Process Planning and Material Management Principles

1.1 Functions of Process Planning

MS

• Determining the most economical manufacturing process to be used.
• Establishing the sequence of operations for each component.
• Deciding which parts to manufacture internally and which to purchase from external suppliers.
• Determining the blank size of raw materials for processes.
• Preparing a bill of materials for all product components to initiate procurement.
• Determining orders for necessary materials and machine tools for production.
• Determining the tooling and gauges required for manufacturing parts.
• Determining necessary inspection equipment and inspection stages.
• Establishing standards for work performance.
• Determining the type of labor required for the work.
• Determining necessary internal changes to simplify manufacturing or reduce costs.

13.3 Tolerance Stack

When individual dimensions are combined such that an unacceptable variation occurs in the overall dimensional relationship, this situation is called Tolerance Stack. When excessive allowable tolerances are combined, it is called Limit Stack. Figure 24 shows examples of tolerance and limit stacks. Cubes are machined to 1.000 ± 0.005 inches. If two cubes are stacked, it is desired that their combined height does not exceed 2.000 ± 0.005 inches. Figure 24(a) shows the limit stack situation, where the maximum allowable dimension on the cubes is 2.010 inches.

2. Make or Buy Decision

The decision to make or buy an item should be taken by top management before actual requirements arise. Factors influencing the buy decision include:

1. Volume Required: When only a few units are needed, buying from an outside supplier is more economical. This is especially important for large-scale manufacturers where interrupting a large production setup for only a few units is inefficient.
2. Economic Profit: The decision to buy depends largely on comparing known costs with estimated costs. Costs charged by the vendor are known costs, while manufacturing costs are unknown. In most situations, known costs are more reliable than estimated costs.
3. Life of Product: When a part is used for a short time, only a small lot size is required. It is better to buy such parts from outside rather than manufacturing them on a large scale.
4. Quality Consideration: Compare the quality of the purchased item with the quality the company can achieve. If there is no possibility of wide variation in quality, quality consideration is not very important. If there is significant variation, quality must be considered. Production facilities should also be kept in mind when making the decision.
5. Degree of Production Standardization: Highly standardized products (screws, nuts, washers, switches, valves, etc.) should be purchased, as it is more economical to buy them even if the company has the necessary equipment and labor.

Dimensional Analysis

The purpose of analyzing the primary part print is to determine what the product designer requires for the final product. In the initial stages of analysis, the base process and main process are determined from the print. Functionally important areas and critical areas of the manufacturing process are both identified. Then, the nature of the work to be performed is studied through the relationship of the various surfaces of the part. This is necessary so that the process engineer has complete knowledge about the job.

Today, process engineers face more technical problems in manufacturing than before. This is due to the demand for high quality and reliability in large-scale production, and the process engineer must also keep manufacturing costs reasonable. Therefore, the process engineer must have knowledge of the dimensional characteristics of the product in addition to the part print, so that high-quality products can be manufactured.

Demand Behavior

12.1 Types of Dimensions

A definitive geometric shape is required so that it can be measured. Before re-manufacturing a part, its surfaces are defined. A flat surface is the best surface for measurement because other surface forms can be easily measured from this flat surface. Cylindrical surfaces are defined by a center line. Since the center line shown on the part print is an imaginary line on the workpiece, locating a hole is difficult by measuring between flat surfaces. Since the level of difficulty in measuring basic geometric shapes can vary, a simple dimensional system should be used whenever possible. For practical purposes, the relationship of surfaces to each other should be rectangular or angular. Angular dimensions should be avoided as much as possible. Figure 7 shows the dimensioning of a part surface using rectangular coordinates and angles.

Angular relationships are generally measured by coordinates and abscissa. There are several reasons for this:

1. Because angles cannot be measured with precision in the form of linear dimensions. Therefore, rectangular tolerance should be used to ensure better control over dimensional tolerance.
2. The possibility of transformation error is always present. Since most angular dimensions are converted into coordinates, the process engineer should use the coordinate method for dimensioning to avoid unnecessary calculations and potential errors.

Demand Behavior

Demand sometimes behaves in an irregular manner and at other times follows a trend or repetitive pattern. Demand behavior is mainly of three types:

1. Trend
2. Cycle
3. Seasonal pattern

• Trend: The gradual and long-term fluctuation in demand is called a trend. Trends are often the starting points for developing forecasts. Figure 2(a) shows the trend of demand. It shows irregular fluctuations in demand. These irregular changes cannot be predicted.
• Cycle: A cycle is a fluctuation in demand that repeats itself over a long period (more than one year). Figure 2(b) shows the demand cycle.
• Seasonal Pattern: A seasonal pattern is an oscillating movement in demand that is periodic and repetitive. This pattern is related to the season. For example, the increased demand for warm clothing in winter. The seasonal pattern is shown in Figure 2(c). Figure 2(d) shows the combination of the seasonal pattern with the trend pattern.

