variabel pembentukan.ppt

VARIABEL PEMBENTUKAN

Variabel Proses Pembentukan • Variabel Bebas:

di mana Perencana dapat mengontrol langsung dan variable-veriabel tersebut biasanya dipilih atau ditentukan ketika proses set-up

• Variabel Tak Bebas:

adalah konsekuensi dari pilihan variable bebas

Variabel BebasTerdiri dari:1. Material Awal: sifat kimia dan persyaratan

sifat dan karakteristik bahan. Dasar pemilihan: kemudahan fabrikasi, dibatasi oleh bentuk/sifat produk yang diinginkan.

2. Geometri mula dari benda kerja; dipilih dari varitas bentuk yang ada atas pertimbangan ekonomi

3. Geometri perkakas / cetakan; sangat berpengaruh karena system perkakas akan memproduksi dan mengontrol aliran logam

4. Pelumasan; untuk mengatasi gesekan50% daya yang diberikan untuk mengatasi gesekanFungsi pelumas: melumasi, pendingin, pembatas panas, pencegah korosi, senyawa perusak.Yang perlu diperhatikan: tipe pelumas, jumlah yang harus diberikan, dan metode pemberian

5. Suhu Awal Pembentukan baik untuk benda kerja dan perkakas

beberapa material memepunyai variasi yang besar terhadap terhadap sisfat dasar akibat temperature, Karena itu pilihlah amatilah posisi yang sesuai agar berhasil dgn baik dan tidak rusak saat mengerjakannya.

6. Kecepatan operasi: mempengaruhi efektivitas pelumas, gaya yang diperlukan untuk operasi, waktu tersedia untuk pndah panas

7. Jenis proses deformasi

sepeti : rolling , Forging

Variabel Tak Bebas1. Gaya/ daya yang diperlukan2. Sifat material produk; perhatian konsumen

pada bentuk dan sifat material akhir sehingga perencana harus pandai memilih material awal dan memprediksi pengaruh proses dalam mengubah sifat tersebut.

3. Exit / final temperature4. Surface finish (permukaan akhir produk) dan

kehalusan permukaan 5. Sifat aliran bahan

Korelasi variabel bebas dan variabel tak bebas

• Pengalaman• Penelitian• Pemodelan proses

Experience

this requires long time exposure to the process and is generally limited to the specific materials, equipment and products encountered in the realm of past contact.

Experiment while possibly the least likely in error direct experiment is both time consuming and costly.

Process modeling

here one approaches the problem with a high speed computer and one or more mathematical models of the

process numerical values are provided for the various independent variables and the models are used to compute predictions for the dependent variables

Variabel Friksi (Geskan) dan Lubrikasi (Pelumasan)

• Beberapa proses 50% energi input digunakan untuk mengatasi gesekan

• Kehalusan pemukaan Hasil proses dan kepresisian produk dipengaruhi oleh friksi

Pelumasan Pada gear, bearing, journal dan komponen sejenis, kondisi friksi melibatkan:

1. Dua permukaan dengan material dan kekuatan serupa di bawah beban elastik di mana salah satu komponen mengalami perubahan bentuk permanen

2. Keausan berbentuk lingkaran yang menghasilkan kesesuaian permukaan

3. Umumnya suhu rendah sampai sedang

Gesekan • Menurut teori friksi modern: “permukaan yang rata

tidaklah rata”, namun ada kekasaran. • Bila dua permukaan berinteraksi, kontak permukaan

dibangkitkan untuk mengatasi beban yang diberikan.1. Bila beban ringan hanya 3 titik yang kontak2. Bila beban naik luasan kontak naik3. Bila beban tinggi seluruh permukaan kontak4. Bila beban dinaikkan lagi maka luasan permuakaan

kontak tak akan naik lagi dan friksi tetap

Slope (gradien)

P

Tekanan kontak, P

Friksi, FF

Pengaruh tekanan kontak terhadap gesekan

Umumnya dengan penambahan temperatur akan mengurangi kekuatan, keuletan dan menambah regangan pemanasan.

Pengaruh panas ini mempermudah perencanaan dalam pembentukan

Proses panas>< proses dingin

In general, an increase in temperature brings out a decrease in strength, an increase in ductility, and a decrease in the rate of strain hardening – all effects that would tend to promote ease of deformation.

