Milling MCQ Quiz - Objective Question with Answer for Milling - Download Free PDF

Explanation:

Climb Milling

Definition: Climb milling, also known as down milling, is a milling process where the direction of the cutter rotation is the same as the feed direction of the workpiece. In this process, the cutter engages the workpiece at the top of the cut and removes material in a downward direction, moving with the feed of the workpiece.

Working Principle: In climb milling, the cutter rotates in the same direction as the feed. As the cutter tooth starts to engage the workpiece, it cuts from the surface to the bottom, which means the chip thickness decreases from the beginning to the end of the cut. The cutting force is directed into the workpiece, which helps to hold the workpiece against the machine table.

Advantages:

• Improved Surface Finish: The cutter in climb milling provides a cleaner cut as it shears the material, leading to a better surface finish.
• Longer Tool Life: Due to the direction of cutting and the way the material is removed, the tool experiences less friction and wear, extending its operational life.
• Better Chip Evacuation: The chips are thrown behind the cutter, making chip evacuation easier and reducing the chances of re-cutting chips.
• Reduced Work Hardening: Climb milling reduces work hardening of the material because the cutter engages the material with less friction.

Disadvantages:

• Requires Rigid Setup: The process can cause the workpiece to be pulled into the cutter, demanding a more rigid machine setup to prevent backlash and ensure precision.
• Not Suitable for All Materials: Certain materials, especially those that are harder or brittle, may not be suitable for climb milling due to the aggressive nature of the cut.

Applications: Climb milling is widely used in CNC machining, especially for finishing cuts and precision work where a high-quality surface finish is desired. It is also preferred in scenarios where tool life is a critical factor, such as in high-production environments.

Explanation:

Straddle Milling

Definition: Straddle milling is a machining process in which two side milling cutters are mounted on an arbor and are used simultaneously to machine two parallel surfaces on a workpiece. This method is employed to achieve precise and symmetrical cuts on both sides of the workpiece, ensuring that the surfaces are parallel to each other.

Working Principle: In straddle milling, the workpiece is held securely on the milling machine table, and the cutters are spaced appropriately to match the desired width of the cut. As the cutters rotate, they remove material from both sides of the workpiece at the same time. This simultaneous cutting action helps in maintaining the parallelism and accuracy of the machined surfaces. The depth of cut and feed rate are carefully controlled to achieve the desired finish and dimensional accuracy.

Advantages:

• High efficiency as both sides of the workpiece are machined in a single pass.
• Ensures parallelism and uniformity between the machined surfaces.
• Reduces machining time and increases productivity.
• Improves dimensional accuracy and surface finish.

Disadvantages:

• Requires precise setup and alignment of the cutters.
• Limited to machining workpieces with parallel sides.
• High initial cost of specialized tooling and fixtures.

Additional Information 1. Gang Milling: Gang milling is a process where two or more milling cutters are mounted on the same arbor and are used to machine different surfaces simultaneously. This method is efficient for machining complex shapes and features in a single pass. However, it differs from straddle milling as it is not specifically focused on machining parallel surfaces.

3. String Milling: String milling is not a standard term in machining processes. It might be a typographical error or a misinterpretation of other milling methods. Therefore, it is not applicable in this context.

4. Side Milling: Side milling involves the use of a side milling cutter to machine vertical surfaces on the workpiece. While it is similar to straddle milling in terms of machining vertical surfaces, it differs as it typically involves using a single cutter rather than two cutters mounted with spacing to machine both sides simultaneously.

Explanation:

Reason for Choosing Climb Milling

Definition: Climb milling, also known as down milling, is a milling process where the direction of the cutter rotation matches the feed direction. This method is widely chosen for various reasons, particularly in achieving a better surface finish on the workpiece.

Working Principle: In climb milling, the cutting tool engages the workpiece at the thickest part of the chip and exits at the thinnest part. This means the cutting force is directed downwards, which tends to push the workpiece into the table. This minimizes the risk of the cutter lifting the workpiece, resulting in a smoother finish.

Advantages:

• Better Surface Finish: The primary reason for choosing climb milling is the superior surface finish it provides. Since the cutting force helps press the workpiece into the table, there is less vibration and deflection. This results in a smoother and more accurate finish on the workpiece.
• Reduced Work Hardening: Climb milling reduces the amount of heat generated during the cutting process, which in turn reduces the likelihood of work hardening. Work hardening can adversely affect the surface finish and make subsequent machining operations more difficult.
• Improved Tool Life: Because climb milling produces less heat, the cutting tools experience less thermal stress, which can extend their lifespan. Additionally, the consistent engagement of the cutter teeth with the workpiece can result in more uniform wear on the tool.
• Reduced Cutting Forces: In climb milling, the cutting forces are generally directed downwards, which can help in reducing the load on the machine spindle and bearings. This can lead to longer machine life and reduced maintenance requirements.

Explanation:

Up Milling

Definition: Up milling, also known as conventional milling, is a machining process where the direction of the cutter rotation is opposite to the direction of the workpiece feed. This method contrasts with down milling (or climb milling), where the cutter rotation and workpiece feed are in the same direction.

