Attention, all plasma table shoppers

Everything you need to know before you purchase a CNC plasma cutting machine

THE FABRICATOR® JUNE 2007

June 12, 2007

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CNC plasma cutting machines are more affordable and easier to operate than ever before. By knowing the right kind of hardware and software makes sense, a fabricator can choose the equipment that makes the most sense for his operation.

The process of choosing a CNC plasma cutting system is much more difficult today than it was 10 years ago, when systems were fewer but more expensive. By 2000 several progressive manufacturers had developed low-cost CNC plasma cutting systems that could run off of a PC.

Hardware and software developers made huge contributions with new, affordable, easy-to-use products. By 2004 it was becoming a chore for the first-time buyer to sort out the increasingly wide range of prices and new models on the market. Knowing about the different CNC plasma cutters available to you and what they are suited for will help you make the best investment for your capabilities.

First, you have to organize the market choices into three categories. One of these categories will have the right CNC plasma cutter for you. These categories, which are organized by price, do not necessarily indicate the level of quality and performance each provides. The price for each group is based on systems with a 5- by 10-foot or smaller cutting surface.

The first group comprises CNC plasma cutting systems that cost $25,000 or less, which have contributed greatly to the profitability of the small shops. They are available as complete plug-and-play systems or as kits. When you get a system from this category, it is important to use it for its intended purpose, which generally is light-duty industrial and artisan applications.

The second category of plasma cutting machines includes units that are priced at $60,000 or less. Some would argue that this group of machines includes the best of both worlds. Like the machines in the first group, they can give a lot of bang for the buck. They are engineered to perform well in demanding applications, up to and including the midindustrial market.

The best systems in this group are built to work hard; even the small tables can weigh up to a ton. They are capable of running one to two full shifts per day continuously with little maintenance. Some come with excellent industrial-grade software, hardware, and drives. Small shops with up to about 20 employees are the largest consumers of the machines from this category, which can handle anything from HVAC-gauge material to thick plate. Machines in this group usually are backed by good customer support, including live, on-screen assistance. This category is a good place for the growing business to start.

The third category includes large, feature-rich, multihead systems with price tags of between $60,000 and $200,000 or higher. These units, which have plenty of features and are smooth, accurate, and easy to operate, are at home in high-production shops. They also come with excellent customer support options. While the biggest and best are in this group, there are some pitfalls as well. Even in these high-end machines, some of their technology is outdated.

Every CNC plasma cutting machine is made up of three independent systems. Let's explore them further.

Mechanical Systems

The mechanical system on a plasma cutting machine includes the drive, the gantry with a torch height mechanism, and the cutting table. These components are not easily upgraded, so it is important to choose them wisely.

Rack-and-pinion gear drives are the most common. The rack gear runs the full length of each axis and the spur gear travels along it. Both are subject to high wear from metallic plasma dust. High-quality machines have a spring-loaded pinion gear to keep it running tight against the rack gear, otherwise the drives will develop play and backlash through use.

Some designers in the industry are advocates of using geared-timing belt drives because they cost less than rack-and-pinion gear drives, have zero backlash, and have long life. Because no force develops between the tool (torch) and the material, and the plasma cutting environment is so dirty, industrial geared-timing belts may be a good choice.

Sometimes a gear-reduction transmission can develop a bit of backlash. A little play in the transmission is not a deal breaker. It may transmit less than 0.001 inch of play at the torch. Also, most controllers can be set to compensate for a little backlash.

The drives should be enclosed or protected in some way. They should not be too exposed to the plasma dust and debris. Also, the drives shouldn't be vulnerable to getting hit by a falling piece of plate or an errant forklift. Basically, you are looking for maintenance-free operation.

Plasma cutting is all about time and accuracy, and the two most significant factors in saving time are material handling and software. You might think that cutting speed is a major factor. Yes, you can save seconds and minutes by tweaking your feed rate, but you can save hours in a week with good software and good material handling procedures.

