Sheet Metal Car Parts (Part 2)

Mark Corbin working in the GM plant

Mark Corbin working in the GM plant (photo courtesy of Mark Corbin)

Part 2, by Mark Corbin

In part 1, I covered the draw die. At my plant we had presses from 50 ton portable gap presses to the 2,000 ton floor mounted conventional press, plus when the plant was modernized in the mid ‘80’s we got a new addition with 10 transfer presses and several more transfer presses to replace lines of conventional presses. Later on, and just a short time before the plant was closed, we got several more crossbar transfer presses. The plant boasted 2.1 million square feet of floor space and the world’s largest press pit. It’s now 2.1 million square feet of parking lot less the world’s largest two-story deep swimming pool.

A conventional press looks like a giant rectangular box with a slot in the middle. In that slot is the press ram and just below it is the press bed. Dies are slid onto the press bed and bolted in place top and bottom. The largest capacity press that was in my plant, a 2,000 ton conventional press, was used to stamp the largest panels used in GM’s car and truck products. One of the parts the line produced was the whole side of the full-sized “G” van, a part that ran from roof to rocker and from the rear of the front door to the taillights. Dies for that part were too large and heavy for our 80 ton capacity overhead crane to pick up complete. So we had to remove the top half of the die, transfer the bottom half to the press, then set the top half onto it, and drag it into the press.

Once a flat sheet is formed into a draw panel, as much of the excess metal as possible has to be trimmed off in the second die in the line, a trim die. Also sometimes holes are punched (piercing) is done as well. Trim dies consist of a male adapter on the lower half which is the shape of the draw panel, but ending at the expected trim line. Scrap cutters are offshoots from the trim line and are also shaped to accommodate the draw panel as it rests on the adapter. The trim line along the edge of the panel adapter is inset with blocks of water-hardening steel with a vertical cutting edge along the trim line. Scrap chutes surround the adapter under the trim edge in order to duct the scrap steel off the die and down the scrap holes in the floor. The scrap (called off-fall) drops off into a conveyor that takes it to the bailer house in another building.

The upper half of the trim die is a stripper pad that, when the die closes, holds down the panel just inside the trim line and strips the panel off the upper trim steels that surround it. The pad is mounted on springs that provide that push when the die is opened. The trim steels that are surrounding it are composite water hard and soft steel cutting blocks that match the trim line steels on the lower. In order to trim through the steel sheet panel, the trim line has to match upper and lower steel cutting edges within thousands of an inch. They actually shouldn’t rub against one another, but instead have clearance which is a percent of the sheet steel thickness. This is because in cutting through the sheet steel, the cutting edge only cuts part way through the metal, then tears through the rest of the way at an angle. If the cutters are too close, this angle causes the panel to stick inside the cutter and then pulls back and distorts the metal panel when the die opens. If the cutters have too much clearance, it forms a burr on the panel edge. Either condition is unacceptable, so the clearance must be right, down to the thousands of an inch, even though the die is opened several feet in the air.

So if these dies must be held accurate within the thousands of an inch, yet open several feet in the air, how is that assured? The answer is that each half of the die contains wear plates in each corner of the die that guide the two halves (called a die shoe) together. This is true of all dies, including the draw die. If needed, further accuracy is achieved with guide pins and sleeves that are precision made and installed along with the guide plates in the corners of the die shoe.

After the draw and trim die, possibly more trimming and/or piercing, and operations like flanging (reforming the shape), may be needed. While any variety of operations can be performed in any one die, some operations may get in the way of other operations, hence the need for multiple dies in any given line. Thus some parts, like quarter panels, need up to 8 dies to completely form the part so it’s production ready. Those operations include but are not limited to trimming off excess metal, punching (piercing) holes, reforming (flange dies) areas that were not or could not be completely formed by the draw die, and adding such extra features as run numbers and dimples.

A transfer press can take the place of several conventional presses, as it (and its similar crossbar press) mounts several dies onto one or more shared press beds and rams, which open and close the entire string of dies in unison. The press beds roll out of the sides of the press, allowing die changes to take less time, which increases production time. Gap presses are floor mounted or portable presses of low tonnage capacity and are used to stamp small parts or perform some operations for parts like quarter panels, where the die works small areas like a window channel or trunk trough. The gap press looks like a giant “C” clamp.

In transfer presses, though, any motions from any direction other than vertical have to use a die cam to change the direction of work. Conversely, conventional press lines allow the panel to be turned and/or rotated between dies as necessary in order to provide the operation a vertical stroke in the die. This cuts back the need for using die cams, which are like smaller dies within the die. Often the cam is a double sliding cam, and often resembles a baby in a cradle. The cradle slides into position and the baby does the work against the end of the cradle. These motions are driven by a wedge that is mounted on the opposite die section (top or bottom) that drives the moving parts of the cam as the die closes, while springs return the parts to their starting position when the die opens.

Dimples are often added to denote the location of optional holes that may be drilled for later additions of components of options like air conditioning. They are little round pointed spikes of hardened steel and held in place with a threaded collar that is screwed into a blind hole. Run numbers are lettered and numbered hardened small steel blocks that are held in a slot with a keeper or holder screwed in place. Their purpose is to put an identifying code into the part, usually consisting of a partial part number, run date, and stamping plant ID, all of which is used for inventory control. Virtually every sheet metal part gets such markings at some point, though it may be trimmed off during the car assembly process. If you see some numbers stamped into a place like a rain gutter along a deck lid opening, that is probably what you are seeing.

Finished parts are stacked in open baskets or special purpose racks just for that particular part. Once filled, the rack or basket is tagged and sent off to wherever the parts are used, be that temporary inventory storage, a production line, or shipping to another plant. My plant was strictly a stamping and sub-assembly plant, so everything we made had to be shipped somewhere. My impression is that when GM went through the bankruptcy, our reputation for being able to run anything, to do it with the best productivity of any plant, and to deliver parts that were of the best quality in the corporation, all didn’t matter to the bankruptcy bean counters. All they looked at were the shipping costs that were incurred on every part we made, which is why they closed the plant.