Working Principle: In up milling, the cutter rotates against the direction of the feed, meaning the cutting edge engages the material at the bottom of the cut. The chip thickness starts at zero and increases as the cutter moves along the surface. This process typically results in a rougher surface finish compared to down milling.

Key Characteristics:

• Cut Starts on Machined Surface: In up milling, the cutting action begins on the previously machined surface and moves upward against the feed direction.
• Feed Opposite to Rotation: The feed direction is opposite to the rotation of the cutter, which is a defining feature of up milling.

Advantages:

• Better for roughing operations due to the increased strength and stability of the cutting tool.
• Less risk of tool damage as the cutting forces are lower at the start of the cut.

Disadvantages:

• Higher cutting forces can lead to more tool wear and potential deflection.
• Less efficient material removal compared to down milling, as the cutting process is more labor-intensive.

Applications: Up milling is commonly used in situations where a rough surface finish is acceptable or where the material is difficult to machine. It is also preferred in manual milling operations and for milling operations on less rigid setups.

Explanation:

Milling Cutter Operations

Definition: Milling cutters are rotary tools with multiple cutting edges used in milling machines or machining centers to perform various machining operations. These operations include cutting, shaping, and removing material from a workpiece. Milling cutters come in various shapes and sizes, each designed for specific tasks.

Operations Performed by Milling Cutters:

• Keyways: Milling cutters can create keyways, which are slots or grooves cut into a shaft and hub to secure a component with a key. This ensures that the component does not rotate relative to the shaft. Keyways are essential for transmitting torque between the shaft and the mounted component.
• Screw Threads: Milling cutters can also be used to mill screw threads. This process involves creating helical grooves or threads on the surface of a cylindrical workpiece. Thread milling is a versatile operation that allows for the production of both internal and external threads with high precision.
• Spur Gears: Milling cutters can manufacture spur gears, which are gears with straight teeth that are parallel to the axis of rotation. Spur gears are used to transmit motion and power between parallel shafts. The milling process involves cutting the gear teeth with high accuracy to ensure proper meshing and smooth operation.
• Splines: Milling cutters can cut splines, which are ridges or teeth on a drive shaft that mesh with grooves in a mating component. Splines are used to transmit torque and rotational motion between the shaft and the connected part. This operation is crucial for ensuring a secure and efficient power transmission.

Concept:

Table speed in mm/minute = ft × Z × N

where, N = RPM, Z = no. of teeth, ft = Feed per tooth

Calculation:

Given:

Z = 10, N = 150 rpm, ft = ?, f_m = 400 mm/min

Table speed in mm/minute, 400 = 150 × 10 × ft

Explanation:

Centre Lathe → Knurling

Milling → Slotting

Grinding → Dressing

Drilling → Counter-boring

Knurling

Knurling is the operation of producing a straight-lined, diamond-shaped pattern or cross lined pattern on a cylindrical external surface by pressing a tool called knurling tool. Knurling is not a cutting operation but it is a forming operation.

A lathe is used for many operations such as turning, threading, facing, grooving, Knurling, Chamfering, centre drilling

Counter - boring

Counter - boring is an operation of enlarging a hole to a given depth, to house heads of socket heads or cap screws with the help of a counterbore tool.

Dressing:

When the sharpness of grinding wheel becomes dull because of glazing and loading, dulled grains and chips are removed (crushed or fallen) with a proper dressing tool to make sharp cutting edges.

The dressing is the operation of cleaning and restoring the sharpness of the wheel face that has become dull or has lost some of its cutting ability because of loading and glazing.

Slot Milling:

Slot milling is an operation of producing slots like T - slots, plain slots, dovetail slots etc.

Explanation:

Milling is a process of producing flat and complex shapes with the use of a multi-point (or multi-tooth) cutting tool.

• The axis of rotation of the cutting tool is perpendicular to the direction of feed, either parallel or perpendicular to the machined surface.
• Milling is usually an interrupted cutting operation since the teeth of the milling cutter enter and exit the workpiece during each revolution.

There are two basic types of milling operations:

Down milling: 

• It is also called as Climb milling.
• When the cutter rotation is in the same direction as the motion of the workpiece being fed.
• The cutting force is maximum at the beginning and minimum at the end of the cut.
• In down milling, the cutting force is directed to the work table, allowing thinner parts to be machined without susceptibility to breakage.
• A better surface finish is obtained.
• Climb milling typically has lower forces involved for a couple of reasons:
  • Chip thickness: Starting the cut with a thin chip that increases in size results in less contact area between the tool and the work, reducing the initial peak in cutting force.
  • Cutter engagement: With the workpiece and cutter rotating in the same direction, the cutter teeth are effectively sliding into the workpiece at a lower speed. This reduces the cutting forces compared to the faster engagement in conventional milling where the work and cutter are moving in opposite directions.

Up milling:

• It is also called as Conventional milling.
• In which the workpiece is moving towards the cutter, opposing the cutter direction of rotation.
• The cutting force is minimum during the beginning of the cut and maximum at the end of the cut. In up milling, the cutting action tends to lift the workpiece and hence,
• High rigidity of the machine tool is required in this milling operation.
• A proper fixture is required in this operation.

