Today’s blog post features an excellent article from the Fab Shop Direct Magazine. In this installment, Jim Colt of Hypertherm delves into the world of CNC plasma cutters and shares his thoughts on what to look for in your next machine.
This is a question I’m often asked, in my position working for a major manufacturer of plasma cutters. My company produces the plasma cutting tool as well as other peripheral devices (CNC controls, height controls, CAM software, etc.) that are used on CNC cutting machines, but does not produce the machine itself, as there are over 100 manufacturers of these machines worldwide. The question is often asked in the context: “which is the best CNC plasma machine” so I always have to ask a few questions in order to find out more details regarding the intended use of the machine.
There are a wide variety of plasma cutters available for CNC plasma cutting, with power levels from 30 A all the way to 1000 A, and thickness capabilities from 26 gauge to 6-1/4 in. on mild steel, stainless steels and aluminum. Of course there are many different cutting applications and productivity and accuracy requirements, as well as the need to keep production costs low. So which machine is the best?
If you were looking for a CNC plasma machine 15 to 20 years ago you would only find high end industrial machines that were designed for 3-shift per day operations in steel service centers and job shops, and in fab shops in the heavy-equipment and barge and shipbuilding industries. These machines were expected to operate on a tough shop floor for 20 or more years, producing thousands of tons of metal parts at high production rates with minimal maintenance, and these machines all had industrial prices that started near $100k and up, depending on options and size. Typically a CNC plasma setup of this nature would have from 1 to 4 plasma torches, up to 16 oxy-fuel torches and could cut plate from 5 ft. x 10 ft. sizes up to 40 ft. x 80 ft. plates for large shipyard applications.
Fast forward to today’s technology. Using standard office PCs or laptops, low cost stepper drives and electronics, as well as a wide variety of “off the shelf” technology in the form of bearings, linear motion ways, gear reduction units and rack and pinion components, machines are now available from a 2 ft. x 2 ft. to 5 ft. x 10 ft. configuration for under $20k. In fact, there are a few companies offering stripped down versions of a 2 x 2 CNC cutting machine that sell in the $3k range. While these low cost tables are not going to perform as well or last as long under a 3-shift per day high -productivity environment, they have opened a lot of doors for small and medium shops — even hobbyists that had to get their plasma cutting done from outside sources, and that could afford the equipment.
So, still the question about which machine is the best needs to be answered, based on the actual needs of the end user. Certainly a one-man shop does not need a $100k industrial machine, and a shipyard is probably not going to see much value in a lightweight 2 x 2 CNC machine for $3k. The point of this article then is to help clarify the differences in performance, cut quality, and long term up time and reliability that will be necessary in the purchase and ownership of one of these machines.
Every CNC machine from the lowest cost and smallest to the largest shipyard machine must have some form of the following components.
- CNC control. The brains of the entire machine, converts cutting program into electrical signals that direct the direction and speed that the machine cuts at. Also signals the plasma cutter, height control and other peripherals how and when to operate.
- Mechanical components. Each machine has to have moving components such as a gantry (long axis), a torch carriage and a Z-axis (up and down) that manipulate and move the plasma torch to produce the cut parts desired.
- Fume Control System. Plasma cutting generates a lot of fumes and smoke. Every machine requires either a downdraft fume control or a water table control.
1. Allows for automatic control of the 3 necessary modes of height for excellent consumable life and cut quality.
Pierce height being correct is absolutely critical to the life and long-term cut quality of the torch nozzle (tip). One pierce too close will permanently affect cut edge angularity, dross, and overall edge quality. Finds the surface of the material and then retracts to the proper pierce height.
2. Once the pierce height is located accurately, the torch then moves down (after pierce delay timer times-out, allowing for a full pierce) to the proper cut height .
Cut height is critical to cut edge angularity, dross and warpage. Too low will cause plate collisions, too high will widen the kerf, increase edge angularity, and cause dross and warpage.
3. Once the X- and Y- motion get up to speed (after the height control delay timer times out) the arc voltage feedback control takes over to maintain the proper torch to work distance, controlled by the arc voltage setting.
