The elements of motion quality
Whether you are purchasing a new CNC cutting system, building your own, or upgrading an existing piece of equipment, the motion capability of the system is a significant factor in your overall success. In fact, machine motion is on par with selecting the optimal cutting system when it comes to part quality and consistency. Cutting features and application capabilities – True Hole® and True Bevel for example - are also influenced by machine motion. For machines already on the floor, it’s important to know which applications each was designed to support.
What determines motion quality?
Plasma systems and other cutting machines are usually designed around certain functional considerations:
- Applications — Will the system be used for thick plate, HVAC, bevel, pipe, multi-process, structural elements, etc.?
- Productivity needs — Will the system need to support multiple cutting stations, multiple plates, fast traverse, or other manufacturing requirements?
- Desired cut quality — Will the system be used to produce precision parts, bolt holes, slots, notches, or a combination of several types of work?
This is not to say that one machine can't do a number of things well. Rather, the point is that you’ll need to work with your machine builder to configure the right machine for your cutting needs.
Motion quality isn’t an absolute. Different levels of quality are needed for different functions and to achieve different levels of cut quality and productivity. Lesser motion quality (typically at a lower price) may be acceptable in some instances, while superior quality is required in others. The motion quality of your machines should be appropriately matched to your business needs.
On the mechanical side, a cutting machine may be outfitted with one of a number of different types of guideways. V-type, bar stock, and linear are just a few commonly used guideways. You will want to understand the differences between them. For example, linear ways with sealed bearings typically have higher friction than a roller bearing and wheel but in normal use are more consistent and provide the best, most accurate cut quality with little to no maintenance.
One good indicator of the quality of the mechanics of a machine is how easy it can be pushed with the drives disengaged after overcoming the machine inertia. Poorly designed or poorly manufactured machines are difficult to push. This indicates that more power is required to accelerate, maintain speed and change direction. While "reducing friction" and minimizing the amount of force needed to drive the gantry are historically considered critical design elements, today machine builders recognize that for superior cut quality it's more important to have a drive system that is properly sized, with motors and gearing that can deliver consistent motion.
Rigidity, another common benchmark, is also easily misunderstood. In some cases, a machine can actually be too rigid. If it's too stiff, the machine is likely to be heavy and difficult to accelerate. On an overly rigid machine, boxes, torch leads, and cables can be shaken when entering and exiting sharp corners, and the resulting vibration can be seen in the cut parts. Where rigidity is desirable is in the mechanical connections between the motor, gearbox and rack or ball screw. The more rigid these connections, the easier it will be to accelerate the machine.
The type of drive system used on the machine is also important. On entry level, DIY, and light production machines, less expensive "open loop" systems are commonly used because the quality is good enough for the job. These systems have no feedback loop so it is not possible to adjust either speed or position during cutting. Closed loop systems, which can adjust the speed or the position based on machine feedback, are generally used in industrial, commercial and production environments. This enables a more predictable cut, resulting in higher precision parts and greater part-to-part and nest-to-nest consistency.
Analog or digital?
Analog drives are still widely used and are capable of very high precision. However, the current trend is toward digital automation standards such as SERCOS (Serial Real-time Communication System), EtherCAT® (Ethernet for Control Automation Technology) and PROFINET (Process Field Net). This is not so much driven by machine motion but is due to increased emphasis on factory automation and universal solutions for a wide range of applications.
Both analog and digital drives can offer feedback devices with extremely high resolution. In some cases, it is possible to achieve more than a million counts per inch of travel - resulting in extremely high cutting precision.
Right-sizing the drive and motor
No matter which drive technology or brand is used, the machine builder needs to properly size the drive and motor for the machine. In general, heavier machines will require more powerful motors and drives. Inertia must be factored in. When changing direction the motor needs enough power to overcome its own inertia, to reverse the drive train, and overcome the inertia of the machine. In a properly designed system the power of the drive will match the force required to overcome inertia and accelerate the mass of the machine.
Gearing can be used to gain mechanical advantage. Gear reduction has the effect of increasing the effective torque of the motor, but at the sacrifice of top end speed. The design goal is to use a cost effective drive/motor size with a gear reduction that achieves target acceleration rate and velocity for your application. With plasma, that's usually 20-40 mG acceleration rate and a traverse speed of approximately 1,000 inches per minute.
Important points related to machine motion and cutting process
Older cutting tables, originally designed for earlier generation plasma systems, are often retrofitted with newer plasma systems -sometimes with disappointing results. Parts coming off the retrofitted table might actually look worse with the new plasma system! What’s going on here?
It’s possible that the older plasma system was masking vibrations or mechanical problems on the table which only now show up because of the new plasma system's finer cut quality. Think of the older plasma system as a large-tipped felt marker, and the new plasma system as a fine-point mechanical pencil. If there are subtle vibrations in the table, you’re less likely to notice them with the wide-edged marker. This can also happen when you make the jump from oxy to plasma cutting on the same table, or upgrade an older plasma system to a newer HyPerformance® system. In any case, your machine builder can advise you on the best solution.
Other factors affecting motion quality
An important but often understated point is how the cutting machine will hold up to industrial use. It might be difficult to distinguish the cut quality of a high-end machine and entry-level machine when both are brand new. However, differences in construction and cut quality will likely become evident with time and use.
Lost motion or "backlash" can result from the use of lower-quality gearboxes, poor pinion engagement (on a rack and pinion system), excessive wear, or poor maintenance of the pinion and/or gear rack. Ball screw systems offer a more robust alternative, but have a length limitation.
The CAD and CAM system can also impact machine motion. Scanned images or artistic parts frequently require editing before uploading to the cutting system. CAM systems help by filtering and smoothing out the program prior to it reaching the CNC.
The geometry of the part may play a role in machine motion, although this is more a function of the control than the motion itself. Understanding how certain geometries behave on a given machine can determine how the parts should be drawn, and ultimately what is possible on a particular machine. Very good controls are able to deal with conditions that can cause others controls to bang and vibrate. If the control isn't up to the challenge, simply slowing down the machine or using corner loops may help clean up a part.
Finally, proper machine maintenance is critical. Mechanical parts wear out. Torch collisions and other issues occur. Keeping ahead of these issues with regular scheduled maintenance will help to ensure good machine motion over the life of the cutting table.