Principles of chip-breaking

 In respect of convenience and safety, closed coil type chips of short length and ‘coma’ shaped broken-to-half turn chips are ideal in machining of ductile metals and alloys at high speed.

The principles and methods of chip breaking are generally classified as follows:

· Self breaking: This is accomplished without using a separate chip-breaker either as an attachment or an additional geometrical modification of the tool.

· Forced chip breaking: by additional tool geometrical features or devices.

(a) Self breaking of chips

Ductile chips usually become curled or tend to curl (like clock spring) even in machining by tools with flat rake surface due to unequal speed of flow of the chip at  its free and generated (rubbed) surfaces and unequal temperature and cooling rate at those two surfaces. With the increase in cutting velocity and rake angle (positive) the radius of curvature increases, which is more dangerous. In case of oblique cutting due to presence of inclination angle, restricted cutting effect etc. the curled chips deviate laterally resulting helical coiling of the chips.

The curled chips may self break:

By natural fracturing of the strain hardened outgoing chip after sufficient cooling and spring back  . This kind of chip breaking is generally observed under the condition close to that which favours formation of jointed or segmented chips.By striking against the cutting surface of the job, mostly under pure orthogonal cutting.By striking against the tool flank after each half to full turn.

The possibility and pattern of self chip-breaking depend upon the work material, tool material and tool geometry, levels of the process parameters and the machining environment (cutting fluid application) which are generally selected keeping in view the overall machinability.

(b) Forced chip-breaking

The hot continuous chip becomes hard and brittle at a distance from its origin
due to work hardening and cooling. If the running chip does not become enough
curled and work hardened, it may not break. In that case the running chip is forced to
bend or closely curl so that it breaks into pieces at regular intervals. Such broken
chips are of regular size and shape depending upon the configuration of the chip
breaker.

Chip breakers are basically of two types :

• In-built type

• Clamped or attachment type

In-built breakers are in the form of step or groove at the rake surface near the cutting edges of the tools. Such chip breakers are provided either

• after their manufacture – in case of HSS tools like drills, milling cutters, broaches etc and brazed type carbide inserts
• during their manufacture by powder metallurgical process – e.g., throw away type inserts of carbides, ceramics and cermets.

The basic principle of forced chip breaking is schematically shown in Fig. 7.2 when the strain hardened and brittle running chip strikes the heel, the cantilever chip gets
forcibly bent and then breaks.

some commonly used step type chip breakers:

• Parallel step
• Angular step; positive and negative type
• Parallel step with nose radius – for heavy cuts.

Groove type in-built chip breaker may be of

• Circular groove or

• Tilted Vee groove

 The unique characteristics of in-built chip breakers are:

·         The outer end of the step or groove acts as the heels that forcibly bend and fracture the running chip.

·         Simple in configuration, easy manufacture and inexpensive

·         The geometry of the chip-breaking features are fixed once made (i.e. cannot be controlled)

·         Effective only for fixed range of speed and feed for any given tool-work combination.

(c) clamped type chip-breaker

Clamped type chip breakers work basically in the principle of stepped type chip- breaker but have the provision of varying the width of the step and / or the angle of the heel. Figure 7.5 schematically shows three such chip breakers of common use :

·         With fixed distance and angle of the additional strip – effective only for a limited domain of parametric combination

·         With variable width (W) only – little versatile

·         With variable width (W), height (H) and angle (β) – quite versatile but less rugged and more expensive.

Need and purpose of chip-breaking

Continuous machining like turning of ductile metals produces continuous chips, which leads to their handling and disposal problems. The problems become acute when ductile but strong metals like steels are machined at high cutting velocity for high MRR (material removing rate) by flat rake face type carbide or ceramic inserts. The sharp edged hot continuous chip, that comes out at very high speed-

·         becomes dangerous to the operator and the other people working in the vicinity

·         may impair the finished surface by entangling with the rotating job

·         creates difficulties in chip disposal.

Therefore it is essentially needed to break such continuous chips into small regular pieces for

·  safety of the working people

·  prevention of damage of the product

·  easy collection and disposal of chips.

Chip breaking is done in proper way also for the additional purpose of improving machinability by reducing the chip-tool contact area, cutting forces and crater wear of the cutting tool.