A properly functioning height control will allow you to program and nest hundreds of parts and walk away from the machine while it is cutting. This is what CNC machines are supposed to do!
If you are cutting one part at a time and don’t mind babysitting the machine continuously during cutting, and can adjust the pierce height for every pierce, stop, then adjust the cut height, then you can get along without height control. Many do. However, the height control makes these machines truly automated and pay for themselves much more quickly. There are no industrial grade machines available on the market without height control.
On Industrial machines the CNC control is basically an industrial grade computer with internal motion and machine control software, and a variety of inputs and outputs that can control all of the drive motors, plasma cutters, height control and any other tools that might be mounted on the cutting machine. This device is definitely the brain of the machine that takes its orders from both the machine operator as well as the CAM (computer assisted manufacturing) software. It converts these orders to perfectly timed electrical signals that control all functionality.
Industrial CNC controls are designed to be extremely robust, living in long-term dirty, rough, and electrically noisy environments. This adds cost and complexity. On most lower-cost CNC plasma machines, a common personal computer or laptop is used as the brains of the CNC control. While these computers (designed for a relatively easy life in controlled environments) would be expected to have issues on a busy production environment, including dirt and dust but also electrical noise interference issues, they do have the capability to run the motion and limited input and output requirements on many small machines, that are equipped with a modern air plasma cutting system. Typically if you see a PC or Laptop controlled CNC plasma machine, it will be of lighter, lower-cost design and construction, ideal for the hobbyist, prototype shop, or even small shops with low production demands.
It is recommended that PC/Laptop-based cutting machines are used only with plasma cutters that use “Blowback” torch starting technology, as opposed to a plasma system that uses high-frequency, high-voltage or capacitive starting. Blowback starting, used on newer technology air plasma systems (originally developed by Hypertherm,) produces a lower level of electrical noise interference and works well with computers.
Of particular importance when choosing a CNC control on any machine is the ease of use of its operating software and human interface. While many lower-technology machines use a standard office keyboard and mouse to control functionality, the higher-end machines have adapted to touchscreen controls, making the operator’s job much more intuitive. Also, look at the different operating softwares that are used on CNC controls. Many are adapted from machining processes such as routing and milling, and are rather clunky when used for plasma cutting. Software and human interfaces that were designed specifically for plasma cutting tend to be much easier to learn and to use on a day-to-day basis.
There are many choices here. Each machine will have moving parts: the gantry or rail axis , the torch carriage or cross axis, and the Z axis (the up down axis used to control torch height). On large industrial machines that are expected to last for decades under 3 shift, highly productive use, expect everything to be heavy, precise and overbuilt – read, expensive. On entry level, low-cost machines, expect all of these same components to be small, lightweight and of much lower construction cost. By using lightweight components on the smaller machines, money is saved in drive motor, electronics, gearing, etc., allowing the system to perform at plasma cutting speed ranges with reasonable accuracy.
The vast majority of machines under about $20k will use stepper motors as the drive systems, while larger and more costly industrial systems will use servo drives. Both drive-motor technologies have the ability to move the machines with very good precision, although modern, properly sized servo drives will generally have a wider speed and torque capability, which improves the ability of a plasma torch to cut very thin materials at speeds of over 400 in. per minute, as well as thick materials at speeds in the 10 ipm or less range.
Steppers and their associated drive electronics are simpler and lower cost as compared to servos, a primary reason they’re often used on lower cost PC or laptop controlled CNC plasma machines. Servos can be more intuitive for machine operators as they use encoder feedback to the CNC, providing less likelihood of lost positioning on the cutting table in the event of a collision with a tipped-up part, a power failure or other cutting anomalies that can occur.
Regardless of the power level or type of plasma cutter you install on a CNC cutting machine, there must be a fume control system. The two basic types used for plasma cutting are downdraft fume controls and water-table-based fume control systems.