Temperature Concerns

Telaah Suhu Proses forming diklasifikasikan menjadi:• Hot working (Proses panas); deformasi dilakukan di

bawah kondisi temperatur dan laju strain (regangan) di mana rekristalisasi terjadi dan simultan dengan deformasi. Untuk mencapai ini, suhu deformasi biasanya di atas 0.6 kali titik cair material pada skala suhu absolut (Kelvin atau Rankine)

• Cold working (Proses dingin): adalah deformasi di bawah kondisi proses recovery tidak aktif. Biasanya suhu kerja kurang dari 0.3 kali suhu leleh benda kerja

• Warm working adalah deformasi di bawah kondisi transisi (yakni suhu kerja antara 0.3 dan 0.6 kali suhu leleh).

Hot working• Hot working didefinisikan sebagi deformasi

plastis logam di atas suhu rekristaliasinya. Yang perlu diingat bahwa beda material beda suhu rekristalisasinya. Misalnya tin / timah putih (Sn) pada suhu kamar, baja pada suhu 2000 0F, tungsten pada suhu sampai 4000 0F belum mencapai daerah hot working.

• Kenaikan suhu berpengaruh terhadap penurunan tegangan yield logam dan meningkatkan keuletannya.

Keuntungan hot working:• Pada suhu hot working, rekristalisasi mengeliminasi efek dari strain

hardening (pengerasan regang) sehingga tidak ada keniakan signifikan dalam kekuatan yield atau kekerasan atau penurunan keuletan.

• Kurva stress-strain sebenarnya mendatar di atas titik yield dan deformasi dapat dipakai mengubah secara drastic bentuk logam tanpa takut akan retak atau diperlukan gaya yang sangat besar.

• Mengurangi atau menghilangkan ketidakhomogenan kimiawi• Pori-pori dapat dilas atau direduksi ukurannya selama deformasi• Struktur metalurgis dapat diubah untuk meningkatkan sifat akhir • Pada baja pada suhu rekristalisasi deformasi terjadi pada struktur

Krista austenit FCC yang lemah dan ulet dari pada ferrit BCC yang kuat dan stabil pada suhu rendah.

Cold working• Cold working adalah deformasi plastis logam di bawah suhu

rekristalisasi. Proses biasanya pada suhu kamar, tetapi penaikan suhu ringan biasa digunakan untuk meningkatkan keuletan dan mengurangi kekuatan.

Keunggulan cold working dibanding hot working• Tidak diperlukan panas• Permuakan akhir yang diperolehlebih bagus • Kontrol dimensi lebih bagus sehingga sedikit/tidak memerlukan

pemesinan lanjutan• Produk memiliki kemampuan reproduksi dan mampu tukar yang

lebih bagus• Sifat kekuatan, kelelahan dan keausan ditingkatkan melalui strain

hardening • Sifat terarah dapat diberikan• Problem kontaminasi diminimisasi

Kelemahan cold working

• Diperlukan gaya yang lebih besar untuk memulai dan menyelesaikan proses cold work

• Diperlukan perangkat yang lebih berat dan lebih kuat• Kurang keuletan• Permukaan logam harus bersih bebas sisik• Anneal antara mungkin diperlukan untuk mengkompen-

sasi hilang keuletan yang menyertai strain hardening• Pemberian sifat yang terarah mungkin merusak• Tegangan sisa yang tak diinginkan mungkin diproduksi

Sifat logam pada cold working

• Kesesuaian suatu logam untuk dicold work ditentukan oleh sifat keuletan, di mana sifat ini merupakan konsekuensi langsung dari struktur metalurgis. Kemudian proses cold work mengubah struktur logam dan pada akhirnya mengubah sifat keuletan produk

Kurva tegangan-regangan pada baja karbon

rendah (kiri) dan baja karbon tinggi (kanan)

• Deformasi elasis sd X1• Deformasi plastis dr X1 sd X4 • X4 material putus• Besar dari titik yield (X1) yang

menentukan gaya yang diperlukan untuk memulai deformasi permanen,

• X1 ke X4, yang menunjukkan jumlah deformasi plastis (atau keuletan) yang bisa dicapai tanpa patah.

