Production of screw threads by machining

Production of screw threads by machining

Machining is basically a removal process where jobs of desired size and shape are produced by gradually removing the excess material in the form of chips with the help of sharp cutting edges or tools. Screw threads can be produced by such removal process both manually using taps and dies as well as in machine tools of different types and degree of automation. In respect of process, machine and tool, machining of screw threads are done by several ways :

Thread cutting by hand operated tools

Usually small threads in few pieces of relatively soft ductile materials, if required, are made manually in fitting, repair or maintenance shops.

• External screw threads

Machine screws, bolts or studs are made by different types of dies which look and apparently behave like nuts but made of hardened tool steel and having sharp internal cutting edges. the hand operated dies of common use, which are coaxially rotated around the premachined rod like blank with the help of handle or die stock.

Δ Solid or button die : used for making threads of usually small pitch and diameter in one pass.

Δ Spring die : the die ring is provided with a slit, the width of which is adjustable by a screw to enable elastically slight reduction in the bore and thus cut the thread in number of passes with lesser force on hands.

Δ Split die : the die is made in two pieces, one fixed and one movable (adjustable) within the cavity of the handle or wrench to enable cut relatively larger threads or fine threads on harder blanks easily in number of passes, the die pieces can be replaced by another pair for cutting different threads within small range of variation in size and pitch.

Δ Pipe die : pipe threads of large diameter but smaller pitch are cut by manually rotating the large wrench (stock) in which the die is fitted through a guide bush

• Internal screw threads :

Internal screw threads of usually small size are cut manually, if needed, in plates, blocks, machine parts etc. by using taps which look and behave like a screw but made of tool steel or HSS and have sharp cutting edges produced by axial grooving over the threads.Three taps namely, taper tap, plug tap and bottoming tap are used consecutively after drilling a tap size hole through which the taps are axially pushed helically with the help of a handle or wrench.Threads are often tapped by manually rotating and feeding the taps through the drilled hole in the blank held in lathe spindle

The quality of such external and internal threads will depend upon the perfection of the taps or dies and skill of the operator.

Machining screw threads in machine tools

Threads of fasteners in large quantity and precision threads in batches or lots are produced in different machine tools mainly lathes, by various cutting tools made of HSS or often cemented carbide tools.

Machining screw threads in lathes

Screw threads in wide ranges of size, form, precision and volume are        produced in lathes ranging from centre lathes to single spindle automats.Threads are also produced in special purpose lathes and CNC lathes including turning centres.

External threads :

External threads are produced in centre lathes by various methods :

Δ Single point and multipoint chasing, This process is slow but can provide high quality. Multipoint chasing gives more productivity but at the cost of quality to some extent

Thread milling:

This process gives quite fast production by using suitable thread milling cutters in centre lathes as indicated. The milling attachment is mounted on the saddle of the lathe. Thread milling is of two types;

.

 Long thread milling

Long and large diameter screws like machine lead screws are reasonably accurately made by using a large disc type form milling cutter

 Short thread milling

Threads of shorter length and fine pitch are machined at high production rate by using a HSS milling cutter having a number of annular threads with axial grooves cut on it for generating cutting edges. Each job requires only around 1.25 revolution of the blank and very short axial (1.25 pitch) and radial (1.5 pitch) travel of the rotating tool

 

Rotating tool

Often it becomes necessary to machine large threads on one or very few pieces of heavy blanks of irregular size and shape like heavy casting or forging of odd size and shape. In such cases, the blank is mounted on face plate in a centre lathe with proper alignment. The deep and wide threads are produced by intermittent cutting action by a rotating tool. A separate attachment carrying the rotating tool is mounted on the saddle and fed as usual by the leadscrew of the centre lathe. the principles of threading by rotary tools. The tool is rotated fast but the blank much slowly. This intermittent cut enables more effective lubrication and cooling of the tool.

Internal threads :

Internal threads are produced in centre lathes at slow rate by using;

Δ Single point tool

Δ Machine taps

Δ Internal thread milling

Δ Internal threading by single point chasing

Internal threads in parts of wide ranges of diameter and pitch are accurately done in centre lathes by single point tool, as in boring.Multipoint flat chaser is often used for faster production.

Δ Internal threading by taps

Internal threads of small length and diameter are cut in drilled holes by different types of taps;

Δ Straight solid tap – used for small jobs

Δ Taps with adjustable blades – usually for large diameter jobs

Δ Taper or nut taps – used for cutting threads in nuts.

Δ Internal thread milling cutter

Such solid cutterproduces internal threads very rapidly, as in external short thread milling, in lathes or special purpose thread milling machine.

∗ Machining threads in semiautomatic lathes

Both external and internal threads are cut, for batch or small lot production, in capstan and turret lathes using different types of thread cutting tools;

Δ External threads in capstan lathe by self opening die and single or multipoint chaser in turret lathe

Δ Internal threads of varying size by collapsible tap.

The self opening die is mounted in the turret and moved forward towards the rotating blank. At the end point, when the turret slows down and is about to stop or reverse, the front position of the die gets pulled and open automatically to enable free return of the die without stopping the job – rotation. The thread chasers may be flat or circular type as shown.

In a collapsible tap,, the radially raised blades collapse (move radially inward) and the tap returns (along with the turret or saddle) freely from the threaded hole after completing the internal thread in one stroke.

∗ Machining threads in automatic lathes

Small external threads for mass production of fasteners are produced by machining in single spindle automatic lathes or similar but special purpose (threading) lathes using solid die. The die is mounted on the coaxially moving turret or sliding attachment in turret lathes and SPM respectively.

In turret lathe, the solid die is returned by reversing the job rotation, and in the special purpose machine, the die is freely returned by rotating the die slightly faster than the job and in the same direction.

 

∗ Machining screw threads in drilling machine

Drilling machines are used basically for originating cylindrical holes but are also used, if needed, for enlarging drilled holes by larger drills, counterboring, countersinking etc. Internal threads of relatively smaller diameter, length and pitch are also often produced in drilling machines by using tapping attachment with its taper shank fitted axially in the spindle bore.