1568 LNote Screw Threads Final

8/13/2019 1568 LNote Screw Threads Final

1/60


2/60


3/60

Unified national screw thread was adopted in 1948

Used in United states, Great Britain, Canada.

Previously use was the American standard. There are three common thread classes:

Class 1 : Loosest fit & broadest dimensional tolerance

Class 2: Most common, closest fits & tolerances

Class 3: More precise, used for critical applicationsThe letter A: External thread

B: Internal thread


4/60

Standard method for designating screw thread

- 20 UNC 3A

Which is nominal size(in), no of threads perinch(pitch), thread series designation, thread class

designation respectively

Common designation

UNC (coarse)

UNF (Fine)

UNEF (extra fine)

UNS (special)


5/60

Is the most efficient form for the transmission ofpower, but more expensive and has been supersededby the Acme thread.


6/60

Are also used for power transmission and are easier tomanufacture than square threads, but the powertransmission capabilities are a little lower.

Some valve stems and many lead screws use thisthread system


7/60

Transmits power in one direction with virtually thesame efficiency of a square thread but are relativelyeasily produced because of the tapered backside of thetooth form. They are used in military applications and

when tubular members are screwed together.


8/60


9/60


10/60

TWO TYPES

a. Coarse type

These are used wherein quick and easy assembly is

important. b. Fine threads

These are used when design requires increasedstrength or reduced weight.

Screw threads are also used to control positionaccurately as in the case of steering mechanism.


11/60

Has a wide range of sizes.

0.3 mm diameter threads with 140 threads percentimeter are used in watches.

Besides this 8.5 mm pitch is used for threading600mm pipes.


12/60

Used for wood , sheet metal and other soft material.

Differs from that used in machine screws.

Shank is normally tapered.


13/60


14/60


15/60

A button die for external threads must be employed byhand.

Used when

1. A limited number of small- to medium size threadsare to be cut.

2. Accuracy of the thread lead in relation to the threadaxis is not essential.

3. There is a expense constraint.


16/60


17/60

A single point tool having profile corresponding to theprofile of the thread is used for generating the thread.

Internal & external threads can be produced.

It is used when1. Work piece is too large in diameter

2. Pitch is too coarse

3. The material is too difficult to machine


18/60


19/60

Most effective and popular means of threading. They have wide ranges and made in a no: of models

and sizes for applications to many machines.

They have four to five insert form cutters

The head is fed axially from the end of the work, thethreads are cut.

Once engaged, the head is self feeding at the rate ofthread lead.

Can be used from low to high production levelsdepending on the circumstance.

It costs lesser than thread rollers.


20/60


21/60

This process uses a form-milling cutter which machines thethread form as the work piece revolves.

It can be applied both internally and externally and can beused to produce most threads forms regardless of whether

they are straight or tapered. Minimum internal thread size determines the diameter of

the cutter. Cutter should not exceed the one-third of thehole diameter.

Thread milling is slower than die cutting, it is often

necessary that a thread be milled because of a coarse pitch,large or odd shaped parts, high helix angle extremely longthread lengths etc.


22/60


23/60


24/60

The tap rotates and is fed axially into the work toproduce internal threads.

The operation can be carried out by hand or with

drill presses, lathes, automatic screw machines orspecial tapping machines.

This process involves the use of cylindrical fromcutter , a tap which has multiple cutting edges.

They are of 2 types Solid and collapsible taps. Solid taps have diameter ranges from 1.2 to 150mm

Collapsible taps are limited to 32mm to 600mmdiameters.


25/60


26/60


27/60

Two types used in the production of screw threads are 1.Center-type grinding and 2. centerless cylindrical grinding.

----single rib form wheels

( low production quantities)

------- Centre type grinding

----multiple rib form wheels(mass production)

-----multiple rib form wheels -------- centerless cylindrical

(high production quantities) grinding

Work axial motion wheel. as the wheel rotates.


