High Production Machining Centres
Posted : September 29, 2004 at 8:18 pm [IST]
Machining centers are entering the high production facilities of automotive plant that were earlier the domain of only dedicated transfer machines with special purpose unit heads that perform total machining on a part in sequence with automated work handling in between. The traditional transfer lines are highly expensive. In case of failure in market prediction regarding volume, the investment may be disastrous for a company, as the transfer lines demand a minimum volume of production for a number of years for getting the required return on investment that is made much before even the first part is produced. Even in day-to-day operation, transfer line requires a lot of blocked work-in-process inventory. Additionally, any minor breakdown in system stops the assembly and holds the assembly. Traditional transfer line is economical only with a production of 150,000 to 200,000 parts per year and above. Present trends of changing customer demands, competition, fast innovations requiring changes in product and in turn, in parts do not justify the use of dedicated equipment or lines. High-speed machining centers have brought CNC flexibility with transfer line level of speed at much lower investment on equipment and engineering. In an ideal case, the production may be started with just one machining center to carry out all operation and then to buy more machines as production requirements increase.
Present trend is to start with a manufacturing cell- a group of minimum number of the high speed machining centers that complete machining of all features. Breakdown of one of the machining centers does not affect the assembly significantly. The cell may include automatic work handling or the work handling may get added later with higher level of production. Similarly, the hydraulic or vacuum type fixtures may replace standard manual tombstone type fixtures used for initial production. As the production picks up, the cell can be expanded or multiplied. At a level of say, 200000 per annum, dedicated machining line may take over, if found economical by that time. At that stage, the cell/s can either jack up the productivity of the dedicated line, if required or switch over to some other part. If the machining centers in the cell have been of the modular design, it can become part of the transfer line. Auto manufacturers are today learning to break a production requirement of even 400,000parts/year requiring expensive and inflexible transfer lines into four cells of 100,000parts/year each. With high speed machining centers, even machining processes such as surface grinding and honing are being integrated on the same machine.
In a Ford plant, a line with high speed machining centers provides the same output as a flexible transfer line at about two-thirds of the total investment. Ford engineers reduced floor-to-floor production time for precision machining of clutch housings by 60%. The substantial shorter changeover times give a decided advantage to Ford when it comes to model change. Most of these machines are modular and easy to move in and out to and from their locations as and when required. Analysis of the multi-spindle machines used in the conventional lines proves that 85% of machines have a head with 10 or fewer spindles and 96% of hole-diameters are 10 mm or less.
A machine tool manufacturer with experience in automobile industry believes optimal production for high speed machining centers is about 1200-1500 cylinder heads and about 1500-1800 gearbox housing per day in two shifts. In specific applications, many methods are being used to cut down the non-cutting time in production setup to improve upon the cycle time to increase production. One of the methods may be the combination tools engineered for specific features or special tool such as thriller that can perform in a single operation what usually calls for a drill, chamfering tool, and tap or thread mill. While reducing the number of tools, it eliminates tool change between operations together with improved quality in many cases. Thriller tools are extensively used in threading operations on aluminum and cast iron engine and transmission parts.
Machining centres have brought flexibility in manufacturing of mechanical parts at all volumes. And the machine tools are now agile enough and moving towards virtual machining systems.
- Indra
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