You Ever Wanted to |
Know about Etching Presses
It's So Simple, Why Is It So Complicated?
But let's not get ahead of ourselves. Let's let Graves explain the issues starting at the beginning and going into all the detail he needs. We're not in a hurry and we do admit, there is a lot we need to know about etching presses. Over to you, Mr. Graves:
Prints Get onto the Paper
However tungsten carbide is not used for plates, but is made into tools, abrasives, and even jewelry like tungsten rings. It's become quite the popular substance due to it's high resistance to scratching. Might I mind you that tungsten carbide is not a solid metal, but is technically a ceramic.
Greasy printer's ink is wiped onto the plate. After the surface is carefully wiped, ink remains in the lines and tones of the plate. Great pressure is needed to make a print because the paper must be squeezed into the incised image.
To obtain satisfactory results application of pressure to the plate must be perfectly even in both dimensions. There are two basic methods by which this can be accomplished: 1/ a pair of flat plates which are squeezed together with great force or 2/ a flat plate squeezed between a pair of parallel, rotating rollers.
To squeeze a pair of flat plates together with enough uniform force to print an etching would require an extremely heavy-duty, relatively complex mechanical and/or hydraulic system. The resulting press would be heavy, cumbersome and expensive. There have been several attempts at this but none was practicable for intaglio printing. On the other hand, this approach is used to great effect for relief images because ink is printed from a raised surface onto relatively smooth paper, so the necessary pressure is much less.
It is much less complicated to use a supporting plate placed between a pair of rollers to generate the large pressures needed to force paper into the inked image. While forces sufficient for any particular plate, blanket and paper are bound to vary it has been estimated that etching presses should be able to exert about 1,500 pounds per inch of roller length. For example a 30 inch wide press should be able to withstand a total of 45,000 pounds (20,454.5 kg).
Modern Intaglio Press
Most modern presses used high strength aluminum or hot and cold rolled steel alloys with a least 40,000 pound per square inch yield strength For frames flame cut steel plate (heavy and expensive), structural and "mild" steel is normally adequate if not used in high stress areas. Many older presses used cast iron frames which included the high-stress roller mechanism area. These frames were expensive to machine partly because the casting process uses sand and partly they were large, heavy and required large jigs, one for each side, and special tooling. Perhaps even worse is the fact that cast iron is not very strong and is brittle and therefore easily cracked or broken. Incidentally, If you have ever visited an iron foundry you would discover that it is a very dirty process.
and Roller Bearings
For long lasting, smooth, easy operation all rotating parts should be mounted in self-aligning ball or roller bearings. These bearings should be oversized. It costs very little extra to use the next larger size and the gains in performance and durability are great.
The steel bed, which also provides additional stiffness, may be either hot or cold rolled steel that must be properly heat treated to relieve internal stresses prior to a final grinding process that assures a high degree of parallelism. The finished thickness for a small press about 18 by 24 inches should be no less than 7/8 inch, 1 1/8 inch for a 24 by 36 inch press and no less than 1 1/4 inch for a 30 by 50 inch bed. Bed length and width is determined by the largest size paper you will use. It should be about 6 to 12 inches longer and about 2 inches wider. The felt blankets may be as long as the bed and about 1/2 inch narrower. Experience has demonstrated that these dimensions are adequate, with proper care, to produce a long wearing press bed which can be reground when necessary. Some very old presses used machined, cast- iron beds. They were usually rather thick and were manufactured with hollows for weight savings. Additional life may be obtained by periodically turning the bed over.
The use of high grade, aircraft aluminum for the lower roller eliminates almost all bed distortion or the need to turn the bed over. Fifteen to 30 times the number of printing cycles can be expected when compared to a similarly sized steel lower roller. Some press manufacturers have machined their light-weight rollers with a slight crown in an attempt to compensate for bending. The problem with this approach is that the amount of crown will properly compensate only at one setting. Because print makers use various weight papers, thickness of blankets and plates the settings and the forces are not always the same which modify the amount the roller will bend and may result in uneven pressure on the plate.
Permit us a few questions, maestro...
Is it fair to say that, all else
being equal, a heavier press is a better press?
