Computer- integrated manufacturing: Manufacturing resources planning

Manufacturing resources planning

Manufacturing resources planning (MRP) was one of the first attempts to use computers to aid manufacturing. It originated in the 1960s under the name 'material require­ments planning', and it used the data processing computers of the day to speed up the main aspects of a manufacturer's pre-production paperwork:

• Generating purchase orders for the supply of raw materials and parts required by the manufacturing process.

• Generating works orders instructing the workshops to carry out manufacturing operations to meet orders.

To generate these outputs, the computer requires infor­mation from the following sources:

• The manufacturing schedule, i.e. the long-term produc­tion plan which specifies what finished products have to be produced by what dates. The finished product demand is expressed in numbers of units, while the due date is expressed as a period number.

• The bill of materials, which describes the relationship between the finished product and the parts and assem­blies that make it up.

• The inventory records, which show the current balances of the parts in stock.

In addition, the computer needs to know the lead times for purchasing raw materials and parts, and also the manufac­turing lead times for any made-in parts. The former will usually be held on the inventory records, while the latter will be held on the bill of materials file.

The computer then goes through a process described as a 'time-phased gross to net explosion'. This rather colourful - if somewhat confusing - expression means that the computer:

1 Compares the products specified in the manufacturing schedule with the information in the bill of materials to work out the quantities of parts required.

2 Uses the lead times to calculate the due dates.

3 From the current stock position calculates what quantities are required to be manufactured or purchased.

In the late 1970s MRP was further developed and renamed 'manufacturing resource planning'. It now became an all­ embracing system covering the entire manufacturing plan­ning process, particularly the financial aspects. This means that important factors could be taken into account which were previously ignored, such as the effect of cash flow on the manufacturing plan. For the first time the manufacturing side of the enterprise was integrated with the financial side, and MRP was able to develop a company-wide business plan.

Since the 1970s, the main advances in this technique have been:

• The reduction in the size and cost of computers, so that quite small manufacturing companies can now apply MRP.

• The adoption of interactive processing using on-line terminals, so that staff in production departments can enter data directly into the system and receive instanta­neous responses. This means that they have an up-to­ date picture instead of one which may be a week or more out of date.

Computer-aided design

A computer-aided design (CAD) package uses the power of the computer to do for drawings what a word process­ing package does for text. After MRP, it was the second main way in which IT revolutionized the production process.

Like WP, CAD enables you to:

• Delete, insert, copy, and move things rapidly and easily around on the computer screen.

• Insert existing material stored on disk - such as drawings of parts of sub-assemblies- into your latest masterpiece, so that you are not constantly re-inventing (or rather re­ drawing) the wheel.

• Format your work, so that it is printed or plotted in the colours, line types and so on, that you require.

CAD packages also offer a number of additional and valuable drawing aids:

• The computer's equivalent of geometric tools such as the compass, the ruler, the protractor, and arcs.

• The computer's equivalent of the nib, that allows you to draw different types of thicknesses of lines.

• A grid, so that the start and end points of any lines that

you draw are locked onto a grid of 'graph paper' points on the screen.

• Zoom facilities, enabling you to expand any part of your drawing on the screen and thereby work more accurately.

• Rotating, inverting, and other facilities enabling you to manipulate parts of your drawing in a very flexible way.

• Scaling facilities, enabling you to type in line lengths, angles, and other dimensions at the keyboard, the computer converting these to the required lines on the screen.

• Dimensioning facilities, which automatically calculate and display on your drawing the lengths of any lines that you may have drawn.

The keyboard is not a suitable drawing tool, and so CAD systems normally use a mouse or similar device.

CAD systems can achieve impressive productivity gains in drawing offices, and it is often claimed that a 400%

increase in output can be achieved. This does, however, assume that the draftsman is familiar with the CAD system and is using its capabilities to the full.

To give an example of the speed at which an item can be drawn, consider a spoked wheel. To draw this, it is only necessary to draw a single spoke and then rotate it through 360 degrees about the centre point of the wheel, duplicating the spoke say every 15 degrees. The hub and the rim can then be rapidly drawn using the CAD package's electronic compass. Any bolts or other parts can be called up from a library held on disk and inserted as required on the drawing.

Even greater productivity can be achieved when the CAD system is linked directly to the CAM system, described below. Then, the CAD data can be used directly to control the settings of automatic machines in the factory.