 Chip Formation

Every Machining operation involves the formation of chips. The nature of which differs from operation to operation, properties of work piece material and the cutting condition. Chips are formed due to cutting tool, which is harder and more wearer-resistant than the work piece and the force and power to overcome the resistance of work material. The chip is formed by the deformation of the metal lying ahead of the cutting edge by a process of shear. Four main categories of chips are:

1.       Discontinuous Chips

2.       Continuous or Ribbon Type Chips

3.       Continuous Chip Built-up-Edge (BUE)

4.       Serrated Chips

 Types of Chips

Discontinuous Chips: These chips are small segments, which adhere loosely to each other. They are formed when the amount of deformation to which chips undergo is limited by repeated fracturing. Hard and brittle materials like bronze, brass and cast iron will produce such chips.
Continuous or Ribbon Type Chips: In continuous chip formation, the pressure of the work piece builds until the material fails by slip along the plane. The inside on the chip displays steps produced by the intermittent slip, but the outside is very smooth. It has its elements bonded together in the form of long coils and is formed by the continuous plastic
deformation of material without fracture ahead of the cutting edge of the tool and is followed by the smooth flow of chip up the tool face.
Continuous Chip Built Up Edge: This type of chip is very similar to that of continuous type, with the difference that it is not as smooth as the previous one. This type of chip is associated with poor surface finish, but protects the cutting edge from wear due to movement of chips and the action of heat causing the increase in tool life.
Serrated Chips: These chips are semicontinuous in the sense that they possess a saw-tooth appearance that is produced by a cyclical chip formation of alternating high shear strain followed by low shear strain. This chip is most closely associated with certain difficult-to-machine metals such as titanium alloys,
nickel-base super alloys, and austenitic stainless steels when they are machined at higher cutting speeds. However, the phenomenon is also found with more common work metals (e.g., steels), when they are cut at high speeds. Some of the conditions favorable for continuous chip formation can be summarized as: Ductile work piece material. Small chip thickness. Fine feeds. Sharp cutting edge of the cutting tool. A large rake angle on the cutting tool. High cutting speeds. Using coolant. Causes of discontinuous chips formation Cutting conditions are the main causes for discontinuous chips such as, ·         Very low or very high cutting speed ·         Large depth of cut ·         Low rake angle ·         Lack of cutting fluid ·         Vibration on the machine tool

Tool Geometry

The back rake angle affects the ability of the tool to shear the work material and form the chip which naturally curves into the work due to the difference in length from the outer and inner parts of the cut. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range.

Side Rake along with back rake controls the chip flow and partly counteracts the resistance of the work to the movement of the cutter and can be optimized to suit the particular material being cut. Brass for example requires a back and side rake of 0 degrees while aluminum uses a back rake of 35 degrees and a side rake of 15 degrees.

Nose Radius makes the finish of the cut smoother as it can overlap the previous cut and eliminate the peaks and valleys that a pointed tool produces. Having a radius also strengthens the tip, a sharp point being quite fragile.

All the other angles are for clearance in order that no part of the tool besides the actual cutting edge can touch the work. The front clearance angle is usually 8 degrees while the side clearance angle is 10-15 degrees and partly depends on the rate of feed expected.

Minimum angles which do the job required are advisable because the tool gets weaker as the edge gets keener due to the lessening support behind the edge and the reduced ability to absorb heat generated by cutting.

The Rake angles on the top of the tool need not be precise in order to cut but to cut efficiently there will be an optimum angle for back and side rake.

Tool-holders

By confining the expensive hard cutting tip to the part doing the actual cutting, the cost of tooling is reduced. The supporting tool holder can then be made from a tougher steel, which besides being cheaper is also usually better suited to the task, being less brittle than the cutting-edge materials. The tool holders may also be designed to introduce additional properties to the cutting action, such as

      Angular approach - direction of tool travel.

      Spring loading - deflection of the tool bit away from the material when excessive load is applied.

      Variable overhang - the tool bit may be extended or retracted as the job requires.

      Rigidity - the tool holder can be sized according to the work to be performed.

      Direct cutting fluid or coolant to the work area.

Note that since stiffness (rather than strength) is usually the design driver of a tool holder, the steel used doesn't need to be particularly hard or strong as there is relatively little difference between the stiffnesses of most steel alloys

Tool Materials in Common Use

High Carbon Steel

Contains 1 - 1.4% carbon with some addition of chromium and tungsten to improve wear resistance. The steel begins to lose its hardness at about 250° C, and is not favoured for modern machining operations where high speeds and heavy cuts are usually employed.

