Abstract:

Machine shop estimation for milling operations involves calculating material costs, machining time, and other expenses to determine the overall cost of a project. This includes factors like cutting speed, feed rate, depth of cut, and tool life.

Here's a breakdown of key aspects:

1. Material Cost:

Calculate the volume:
Determine the volume of the raw material needed based on the part's dimensions.
Determine material cost per unit:
Obtain the cost per unit (e.g., per pound, per cubic inch) of the specific material from your supplier.
Calculate total material cost:
Multiply the material volume by the cost per unit to find the total material cost.
Factor in scrap:
Account for material waste during machining, adding a percentage to the material cost to cover scrap.

2. Machining Time:

Cutting Speed (V): The distance traveled by the tool along the workpiece in one minute.
Feed Rate (F): The distance traveled by the tool along the workpiece in one revolution of the workpiece.
Depth of Cut (d): The thickness of metal removed in one cut.
Calculate Machining Time: Use formulas and parameters specific to the milling operation to calculate the machining time for each feature or part.
Consider tool changes: Factor in the time required for tool changes, which can vary based on the complexity of the operation.
Setup Time: Include the time required to set up the machine and prepare the workpiece.

3. Other Costs:

Labor Costs: Calculate the cost of labor required for the milling operation, including operator and setup time.
Machine Costs: Account for the cost of using the milling machine, including depreciation, maintenance, and energy consumption.
Tool Costs: Factor in the cost of cutting tools, considering their lifespan and replacement costs.
Post-Processing: Include any costs associated with finishing, cleaning, or inspection after milling.
Overhead Costs: Add overhead costs like rent, utilities, and administrative expenses.
Profit Margin: Add a profit margin to cover business expenses and desired profitability.

4. Estimation Tools:

Software: Machine shop estimation software can streamline the process, providing tools for calculating material costs, machining time, and other expenses.
Spreadsheets: Use spreadsheets to organize data and perform calculations manually.
Charts and Tables: Refer to charts and tables for standard milling parameters and machining time estimates.

5. Factors Affecting Estimation Accuracy:

Complexity of the part:
Complex parts require more precise calculations and may increase machining time.
Material type:
Different materials require different cutting speeds and feed rates.
Tool selection:
The type and condition of cutting tools can significantly impact machining time and costs.
Machine capacity:
The capabilities of the milling machine, including its speed and power, affect machining time.
Operator experience:
Experienced operators can often complete milling operations more efficiently, reducing machining time.

Here is a complete Chapter 6: Machine Shop Estimation – Milling Operations, including theory, estimation formulas, solved examples, exercises, and questions.

Chapter 6: Machine Shop Estimation – Milling Operations

6.1 Introduction

Milling is a machining process where a rotating multi-point cutting tool is used to remove material from a workpiece. It is one of the most common and versatile manufacturing processes in the machine shop. Milling operations are carried out on machines called milling machines, and they can perform a wide variety of operations such as face milling, slot milling, side milling, and contouring.

6.2 Types of Milling Operations

Face Milling – Cutting is done with the end of the milling cutter.
Peripheral Milling – Cutting occurs along the periphery of the cutter.
Slot Milling – Used to produce slots using a single or multiple cutters.
Side and Face Milling – For milling both the face and the side simultaneously.
Straddle Milling – Two cutters are used to machine two parallel surfaces.
Gang Milling – A combination of more than one cutter mounted on the arbor to machine several surfaces simultaneously.

6.3 Milling Parameters

To perform estimation for milling operations, the following parameters are essential:

Cutting Speed (V): Surface speed of the cutter, in m/min.
Feed (f): Distance the work advances per tooth, in mm/tooth.
Feed Rate (F): Product of feed per tooth, number of teeth, and spindle speed.
Depth of Cut (d): Thickness of material removed in one pass, in mm.
Width of Cut (w): Width of cutter or milled surface.
Spindle Speed (N): Revolutions per minute of the cutter.
Time of Cutting (Tc): Time taken to machine a surface.

6.4 Milling Time Estimation Formula

To estimate machining time for a milling operation, we use the following general formula:

Spindle Speed (N):

Where:

= Cutting speed in m/min
= Diameter of cutter in mm

Feed Rate (F):

Where:

= Feed per tooth in mm/tooth
= Number of teeth on cutter
= Spindle speed in rpm

Machining Time (Tc):

Where:

= Length of cut in mm
= Feed rate in mm/min

6.5 Solved Examples

Example 1: Face Milling

A face milling operation is to be performed on a 250 mm long mild steel plate using a 100 mm diameter cutter with 8 teeth. The cutting speed is 60 m/min, feed per tooth is 0.1 mm/tooth. Calculate the time required for machining.

Solution:

Given:

m/min
mm
mm/tooth
teeth
mm

Step 1: Calculate Spindle Speed :

Step 2: Calculate Feed Rate :

Step 3: Calculate Machining Time :

Answer: The time required for machining is approximately 1.64 minutes.

Example 2: Slot Milling

A slot of 200 mm is to be milled using a cutter of diameter 50 mm having 6 teeth. The cutting speed is 70 m/min, and the feed per tooth is 0.12 mm. Estimate the machining time.

Solution:

Given:

m/min
mm
mm/tooth
mm

Step 1: Spindle Speed (N):

Step 2: Feed Rate (F):

Step 3: Machining Time (Tc):

Answer: The estimated time is 0.62 minutes.

6.6 Factors Affecting Milling Time

Material of Workpiece and Cutter
Cutting Parameters (Speed, Feed, Depth of Cut)
Cutter Condition (Sharpness, Wear)
Machine Condition and Power
Coolant Usage

6.7 Practical Tips for Estimation

Always consider overtravel and approach distance in cut length.
Select appropriate cutting parameters based on material and tool specs.
Account for setup and tool changing time separately.
Use manufacturer's data for tool performance and feed/speed recommendations.

6.8 Exercises

Exercise 1:

A 150 mm slot is to be milled using a 60 mm diameter cutter with 5 teeth. If the cutting speed is 65 m/min and the feed per tooth is 0.15 mm, calculate the machining time.

Exercise 2:

Determine the time to face mill a surface of 180 mm using a 75 mm cutter with 7 teeth. Cutting speed is 50 m/min and feed per tooth is 0.1 mm.

Exercise 3:

Calculate feed rate and machining time for a 300 mm long face milling operation with a 125 mm diameter cutter, 10 teeth, at 80 m/min cutting speed and 0.2 mm/tooth feed.

6.9 Short Answer Questions

What is face milling and how does it differ from peripheral milling?
Define feed rate in milling operations.
Why is overtravel considered in length estimation?
List the factors affecting milling machining time.
Write the formula to calculate spindle speed and explain each term.

6.10 Long Answer Questions

Explain the procedure for estimating milling operation time with an example.
Differentiate between various types of milling operations with sketches.
Describe how cutting speed and feed affect the surface finish and tool life.
Discuss the practical considerations while estimating time in a machine shop.

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