore, starting points for fatigue failures.

Rolling also leaves the surface layers of the thread, particularly those in the roots, stressed in the compression. These compressive stresses must be overcome before the tensile stresses can be built up, which alone, can cause fatigue failures. This increase in root hardness, up to 30%, adds considerably to the parts resistance to fatigue.

DRUNKENNESS :- The control of drunkenness is dependent upon using dies with correct lead angles and

upon careful matching and proper feeding and positioning of the blank in relation to the dies.

ROUNDNESS :- Roundness is dependent upon the roundness and uniformity of

hardness of the blank and upon the rate of application and release of the die pressure. If the dies are designed and setup to apply and release pressure gradually and uniformly, close tolerances on roundness may be steadily maintained.

BENEFITS OF THREAD ROLLING

Reduced Threading Costs Per Piece. Dies, Blanks, Set-up and Machine are the variables which contribute to thread rolling costs. Using Hi-Life Thread Rolling Dies eliminates the die variable from thread rolling operations thus reducing threading costs. Production tolerance goes to the thread of the product being rolled rather than being lost to the dies themselves.

Installation is made easy because of the accuracy of Hi-Life dies which are made to closely controlled limits of squareness in adjacent surfaces and parallelism of all edges to less than 0.01 per 25mm (0.0004” per inch). This ensures that dies rest on a true flat base, make initial contact with the screw blank to be rolled at its starting end, and presents a threaded surface square to the blank.

Reduced setup time, minimised shimming, elimination of drunkenness and extended die life due to squareness of roll-on and roll-off in Hi-Life dies. At the start of the threading cycle the screw must roll in a path parallel to the top surface of the die rather than parallel with the thread helix of the die. When the screw blank makes initial contact with the stationary die, a smooth start is needed in order not to tip the blank. Otherwise, a drunkenness of thread or other thread starting problems may result. Squareness of roll-on is especially important in today’s high-speed flat die machine where misalignment problems may become pronounced.

EQUIPMENTS AND TOOLS:-

Thread rolling Equipment –Most of the threads produced today are rolled on threads rolling machines, automatic screw machines and automatic lathes. The thread rolling machines use flat and cylindrical die, while the automatic screw machines uses cylindrical thread rolls.•Reciprocating (flat die) machinesFlat dies are used in reciprocating type of thread rollers including bolt making type machines. These machines are made in a number of sizes, each for limited diameter range and with specified length of die.Although taper pipe threads are most commonly rolled on cylindrical die machine, reciprocating machines are used for rolling pipe plugs in some limited case.•

Rotatory planetary machine These recently developed die have one central rotary die on a fixed axis and one

or more stationary concave segment dies located to the outside of the rotary die.These machines are made in several sizes, with limited diameter ranges. Each size machine has a specified length of segment die. Speed may be varied to accommodate the rolling of different kinds and hardness of materials.

Cylindrical die machine Cylindrical die machine has wide ranges of speeds and feeds, and are widely used

for in-feed rolling with appreciable axial movement of the blank during rolling.because of their circular shapes there are no limitations on the no. of work revolutions provided for the rolling of a thread or the rate at which these dies feed into the work.the speeds for rolling depends on the type and size of work, material and equipments used. High production rates can be maintened on cylindrical machines when rolling harder materials by increasing die speeds when a reduced rates of penetration is required.

Screw machinesRolling threads by in feed method at high production rates on automatic screw machines and automatic lathes is done with attachments using one or two thread rolls. The thread is rolled very rapidly on one pass with relatively few revolutions of the work spindle required.When rolling with one thread roll, the pressure is usually directly against the spindle and the blank. when two opposed rolls that form the thread between the rolls as they straddle the blank, the direst pressure on the spindle and blank is considerably less than the pressure exerted by one roll

SELECTION OF MATERIALS FOR ROLLING:

Straight carbon steels, structural alloys steels, high speed steels, nonleaded brasses , and copper constitutes the majority of the materials used in producing parts calling for rolled threads and forms.