For good material handling practices, the table should be easy to load and off-load. The gantry should be parked off of the cutting surface, so that a forklift can come straight in when loading heavy plate. The table should be able to handle a full sheet with no built-in obstructions. Also, most operators want the grates to be flush or a bit higher than the frame, which makes it easier to off-load heavy parts.

The cutting table must be able to handle a lot of weight, even if you are not planning to cut heavy plate. If you need to cut plate that is too thick for your plasma power supply, you can slide the plasma torch out of the housing and slide in an oxyfuel torch (they are all interchangeable). Now you are able to cut 2- to 4-in. material, and if you bought the right system, the controller will interface with on/off solenoids on the oxyfuel torch.

Check for play and flex by wiggling the gantry and automatic torch height control mechanism in several places while the machine is stationary. Look to see that the linear rails are secured along their full length. Be sure to run the machine through a few tests at different speeds with the plasma cutter turned off. Watch and listen. Everything should be snug and run smoothly and quietly. Stepper motors, as opposed to servomotors, make a buzzing sound that has nothing to do with the mechanical elements of the system.

Automatic torch height controls (ATHC) can be problematic. A good-quality control can be expensive, so it is important to give it a good workout, especially if you are looking at a lower-priced machine. An adjustable pierce height will significantly increase the consumable life and enable you to pierce and cut much thicker material. Also, make sure the ATHC does not have any sensors or wires sticking out that can get damaged.

The linear guide wheels on each axis are keys to ensuring smooth and accurate motion control. V-groove wheels with sealed bearings are desirable, and they should be spaced widely for increased stability. Cam followers are rightly used sometimes in group 1, but not often in groups 2 or 3.

CNC plasma cutting creates smoke that contains unhealthy black metallic dust as well as ozone. Smoke extraction systems usually come with the machines in group 2 and 3, in the form of a waterbed or a plenum/fan setup. Waterbeds have the added benefit of cooling the material during cutting. Both systems will extract smoke if they are set up correctly.

Hardware

Hardware comprised of the electronic system that controls the transmission (belt or gears) that moves the torch. Of course, this system must operate without a glitch and be maintenance-free. The computer and controller, cables, motor, and any remote control device work together to make the plasma torch do exactly what you have specified in the software program.

Overall, a closed-loop servo system is better than an open-looped stepper. Like the mechanical components, the hardware is not easily upgraded.

Some controllers cannot be upgraded with the latest CAD/CAM software. CNC software is improving in performance and price, and you will want to be able to upgrade as your business grows. Don't get stuck with a machine that can't be upgraded.

Software

There are actually three different software programs in your plasma cutting system: CAD, CAM, and controller.

CAD software is used to draw the parts. If you are a heavyweight with computers, you may be accustomed to AutoCAD®. Other programs available include AutoSketch® and CorelDRAW®. If you don't have any CAD experience, don't worry about it. It is getting easier and easier to learn. And that's what customer support is for. You can be drawing simple parts in an hour.

Sometimes your customer will e-mail you a nice drawing saved in a DXF file format, but sometimes a customer will shuffle into your shop, butt out his smoke, and make a sketch on your floor with a piece of soapstone. You will really be able to dazzle the old boy when you step up to your monitor and redraw his floor art in the form of a perfect CAD drawing. Remember to save your masterpiece as a DXF file.

CAM software is used to make the CAD drawing ready for cutting. It is important to make sure that the CAM software that comes with your system is easy to operate and has enough features. With a few clicks of the mouse, you should be able to:

  • Create lead-ins and lead-outs so that the torch starts and stops cutting on the waste side of the profile.
  • Set the tool offset so that the torch cuts on the edge of the line.
  • Perform autonesting.
  • Set the cutting and transverse speeds, pierce and cutting height, and cutting order.

Controller software uses the CAM file that you created to direct the controller to cut your part. It also enables you to manually override the operation with commands like e-stop, back up, and go.



Brian Gallup


FastCut CNC Inc.
2841 Bowers Place
Kamploops, BC V1S 1W5
Canada
Phone: 866-339-3377

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The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.

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