Kesimpulan:1 Baja karbon rendah: • jumlah deformasi yang bisa dilakukan lebih banyak• Keuletan yang lebih besar • gaya yang lebih kecil diperlukan untuk memulai dan

melanjutkan deformasi.2 Baja karbon tinggi:• memiliki koefisien regangan lebih besar. • mengalami kenaikan kekuatan yang lebih besar untuk

sejumlah cold work yang sama. • akan lebih menarik untuk operasi pemotongan dan • lebih mudah untuk dimesin.

Cold-working Processes

Major Cold-Working Operations…

Ocfemia, Cathrine Joy


• Bending

• Squeezing

• Drawing

• Shearing

• Presses


Squeezing ProcessesClassifications of Squeezing Processes…

Rolling

Swaging

Cold Forging

Extrusion

Sizing

Riveting

Staking

Coining

Peening

Burnishing

Hubbing

Thread Rolling


Squeezing Processes

ROLLING

process used in sheets, strips, bars, and rods to obtain products that have smooth surfaces and accurate dimensions; most cold-rolling is performed on four-high or cluster-type rolling mills


Squeezing Processes

SWAGING

process that reduces/increases the diameter, tapers, rods or points round bars or tubes by external hammering


Squeezing Processes

COLD FORGING

process in which slugs of material are squeezed into shaped die cavities to produce finished parts of precise shape and size.


Squeezing Processes

EXTRUSION

process which is commonly used to make collapsible tubes such as toothpaste tubes, cans usually using soft materials such as aluminum, lead, tin. Usually a small shot of solid material is placed in the die and is impacted by a ram, which causes cold flow in the material.


Squeezing Processes

SIZING

process of squeezing all or selected areas of forgings, ductile castings, or powder metallurgy products to achieve a desired thickness or precision


Squeezing Processes

RIVETING

process where a head is formed on the shrank end of a fastener to permanently join sheets or plates of material;


Squeezing Processes

STAKINGprocess of permanently joining parts together when one part protrudes through a hole in the other; a shaped punch is driven into the end of the protruding piece where a deformation is formed causing a radial expansion, mechanically locking the two pieces together


Squeezing Processes

COINING

process where metal while it is confined in a closed set of dies; used to produce coins, medals, and other

products where exact size and fine details are required, and thickness varies about a

well-defined average


Squeezing Processes

PEENING

process where the surface of the metal is blasted by shot pellets; the mechanical working of surfaces by repeated blows of impelled shot or a round-nose tool


Squeezing Processes

BURNISHING

process by which a smooth hard tools is rubbed on the metal surface and flattens the high spots by applying compressive force and plastically flowing the material


Squeezing Processes

HUBBING

process to used to form recessed cavities in various types of female tooling dies. This is often used to make plastic extrusion dies in an economical manner


Squeezing Processes

THREAD ROLLING

process is used for making external threads; in this process, a die, which is a hardened tool with the thread profile, is pressed on to a rotating workpiece

Glassl, Gerlli

Bending- the plastic deformation of metals about a linear axis with little or no

change in the surface area

- the purpose of bending is to form sheet metal along a straight line

parts manufactured by bending

center of the sheet -metal is thinned somewhat at the bend

- where the material cannot compensate by the sideways contraction of the free edges

outside: stretched inside: compressed

Glassl, Gerlli

Bending

The bending operation can be considered as a system with six components:

Product/Workpiece Equipment/ToolDeformation zone Interface EnvironmentDie

Glassl, Gerlli

BendingSpringback - the elastic recovery of the material after unloading of the tools

To compensate with the unbending action of the springback, the metal should be slightly overbent.

Glassl, Gerlli

Bending

Rotary Bending

Double Die Bending

Wiping Die Bending

Glassl, Gerlli

Bending

Classifications of Bending Processes…

Roll Forming

Roll

Seaming

Straightening

Draw and Compression

Flanging

Angle

Glassl, Gerlli

Bending

Angle Bending

Bar folder

Glassl, Gerlli

Bending

Press Brake Bending rubber

Roll Bending • bending where plates, sheets and rolled shapes can be

bent to a desired curvature

Roll bending toll can bend plate up to 6 inches thick

Glassl, Gerlli

Bending

Drawing and Compression

Drawing Bending Compression Bending

- workpiece is clamped against a bending form and the entire assemble rotates to draw the workpiece across a stationary tool

- the bending form remains stationary and the pressure tool moves along the workpiece

