The Power of the Press
Leipzig, Germany – If you want to see the most modern metal pressing operation in the world you have to travel to Leipzig, in the state of Sachsen, and head northeast of the city center to the BMW Werk Leipzig (take the number 16 tram, Messe-Gelaende, from the Leipzig Hbf and then the 86 bus, it will get you right to the front door). However you get there, the pressing operation at Leipzig is an eye-opener.
Car bodies have been constructed using various methods over the last 100 or so years. Initially metal panels were hand formed (with wooden bucks as templates for shape) using planishing hammers and English wheels and then attached to wooden frames. Morgan still builds some of their sports cars using that method. The problem with that process is it’s hugely time consuming. Not just the time it takes to beat the panels, but also the time required for finishing the body shell. Thanks to the wooden frame the metal skin is stretched over, baking the paint is a non-starter, which requires additional time (and exposure to potential contamination) before the paint cures.
The sheet metal press has been around since about 1860 but it wasn’t used initially in the production of automobile bodies. However, the efficiency and solidity of a sealed metal automobile body was hard to overlook. That was one of the reasons metal pressing for automobile bodies was undertaken by Hale & Kilburn of Phildelphia for the 1912 Huppmobile. The general manager of Hale & Kilburn was Edward G. Budd and he went on to found the eponymous company that revolutionized the use of pressed steel panels for automotive bodies.
At one time, before WW II, BMW was importing metal-over-wood bodies from Sindelfingen (yes, Daimler-Benz) but eventually started getting bodies from the German Budd company, Ambi Budd. These were all metal bodies built from welded pressed panels. The blank steel was pressed between a positive and negative die for a particular body panel and then stitched together through welding into a three dimensional structure.
The key to effectively running a mechanical pressing process is to use the presses tremendous force as delicately as possible. The blank panel is placed into the dies precisely and subjected to the force, expressed as tonnage, of the press. Get the force wrong, at one extreme, and you crack or split the panels, or, on the other extreme, incompletely form them. And some panels require multiple pressing operations to end up in a finished state. All along the way there is a possibility of
dimensional errors being introduced. And contamination is a concern. If you want to ruin a set of dies, throw a teaspoon of fine grain sand into the dies before a pressing cycle starts.
Until recently pressing was done exclusively using hydraulic or flywheel presses. A flywheel press transfers energy from the flywheel to a connecting rod that moves one die up and down in the press. It can apply the bulk of it’s force somewhere before bottom dead center (because, like a piston, the die will slow and stop its travel at dead center – and less energy is transferred in a flywheel press the slower the speed is). Flywheel presses also operate at inherently one speed. Further control of speed throughout the stroke of the press is fixed.
Here’s an animation of a flywheel press in action (complete with snappy background music):
And that’s where the servo press comes in. Direct coupling a servo motor, rather than delivering energy through a flywheel, allows a precision and delicacy to the stamping process previously unobtainable.
Multiple functions can be accomplished thanks to the ability to vary speed and stroke (including reversing the direction) of the press. It’s more energy efficient and reduces the total number of presses in the shop needed to accomplish the same amount of work as a given set of flywheel presses require.
In addition, the servo press maintains torque and energy throughout its operation. A flywheel press has to recover energy after each stroke and subsequently needs greater cycle time between strokes to recover the energy needed to perform work, slowing the operation.
Here’s an extended video demonstration of a servo presses capabilities (get a good laugh at the narrator’s hat – such a lovely chin strap):
BMW Werk Leipzig has installed one big six bay Schuler ServoDirekt automated press. Blanks are fed in using robot feeders, and are transferred from position to position using robotic arms and emerge at the other end stamped to perfection. BMW is in the process of installing another Schuler ServoDirekt press line in a new hall at Leipzig.
Some panels still have to be built from separate stampings, especially those whose shapes can’t be formed inside a die. Those pieces are welded up – but BMW uses laser welding to minimize the time required to perfect the panel surface. (A typical welding operation would require going over the welded seam to remove imperfections – height differences – in the panels join.)
The benefits of the servo press and the ability to quickly change dies pay dividends when you want to make small batches of any given part. The ability to quickly change dies minimizes down time and allows for less inventory of any given part being kept in stock. If you had to wait hours before completing a die change, you would be less apt to change dies. That may be fine if you’re building a million of any given part, but BMW doesn’t build at that scale.
Quick die changes are critical to BMW and some die changes – moving the forms for the blanks out of the press and replacing them with other dies – can be accomplished in three minutes with an average of under six minutes to change dies. That’s quick. It allows BMW to keep about two days worth of panels in stock for some vehicles and they change dies eight or nine times a day. Keeping the dies close to the press helps speed die changes (and keeping the floors in the press plant almost surgically clean is also a major benefit).
The press is also quite reliable – BMW runs the press 138 hours a week, allotting only 30 hours for downtime and any necessary maintenance. This allows BMW to build panels just in time. It also allows them to build niche vehicles, without quick die turnaround and efficient stamping operations they would be constrained to building fewer variants of a given architecture.
I watched the press produce inner supports and outer skins for hoods. The blanks were loaded in parallel into the press using what are called crossbar feeders, robotic arms. They are placed and optically checked for alignment prior to pressing and, as one operation completes, the pressed panel is swept out and up towards the follow on die while the panel to be pressed is placed from above onto the die. They can stamp 17 parts per minute in each die set and there’s a steady stream of parts emerging from the end of the press. The Schuler press can swallow a 4.6 meter by 2.4 meter die – think an SUV floor pan – if necessary or, as I observed, stamp multiple pieces simultaneously.
The capabilities of the servo press and quick die changes enable BMW to keep a tight control over panel quality – never stamping big batches at a time – and allows them to build variants of vehicles that a less efficient and slower production process would preclude.
The Leipzig pressing operations are the model for what BMW will do in China. It also tells us that metal paneled BMWs will be around for some time to come, you don’t undertake an investment on this scale to be dispensing with it in the near future. And the remarks I heard a BMW board member make regarding wanting to do all pressing in-house resonates loudly when you see how well it’s done in Leipzig.
Thanks to the folks at BMW NA, especially Mr. Monty Roberts, and to BMW AG’s Mr. Jochen Mueller and Dr. Alex Sencar for their time and kindness shown to this nosy American.
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The Power of the Press