27. Production Planning and Control
27.1 Overview
• A design must be converted to a process plan before it may be produced.
• But, if we have thousands of process plans, and hundreds of customer orders, with dozens of parts in each, which machines do we use when to make the products? What parts do we need?
• Traditionally jobs have been scheduled on a first come, first served basis. This resulted in a lineup of various jobs waiting to be done at each work center.
• When jobs are not scheduled efficiently, we often will get jobs sitting half completed, while we wait for simple parts to be processed. This costs money, wastes time, takes up floor space, makes the customer unhappy, etc.
• Eventually computers were used to figure out how to schedule jobs so that parts were made before they were needed, and so that work was done on time.
• As computers were used more it also became obvious that strict schedules were a nice idea, but they don’t work. A schedule is only valid until the first breakdown.
• Newer control programs called Production Planning and Control (PPC) systems were used to generate schedules, and fix problems that came up.
• Most systems, manual, and automatic either push, or pull the work through the factory. If the work is pushed, then customer orders tend to drive the production. If the work is pulled, the factory often tries to satisfy some continuous demand, and when things are about to run out, more is produced.
• Regardless of which system is used, Scheduling is not exact, and never optimal, but you can get a near optimal schedule with the right tools and methods.
• Some of the traditional Production, Planning and Control subject include,
1. Forecasting - Estimating the production demands using a horizon of a few month to a few years for long range planning.
2. Production Planning - Matching needed production to available resources.
• Note: This is more of a CIM topic.
27.2 Scheduling
• We often know well in advance what has to be produced
• We can use computer programs to come up with a ‘near perfect’ schedule for all jobs, ahead of time.
• These methods at the present time are not well enough developed to handle sudden disruptions on the shop floor (See next section on Shop Floor Control).
• Schedules are often made up weekly
*************** ADD DETAILS FOR MRP I and MRP II
27.2.1 Material Requirements Planning (MRP)
• This is one very popular approach to planning
• Uses Master Production Schedules to determine how much of each product should be produced within given periods. Master Production Schedules are based on customer, or projected demand.
• The elements used by MRP to plan are,
- Master Production Plan (Schedule)
- On-hand inventories
- Bill of Materials
- Current of Purchased and Manufactured Orders
- Rules for each part produced (including WIP)
• The rules about each step in production include,
- Lead time
- Order quantity per final part
- Scrap rate
- Buffer stock quantity
- etc.
• MRP then tries to determine quantities required using the data input from the users, and a set of rules, such as,
- Fixed Order Quantity - Product are produced as required using a prespecified lot size.
- Economic Order Quantity - The cost of carrying inventory is weighed off against the cost of setup for one production run.
- Lot for lot - Lots are produced as required, any batch size.
- Fixed-period Order Quantity - Produce parts to cover more than a single order.
• Lot sizes required are subtracted from available stocks.
• The required production quantities are used to order from suppliers, etc, while considering lead times, and delays.
• You should note that this approach is concerned more with inventory minimization than with utilization of machines.
• While this system can lead to easy production scheduling, it is susceptible to errors in BOMs, routings, etc.
• Advantages,
- improved Customer Service
- better Scheduling
- reduced inventory
- reduced component shortages
- reduced manufacturing costs
- reduced lead times
- higher production quality
- less scrap, and rework
- higher morale in production
- improved communication
- improved plant efficiency
- improved competitive position
- improved coordination of marketing and finance
• MRP II (Manufacturing Resources Planning) - A closed-loop MRP system that, at a minimum, includes detailed capacity analysis (see next section). Some MRP II systems include the business plan in the closed-loop system.
27.2.2 Capacity Planning
• While MRP is concerned with determining how much should be produced, it is not concerned with how to produce it.
• Capacity planners attempt to determine how to assign jobs to machines, people, etc.
• Information used by capacity planners includes,
- Planned orders (from MRP)
- Orders in process (order status)
- Routings, including setup and run time (from process plans)
- Available facilities
- Workforce availability
- Subcontracting potential
• There are some strategies used by the Capacity Planner to Assign jobs to machines,
- Splitting of lots (batches) across identical machines
- Splitting of lots to expedite a smaller quantity
- Sequencing of lots to minimize setup times
- Alternative routings that require different resources
- Loading a facility by weight, volume, etc. (eg. heat treating)
• After jobs have been assigned to machines, the capacity of the machines must be considered.
27.3 Shop Floor Control
• No factory is perfect, and a schedule can become invalid at any time because of,
- Machine breakdown
- Sudden material shortage
- Workforce vacancy
- Tool breakage
- etc.
• What to do about it,
- Wait and See
- Try to find alternative production plans/parts
- Ask engineering for replan
- Bump other jobs
- ?????
27.3.1 Shop Floor Scheduling - Priority Scheduling
• Instead of scheduling before production (MRP and Capacity planning), a manufacturer may opt to do scheduling on the fly.
• Some of these methods include,
- Earliest operation due date - soonest date. This gives time until due, but ignores processing time.
- Order Slack - soonest date minus processing time. This gives the amount of time to play with.
- Shortest operation first - Do the quickest jobs first. This just clears out WIP faster.
27.3.2 Shop Floor Monitoring
• It is important to know what is happening on the factory floor.
• To do this we must pay attention to obvious problems like machine operation, and hidden problems such as quality, and production quantities.
• Inspection covers a number of areas,
- Inspection of raw materials
- Inspection of manufactured product
- preprocess
- in-process
- post process
- Inspection of production process parameters
- tools
- fixtures
- production machinery
- Verification/calibration
- inspection fixtures
- Inspection gauges
- Inspection machinery