28/60

For center type grinding, the number of passes required tocomplete the work varies from 1 to 5 or 6

The factors determining the number of passes are:-

1. the material specifications

2. the form of thread

3. the length of thread 4. the quality of thread

For centerless grinding the number of pass for finishing the

part is normally 1. In the process, the work is 1stsized to the required diameter

and then the threads are formed.


29/60

Centerless ground threaded parts include :-

1. set screws

2. studs 3. threaded bushings

4. threaded size adjusting bushings

5. threaded gauges

6. worm gears

7. powdered iron screws

8. self threading insert bushings

Setup time for hand operations ------ 0.5 to 1 h

Setup time for automatic operations-------1.5 to 2 h


30/60


31/60

process that forms the thread into mirror image of theroller.

depends on the plasticity of the base material to bedeformed.

leaves the shape of the thread permanently into theworkpiece blank.

the thread shape is imparted on the workpiece blank

by moving the parent material. A key factor of this movement is the depth or root of

the thread.


32/60

roller displaces the material that will become the root.

roller position holds the thread pitch diameter to apredetermined size.

displaced material actually lengthens the workpieceblank.

Undersized diameter will not allow material to fullyflow in the roller dies resulting in undersized threads.

over sized blanks exerting undue pressure on therollers and head, can have a damaging effect for thethread rolling unit.


33/60

There are Three Types of Thread Rolling Process:

Axial Thread Rolling

a thread is created by moving the axial thread rollerfrom the tail stock end of the turning center along theworkpiece blank centerline. The diameter of the axialhead ranges from 0.06 to 9 inches in diameter.


34/60

Tangential Thread Rolling

In this process the tangential roller head makes thread by

approaching the workpiece blank from its sides. Manual operations are not possible in tangential thread rolling.

Mechanical or servo feed is required.

Tangential thread rollers roll threads by pushing two fixedparallel rolls onto the rotating component at a controlled feed

rate. rolls make tangential contact with the workpiece blank diameter

fast and precise process,which is burr free

complete thread in 15 to 30 revolutions of the workpiece blank.


35/60

Radial Thread Rolling

Use of two or three rolls to form a thread in one rotation of theworkpiece blank.

Thread roller are ground eccentrically. Thread form is progressive, starting with a flat on each role. allows the workpiece blank to be positioned in between the rolls

and the finished threaded part to leave it without damaging thethreads. same principle applies where two roll head is used.

Usually the working range of the thread rolling head is 1/16 to 2 inches in diameter.


36/60

Advantages of Thread Rolling Process :

Material Saving : Depending on the size and shape of

the thread being rolled considerable savings can bemade, which can add up significantly on largeproduction run.

Increased Tensile Strength

Better Surface Finish.


37/60

Factors that go in selecting the right ThreadRolling process.

Type of Thread to be Rolled.

Major Diameter.

Pitch and Root Depth.

Properties


38/60


39/60


40/60

The desired thread is formed in the metal underpressure and the grain fibres, as in good forging, followthe contour of the thread.

These grain fibres are not cut away as in conventionaltapping.

The cold forming tap has neither flutes nor cuttingedges and therefore, it produces no chips and cannotcreate a chip problem.


41/60

The resulting thread has a burnished surface. Care must be taken to minimize surface damage to the

hole when tapping materials which are prone to workharden.

This may be accomplished by using sharp drills,correct speed and feeds. Surface damage may causetorque to increase to a point of stopping the machineor breaking the tap.


42/60

Cold forming taps have been recommended forthreading ductile materials. Examples of materialclasses which have been tapped are: Low carbon steels

Leaded steels Austenitic stainless steels

Aluminium die casting alloys (low silicon)

Wrought aluminium alloys (ductile)

Zinc die casting alloys Copper and copper alloys (ductile brasses)


43/60


44/60


45/60


46/60

Usually the end use of the workpiece or considerationsother than threading dictates the selection of material.

Producing threads on free cutting materials will result

in higher production at lower machining and toolcosts.

Soft non free machining metals are difficult to threadsince they produce stringy chips which weld to the

cutting edge. Mostly metals selected on the basis of cost- more

expensive, more time, higher tool cost , lowers tool lifeand increases downtime for tool change.


47/60

Materials suitable for threading follows those suitablefor most machining.

Brasses and bronze cut better , plus can be threaded at

higher speeds Free machining steels cut better than unleaded or non

free machining grades.

As carbon content increases and additives such as

chromium, molybdenum are added machinabilitydrops rapidly.

Cast aluminium not preferred due to abrasiveness andcan cause excessive tool wear


48/60

Cast iron is brittle and presents a problem of

maintaining a good form on the crest of the thread. Low carbon steels such as 1010 and 1020 grades, while

soft enough for machining, tend to tear- thus difficultto obtain good finish.

In steels its difficult to cut threads BHN < 160;; dueto difficulty in breaking the chips in soft steel.

Difficult to machine materials can be moreadvantageously threaded by thread milling


49/60


50/60

Materials generally suitable for grinding operations aresatisfactory also for ground threads.

Most preferred materials- Hardened steels and anymetal that can be heat treated above Rc 33 beforethreading.

Aluminium and comparable soft materials are notpreferred since they are the most difficult to grind.


51/60

Different properties are required for thread formingthen for cutting.

Materials suitable for thread cutting may not besuitable for thread rolling or cold form tapping.

Factors which promote thread formability are Low Hardness

Low yield point

Fine grained microstructure

Freedom from work hardening


52/60

Leaded and sulpherized steel do not work well withthread rolling;; usually preferred using thread cutting

Thread rolling not recommended for materials withhardness gfreater than Rc 32.

Cold form tapping requires even greater coldworkability than external thread rolling.

Usual materials used are brass, copper, aluminium and

low carbon steels


53/60


54/60

Thread relief or undercut prevents incomplete threads andtool breakage.

Internal threads need some unthreaded length for chipclearance.

60 or 65% of thread height is required for adequate threadstrength. These can be machined more easily.

Thread length should be minimized for longer tool life andlesser machining time.

External threads should have chamfer and internal threadsshould have countersink at their ends.


55/60

Slots, cross holes etc shouldnt be there where theyintersect the thread. These result in burrs.

Tubular parts should have enough wall thickness towithstand the pressure of the cutting action.

Tolerances closer then the required should not bespecified.

Threads to be ground should not have sharp corners at theroot.

Coarse threads are more economical.


56/60


57/60

Dimensional Factors and Tolerances

Accuracy and conditions of tooling and equipment are the key

factors of all thread-making processes. The skill of the worker, the suitability of the material and the

feed rate of the threading tool have influence. Class 4 and 5 Threads can be easily produced by methods like

hand dies, thread-point cutting, tapping etc.

Generally hand dies are employed for Class 1 and 2 threads. The outside, pitch and root diameters can be restricted to +0.025

to -+0.025mm when thread milling is employed. Thread milling is most preferred when the work piece has

machinability limitations.

The surface finish of rolled threads is superior to that of cutthreads. The piece-to-piece accuracy of rolled threads depends on the

consistency of the blank diameter and uniformity of thematerial.


58/60

Geometric tolerances define the shape of a feature

as opposed to its size. There are 3 types:

Form tolerances: straightness, circularity, flatness,cylindricity;

Orientation tolerances; perpendicularity,parallelism, angularity; and

Position tolerances: position, symmetry,

concentricity.


59/60

A geometric tolerance is prescribed using a feature


60/60

g p gcontrol frame.

It has three components:

the tolerance symbol,the tolerance value,

the datum labels for the reference frame.

Material condition modifiers define the condition at

which the tolerance is to be applied. If the maximummaterial condition is specified, then there is a bonustolerance associated with a decrease in material.

The form of a feature is assumed to be perfect at itsmaximum material condition.

If no material condition is specified, then it isregard less of feature size.

1568 LNote Screw Threads Final

Documents