During metal cutting, low feed and high cutting speed is considered to obtain ________.

Question

Prepp Team · Accepted Answer

Understanding Metal Cutting Parameters: Feed and Cutting Speed

Metal cutting is a manufacturing process where excess material is removed from a workpiece to get a desired shape and size. This is done using a cutting tool. The outcome of the metal cutting process, such as the quality of the finished surface and how fast material is removed, depends heavily on the chosen cutting parameters. Two important cutting parameters are feed rate and cutting speed.

Impact of Feed Rate on Surface Finish

The feed rate in metal cutting refers to the distance the cutting tool advances into the workpiece per revolution of the workpiece (in turning) or per stroke of the tool (in shaping/planning), or per unit time (in milling). It directly influences the thickness of the chip being removed and the texture left on the machined surface.

A high feed rate means the tool moves faster across the surface, removing more material per pass. This leaves behind larger tool marks or ridges, resulting in a rougher surface finish. It typically leads to a higher metal removal rate.
A low feed rate means the tool moves slower across the surface, removing a thinner chip. This leaves behind smaller tool marks or ridges, resulting in a smoother, better surface finish.

Therefore, a low feed rate is crucial for achieving a good surface finish during metal cutting operations.

Impact of Cutting Speed on Surface Finish

Cutting speed is the speed at which the cutting edge of the tool passes over the material. It is usually measured in meters per minute (m/min) or feet per minute (ft/min).

High cutting speed, when combined with appropriate tooling and cutting conditions, can help in achieving a cleaner cut and reducing the formation of a built-up edge (BUE) on the tool. A stable or reduced BUE contributes to a better surface finish. High speeds also often require more rigid machine setups and specific tool materials capable of withstanding the heat generated.
Low cutting speed can sometimes lead to less stable cutting, potentially more BUE formation (depending on the material and tool), and a poorer finish compared to high speeds, especially when aiming for high precision.

When aiming for a good surface finish, a combination of low feed rate and high cutting speed is often preferred. The low feed rate is the primary factor determining the roughness, while the high cutting speed supports the finishing process by promoting cleaner cutting action.

Analyzing the Options Based on Metal Cutting Principles

Let's evaluate the given options in the context of low feed and high cutting speed:

High metal removal rate: Metal removal rate (MRR) is proportional to cutting speed, feed rate, and depth of cut. MRR $\propto$ Speed $\times$ Feed $\times$ Depth of Cut. While high speed increases MRR, low feed rate significantly reduces it. To achieve a high MRR, you typically need high feed and/or high depth of cut along with sufficient speed. Low feed fundamentally contradicts the goal of high MRR. So, this option is incorrect.
Dry machining: Dry machining is a method of cutting without using cutting fluids. It is a technique or condition of machining, not a direct outcome of using low feed and high cutting speed. While certain parameter combinations might be more suitable for dry machining, the parameters themselves do not guarantee or result in dry machining. So, this option is incorrect.
Surface finish: As discussed, low feed rate is the primary parameter for achieving a good surface finish by reducing the tool marks. High cutting speed, when used with suitable tools and conditions, complements this by promoting a cleaner cut. Therefore, low feed and high cutting speed are conditions often used to obtain a good surface finish. This option is correct.
Use of soft cutting tool: The choice of cutting tool material (soft vs. hard) depends on the workpiece material, cutting speed, feed rate, and desired tool life and surface finish. High cutting speeds typically require harder tool materials (like carbides, ceramics, CBN) that can withstand high temperatures and wear. Using a soft cutting tool is generally not associated with high cutting speeds and is not an outcome of using low feed and high speed. So, this option is incorrect.

Based on the analysis of the effect of feed rate and cutting speed on the outcome of metal cutting, low feed and high cutting speed are employed to achieve a good surface finish.

Parameter Combination	Likely Result
High Feed, High Speed, High Depth	High Metal Removal Rate
Low Feed, High Speed, Low Depth	Good Surface Finish
Low Speed, High Feed	Potential for Built-up Edge, Rougher Finish

Revision Table: Metal Cutting Parameters and Results

Parameter	Effect on Metal Removal Rate	Effect on Surface Finish
High Feed	Increases MRR	Worse (Rougher)
Low Feed	Decreases MRR	Better (Smoother)
High Speed	Increases MRR	Better (if tool/setup suitable)
Low Speed	Decreases MRR	Worse (potential BUE, tearing)
High Depth of Cut	Increases MRR	Generally Worse (unless finishing pass)
Low Depth of Cut	Decreases MRR	Generally Better (finishing passes)

Additional Information on Metal Cutting

Beyond feed rate and cutting speed, other factors also influence the metal cutting process and the resulting surface quality:

Depth of Cut: This is the amount of material removed in a single pass of the tool. A larger depth of cut increases the metal removal rate but can negatively impact surface finish, especially in roughing cuts. Finishing cuts typically use a small depth of cut.
Tool Geometry: The shape of the cutting tool, including rake angles, clearance angles, and nose radius, significantly affects chip formation, heat generation, and surface finish. A larger nose radius generally improves surface finish but can also lead to vibration.
Tool Material: The material of the cutting tool (e.g., HSS, carbide, ceramic, CBN) determines its hardness, wear resistance, and ability to withstand high cutting temperatures. This is crucial for maintaining a sharp cutting edge and achieving good finish, especially at high speeds.
Cutting Fluid: Cutting fluids (coolants and lubricants) are used to reduce friction, dissipate heat, and flush away chips. Proper use of cutting fluid can improve tool life and surface finish, although some operations are performed dry.
Machine Tool Rigidity: A rigid machine tool minimizes vibrations and deflection during cutting, which is essential for achieving tight tolerances and good surface finish, particularly with high cutting speeds and challenging materials.
Workpiece Material Properties: The hardness, strength, and thermal conductivity of the material being cut influence chip formation, tool wear, and the resulting surface finish. Some materials are inherently more difficult to machine to a high finish.

Understanding how these parameters and factors interact is key to optimizing metal cutting processes for desired outcomes like high metal removal rate or excellent surface finish.