When demand behavior does not follow any pattern, it is called irregular demand behavior.

Objectives of Material Management

The main objectives of material management are:

1. Minimizing the cost of materials.
2. Ensuring a uniform flow of materials for production.
3. Ensuring the right quality at the right price.
4. Establishing good relationships with suppliers.
5. Receiving materials in a safe and good condition.
6. Issuing materials upon receipt from the appropriate authority.

4.2 Statistical Methods

These methods are very complex and are used for long-term forecasting. In this method, demand is forecasted based on historical and cross-sectional data. Historical data refers to past data obtained from various sources, while cross-sectional data is collected through market surveys.

1. Time Series Analysis: In this method, the trend of future consumption is determined by analyzing historical consumption data, and this method is the most important technique for business forecasting. In this, the four components affecting the time series, such as (i) long-term trend, (ii) seasonal variation, (iii) cyclical variation, and (iv) random variation, are separated.

• (i) Long-term Trend (T): This refers to long-term changes resulting from a general trend.
• (ii) Seasonal Variation (S): This refers to short-term seasonal patterns or changes in social habits.
• (iii) Cyclical Variation (C): This refers to changes occurring in the industry during business fluctuations.

Break-Even Chart (BEC)

The concept of break-even can be well analyzed by the break-even chart. The break-even chart is a graphical representation of sales and costs at various levels of output. This chart is used to study the change in profit/loss at various levels of sales, as well as to study the Cost-Profit Volume Relationship.

2.1 Uses of BEC

BEC is used in the analysis of the following situations:

1. Analyzing profit and expenses.
2. Knowing the cost-volume-price relationship.
3. Determining the Break-Even Point (BEP).
4. Knowing the effect of changes in sales on production cost and profit.
5. Analyzing problems related to manufacturing economics.
6. Studying profit volume and margin of safety.
7. Determining the optimum level of production.
8. Comparing actual sales and profit with the budget.

Computerized Assembly Line Balancing

If the assignment of tasks at a workstation increases, it becomes very difficult to balance the line using heuristic methods. In this situation, line balancing is done with the help of computer software. Currently, the following balancing programs are being used in computerized assembly line balancing:

1. ASYBL (Assembly Balanced Line Configuration)
2. COMSOAL (Computerized method for sequence of operations in assembly line)

These software programs can balance more than 100 tasks in a line.

Importance of Material Management

Material management is an important function of an organization that covers the purchase, storage, replenishment of stock, and maintenance of various types of materials for the production process. Thus, material management is considered important for the transformation process in a production process. It includes making necessary plans and programming after purchasing the required quality and specified material and capital goods at the appropriate price and time. It is related to updated information, store and stock control, inspection of material received in the enterprise, transport, and material handling. A survey conducted by the Directorate of Industrial Statistics during 1954-57 revealed that the average material cost is 64% of the sales price. Thus, 36% cost is incurred for wages and salaries, overheads, and profit. These figures show the importance of material management. The cost of carrying material inventory should be 20% of the material cost.

4.1 Objectives of Purchasing

The main objective of the purchasing function is to ensure the continuity of supply of raw materials, sub-contracted items, and spare parts, and to reduce the final cost of finished goods. The main objectives of the purchasing department are:

1. Providing materials, suppliers, and equipment at the minimum possible cost.
2. Ensuring a continuous flow of production.
3. Developing alternative sources of supply.
4. Increasing asset turnover.
5. Establishing good relationships with suppliers.
6. Training personnel.

Vendor Development

Vendor development refers to the development of products/product parts and unfinished products to make the business better in aspects like finance, manufacturing, growth, relationship, and supply chain. Before taking the decision for vendor development, the following reasons should be considered:

1. Availability of material in the market.
2. Quantity to be purchased.
3. Pattern of purchasing.
4. Time availability for obtaining materials.
5. Economic feasibility of the material.
6. Capacity of suppliers.
7. Location of suppliers.
8. Creditability of the supplier.

6.1 Analysis

Vendor analysis is the evaluation of a vendor's strengths and weaknesses, which is related to their ability to meet the needs of the person or institution demanding their goods and services. When a company needs to select a new vendor or review existing vendors, the supplier is evaluated by the purchasing department through weightage determined by measuring their performance. These criteria are:

1. Cost Price
2. Quality
3. Delivery

While evaluating the quotations of suppliers, the following factors should be considered:

• (i) Cost: Price, transport cost, installation cost, tooling and handling cost, sales tax, and product duty, cash discount, etc., are included in the cost factor.
• (ii) Delivery: Routing and FOB are important in determining the point at which goods go from the vendor to the buyer.