High Speed Steel (H.S.S.)

Steel, which has a hot hardness value of about 600° C, possesses good strength and shock resistant properties. It is commonly used for single point lathe cutting tools and multi point cutting tools such as drills, reamers and milling cutters.

Cemented Carbides

An extremely hard material made from tungsten powder. Carbide tools are usually used in the form of brazed or clamped tips. High cutting speeds may be used and materials difficult to cut with HSS may be readily machined using carbide tipped tool

USE OF A SHEET METAL TO BUILD UP A MUG(PART 3 & LAST PART)

Calculation:

We had to make a mug where diameter is height of the body 4” meanwhile the length of the handle is 7”.

i)           Circumference:

                For dia = 4”xπ + allowance

          Horizontal = 4”×π + (1/4” +1/4” clearance)

                              = 12.56 + (1/2” clearance)

                              = 13.5”

         For vertical = height + clearance

                              = 4” + (1/4” + 1/4”)

                              =4.5”

    Handle length = 7” + (1/4” + 1/4” clearance)

                              = 7.5”

    The upper part of the handle = 1” + (1/4” + 1/4” clearance)

                                                         =1.5”

Operation for making a mug:

          First we cut the sheet from sheet plane. It is wide. It was cut by hand share. Making clearance by a divider all around the taken sheet. We took the sheet for bonding the extra portion of the sheet. We made the sheet round in a definite pipe. We made the round sheet with ahiset by straining a metal.

          We cut a sheet in dia too lower part of a mug. After bonding it to connect with upper part of the mug. The extra portion is connected by striking with upper part of the mug. Then the extra portion is connected by striking it with hammer.

           Then we made a hole in the mug. In the hole, we joint resist to make the mug strong.

 Performance:

The job in workshop at sheet metal helps us to aware about making a simple mug in broad sense. But in practice we come to know about the use of different hand tools & mechanical tools in sheet metal shop. The most important thing is that we have broad idea about the sheet metal shop in manufacturing the world.

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Conclusion:

                  One of the characteristics of the skilled worker is the way in which he selects & uses the tools of his trade. It is essential to know how to select & proper use both the hand 7 mechanical tools of the sheet metal trade. Therefore, a little work in workshop that we have had an important significances. I think all B.Sc. Engineers must have a fair idea about the manufacturing world to take the first step toward becoming a successful sheet metal craftsman.

USE OF A SHEET METAL TO BUILD UP A MUG(PART 2)

Stakes:

A variety of stakes entirely useful but makes shift applicanears can often be made to answer a special purpose for which no. of inch is used as a support for sharp straight binding for similar works for on curved shapes there is the half moon stake.

Sheet gauge:

The work of measuring sheet is much more important in metal work. Then in work doing to grater constancy of the material & the development of increasing to a very high degree of perfection is of the almost temperature in modern industrial organization. Steel metal shop are used some machine also.

i)        Sheet bonding machine

ii)      Grillating share machine

iii)    Bond share machine

 Raw material:

i)         Galvanized sheet metal

ii)       Rivits

i)           Galvanized sheet metal:

Galvanized sheet metal consists of soft sheet metal coated with zinc, usually the steel is dipped in an acid both for clearing & then is dipped in zinc.

i)           Rivits: 

Black & timed rivits from the soft to extra large 12-16 are extremely to join the sheet in manufacturing any special job.

Soldering:

Soldering is a quick & useful process of making joints in light articles from steel, copper & brush & forewire joints such as occur in electrical work. It should not be required where much strength is required.

Fluxes:

        To assist in manufacturing necessary elements, a flux must be used to protect the clearance surface from temperature & atmospheric action. The process which is taking place of the fluxes is two kinds:

i)        This which not only protect the surface but also play an active chemical part in cleansing it.

ii)      Those which nearly protect a property by clearing surface.

 Making joint by soldering:

    A soldering joint may be made by heating & fluxieting the parts to be gained & adding the solders. By dipping the previously fluxed parts into a path of mother solder by the use of a soldering iron. For general turn of work the use of iron & copper. The metal is used :

i)        Because it is a very good conductor in conducting heat from itself to the metal of the joint.

ii)      It’s readily alloys with tin7 this facilities the apparatus of the coating the end of the iron with a large of solder known as to zinc, the iron to a good temperature. But less aired heat quickly clearing its & with a fit on energy, dipping it in the flux & other condition are right the iron well taken or their flism of solder & it is held against the struck of a soldier a certain amount in the formate globute well where to it.

