Schedule design decisions provide information on how much to produce and when to produce. Production quantity decisions are referred to as lot size decisions. Determining when to produce is referred to as production scheduling. It is also important to know how long the production will continue, which can be determined from market forecasts.

Schedule design decisions impact the following:

Schedule planners need to interface continuously with marketing and sales personnel and with the customers in order to provide the most accurate information to facilities design planners.

Process design determines the specific equipment types required to produce the product. Schedule design determines the number of each equipment type needed to meet the production schedule. Specification of process requirements occurs in three phases:

Phase 1: Determines the quantity of components that must be produced, including scrap allowance, in order to meet the market estimate.

Phase 2: Determines the equipment requirements for each operation.

Phase 3: Determines the overall equipment requirements by combining the operation requirements established during Phase 2.

Scrap Estimates:

The market estimate specifies the annual volume to be produced for each product. To produce the required amount of product, the following formula must be kept in mind:

R = N + S

Where

R = Required amount of product

N = Market estimate

S = Scrap estimate

The production capacity must be planned for the production of scrap. Otherwise, the market estimate will not be met. Scrap is the material waste generated in the manufacturing process due to geometric or quality considerations. For example, scrap due to geometry is generated when a rectangular plate is used to create a circular component. Scrap due to bad quality is generated by errors in machining or assembly operations. An estimate of percentage of scrap that will be expected from each operation must be made. The estimate can be based on historical data or estimated from similar operations.

Let P_j represent the percentage of scrap produced on the j^th operation, T_j the desired output of nondefective product from operation j and I_j the production input to operation j.

T _j = I _j - P _j I _j

Or

T _j = I _j (1- P _j )

The input requirement for a specific operation is then:

I _j= T _j / (1- P _j )

If a part is produced in n number of operations, the expected number of units to start into production would be:

I ₁= T _n/ (1- P ₁ ) (1- P ₂ )....... (1- P _n )

Where T _n is the market estimate.

A product has a market estimate of 50,000 components and requires three processing steps (turning, milling, drilling) having scrap estimates of P₁ = 4 %, P₂ = 2 % and P₃ =3%. Calculate the number of components needed to start the production.

The market estimate is equal to the output required from the last process. In this case the last process is drilling, which is process # 3. Therefore, the input to the last process should be:

Between operations 2 and 3, the output of operation 2 equals to the input to operation 3. Therefore, the number of components to start operation 2 should be:

The output of operation 1 is equal to the input for operation 2. The number of components to start the first operation should be:

This means that we need 54,790 components to start the production of parts which will generate 50,000 good parts at the end of the last operation. In other words, by supplying 54,790 components at the beginning of the production, the market estimate of 50,000 will be met at the end of the production.

The calculations can be combined by using the following formula:

Summary of production requirements:

The quantity of equipment required for an operation is referred to as the equipment fraction.

The following equation can be used to determine the equipment fraction:

where,

F = Number of machines required per shift

S = Standard time (minutes) per unit produced

Q = Number of units to be produced per shift

E = Actual performance of the machine(% of standard time)

H = Amount of time (minutes) available per machine

R = Reliability of the machine (% up time)

Example:

Standard machining time for a part on a milling machine is 4.5 minutes/part. In an 8-hour shift 400 parts are to be produced. During the machining operation, parts are made at a rate which is equal to 90 % of the standard machining time (E=90%). The machine will be operational 85 % of the time (R=85%). How many milling machines are required?

S = Standard machining time = 4.5 minutes

Q = Number of units to be produced per shift = 400 parts

H = Amount of time available per machine = 8 hours = 480 minutes

E = Actual performance of each machine = 0.90

R = Reliability of each machine = 0.85

The determination of total equipment requirement is not a straightforward determination procedure. The following example illustrates the total equipment specification:

Assume that three different types of operations are performed on a machine during a shift. The machine fractions required for each operation is given in Table 1.

Table 1. Total equipment requirement

By looking at this table we can see that a minimum of four and a maximum of six machines are required. However, with no other information, a decision on how many machines to be purchased cannot be made. The following additional information is to be provided, before the total number of machines can be determined: