Making a clear-cut decision

Choosing automated cutting equipment for small, medium-sized shops

PRACTICAL WELDING TODAY® MAY/JUNE 2005

May 10, 2005

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Deciding what automated cutting equipment is best for your small or medium shop depends on many factors: part size, part thickness, part accuracy, parts quantity, computer skills, and machine payback, or return on investment.

Read any metalworking trade magazine and you routinely will see introductions of new cutting machinery. You'll also see machines advertised and the tradeshows that will feature them.

As the owner of a small or medium-size shop, how do you decide which equipment is best? How can you be sure you're getting the best value? You have to consider several factors.

Key Issues

Given the range of equipment available, you may find it difficult to make a choice, but answering a few key questions will help steer you in the right direction:

  • What size parts do I plan to cut?
  • How thick are the parts I want to cut?
  • How accurate are the parts that I want to cut?
  • How many parts do I plan to cut?
  • What are my computer skills?
  • How quickly do I want to pay for the machine?

Notice that none of these questions asks how many torches you would like, what type of machine you want, or how much you would like to spend. The answers to these questions will come naturally after you consider the key factors. Let's see how these factors will help you determine the kind of cutting machine you will need.

Part Size. What is the largest part you want to cut? Will you be cutting many small parts or several large parts? Most parts used in manufacturing are relatively small, about 2 inches by 6 in., up to 24 in. by 48 in. You can cut these parts from plates or sheets that are 4 feet by 8 ft. or 5 ft. by 10 ft. Another common size is 6 ft. by 12 ft. These plates are readily available and typically are the most economical.

This leads to how you'll handle the plate. A 1/8-in. (10-gauge), 4-ft. by 8-ft. steel sheet weighs 152 pounds. Two people can pick this up. But a 1-in. plate that's 5 ft. by 10 ft. weighs 4,680 pounds. This requires some form of automated lift. Plates of this size also can be moved into position with a forked truck. Difficulty moving large plate will be a factor in determining part sizes.

Part Thickness. Parts can range in thickness from thin, 20- to 26-gauge sheet metal for ductwork, up to 6, 8, or 10 in. thick. Generally, most parts are between 1/8 in. and 1-1/2 in. thick. The metal thickness determines the most efficient cutting process. Two cutting methods are plasma arc and oxyfuel.

Plasma arc cutting uses the power of an electric arc to cut through metal. It can cut any conductive metal, including steel, stainless steel, and aluminum, from gauge to 1 in. thick. If your part's thickness falls within this range, a cutting machine with a plasma torch is suitable.

Oxyfuel cutting uses the chemical reaction between oxygen and iron to produce a cut. This process can cut carbon steel thickness from 1/8 in. to 10 in. or more. Without special equipment, it can't cut stainless steel or aluminum. An oxyfuel torch is suitable when the metal is 1 in. to 6 in. thick. The speed of an oxyfuel torch typically is lower than that of plasma, particularly when cutting metal that's less than 1 in. thick.

Part Accuracy. The cutting process typically starts with a drawing of the part. The drawing shows part tolerances, which can range from ±1/16 in. or more down to thousandths of an inch. Cutting machines can cut parts with dimensional accuracy within 1/64 in. to 1/6 in. You can improve these accuracies by doing a test cut and then adjusting the cutting parameters.

Part Quantity. The important detail to know regarding part quantity is how many identical parts must be cut. You can cut large numbers of the same part with multiple tools working at the same time. This helps increase the machine's productivity. However, if the work load consists of many different parts, they must be cut one at a time, and you have little reason to need multiple sets of the same tool on the machine.

Computer Skills. All modem cutting machines use a CNC. The simplest controllers have a non-PC-based unit with hardwired instructions. More elaborate machines use PC-based controllers, usually with a simplified Windows®-based environment. This means functions such as file management, movement through screens, and locating and using routines follow Windows logic. The machine operator must be familiar with these routines. The non-PC-based controllers, however, may require fewer steps to perform common routines.

Machine Payback. This is called return on investment. Once you have determined the size and number of parts to cut, you'll know how elaborate, and expensive, the cutting machine will be. Now you'll be able to calculate a return on investment. The payback period should be three to five years. If it isn't, it may be because the machine you selected is too expensive.

Machine Selection

After considering these factors, you'll be able to choose a machine configuration. Think about the same factors discussed previously, but this time think about them in terms of how they affect equipment.

Part Size. Select the maximum plate size you want to cut. The width of the plate usually is placed between the rails or tracks of the cutting machine, so this determines the width of the machine. With the exception of light-duty machines, cutting widths of 6, 8, or 10 ft. are possible, so the smallest machine will be able to accommodate all of these plate widths. The cost difference between a 6-ft. machine and an 8-ft. machine usually is small, so you should choose the widest machine that will fit in the space available.

Part Thickness. The thickness of the parts to be cut determines the process. As previously stated, parts less than 1 in. thick are best cut with a plasma torch; those thicker than 1 in. with an oxyfuel torch. If your requirements include all of these thicknesses, you should select a machine with one or more of each tool.

Part Accuracy. The practical part accuracy range for a cutting machine is from 1/64 to 1/16 in. If the parts fall at the low end of this range, then the cutting machine must be able to compensate for positioning errors, including gear rack error, backlash error, and general machine motion. This compensation capability will add to the machine's cost.

If the part tolerance is more generous, you can choose a machine built for general fabrication at a lower price.

Part Quantity. The number of parts to be cut determines the number of tools required and, with plasma, the power of the tool. Usually up to two plasma cutting torches can be installed on a cutting machine. This means that it can cut two identical parts simultaneously. However, six or more oxyfuel torches can be put on the same machine. This means that six instead of two identical parts can be cut at the same time. Although oxyfuel cutting generally is slower than plasma cutting (particularly when the part is thinner than 1 in.), six torches will cut more parts per hour than one plasma torch.

Computer Skills. Even a machine with a PC-based CNC system is relatively inexpensive, so unless price is an overriding factor, you should consider a machine with a computer-based control system. Some control features are communication with a host computer, onboard part programming, and axis compensation.

Return on Investment. Now that you have determined the machine configuration you need, it's time to establish specifications. Contact several machine suppliers and obtain quotes. These prices will enable you to see if the return on investment is acceptable. If it's too high, then you may need to review the specifications—perhaps you should select a lower-cost plasma cutting machine or fewer oxyfuel torches. This should help you get the price in line.

Another way to look at this is to plan for the future. Today you may need a machine with only modest capabilities, but future business might require a higher-production unit, justifying a larger expense.

Another way to look at this is to plan for the future. Today you may need a machine with only modest capabilities, but future business might require a higher-production unit, justifying a larger expense.

Robert Ludwigson is product manager, MG Systems and Welding Inc., W141 N9427 Fountain Blvd., Menomonee Falls, WI 53051, 262-255-5520, fax 262-255-5542, bludwigs@messer-mg.com, www.messer-mg.com.



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