Presentation on theme: "Thread Systems Unified national screw thread was adopted in 1948 Used in United states, Great Britain, Canada. Previously use was the American standard."— Presentation transcript:
Thread Systems 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 applications The letter A: External thread B: Internal thread
Standard method for designating screw thread ¼ - 20 – UNC – 3A Which is nominal size(in), no of threads per inch(pitch), thread – series designation, thread class designation respectively Common designation UNC (coarse) UNF (Fine) UNEF (extra fine) UNS (special)
Square thread form Is the most efficient form for the transmission of power, but more expensive and has been superseded by the Acme thread.
Acme Thread Are also used for power transmission and are easier to manufacture than square threads, but the power transmission capabilities are a little lower. Some valve stems and many lead screws use this thread system
Buttress Thread Transmits power in one direction with virtually the same efficiency of a square thread but are relatively easily produced because of the tapered backside of the tooth form. They are used in military applications and when tubular members are screwed together.
AMERICAN STANDARD TAPER PIPE THREAD
METRIC SCREW THREADS
TWO TYPES – a. Coarse type These are used wherein quick and easy assembly is important. b. Fine threads These are used when design requires increased strength or reduced weight. Screw threads are also used to control position accurately as in the case of steering mechanism.
Has a wide range of sizes. 0.3 mm diameter threads with 140 threads per centimeter are used in watches. Besides this 8.5 mm pitch is used for threading 600mm pipes.
SELF TAPPING SCREWS Used for wood, sheet metal and other soft material. Differs from that used in machine screws. Shank is normally tapered.
THREAD MAKING PROCESSES: THEIR APPLICATIONS AND ECONOMICS
HAND DIES A button die for external threads must be employed by hand. Used when 1. A limited number of small- to medium size threads are to be cut. 2. Accuracy of the thread lead in relation to the thread axis is not essential. 3. There is a expense constraint.
SINGLE POINT THREADING A single point tool having profile corresponding to the profile of the thread is used for generating the thread. Internal & external threads can be produced. It is used when 1. Work piece is too large in diameter 2. Pitch is too coarse 3. The material is too difficult to machine
Thread Cutting Die Heads 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, the threads are cut. Once engaged, the head is self feeding at the rate of thread lead. Can be used from low to high production levels depending on the circumstance. It costs lesser than thread rollers.
Thread Milling This process uses a form-milling cutter which machines the thread form as the workpiece revolves. It can be applied both internally and externally and can be used 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 the hole 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 long thread lengths etc.
Tapping This process involves the use of cylindrical from cutter, a tap which has multiple cutting edges. The tap rotates and is fed axially into the work to produce internal threads. The operation can be carried out by hand or with drill presses, lathes, automatic screw machines or special tapping machines. 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 600mm diameters.
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….
For center type grinding, the number of passes required to complete 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 1 st sized to the required diameter and then the threads are formed.
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 to 1 h Setup time for automatic operations to 2 h
Thread rolling process that forms the thread into mirror image of the roller. depends on the plasticity of the base material to be deformed. leaves the shape of the thread permanently into the workpiece 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.
roller displaces the material that will become the root. roller position holds the thread pitch diameter to a predetermined size. displaced material actually lengthens the workpiece blank. Undersized diameter will not allow material to fully flow in the roller dies resulting in undersized threads. over sized blanks exerting undue pressure on the rollers and head, can have a damaging effect for the thread rolling unit.
There are Three Types of Thread Rolling Process: Axial Thread Rolling a thread is created by moving the axial thread roller from the tail stock end of the turning center along the workpiece blank centerline. The diameter of the axial head ranges from 0.06 to 9 inches in diameter.
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 fixed parallel 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.
Radial Thread Rolling Use of two or three rolls to form a thread in one rotation of the workpiece 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 the threads. same principle applies where two roll head is used. Usually the working range of the thread rolling head is 1/16 to 2 ½ inches in diameter.
Advantages of Thread Rolling Process : Material Saving : Depending on the size and shape of the thread being rolled considerable savings can be made, which can add up significantly on large production run. Increased Tensile Strength Better Surface Finish.
Factors that go in selecting the right Thread Rolling process. Type of Thread to be Rolled. Major Diameter. Pitch and Root Depth. Properties
COLD FORM TAPPING
WHAT IS COLD FORM TAPPING:- The desired thread is formed in the metal under pressure and the grain fibres, as in good forging, follow the contour of the thread. These grain fibres are not cut away as in conventional tapping. The cold forming tap has neither flutes nor cutting edges and therefore, it produces no chips and cannot create a chip problem.
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 work harden. This may be accomplished by using sharp drills, correct speed and feeds. Surface damage may cause torque to increase to a point of stopping the machine or breaking the tap.
Cold forming taps have been recommended for threading ductile materials. Examples of material classes 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)
Suitable Screw Thread Materials
Material Selections Usually the end use of the workpiece or considerations other than threading dictates the selection of material. Producing threads on free cutting materials will result in higher production at lower machining and tool costs. Soft non free machining metals are difficult to thread since 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 life and increases downtime for tool change.
Cut Threads Materials suitable for threading follows those suitable for 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 machinability drops rapidly. Cast aluminium not preferred due to abrasiveness and can cause excessive tool wear
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 difficult to obtain good finish. In steels – its difficult to cut threads BHN < 160;; due to difficulty in breaking the chips in soft steel. Difficult to machine materials can be more advantageously threaded by thread milling
Ground Threads Materials generally suitable for grinding operations are satisfactory also for ground threads. Most preferred materials- Hardened steels and any metal that can be heat treated above Rc 33 before threading. Aluminium and comparable soft materials are not preferred since they are the most difficult to grind.
Thread Forming Different properties are required for thread forming then for cutting. Materials suitable for thread cutting may not be suitable 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
Leaded and sulpherized steel do not work well with thread rolling;; usually preferred using thread cutting Thread rolling not recommended for materials with hardness gfreater than Rc 32. Cold form tapping requires even greater cold workability than external thread rolling. Usual materials used are brass, copper, aluminium and low carbon steels
DESIGN RECCOMENDATIONS FOR SCREW THREADS Thread relief or undercut prevents incomplete threads and tool breakage. Internal threads need some unthreaded length for chip clearance. 60 or 65% of thread height is required for adequate thread strength. These can be machined more easily. Thread length should be minimized for longer tool life and lesser machining time. External threads should have chamfer and internal threads should have countersink at their ends.
Slots, cross holes etc shouldn’t be there where they intersect the thread. These result in burrs. Tubular parts should have enough wall thickness to withstand the pressure of the cutting action. Tolerances closer then the required should not be specified. Threads to be ground should not have sharp corners at the root. Coarse threads are more economical.
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 diameter’s can be restricted to to mm 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 cut threads. The piece-to-piece accuracy of rolled threads depends on the consistency of the blank diameter and uniformity of the material.
Tolerances 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.
Symbols for Geometric Tolerance
A geometric tolerance is prescribed using a feature control 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 maximum material condition is specified, then there is a “bonus tolerance” associated with a decrease in material. The form of a feature is assumed to be perfect at its maximum material condition. If no material condition is specified, then it is regard less of feature size.