There are many options when cutting metal for parts. The original solution of using a saw is nice but rather noisy, messy, costly, in terms of consumables, and therefore outdated.
A stalwart of the small shop or home hobbyist is the oxy-fuel rig. Although still used when cutting steel over two inches thick, this method is not practical for a modern shop.
Plasma, laser and waterjet solutions are the most modern metal-cutting methods. These methods were all developed in the 1960s and have their fair share of proponents and detract- ors. Laser and waterjet cutting has always been associated with mechanical CNC cutting. Plasma has been comfortable with both handheld and mechanical cutting solutions.
“Over the last decade lasers have carved out a portion of the metal shape-cutting workload, particularly on parts with intricate shapes cut from thinner materials,” said a CNC plasma expert.
“However, the predictions of plasma cutting’s demise were premature. Improvements in plasma technology and automation have continued to improve the cost-effectiveness of the systems, the cut quality produced and the range of materials that can be cut.”
Low cost CNC adds options:
A significant development is the introduction of low cost but accurate CNC machines which can mount either handheld plasma torches or torches designed for mechanized plasma cutting. “A 70 or 90 degree torch designed for hand-cutting is sometimes ideally suited to robotic applications particularly when access to the work-piece is limited and there is not sufficient clearance for an in-line mechanized torch,” said the expert.
“Some manufacturers of small X-Y cutting machines design their equipment specifically to use hand torches. This allows their customers to use a single plasma system for both manual and automated cutting. It also allows them to buy standard manual packages which are readily available from welding distributors. To support these applications most manual plasma consoles have provisions for basic automation and upgrade kits available to install a full automation interface.”
“With our handheld systems you can run it in your house if you want to,” said Jim Colt, Strategic Account Manager for Hypertherm.
“Almost all of those systems from Hypertherm have an available mechanized torch or an available interface connector. You can do electrical interface to go with a CNC. There are some companies that actually adapt a handheld plasma torches directly to a mechanized device. There is actually a growing trend for what I call entry-level CNC machines that at are extremely low cost CNC machines that a home-owner can use. I have one in my home shop. I have a PlasmaCAM machine and you can place a handheld torch in them for a very low price you can do CNC cutting in your home or small shop.”
Upgrading from hand-held torches:
While plasma hand torches can be adapted to fit some CNC machines, the preferred, more professional and certainly more accurate method is to use a machine style torch.
“They fit perfect in height controls and if you want to hand cut all major plasma companies make quick disconnects on the power supply to quickly switch back and forth,” according to a CNC machine salesmen.
“Plasma hand torches are not insulated, your welding glove and hand are used as the insulation. Hand torches can be used but they are not designed for machine applications.”
Mechanized torches will, be water -cooled and the power supplies have a longer duty cycle that will benefit a productive shop. However, if you can’t afford a large CNC machine but still want to do some precise cutting than you can’t beat the entry level CNC machines with plasma adapters. Most times space is at a premium in shops and introducing new tool systems means a lot of moving around with the associated down time.
Speed with accuracy:
Accuracy in the cut is always important. CNC plasma is getting more accurate but its main forte is speed, low maintenance and durable consumable life. “You can get some pretty tight tolerances with plasma,” said a CNC plasma specialist.
“As far as the heat goes with the distortion, depending on the material thickness you can turn the amperages down and you can move the torch head around depending on the part.”
For most cutting applications plasma is perfectly adequate, “where the tolerances will be within plus minus twenty-thousandsth of an inch on one inch steel,” said Colt. He cautions that the cut quality is not only dependent on the cutting process.
A lot of this has to do with the machine and software that is driving the, “the cutting process is no better than the motion control that is driving it. The better the CNC machine, the better the robot at maintaining the correct speed. Being able to maintain the correct torch to work distance and being able to turn the plasma on and off at the right points makes a big difference.”
A common complaint about plasma cutting is the dross that forms on the underside of the cut. This is most often seen in handheld plasma cutting and is a result of a combination and amperage and cutting speed. “There is a sweet spot for every material thickness,” explains the plasma specialist.
“The biggest trick is to find that sweet spot. You can almost eliminate the dross on any thickness of material but it is an issue of finding where the sweet spot is. It is a combination of amperage, what output the machine is set at, compared to your travel speed. Too slow or too fast you are going to build up dross.”
Mechanized cutting should completely eliminate dross and its associated secondary clean-up costs. “Dross occurs mostly with handheld. Normally, when people buy a plasma cutter they turn it all the way up and they cut. When you get it on a cutting table dross will occur when the speed of the machine is not fast enough and the machine is turned up all the way. It is a combination of speed and amperage. If you are cutting thin material you have to turn the amperage down and put the speed up. And the dross will be eliminated.”
Another mistake at leaving the amperage all the way up is that the cut will become less accurate.
“High amperage blows out so much base material that not only does your cut get wider but the metal actually folds back and forms the dross,” he said. Connecting even a small plasma cutter to an entry-level CNC machine has lots of advantages for a fabricating shop. The speed and accuracy of the cuts will improve the finished product dramatically. With even the simplest of CNC machines and a plasma cutter, “anything that you can put on a computer you can cut on a cutting table. Any type of the design you can come up with you can do. I have seen very intricate designs done on cutting tables,” said the plasma specialist.
Source: Canadian Metalworking