Process Planning, Scheduling and Control for One-of-a-Kind Production
5.3 Process Planning
5.3.2 Short-term Process Planning
Short-term planning is required to meet the promised or expected delivery date of the customer. In conventional mass-production systems this means producing one or x thousands of the ordered products. Based on the standard (average) production rate, this will take a certain amount of time. However, when planning for mass customisation and particularly one-of-a-kind products, the standard (or average) rate does not work. Depending on the specification (size, shape and options), each product will take a different amount of time. This complicates the planning process, resource availability, material supply and sub-component delivery. Consider the following shop-floor layout from Gienow for one of their window products.
Figure 5.1. Shop-floor layout for a production line and work centres
Delivery-date Confirmation
As orders are entered into the system, the delivery date may be confirmed or may be tentative. If tentative, a process has to be followed to confirm the delivery date so that the planners know when to schedule the order. At Gienow, customers with tentative orders are phoned several days prior to the delivery date and asked to confirm. If the order is not confirmed, it is then tentatively rescheduled for a later delivery date.
Since Gienow uses a fleet of trucks to deliver their (large) products, there is a trucking schedule in place. This means as an order is processed; only dates available to the location of the delivery address are seen as potential delivery dates by the schedulers. During this process and as the various delivery dates come closer to
“today”, the scheduler’s initial task is to load trucks to their potential capacity.
When the time comes to confirm the schedule for the next day for any truck that is not full to near capacity, the scheduler will attempt to fill the truck by contacting customers that are known to have flexibility in their receiving dates.
Line Balancing
Once the initial schedule based on the trucking sequence has been determined, the loading of production lines is reviewed. If lines are under- or over-scheduled to their available capacity, the scheduler’s task is to balance the load on each line. The balancing is achieved by either increasing or decreasing the available capacity by redistributing employee resources or by rescheduling orders to increase or decrease the load on the lines.
As stated earlier, it is essential to have the BOM and routing information for each customised product because this will enable the system to accumulate the standard time for each product on the production lines (by work centre if necessary).
This also means that as the scheduler reschedules orders he/she can see the immediate effect on the loading of lines required to make all of the different products on the order. At Gienow, reports and graphical views are available to help the schedulers in this task.
Figure 5.2 shows an analysis of all orders according to their current scheduled date. The analysis processes detailed orders and shows how many units are to be produced by each production line daily. The analysis also shows how much (standard) time will be required to produce the products. Therefore, the planners are able to see how the production lines are loaded and to perform the balancing operation.
The graph in Figure 5.3 shows how each production line is performing and what the size of the forward load is by day. This helps the planner to see the possible orders to move. The program also has an algorithm that calculates the lead time of each production line.
Lead time is calculated based on the assumption that an order will enter the system on the next production day after being posted in the order-entry program and delivered to site that day plus the maximum lead time days. Under normal circumstances, the lead time is 10 days or 2 weeks. Each week has 5 production days, i.e. Monday to Friday.
Figure 5.2. Forward load by tentative hours
Figure 5.3. Production line graph
Under circumstances when a production line has more than 10 days worth of work, the lead time will be extended. This means that within the lead time there are several days that have more orders than capacity (excess orders). The lead time is calculated under the assumption that all orders in the system that lie within the lead time will be made before any new orders are processed.
The next available production day with available time is found by calculating the excess order time and allocating that time to production days that have free capacity starting at the first planning day. This is done because the next two or three production days are already fully allocated and fixed.
Once the next production day with free time is found, if this is less than 10 days from today, then the lead time is set to 10 days. If this is greater than 10, then the lead time is set to the day after the next available day, allowing for production and shipping to complete their tasks.
The performance of the lead time calculation can be affected by two parameters:
1. A portion of the maximum capacity, expressed as a percentage of maximum capacity or simply recorded as capacity %, can be used to adjust the effective capacity of the production line. If a board is being blanked or reduced within the 10-day lead time without making the day a non- production day in the calendar, then the capacity % can be changed. It can also be reduced to less than 100% in cases where allowance is being made for rush orders or service items.
2. Likewise, the efficiency % gives an indication of the amount of actual hours required to complete a specific amount of standard hours. For example, if the current efficiency of production line V10 is 120%, then 100 standard hours of work should be completed in 100/1.2=83.3 h.
This lead time (for each production line and therefore each product type) is transferred to the knowledge base for the order-entry system to use online.
Sequencing
Each operation will be different and the method of sequencing will, as a result, be different. However, for one-piece flow of customised items it is necessary to have a sequence for each product on each line. In Gienow’s case, the sequence is initially driven by the trucking plan. This plan dictates which truck is going to leave at what time of day. The location that the truck is going to will determine which orders it will carry. Therefore, the major sequence of orders is predetermined by this trucking sequence.
Once this is established, then the orders are first sorted into a sequence that starts with the trucking sequence and then followed by the order number (this ensures that the earliest orders get made first) and then the height of the product (in descending sequence as this helps with production efficiency1and storage on shipping carts – the tallest product is placed at the back). The orders are then organised according to their details and the different products allocated to their appropriate production lines.
This then generates the sequence for each production line for this schedule.
Distributed Manufacturing – Outsourced Components
Part of the planning process involves the customised components that are outsourced. This outsourcing can be to a supplier or member company, either constitutes a part of the supply chain. Depending on the suppliers’ capability to match the JIT (just-in-time) process, different methods of planning have to be used.
1 Generally, products with the longest production time are scheduled first. Also, if a process using a CNC machine can be gradually changed instead of randomly changed, then the process is more efficient.
