Thread manufacturing Processes

- Viral Patel

Introduction: Thread is ridge of uniform cross section in the form of a helix

provided on the outside or inside of a cylindrical or conical surface.

External threads are provided on bolts, screws while internal threads are on corresponding nuts or any machine members.

Functionally fasteners are threaded for holding component securely.

Thread manufacturing is a very important manufacturing activity because of huge demand of high precision fastening devices and power transmission systems.

Elements of Screw thread: Major Diameter Minor Diameter Pitch Diameter Pitch No. of start ( Independent thread

on job surface) Lead hand Form

ISO Metric threadV-ThreadSquare thread British standard WhitworthBritish AssociationAmerican nationalsAcme threadKnuckle Thread

Methods of Thread manufacture:

1. Thread cutting on Lathe2. Thread Chasing3. Die threading4. Tapping5. Thread Milling6. Thread rolling7. Thread Grinding8. Thread casting9. Thread Whirling

Thread cutting on Lathe Principle is to produce helical groove on a cylindrical or conical

surface by feeding tool longitudinally when job is rotating.

Steps :1) Selection of cutting tool 2) Mounting of cutting tool and workpiece 3) Establish relation between longitudinal movement of cutting

tool and work piece rotation.4) Thread Cutting5) Tool feeding and Thread catching

Thread Chasing: It is method of thread cutting with the help of multi point cutting

tool called thread chaser. Chaser types:1) Hand Chaser 2) Threading tool Chaser

Hand chaser is worked manually by hand during operation with provision of suitable support.

Threading tool chaser has two useful forms as Flat type and Circular type. Both are fixed in tool holder and used for cutting thread in large scale production.

This chasers consist Cutting teeth for cutting operation and Sizing teeth for finishing operation.

Arrangement of Thread Chasing on Turret Lathe

In machine, lead screw is rotated by head stock of machine by means of changing gears.

On shaft, feed nut and tool slide are mounted which can be engaged and disengaged with lead screw by hand operated lever.

Die Threading: External threads cutting on cylindrical or taper surface by

means of Solid and self opening dies is called die threading. Dies are look like nuts but it is made of Hardened Steel or HSS

with sharp cutting edges. This dies are coaxially rotate around the blank. Types of Die1) Solid die2) Self opening die

Solid diesSprin

g Die

Split die

Pipe die

In Spring dies elastically reduction in bore is possible and cut threads in number of passes.In Split die , die is made in two pieces. One is fixed and other is moving within cavity of wrench.Pipe dies cut threads on pipes having large diameter and smaller pitch.

Self opening dies

Radial die

Tangential die

Circular

die

It opens automatically when disengage from w/p.These dies are used in Turret lathe and special threading machine. All dies having four sets of adjustable multi point cutters or chasers.Working:

Mounted in turret and moves towards w/p, when measured length completed die gets pulled and open automatically for free returns. video

Tapping: Tapping is Thread cutting process for producing internal threads. Multipoint cutting ( Tap ) is used like die threading and cutting

may be done manually or on machines. In machine , process is similar as manual tapping except feed

being automatic and threads will be high accurate and uniform. During machining features like fast retraction of the tool,

automatic loading or unloading, use of multiple spindles; machining rate will considerably increased.

Taps are classified as Solid taps and Collapsible taps.

Solid Taps: It is one piece construction and

similar to threaded bolt with four flutes to provide cutting edges.

It is used for small diameters and worked manually. So it is also known as hand taps.

Taper tap, Plug tap and Bottoming tap are used as Solid tap.

Taper tap is tapered at starting for six threads to make entry easy.

Plug tap is also chamfered little for few threads and it is used as second tap after removing first tap.

Bottoming tap is used after second tap to cut thread up to the bottom of the hole. Video

Collapsible taps: It is tool which collapse inwards

automatic when thread is completed.

It permits return without unscrewing . So speed of operation is increased.

Due to machining surface finish is good .

The taps are provide with trip plate which set accordingly the length of thread to be cut.

Video

Thread Milling: Threads produced by milling

process. By using form cutter ( single form cutter and multiple form cutter ) threads are manufactured on rotating work-piece.

When milling suitable: For producing internal thread

which are not suitable to cut with tap.

Work on large diameter beyond the capacity of die.

Video…

In case of single form cutter blank is rotating slowly and cutter is precisely moved along the axis of blank.

Threading is completed in one or more passes depends on the depth of the thread.

The multiple form cutter have several annular rows of cutting teeth made by flanks or gashes.

Same as single form cutter , spindle is set parallel to work axis and this process is suitable for threads for small distance.

Thrilling is combined process of threading and drilling by using specialized cutting tool on a CNC milling machine. THRILLING

Thread Grinding The principle of thread grinding is similar to thread milling.

Grinding is done by using specially dressed grinding wheels. Grinding wheel may be of single rib type or multi rib type and

process is suitable for hard materials for which other methods of thread cutting is not possible.

Types of thread grinding 1) Pass over grinding2) Plunge grinding3) Centre-less grinding

Pass over grinding It is similar to threading on

lathe but single point cutting tool is replaced with grinding wheel.

In single wheel rib wheel turns against rotation of work and axial motion is provided with accurate lead screw.

Thread is not completed in single pass. Multiple pass are required to complete thread up to certain depth.

It is also known as transverse thread Grinding.

Plunge Grinding

In Plunge grinding , wheel is having multiple ribs and it is longer than the length of the desired thread.

The Plunge grinding is mostly used for short length threads. Example : Thread on shaft ends.

Method is suitable when production is more important than accuracy.

Centre less thread grinding

This process is used to make headless set screws.

Work is fed between regulating wheel and grinding wheel where threads are fully formed.

Production rate is 60 to 70 (13 mm long) pieces per minute.

Thread Rolling It is cold forming process used for making external threads on

cylindrical w/p. Threads are formed into blank by pressing thread rolling die

against the blank. Faster method of production Skilled labor not required , so it is economical method of thread

making. Chip less operation, no wastage of material. Threads produced by rolling have good quality and high accuracy. Thread rolling increase tensile and fatigue strength of work

material but rolling process is limited to ductile materials. Thread rolling machines:

1) Reciprocating Flat die machine.2) Cylindrical two/three die machine.

Flat die machine Two dies are used. One is

bottom die which remains stationary and another is reciprocating die.

Blank is placed on stationary die and then moving die slides over the blank. Due to such action blank roll between two dies and threads are formed.

Cylindrical die machine Cylinder die machine use

two / three dies. Incase of two die machine as

shown in figure work blank is supported between two dies.

Incase of three die machine one die is move radialy outward for let the work blank enter between die and then closes and rotate to roll threads.

Thread Casting:

In casting and molding threads are directly formed by geometry of cavity in the mold or die.

Thread made by sand casting are rough and not used except sometimes in vises and rough machinery.

Compare to other casting method lost wax method produce more accurate threads with good surface finish but it is costly process .

Due to poor accuracy cast threads are used in such cases where only few threads are required over short length.Example : Thread on Soda bottle

Thread Whirling

Thread Whirling is a form of the Thread Milling process with the exception that the cutters are mounted on the inside of a Cutting Ring or Cutter Holder rather than the outside of a milling tool.

Advantages of Thread Whirling: Deep Threads Faster Setup Increased Productivity Increased Tool Life