1568 LNote Screw Threads Final
-
Upload
ankur-sodhani -
Category
Documents
-
view
244 -
download
1
Transcript of 1568 LNote Screw Threads Final
-
8/13/2019 1568 LNote Screw Threads Final
1/60
-
8/13/2019 1568 LNote Screw Threads Final
2/60
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
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)
-
8/13/2019 1568 LNote Screw Threads Final
5/60
Is the most efficient form for the transmission ofpower, but more expensive and has been supersededby the Acme thread.
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
8/60
-
8/13/2019 1568 LNote Screw Threads Final
9/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
12/60
Used for wood , sheet metal and other soft material.
Differs from that used in machine screws.
Shank is normally tapered.
-
8/13/2019 1568 LNote Screw Threads Final
13/60
-
8/13/2019 1568 LNote Screw Threads Final
14/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
16/60
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
18/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
20/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
22/60
-
8/13/2019 1568 LNote Screw Threads Final
23/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
25/60
-
8/13/2019 1568 LNote Screw Threads Final
26/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
30/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
37/60
Factors that go in selecting the right ThreadRolling process.
Type of Thread to be Rolled.
Major Diameter.
Pitch and Root Depth.
Properties
-
8/13/2019 1568 LNote Screw Threads Final
38/60
-
8/13/2019 1568 LNote Screw Threads Final
39/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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)
-
8/13/2019 1568 LNote Screw Threads Final
43/60
-
8/13/2019 1568 LNote Screw Threads Final
44/60
-
8/13/2019 1568 LNote Screw Threads Final
45/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
49/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
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
-
8/13/2019 1568 LNote Screw Threads Final
53/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
56/60
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
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.
-
8/13/2019 1568 LNote Screw Threads Final
59/60
A geometric tolerance is prescribed using a feature
-
8/13/2019 1568 LNote Screw Threads Final
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.