It would be ideal if all suppliers had the capability to deliver the sub-component just-in-time, but for SMEs (small and medium-sized enterprises) this is not always practical or possible owning to their limited buying power.
Hence, if the supplier does not have JIT capability, the sub-component has to be ordered according to the lead time. The longer the lead time, the less flexible the main production becomes. In a situation where the customer changes the order, and the sub-component has been ordered, there can be additional costs. In the case where the customer and/or scheduler reschedules and delays the production of the order, the sub-component will be delivered early and this can also lead to additional costs, potential damage, and reordering. All this leads to an increase in the cost of the customised product, which is contrary to the goal of mass customisation.
If the supplier does have JIT capability, then when the schedule for the main plant is confirmed, the customised sub-components for each supplier are extracted from the system and communicated to the suppliers using some form of electronic data interchange (EDI). At Gienow this function includes transmitting a computer file with the specification of customised components and details of what to print on a bar-coded label. These labels are then attached by the supplier to the product and delivered to the main plant within a specified period of time (currently less than 24 hours).
Irrespective of the supplier’s capability, all sub-components are controlled by the receiving department. Each day, the receiving department is provided with a list of all sub-components required for that day’s schedule. The receiving department’s task is to deliver the sub-components to appropriate production lines. The bar-coded labels have information that identifies the production lines as well as the specific product. This product identification number is a compound number that also identifies the date of production and the sequence number. In the case study this is defined as “MMDD-nnnn”, where MM is the month, DD is the day and nnnn is the production sequence number.
One of the computers available at Gienow shows all production lines and which sequence number each line has just completed. If necessary the receiving department can use this computer display to determine when the sub-components can be delivered to a line and whether the line has enough space to accept the delivery. If so, the receiving department can deliver the lines’ components at the start of the schedule. For large quantities of sub-components, the receiving department is given a function similar to the one described next in internal components.
Internal Components
There are a number of issues with the production of internally made sub-components and their delivery to various production lines. First, there is a lead time to produce these components. In Gienow’s case, the quantity is so large that it takes the full schedule of time to make them. Therefore, these sub-components have to be made and delivered in the sequence that the main product is being made. Since all sub- components can not be made at the same time, there will be multiple deliveries throughout the schedule. Gienow uses the cart method to deliver sub-components to the production lines. Therefore, these carts have to be identified and the items within them also identified and in the same sequence as the main production lines.
To achieve an even distribution of sub-components to multiple production lines in a JIT mode, it was necessary to develop a specific algorithm. At Gienow, there are two algorithms, one for a flow process and the other for a batch process.
Flow Process
• Determine lines involved, e.g. V10, V20, V30 and V40
• Determine total number of items to be made on each line
• Determine size of delivery batch for each line
• Determine the sequence of delivery to lines, e.g. V40, V20, V10 and V30
• Create sequence of sub-assembly production Table 5.2. Flow method – batch size
Production line # of items Size of batch Sequence
V10 50 5 3 V20 10 2 2 V30 50 5 4 V40 5 1 1
The sample data above would create the balanced sequence given in Table 5.3.
Table 5.3. Flow method – production sequence
V40 items 1 V20 items 2
V20 items 2 V10 items 5
V10 items 5 V30 items 5
V30 items 5 V40 items 1
V40 items 1 …………
V20 and V40 are the slower lines. By delivering the sub-assemblies to these lines early in the production cycle, ensures that waiting does not occur, while the other two lines maintain a steady production rate.
Batch Process
• Determine lines involved, e.g. V10, V20, V30 and V40
• Determine total number of items to be made on each line
• Determine size of cart delivered to each line
• Determine number of carts required for each line
• Determine the number of deliveries
• Determine how many carts are produced for each delivery
• Create sequence of sub-assembly production
The model allows for closer synchronisation of the sub-lines with main lines.
This closeness is very dependent on the product and the manufacturing processes involved. In the case of Gienow, some of the sub-components require a certain
amount of “curing” or “waiting” time before they can be used in the fabrication process. This creates an automatic buffer that means that synchronisation does not have to match exactly.
If closer synchronisation is required, then details of the main production-line schedules are available. This is achieved by using the actual scheduled production numbers for each line instead of assuming that there is an even production rate. This again is caused by each customised item requiring a different amount of time to make. These varying production rates are discussed in Section 5.4. An example of (batch) requirements and calculated production sequence for a sub-component is show in Tables 5.4 and 5.5.
Table 5.4. Batch method – requirements
Production line Work centre # of items # of carts
A00 A05 40 2
V10 V13 587 24
V10 V15 687 28
V20 V23 75 3
V20 V25 286 12
V30 V35 100 4
Table 5.5. Batch method – production sequence
Production line A00 V10 V10 V20 V20 V30
Deliver to A05 V13 V15 V23 V25 V35 Total
1 25 50 50 25 25 25 200
2 – 75 100 – 25 – 200
3 – 75 75 – 25 25 200
4 – 75 75 25 25 – 200
5 15 75 60 – 25 25 200
6 – 75 100 – 50 25 200
7 – 50 75 – 50 – 200
8 – 50 75 25 50 – 200
9 – 62 77 – 36 – 175
Total 40 587 687 75 286 100 1775
The algorithm is a modified version of the minimum-cost-assignment algorithm.
It attempts to provide a smooth delivery of sub-components to each production line that matches its production rate. In Section 5.5, Adaptive Planning and Control, the issue of main lines getting out of synchronisation with each other is discussed. This happens when a production line either gets ahead of schedule or falls behind schedule due to machine downtime or lack of manpower or material resources, which happens in real-life situations.