Explanation:

Milling is a process of producing flat and complex shapes with the use of a multi-point (or multi-tooth) cutting tool. The axis of rotation of the cutting tool is perpendicular to the direction of feed, either parallel or perpendicular to the machined surface.

Milling machines are superior to other machines in terms of accuracy and surface finish.

There are two basic types of milling operations:

Down milling/ Climb milling: 

• The cutter rotation is in the same direction as the motion of the workpiece being fed.
• The cut starts with the full chip thickness.
• The cutting force is maximum at the beginning and minimum at the end of the cut.
• In down milling, the cutting force is directed on to the work table, which allows thinner parts to be machined without susceptibility to breakage, but it requires the optimum holding device as the cutting tool forces the work-piece in the opposite direction of the tool motion. Therefore the gap between the lead screw and the half nut increases which requires backlash eliminator. 
• A better surface finish is obtained.

Up milling/ Conventional milling:

• The workpiece is moving towards the cutter, opposing the cutter direction of rotation.
• The cutting force is minimum during the beginning of the cut and maximum at the end of the cut. 
• The thickness of the chip is less at beginning of the cut and more at the end of the cut.
• As the cutting force is directed upwards, it tends to lift the workpiece from the fixtures.

Explanation:

• Milling is a process of producing flat and complex shapes with the use of a multi-point (or multi-tooth) cutting tool.
• The axis of rotation of the cutting tool is perpendicular to the direction of feed, either parallel or perpendicular to the machined surface.
• Milling is usually an interrupted cutting operation since the teeth of the milling cutter enter and exit the workpiece during each revolution.
• There are two basic types of milling operations:

Down milling: 

• It is also called as Climb milling.
• When the cutter rotation is in the same direction as the motion of the workpiece being fed.
• The cutting force is maximum at the beginning and minimum at the end of the cut.
• In down milling, the cutting force is directed on to the work table, which allows thinner parts to be machined without susceptibility to breakage.
• A better surface finish is obtained.

Up milling:

• It is also called as Conventional milling.
• In which the workpiece is moving towards the cutter, opposing the cutter direction of rotation.
• The cutting force is minimum during the beginning of the cut and maximum at the end of the cut. In up milling, the cutting action tends to lift the workpiece and hence,
• A proper fixture is required in this operation.

Explanation:

Plain milling:

• It is the operation of the production of a flat surface parallel to the axis of rotation of the machine.

​

• It is also called as slab milling.
• Plain milling cutters and slab milling cutters are used to perform this operation.

Face milling:

• Face milling is the operation performed by the face milling cutter rotated about an axis at right angles to the work surface.
• End mills and side & face milling cutters are also used at times to perform this operation.

Explanation:

Feed rate in slab milling operation is given by, 

f_m = f_t × N × Z 

Where f_t is the feed per tooth.

N = Spindle rotational speed (in rpm)

Concept:

Material Removal Rate (MRR) = \(\frac{No.~ of~pieces~×~Volume ~of~ material~removed }{Time~taken~by~tool~to~travel}\)

Volumed removed is, l × b × t

where, l = length of 1 workpiece, b = width, t = thickness

Velocity of cutter is, \(V = \frac{{\pi DN}}{{60}}\)

Calculation:

Given:

D = 25 mm, N = 100 rpm, f = 10 mm/min, l = 205 mm, b = 5 mm, t = 1 mm

Velocity v is, v \(= \frac{{\pi × 25 × 100}}{{60}}\)

= 130.8997 mm/s

Volume of metal removed from one work piece = 5 × 1 × 205 mm³

= 1025 mm³

Time taken to travel 205 mm

\(\Rightarrow \frac{{205}}{{10}}~min = 20.5~min\)

Therefore, material removal rate from one work piece, MRR \(= \frac{{1025}}{{20.5 × 60}}\) = 0.833 mm³/s

Total MRR from 50 work pieces

= 50 × 0.833

= 41.67 mm³/s

Concept:

Time taken to face a work piece is given by

\({T_m} = \frac{L}{{fN}}\)

Here L = D/2, f – cross feed, N – spindle speed

Calculation:

Given:

D = 80 mm, f = 0.3 mm/rev, N = 90 rpm

fN = 27 mm/min, L = 40 mm

Concept:

Feed rate in slab milling operation is given by, 

fm = ft × N × Z 

Where, ft is the feed per tooth, N = Spindle rotational speed (in rpm), Z = Number of teeth in cutter (teeth per rev)

Cutting velocity \(V = \frac{{\pi DN}}{{1000}}\)

Calculation:

Given:

D = 125 mm, Z = 10, V = 14 m/min, f_m = 100 mm/min

\(N\; = \frac{{1000 \times V}}{{\pi D}} = \frac{{1000 \times 14}}{{\pi \times 125}} = 35.67\)

Feed per tooth \({f_t}\; = \frac{{{f_m}}}{{NZ}} = \frac{{100}}{{35.67 \times 10}} = 0.28\frac{{mm}}{{tooth}}\)