Glassl, Gerlli

Bending

Roll Forming- involves the progressive bending of metal strip as it passes through a series of forming rolls

• Any material that can be bent can be roll- formed

Glassl, Gerlli

Bending

Seaming- bending operation that can be used to join the ends of sheet metal to form containers such as cans, pails and drums

Glassl, Gerlli

Bending

Flanging-the process of rolling on sheet metal in essentially the same manner as seaming

Glassl, Gerlli

Bending

Straightening• also known as flattening• opposite of bending

Glassl, Gerlli

Bending

Glassl, Gerlli

BendingRoll- straightening/ Roll Leveling

-subjects the material to a series of reverse bends

-metal is bent back and forth

Stretcher Leveling-sheets are gripped mechanically

Glassl, Gerlli

Shearing- the mechanical cutting of materials without the information of chips or the use of burning or melting

for straight cutting blades: shearingfor curved blades: blanking, piercing, notching, trimming

Aluminum Brass

Bronze Mild steel

Stainless steel

Glassl, Gerlli

Shearing

Classifications of Shearing Processes…

Blanking

Piercing

Shaving

Cutoff

NotchingTrimming

Slitting

Dinking

Glassl, Gerlli

ShearingSlitting

- shearing process used to cut rolls of sheet metal into several rolls of narrower width

used to cut a wide coil of metal into a number of narrower coils as the main coil is moved through the slitter.

Glassl, Gerlli

Shearing

Blanking

during which a metal workpiece is removed from the primary metal strip or sheet when it is punched.

punch-out: workpiece

remaining strip: scrap

Glassl, Gerlli

Shearing

Piercing

punch-out: scrapremaining strip: workpiece

http://www.osha.gov/SLTC/etools/machineguarding/animations/punch2.html

Glassl, Gerlli

Shearing

Lancing- forms either a line cut (slit) or an actual hole

Perforating- piercing a large number of closely spaced holes

Glassl, Gerlli

ShearingNotching-same as piercing- edge of the strip or black forms part of the punch-out perimeter Nibbling- produces a series

of overlapping slits/notches

Glassl, Gerlli

ShearingShaving

- finishing operation in which a small amount of metal is sheared away from the edge of an already blanked part

- can be used to produce a smoother edge

Glassl, Gerlli

Shearing

Trimming

Glassl, Gerlli

Shearing

- punch and die operation used to separate a stamping or other product from a strip or stock

Cutoff

Glassl, Gerlli

Shearing

Dinking

-used to blank shapes from low-strength materials such as rubber, fiber and cloth

Ligon, Jennilyn

Drawing and Sheet Metal Forming

COLD DRAWING• for sheet metal, it refers to the forming of parts where plastic form occurs over a curved axis

• for wire, rod, or tubing, it refers to the process of reducing the cross section of the material by pulling it through a die

• A. Rod, Bar and Tube Drawing

Rod or bar drawing

Die

Rod

If the product cannot be bent or coiled conveniently, straight-pull draw benches are employed on finite-length stock

Ligon, Jennilyn


Tube Drawing

• can be used to produce high quality tubing where the product requires the smooth surfaces, thin walls, accurate dimension and added strength.

Mandrel

Die

Tube

Die

Die

Floating plug

Ligon, Jennilyn


B. Wire Drawing

• involves smaller diameter material

Direction of travel

Starting stock in coil form

Lubrication box

Die

Draw block

Carbide die

Steel casing

Back relief

Land

Approach angle

Entrance angle

Back radius

Ligon, Jennilyn

Drawing and Sheet Metal FormingC. Spinning

• a cold-forming operation in which a rotating disk of sheet metal is shaped over a male form or mandrel

Form attached to the Headstock spindle

Final shape

Steps in spinning

Original blank of sheet metal

Follower held in tailstock

Ligon, Jennilyn


D. Shear Forming

Shear Forming or Flow Turning• modification of the spinning process in which each element of the blank maintains its distance from the axis of rotation• forms cones, hemisphere and similar shapes

Ligon, Jennilyn


Blank

α

tc

tb

Feed Roller

Mandrel

Direct Process

tc will var with th e angle of the particuilar region according to to the relationship: tc = tb sinα

Blank

FeedFinished part

Reverse Process

Ligon, Jennilyn


E. Stretch Forming (Stretch-wrap forming)

• a sheet of metal is gripped by two or more sets of jaws that stretch it and wrap it up around a single form block