A water tray fume control is typically just that: a tray (from 2 in. to any depth) that allows water to touch the bottom of the material being cut. The fine, hot particles of the material being cut are blown by the force of the plasma jet into the water. These particles are cooled and sink in the tray. Without the water, larger particles would fall and smaller particles would drift up when hot and rise in the form of smoke. Much of the smoke is a byproduct of mill scale or oil on the material; some of it is the material. Simple water trays are often found on low cost, under $20k CNC plasma systems because they are simple, effective, and low cost.
A water table is often deeper, and has the ability to raise and lower the water using a simple pneumatic (air) controlled displacement design. It uses an air/water tank at the bottom that gets filled with air, pushing the water into the upper chamber, raising its level in relation to the material to be cut. You can cut underwater (there are some advantages doing this on stainless and at high power levels — over 200 A). Another advantage with a raise/lower water table is that you can adjust the water to a point where it does not splash on the metal, yet still traps an adequate amount of fumes. Raise/lower water tables are more complex and cost more, and yet are very effective fume controls, as well.
Downdraft tables require enough air flow so that the flow rate in the table is powerful enough to move the heavy and light particles through a duct. If your local situation allows blowing these fumes outside, then that is adequate to dissipate the fumes. Typically a 2 x 2 table needs about 1000 cfm, a 4 x 4 about 3500 cfm, and a 4 x 8 about 7000 cfm in order to remove all of the smoke and fumes. That means that during heating season you will be pulling that amount of heat out of your shop. Some better designs use a cold air makeup duct strategically placed near the table so that air flow of the table uses outside air instead of your shop air. Some larger downdraft tables use zoned sections with movable louvers so that the suction is only pulling on the area of the table that the torch is cutting on.
Of course, there are many downdraft tables that use a large self-cleaning filtration system inside the shop. These filter the fumes and smoke and keep all of the heat in the shop. The filter systems, zoned systems and air makeup systems add cost and complexity.
Bottom line on fume control: properly designed water tables and trays as well as downdraft systems, are very effective at controlling the fumes and smoke from the cutting process. Water in contact with most materials (steel) will affect the edge quality in terms of roughness and dross (resolidified metal on the bottom edge), yet often will help control warpage and camber that occurs from heat input on long, thin parts. Downdraft generally allows for best cut quality on steel; however, if you are simply blowing the fumes outside, you may also be removing heat from your shop. Inside filtration systems can add cost and maintenance as well.
CAD and CAM Software
CAD — computer assisted drawing or drafting — lets you draw the exact parts that need to be cut. This electronic drawing of parts is then input to the CAM – computer assisted machining or manufacturing — software, which “post processes” the drawing, adjusting the size to compensate for the kerf width of the plasma cut, adding lead-ins and lead-outs to the drawing so the plasma piercing process has minimal effect on the finished part. The CAM file is then entered directly into the CNC control, where it is converted into electrical signals that control the entire cutting process.
Today, there are CAM software packages that do much more than simply “post process” CAD drawings. CAM for cutting applications is often called nesting software, which is used to post process and accurately position multiple different CAD files, in different quantities, to most efficiently utilize the plate of material being cut. Nesting will efficiently rotate and position parts, and often will generate reports involving costing, plate waste and utilization, and much more. CAM software in some systems today also can control and automatically set virtually every cutting parameter, such as arc current, voltages, gas preflow, cutflow settings, cut speeds, cut heights, pierce heights and more, simplifying the machine operators job, limiting scrap and increasing productivity.
On some low cost machines the CAD and CAM software are combined, allowing for a seamless transition from drawing to cutting. This is often seen as a way to allow everyone in the smaller shops to be able to draw and cut parts right at the machine on the shop floor, and effectively makes the learning curve to operate these machines much shorter.
There is a lot to think about when looking at the wide variety of CNC plasma cutting machines. With over 100 manufacturers worldwide, that should be expected. You can purchase small, 2 x 2 hobbyist machines for under $3k; you can easily find large shipyard machines in excess of a million dollars. Look at your needs in terms of parts thickness, plate sizes, cut-part accuracies and productivity requirements. Of course, look at your budget as well as your cutting needs that may come up a few years down the road, I am quite sure there is a machine that fits. I have not even talked yet about the tool, the plasma cutters which are available for cutting from thin gauge materials to over 6 in. thick.