A simple, logically designed structure and strategic use of high quality aluminum, one third the weight of steel, can result in great weight saving. From the small manufacturer's point it is less expensive to spend a bit more on the machine rather than service a warranty somewhere on the other side of the world! Strength and rigidity are related to materials used and structural design.
Modesty aside, the 30 x 52-inch Elephant Etching Press (which I designed) weighs about 1,200 pounds (545.5 kg). Similar sized press of other brands weigh around 2,400 pounds (1,091.0 kg). The Elephant uses a solid aluminum lower (drive roller) and a hollow 8-inch steel upper roller with a 1-inch wall. Both are supported by 2 15/16-inch self aligning ball bearings. Printing forces are supported by take-up units, not the press frame.
This permits the use of a light weight frame which need only carry the weight of the press bed, about 550 pounds (250.0 kg) carried across at least four idler wheels and the lower roller, and the forces exerted by the drive system. This is far less than the up-to 45,000-pound (20,454.6 kg) printing forces exerted by the rollers. Most conventional presses do not use the same type of take up unit construction as, for example, the Elephant. Those designs depend on the strength frame to resist the printing forces and, therefore, must be considerably heavier. The Elephant press uses a welded steel base to carry the printing section and leveling screws for final alignment.
"Brand X" Press Manufacturers Cut Corners
The structure of inferior press rollers is less substantial, being made of a through shaft and end plates welded into position then machined. The wall thickness of the these press rollers is usually around 5/8 inch. The roller journals and bearings on these presses tend to be relatively small and can crack in the weld areas because they are not properly sized or heat treated.
Iron Is for Lawn Benches and Manhole Covers
Cast iron, also, is weaker in tension than compression. Another weight saving in the design of all my press designs is the use of alloy 6060-T6 aluminum for the lower rollers rather than steel. They are solid because it is less expensive to machine a roller from a solid piece than build one from components. When properly sized they provide sufficient resistance to bending and they allow many, many more operations without bed distortion. The reason for this is that aluminum is more resilient than steel and reduces local contact pressures by a factor of about 27 according to the roller equations.
This works something like a rubber tire. Can you imagine what the roads would look like if we used steel tire on cement roads? The resilience of the tire spreads the load over a considerably larger area.
What's the minimum price you have to pay for a good, solid,
about accessories for the press?
about buying a used press, are there "tests" for checking out the roundness of
its cylinders, etc.?
A slight bow may be corrected by flipping the bed over so that it gradually bends in the opposite direction. To the best of my knowledge the only cure for a twisted bed is, if no more than about a 1/16 inch, is to take it to be reground otherwise it might be cheaper to buy a new one. Adjust the press so that upper roller just barely contacts the bed. Making certain there in nothing on the bed shine a light from behind the roller and observe the line of contact for obvious irregularities as you run the bed through the rollers. Remember, however, that nothing is perfect.
Then you can make a test surface print without blankets. Coat a large litho plate evenly with ink. When making the trial print adjust the press so that upper roller just barely skims over the bed. Without a blanket there may be some uneven areas. Totally blank areas indicate flat spots on the roller or a depression in the bed. You might want to make this test with a sizing catcher and 1/8 inch felt blanket. If you really want to get a good fix on trueness of the cylinders you can buy a dial indicator with a stand or, better still, pay a local machinist to check it. If the bed is warped you will be able to check roundness at only in one place at a time.
It should not be excessively heavy. Intelligent, untrained personnel should be able to install and align presses of less than about 30 by 50 inches with appropriate equipment. Be sure that larger presses can be fit through doorways, passageways or into elevators safely without exceeding the building's load capacities! Presses that can be broken down into two basic parts (a base and printing section containing the rollers, bed and drive) are easier to maneuver.
Presses that are designed as a single unit must usually be taken apart and reassembled by knowledgeable personnel. It is necessary to use professional help to install very large or heavy presses. Because of liability and safety concerns powered etching presses are not commonly manufactured.
(Parting note from World Printmakers: Our sincere thanks to John Maitland Graves and Rembrandt Graphic Arts for this authentic master class in etching presses!)
Illustrations by John Maitland Graves
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