Plasma Cutting - Precision and Accuracy

Precision and Accuracy

A machine is accurate when it does exactly what you want and it is precise when it does exactly what you want over and over again. I am going to talk about CNC (computer numeric control) plasma cutters here because with hand held plasma cutting equipment it is your skill and talent that determines precision and accuracy. With a machine, it is a variety of factors that do not included skill or talent.

Generally, mechanical machining processes lend themselves to easily assign standard tolerance, but that is not so for Plasma Cutting equipment. The actual machine (the cutting table, motors, rails, CNC, and bridge) actually have tolerances that far exceed the tolerances of the plasma cutting process itself. There are many different factors that can influence the precision and accuracy, collectively referred to as the quality, of the plasma cut including: the unit's power settings, the type of consumables, the gases used, type of material, characteristics of the material being cut, gauge (thickness) of the material, the layout of the parts on the plate.

Other factors affecting the cut quality include construction and condition of the rail system, the type of CNC unit and PC used, the sizing of the motors relative to the gear box, the drive systems used, etc. As you can see there are lots of details that can affect the cutting quality of plasma cutting system. So when you are buying a CNC plasma cutter it is best to ask the manufacturer to provide you with cut samples made using the machine and plasma cutter combination that you are interested in. Ask that the samples closely resemble what you will be cutting with the machine. That way you will have an idea of the quality of the cut with different set ups. If the quality is not up to snuff then keep looking. You will be dropping a pretty penny on plasma cutting equipment, so make sure it does exactly what you need with accuracy and precision.

Hold Your Breath, Here Comes a Bridge

Ever hold your breath while you were crossing a bridge. If you have, you're not alone. Either when you were a kid or now watching your own children - seeing if you could hold your breath all the way across the span of a bridge is a common challenge. However, if it weren't for the fastener industry, we might all be holding our breath for a different reason - fear - and not fun. The same holds true for taking a plane to Grandma's house, or tossing the car keys to your daughter. The excellence of fasteners (nuts, bolts, screws, rivets, etc.), used in manufacturing today, allow us to take much for granted.

From the Industrial Revolution to 2 World Wars: It was a long and bumpy road to the levels of standardization and quality that we enjoy today. The Industrial Revolution saw the end of the crude fasteners that had been around since early civilizations when they were employed in carts and agricultural equipment. After hundreds of years of fairly static technological improvement preceding the Industrial Revolution, this new era saw large numbers of screws and bolts produced in a relatively short amount of time, with more consistency, and more precision. By the mid 1700's, the Wyatt brothers in England were manufacturing 150,000 wooden screws a week. By the late 1700's, across the pond in America, companies were also making fasteners.

However, expansion of the industry was difficult due to a lack of standards. Size, thread density, and other factors varied greatly among businesses. Two Connecticut firms established in the 1840's - The Rugg & Barnes Company and the A.P. Plant Company - were the first large American manufacturers to focus solely on making fasteners. Then, as often happens, a large historical event motivated growth and innovation - such an event was the American Civil War. It brought with it a huge demand for machinery - machinery held together by screws, nuts, and bolts. With it came the need for developing an American thread standard. William Sellers entered the picture in 1864. He proposed a uniform system of screw threads which differed from the British (Whitworth) standard in that the tops and bottoms of the threads were rounded rather than flattened. Ultimately, this standard proved to be a superior one, as rounded threads better withstood stress and resisted cracking and breaking compared to the flattened threads of the Whitworth standard. Standards are not always adopted quickly, though, and it would be another twenty years before his system was accepted as the American standard.

Differing American and British standards did cause some problems during the world wars of the 20th century. Field repairs were made difficult by the inconsistencies, but cooperation and temporary measures saw them through. In 1964 the International Organization for Standardization (ISO), announced two universal thread systems: ISO Inch and ISO Metric. The United States is the only country still tied to the inch system.

The center of the industry - American moves west: As the country expanded toward the west, so did the center for fastener manufacturing. Cleveland, Ohio, which was close to the expanding railroads and steel and iron production, became the capital of the fastener industry in America. The industry saw steady growth throughout the 20th century. By 1969 there were 450 companies, 600 plants, and more than 50,000 people employed in fastener production. Nuts, bolts, screws and rivets put meat and potatoes on the dinner tables of many a family. However, the next twenty years would bring steady decline. The increasing availability of less expensive product from overseas cut into demand for American product.

"Bogus Bolts": In 1985, a controversy surfaced with reports of equipment failure and even the loss of life due to faulty, substandard bolts. A U.S. House subcommittee spent 18 months on an investigation and ultimately determined that the faulty and counterfeit bolts were largely foreign-made. This led to the passage in 1990 of the FQA - Fastener Quality Act. This reignited demand for American made fasteners. By 2007, the fastener industry in the U.S. was a $14 billion part of the economy. Competition from foreign manufacturers continues, however, the U.S. maintains its leadership by responding to the need for technologically sophisticated products. The aerospace industry, the medical and food industries, energy producers, and the semiconductor industry all have a requirement for special materials such as A286, Inconel 718, PVDF, or MP35N, as well as for uncompromising quality and strength. The U.S. fastener industry continues to respond to these needs with unsurpassed excellence.