Ligon, Jennilyn

Drawing and Sheet Metal FormingF. Sheet Metal Drawing

Shell Drawing• earliest use was the manufacture of artillery shells and cartridge cases

Shallow Drawing• the process considered when the depth of the product is less than its diameter

Deep Drawing• the process considered when the depth is greater than its diameter

Shrink Forming

•The circumference decreases as the blank diameter is reduced from d1 to the cup wall diameter d’1 since the volume of material must remain constant, the decrease in circumferential dimension must be compensated by an increase in another dimension, such as thickness

d1

d’1

Stretch Forming• the diameter and the circumference increases, with a corresposponding decrease in thickness that can be the cause of subsequent service failures such as premature failure in corrosive environment

d2

d’2

Ligon, Jennilyn


F. Forming with rubber Tooling or Fluid PressureGuerin Process

• based on the phenomenon that rubber of the proper consistency, when totally confined, acts as a fluid and transmits pressure uniformly in all directions

Rubber Ram

BlankForming Block

High-pressure Flexible-die Process (Hydroforming or Flexiforming)

• the rubber pad is replaced by a flexible rubber diaphram backed by controlled hydraulic pressures

Ligon, Jennilyn


G. Drawing on a Drop Hammer

• when small quantities of shallow-drawn parts are required, they can often be made most economically through the use of Low-melting point metal dies and a drop hammer• dies can be directly cast eliminating the expense of machining operations, and when no longer needed, can be cast into other shapes

Wood shims

Work(a)

Die

(b) (c)

Ligon, Jennilyn


H. High-Energy-Rate Forming

• process of forming metals through the application of large amounts of energy in a very short time interval

1. Ironing> process of thinning the walls of a drawn cylinderby passing it between a punch and die whose separation is less than the original wall thickness

2. Embossing> presswork process in which raised lettering or other designs are impressed in sheet materials

Ligon, Jennilyn


PunchTube

Die

Embossing Die

work

3. Supeplastic Sheet Foerming> makes possible the economical production of large, complex-shaped products with compound curves

>major limitation is low forming rate that is required to maintain superplastic behavior

Ligon, Jennilyn


I. Designs and aid for shet Metal FormingStrain analysis, Forming limit diagram

•Quick and economical way of evaluating the severity of deformation in a formed part

Major strain (percent)

Minor strain (percent)

020406080

100120140

60 40 20 0 20 40 60 80

•Major and minor strain=positive >deformation is stretching >sheet metal decrease in thickness•Major strain=positive;minor strain=negative >deformation is drawing >thickness may increase, decrease or stay depending on the magnitudes of the 2 strain

Ligon, Jennilyn

PressesA. Classification of Presses

Mechanical Drive

• provides faster motion and more positive control of displacement

Hydraulic Presses

• produce motion as a result of piston movement, and longer or variable-length strokrs can be programmed within the limitations of the cylinder

Manually operated Presses

•Generally used for very light work such as shearing small sheets

Kick presses Crank>used for most piercing and blanking operations and for drawing

Eccentric or cam drives> Used when only a short ram stroke is required

Knuckle-joint drive> provides a very high mechanical advantage along with fast action

Toggle mechanism> Used principally in drawing presses to actuate the blank holder

Screw-type drives> offer great mechanical advantage coupled witj action that resembles drop hammer

Screw presses> popular in forging industry

Single-slide

Multiple slide

A. Classification of Presses

Mechanical Drive Hydraulic Presses Manually operated Presses

Ligon, Jennilyn

PressesB. Types of Press Frame

> Imposes limitations on the size and type of work that can be accomodated, How that work is fed and unloaded, and the overall stiffness of the machine

Arch Gap Straight Sided

• seldom used today except with screw drives for coining operations

• frames are in the shape of letter C•Most verastile and commonly preffered presses•Provide unobstructive access to the dies from 3 directions and permit large workpiece to be fed into the press

• have frames that consist of a crown, 2 uprights, a base or bed , and one or more moving slide

Ligon, Jennilyn

Presses

C. Press Feeding Device>increases productivity >operators safety is an advantage

• When continuous strip is used it can be fed automatically by double-roll feeds mounted on the side of the press

Major Cold-Working Operations…



• Bending

• Squeezing

• Drawing

• Shearing

• Presses