The following additional hints are given in hope that they may be useful.

a)      Always use an iron as large solder can be handled & on the direction are having it too hot rather than not heat enough.

b)      A better joint can be jointed if the work is rather than cloud.

c)      Iron timing is facilitated by having some bolds of solder in a tin lid with a little spirits & touching both the spirits & the solder of the joint sometime.

d)      Quenching the hot joints in spirits hot wall often affect remarkably through clearing.

Riviting:

              Some plate is low strong to be seamed then place cannot be soldered if the articles are to with the stand heat. In such causes, joints are made by the use of rivits.

               A lap equal to their times to their rivet diameter is sufficient for a single mew hole & the counters should be close together to give a reasonable hold without making a working along the holes.

               In riveting two pieces of joint is jointed together. The required number of holes may be punched in one piece first but it is best to punch are only in the other place & to out the one in place before recording this ensure of confidence of holes throughout.

 Joining the Sheet Metal:

There are two joints, lap & bult which depend entirely on soldering lap of 3/16 inch which is allowed for must work of a small nature & the joint is strong when the solder flows right through the lap is solder bult which is only suitable for right angle joint shape.

                Most work is jointed in a stranger function by being same, it may solder as well. For the ordinary grooved, the edges are turned up & down respectively to a distance 1/8 inch are more the bunch then brooked work shape over a shape of metal are filled together & nommer   done.

                The simplest joint to the end of a complex is a circular lap. The edge of the box is thrown up 1/8 inch on a half moon set down square on a flat & standard is placed.

USE OF A SHEET METAL TO BUILD UP A MUG(PART 1)

One of the characteristics of the skilled worker is the way in which he selects & uses the tools of his trades. The materials things we possess are made from substance which in the first place our own from the earth or nature. Our priority depends on our ability to convert these raw materials into useful articles of consumption. For this reason, this is turned as machine age for its use of machines.

      The productions that may have small or big metal of sheet shops are:

1.      Making household instrument

2.      Making fabricating duet

3.      Making air conditioning duet system

Tools required & description:

1.      Hammer

2.      Ball peen hammer

3.      Spring divider

4.      Scriber

5.      Steel scale

6.      Rivet set

7.      Hand Groover  or Grove liner

8.      Mallet

9.      Hand share

10.  Chisel

11.  Drift Punch

       1.                       Hammer:

 It is essential that all sheet metal workers must have a or different types of hammer. There are types of hammers.

1.      Riveting hammer

2.      Setting hammer

3.      Nail hammer

4.      Ball peen hammer

5.       Raising hammer   

2.                       Ball peen hammer:

The ball peen hammer or machinist’s hammer has a round, slightly curved face & round head like a ball.

                     

3.                       Spring divider:

For circular making on stepping of equal distance a pair of spring dividers is the best.

                                           

4.                       Scriber:

The universal making tools is a conical point of hand sheet is called scriber.

5.                       Steel scale:

It is used for measuring tools. The steel scale is required for accurate layout of work in pattern drafting.

      

6.                       Rivet set:

A rivet set is a common hand tools made of steel. The deep hole in the bottom is used to draw a rivet through metals.

                                            

                                                                        

7.                       Hand groover:

The hand groover is used when grooving a seam by hand. The end of the tool is recessed to fit over the lock, making the grooved seam.

                                         

8.                       Mallet:

The mallet is one of the most abused tools because it is often used to perform operations for which it is not designed. Mallets are properly used where steel hammers would deface the work.

                                           

9.                       Hand share:

It is very useful things for any sheet metal workshop. It is made of metal. It is used to cut the sheet metals.

                   

10.                       Chisel:

                        Chisel is used for cutting & shaping metal. Sheet metal workers use “Flat Cold Chisel” more than the other types. There are three or four types of chisels. As-

i)                    Flat cold chisel

ii)                  Cape chisel

iii)                Diamond point chisel

iv)                Round point chisel

                                              

11.                       Drift punch:

                       It is one of the common tools which are used in steel metal workshop.