Question

Q2) a) (a ) (b) Tool Direction Tool Direction Chip Flow Chip Flow Figure 3. (a) and (b) is the schematic of milling tool, showing the direction of cutting and the direction of chip flow Identify the type of milling operation represented in Figure 3(a) and Figure 3(b)? List at least 3 differences between the milling operations shown in Figure 3 (15 points) b) Why is it difficult to cut a plastic material using a bandsaw? (5 points) c) What is a spring back effect in sheet metal bending? How can we compensate for this effect? (10 points) [minimum 4 bullet points] d) List at least 8 turning operations and in a sentence explain why are they used? (15 points) e) List the types of chips formed in conventional machining process (5 point)

Answer 1

(a)figure 3(a) = Up or forward milling.

figure 3(b) = Down or reverse milling.

Up or forward milling.

• direction of rotation of milling cutter and movement of workpiece will be opposite.
• Because of opposite direction, the force induced is higher,which leads to higher power consumption and tool wear.
• Uncut chip thickness is varying from minimum at begenning to maximum towards end.
• Due to high force there is chance of lift of tool from workpiece.

Down or reverse milling.

• direction of rotation of milling cutter and movement of workpiece will be same.
• Because of same direction, the force induced is lower,which leads to lower power consumption and tool wear.
• Uncut chip thickness is varying from maximumat begenning to minimum towards end.
• no workholding device is required.

(b)It is difficult to cut plastic with band saw because:

• Plastic’s insulative (non-conducting) nature slows the dissipation of heat built up at the cutting interface leading to localized melting, warping and wandering cuts if excessive frictional heat is developed.The feed rate must compensate for the blade speed to avoid over heating. If localized melting occurs, either feed faster or slow the blade speed (if possible).
• Developement of residual stresses.

(c)Spring back effect:During bending in the bend portion at the inside surface the compressive stresses and outside the tensile stresses are induced.At the inside surface the compressive stress induced are greater than flow stress so deformation produced is 100% plasticdeformation ,whereas on the outter surface the tensile stresses induced is greater than the yield stressof the material but still there is some amount of elastic deformation is present in the material after drawing the load ,this elastic deformation present on the outter surface is regaining to the original shape and increasing the radius of bending.this is called spring back.

To avoid spring back:

• The force applied has to be maintained for a longer duration.
• Overbending,angle of bending is increased or reduce radius of bending.
• streatch bending,the method is to apply tangential tension when the sheet metal is bent change the stress state and distribution inside the sheet metal.
• local compression,increase the length of outer plate by reducing the thickness of the outer plate so that spring back of inner and outer layers offset each other.
• multiple bending.

(d)Turning operations:

1. Tapered turning:Taper turning means, to produce a conical surface by gradual reduction or increase in diameter from a cylindrical work piece. This tapering operation has wide range of use in construction of machines. Almost all machine spindles have taper holes which receive taper shank of various tools and work holding devices.
2. Spherical turning:Spherical turning means to produce a ball shape on the work piece.
3. Hard turning :Hard turning means, to turn materials with a Rockwell C hardness greater than 45. It is typically performed after the work piece has been heat treated.The process is intended to replace traditional grinding operations. Hard turning is appropriate for parts requiring roundness accuracy of 0.5-12 micrometers

4. Facing:Facing in the context of turning work involves moving the cutting tool at right angles to the axis of rotation of the rotating workpiece. This can be performed by the operation of the cross-slide, if one is fitted, as distinct from the longitudinal feed.

5. Parting:parting off or cutoff, is used to create deep grooves which will remove a completed or part-complete component from its parent stock.

6. Grooving :Grooving is like parting, except that grooves are cut to a specific depth instead of severing a completed/part-complete component from the stock. Grooving can be performed on internal and external surfaces, as well as on the face of the part.

7. Boring:Boring is the process of enlarging a hole that has already been drilled (or casted), by means of a single-point cutting tool.Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole. Boring can be viewed as the internal-diameter counterpart to turning, which cuts external diameters.

8. Drilling:Drilling is the process used to remove material from the inside of a work piece. This process utilizes standard drill bits held stationary in the tail stock or tool turret of the lathe.

9. Reaming:Reaming is the sizing operation that removes a small amount of metal from a hole already drilled. It is done for making internal holes of very accurate diameters.

10. Threading:Both standard and non-standard screw threads can be turned on a lathe using an appropriate cutting tool.

(e)Typs of chips formed in conventional machining:

1. Discontinuous Chips:Main reason of discontinuous chip formation is the deformation due to repeated fracturing.During machining of hard and btittle workpiece because of lower toughness the energy wave can propogate upto the surfaceeasily due to this chips will become discontinous.Hard and brittle metals like C.I , bronze and brass are prone to produce discontinuous chips.
2. Continuous Chips:.During machining of soft workpiece because of higher toughness when the energy wave is propogating through the material it is absorbed by the workpiece and dissipearing somewhere in the middle so the continuity of the chip will be maintained.
3. Continuous Chip Built-up-Edge :These are very similar to the continous type chips but it is not as smooth as those. These chips are seen in the workpieces with poor surface finish. These chips protect the cutting edge from wear because of the chips movement. The tool life increases by the action of heat.During machining microchips are produced and moving along the rake face of the tool so that they exposed to the higher temperature environment, due to this the microships will get adhered to rake face of the tool called as built up edge formation.