In last month’s column, I covered the options for spindles, axes and tables on a CNC router. Now let’s examine the inner workings.
Motors and position
The motion of the spindle on the gantry (a moving bridge), and the motion of the gantry along the table is driven by specialized motors. These are either stepper or servo motors, and the differences are worth considering before making the decision to buy.
Stepper motors are usually operated in an “open-loop” where the CNC router controller commands the motor to move a “step”, and then assumes that the command was followed.
Servo motors operate in “closed-loop” where the commands to move are monitored by encoders and adjustments are made depending on the feedback.
Stepper motor systems are generally less costly. Machines with stepper motors can function quite well, but if a step is unable to be completed, stepper motor systems can lose track of where they are and won’t cut as intended from that point forward in the job. This may happen if the cut becomes unusually heavy, such as when encountering a knot. The operator needs to realize what’s happening and reset the machine.
Servo motor systems don’t usually lose track of where they are because they make real-time adjustments to stay on track.
How do they track their location? The simplest system is the use of limit switches. At powerup, the controller carefully moves the spindle on the gantry, and the gantry on the table, until a switch on that axis is tripped. This technology offers pretty good repeatability – perhaps within 1/64th”. An improvement on limit switches is the use of proximity switches, which are sensors that can detect limits without physically touching objects. These offer higher repeatability.
The next step up is absolute encoders. These are sensors that don’t require a fixed starting point to discover their own location. They have a unique code for every position, so they can constantly read the environment. That means they don’t need to go through the homing process every time they are switched off and then on again.
So, that tells us how the machine knows where the spindle is. But how does it know where the business end of the tool is?
On entry-level machines, the operator goes through a manual process of touching off the tool to establish “z-zero” (where the end of the tool is in the Z direction). This process can be done with a touch-off pad, where the machine runs through a routine initiated by the operator to determine the position of the end of the tool. On more advanced machines with ATC, the touch-off pad and routine can be automated so it kicks in whenever a new tool is installed in the machine.
Need for speed
Manufacturers often publish a number indicating how fast its machines are. There are two commonly published specifications: rapid speed and cutting speed. The rapid rate is how fast the spindle can move in the X and Y axes when it’s not cutting (when it’s on its way to the first cut position, so accuracy is not a concern). The cutting rate is the fastest rate recommended when the tool is in contact with the part and accuracy is required.
Note that the rate of vertical travel is yet a different specification. Published rates are for long, straight moves. Short and curved moves require acceleration and/or deceleration, which will result in much slower motion than indicated by the published specifications.
What is important to you? Time is money, so you probably care about how fast your jobs will cut. More powerful motors and more sophisticated control systems will probably result in faster jobs, but that doesn’t always translate to a higher cutting rate. Moving fast doesn’t always deliver great results. For example, stiffness and deflection of the CNC router frame are rarely discussed, but they can have a significant impact. Industrial-class machines are fabricated from steel weldments that deflect very little with cutting forces, but those are more costly to fabricate. Less expensive machines are often fabricated with bolt-together elements, and often use aluminum extrusions. Stiffer frames enable more accurate cutting at higher feed rates, while minimizing chatter and other deflection related issues. To deal with the issues introduced by deflection, jobs must be run with lighter cuts and slower feed rates. For production shops, a more rigid frame will deliver the highest levels of production.
In addition to the frame, there are additional mechanical components that influence stiffness, deflection and precision. Guide rail systems on high-end machines use precision contour rails that can carry heavier loads with less deflection. Those linear bearings require very precise and stiff mounting surfaces. Round linear bearing systems are not as stiff, but they are less fussy about the support structure. So, round guide rails are a good choice for intermediate CNC router tables. Lower cost machines may use simple rollers on extruded rails.
The drive systems also influence stiffness, accuracy and speed. Helical rack and pinion systems are found on large, high-speed industrial systems, while ball screws are found on mid-size to lower-end systems. Belt drive systems are found only on the lowest cost systems. Ball screws are common across most machines for the Z-axis motion, because they are a good choice for a shorter range of motion. On the higher-end machines, an air cylinder will be used to counterbalance the weight of the spindle. Smaller machines are able to use a single drive to move the gantry. Larger and heavier duty machines will use a dual drive system, with motors and rack-and-pinion drives on both sides of the table.
Another aspect of speed is material handling. To achieve the highest production, the spindle should obviously be cutting as much of the time as possible. If jobs are cutting efficiently, it’s not unusual to find that more time is spent loading and unloading parts and waste than is spent actually cutting parts. Manufacturers have addressed this challenge in a variety of ways. Some machines have two moving beds to enable pendulum processing, where one bed is being unloaded and re-loaded while the other is cutting. Numerous manufacturers offer equipment to automate loading sheet goods while sweeping parts and waste off the other end. They are effectively turning the operators into parts handlers, so that the spindle can transition quickly from machining one sheet to the next.
Smaller CNC routers are usually powered by 120-volt service in North America. Mid-size machines will require 220-volt service, while the larger and industrial-class machines will require some form of 3-phase power. The cost of 3-phase power or a 3-phase converter needs to be factored into your considerations – not just for the machine, but also for the related machines such as dust collectors and air compressors, and especially the vacuum pumps.
Software and other considerations
The CNC’s controller translates data from a computer into instructions that tell the machine what to do. The controller and a software program work together to interpret G-code and to issue commands that determine how the spindle moves.
Some manufacturers provide proprietary controllers that use proprietary code instructions. Others use controllers that use ISO-standard G-code. It is important to understand how the CAD/CAM software on your computer will interface with the controller that is going to be provided with the CNC router you are considering. CAD is used to design parts, and CAM essentially turns the design into instructions.
A CNC post processor is software that converts instructions that were created in a CAM system into programs that can be read by a machine’s controller. Post-processors for controllers using industry standard G-code are often provided at no cost with many CAD/CAM packages, while post-processors for proprietary systems may only be provided by that manufacturer’s own CAD/CAM package. The latter may carry a hefty price tag, so ask lots of questions.
Some CNC controllers are based on a PC platform with additional motor-controller hardware added. These use an operating system such as Microsoft Windows and add special-purpose software to interpret the G-code and control the motors. The more industrial of these controllers use a special version of Windows that is designed and customized for embedded systems. This insulates the system and the user from system operations inappropriate to machine controllers, such as automatic system upgrades running in the middle of a CNC router job. Some of these controllers run the same version of Windows found on home PCs. In that case, it is advised that Windows be configured to minimize unwanted disruptions. The least expensive controllers are often termed hand-held. These provide a minimal display and a set of proprietary keys. The handheld controllers are intended primarily to just run the program. They don’t allow code editing or graphic simulation, and they provide little information about a running program. The programming of the G-code will be done elsewhere, so something like a PC is likely to be needed anyway. The hand-held controllers’ advantage is that they’re inexpensive.
Skills with computers are a pre-requisite for producing parts on a CNC router. Those include CAD, CAM and controller familiarity. Before purchasing, a shop should plan to have these skills available to put the CNC router to productive use.
One should also think about dust collection. Operating with an underpowered or poorly configured collection system is a hazard when cutting materials such as MDF. Automatic tool changers and some fixturing clamps require compressed air. Some CNCs also use compressed air cylinders to offset the weight of the spindle. Nesting and pod and rail holding devices require a sizeable vacuum pump.
This is the second of a two-part series. Part one was published in the February 2021 issue and is available at www.woodshopnews.com.