Selection of materials often depends upon an optimum combination of rollability and machinability of the material.

Carbon Steels >alloy consisting of iron andcarbon. >Straight carbon steels with standard allowable quantities of phosphorous and sulphur are perhaps

ideal material to roll. >Excellent finished are obtained with the minimum rolling pressure. >Free cutting steels having sulphur of 0.09% to 0.33% (but the finish is not quite the same as

straight carbon) >Finish is good on standard 60o threads but not good on threads with 29o. Leaded Steels >addition with carbon as well as standard alloy steels often made to improve machinability of screw

stock. >Rollability of these steels is good on standard 60o threads

Alloy steels Nickel : is the amount of 3.25% to 3.75% such as in the 2330 or 2340 nickel steels

contributes to strength and ductility. Requires more revolutions to obtained the desired displacement because the greater

tendency to work-harden,power requirements slightly increased. Manganese: Steels containing manganese in the amounts of 1.5% to 2% It increase the resistance of the material to rolling, materials rolls well and good finished

are obtained. Manganese steels behave very much the same as nickel steels. Nickel Chromium: Structural steels such as 3120 or 3140 containing 1% to 4% nickel

and 0.5% to 1.75% chromium. Commonly used for shafts and various machinery parts. It gives good surface finished. Molybdenum: in the amount of 0.15% to 0.3% is usually combined with chromium or

nickel to form high grade structural steels such as 4140,4340,4640 and 4820. This materials is always used in the hardened state and very frequently the rolling is

done after heat treatment. Nitriding steels Nitriding steels are suitable for rolling prior to nitriding and behave very similarly to

molybdenum steels.

Stainless steels Stainless steels of the 400 series may be heat treated and for the most part do not work

harden as much as the 30-0 series of the stainless steels. The rolling of materials results in an excellent finish. However considerable pressure is required and a slight to moderate seam in the crest

may be detected. Nonferrous Metals among all nonferrous materials available Copper and many copper base alloys posses

excellent cold working properties and are good materials for rolling. Brasses such as Commercial Bronze,Cartridge Brass and Yellow Brass are prefffered for

rolling. Free cutting brasses with not more than 15% reduction in area to finish size after the last

anneal are recommended for thread rolling. Heating aluminum alloys increases ductility and this may be helpful when required to

roll low ductility alloys. Titanium Most common of the alloys used in threaded fasteners are the aluminum 4%, manganese

4% and aluminum 6%, vanadium 4% alloys. These alloys are of about 33 to 38 Rockwell C hardness and work harden very rapidly Titanium are used almost exclusively for aircraft applications.

ROLLABLITY OF MATERIALS It is the property of the material to be rolled or to form EXTERNAL threads in it

during thread rollin with help of a die. Rollablity depends on following characteristics of the material:

:

1.RESISTANCE TO PLASTIC DEFORMATION: Higher the resistance, better is the thread.As this operation is based on cold forming .Hence the material must posses high resistance to plastic deformation.It depends on properties like, hardness,yeild point,frictional resistance developed between rollers and job.

2.BEHAVIOUR DURING DISPLACEMENT : Different material displaces differently during thread rolling. If the material has a high degree of internal friction,it will be more difficult to move the surface layers over the deeper layers and hence, the penetration wont be that deep.This leads to the formation of the folds in the thread of varying degrees which is not visible to our eyes.

3.DUCTILITY OF THE MATERIAL : Ductility of the material plays an important role in giving a smooth surface finish. Materials having high ductility gives the best surface finished product.

APPLICATION OF THREAD ROLLING

Thread Rolling produces rolled threads on a wide variety of parts. Often, rolled threads are required by design because of their superior tensile, shear, and fatigue strength

There are different types of threads:- metric thread , acme thread , square thread , buttress thread etc which are used according

to our needs. Thread rolling is basically used for making threads in screws and bolts Metric- general uses , Square-Ideal thread for power transmission , ACME-Stronger than

the square thread , Buttress-Designed to handle heavy forces in one direction.