What You Need To Know Before You Invest In A CNC Router

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CNC Controls are made up of two parts; a control computer & control software. Controllers translates the code (g-code) from the tool-path program into motion. It sends the code via electronic signals on to stepper or servo motors that move the various axis of the machine. In addition to motion commands, controllers provide: I/O capacity, backlash compensation, jogging routines, positioning error correction and so on.

It is worth noting that a control is likely the most complex of all of the technologies involved in the makeup of a CNC system. The control plays an integral part in system performance as it relates to accuracy and smoothness of motion, which has a direct impact finish quality.

Techno Servo Control

The Techno servo control is designed for continuous heavy-duty applications and features a comprehensive set of capabilities, an easy to use interface and is seamlessly integrated with Techno machines. The control system is available in an on-board unit, or optionally as a PCI Servo Card designed to be inserted into an IBM PC or compatible computer, and supports up to (4) axis of simultaneous motion.

It’s worth noting that Techno is one of a select few CNC router manufacturer that designs and builds its own control. Techno is not reliant on third-party NC controller suppliers and assumes full responsibility for their entire machine, and has the in depth knowledge to do so effectively.

Techno Control Features

Closed Loop Operation – the system servomotors have constant feedback from an optical encoder. This device sits on the back of the motor and keeps the controller informed of how far the motor has actually moved and at what velocity. This constant feedback is used to correct any discrepancy between a desired and an actual position, and between a desired and an actual velocity.

Built-In Text Editor – enables fast shop floor modifications of G-Code programs.

Toolpath Preview – enables a fast shop floor graphic verification of toolpath motion prior to program execution.

Jog Routines - continuous and step jogging functionality to position a tool to any desired location.

Machine Feed/Plunge Rate Override – allows modification of toolpath feed and plunge rates on the fly (without editing G-Code file). This feature is especially handy when trying to arrive at optimum cutting speeds for unfamiliar materials.

Mechanical Homing Locations – enables user to establish a repeatable home position at any corner of an X,Y, Z table.

Fixture Offsets – enables up to 10 specific table locations (reference points) to be stored in control memory. These stored locations are commonly used to expediently direct the cutting head to the correct starting position and orientation relative to toolpath program and fixture locations.

Machine Diagnostics – Thousands of Techno CNC systems are currently providing dependable service throughout the world. However, on those rare occasions when problems arise, the control system’s diagnostics provide an in-depth set of functions to identify the specific nature of the difficulty. In conjunction with Techno’s phone support, diagnostics enables expedient problem isolation and ratification.

Inputs/Outputs – The control supports16 inputs in 2 groups of 8, and 8 outputs in 1 group of 8 – each group of 8 can be independently operated at 5V, 12V, or any user-selected voltage up to 24V. This I/O capacity enables a wide range of peripheral devices such as coolant units, pneumatic clamps and so on to be operated under system control

User Friendly Directory Structure – the Techno control features a familiar windows naming and directory structure, enabling part programs to assigned alphanumeric names and grouped in specific directories by design, type of part, finished product, customers, and so on. With the advent of mass program storage it is possible that a shop can store thousands of different programs. When both letters and numbers can be used to name part programs and those programs can be stored within a categorized directory structure, it is much easier to locate the programs you want.

Network Interface – The Techno control can be easily integrated with existing networks

Interpolation – The control support both linear and circular Interpolation

Emergency Stop and Program Pause – The control system features easily accessible emergency stop and program pause buttons. In case of tool bit failure, the pause function is commonly used in conjunction with the system’s ability to accurately backtrack to the point that tool failure occurred and recommence with cutting from that point forward.

Backlash Compensation – enable each individual axis on a Techno system to be fined tuned for accuracy.

Up to (4) Axis Control – The Techno control supports up to (4) axis simultaneous motion.

For those un-familiar as to how axis designations relate to machining routines: 2 ½ axis is identified as cutting at a constant depth on a given plane, 3 axis is commonly identified as cutting at a constantly changing depth on a given plane (associated with 3-D carving, & 3-D modeling) and 4 axis is commonly associated with a system that support a rotary table in addition to X,Y,Z motion.

Free Software Updates – The Techno Control Interface Program is periodically updated to provide improved functionality. Updates are available to Techno users at no charge and are easily downloadable via the internet.

Standard Industry Codes – The control processes industry standard “G” and “M” code toolpath files, and is compatible with virtually every major CAM package.

Servo/Stepper Comparison

Stepping motors were used in early CNC routers primarily because they were substantially lower in cost than other drives at the time. Properly engineered, they performed well in less demanding applications. But, as the cost of DC and AC servo drives came down, stepping motors lost favor. Their greatest advantage, cost, was no longer as significant and the technical effort required to properly apply them was greater than with the alternative drives.

Today, a number of lower cost machines have again begun using stepping motors. In theory, a stepper motor is a marvel in simplicity. It has no brushes, or contacts. Basically it's a synchronous motor with the magnetic field electronically switched to rotate the armature magnet. Most stepper based systems do not utilize a position feedback loop and are termed “open loop” systems. The control commands a motor position and then simply assumes that the machine follows the command.

Stepper motors features a number of praiseworthy characteristics which include: excellent repeatability, outstanding low speed torque, simple/rugged construction and a lower cost than other motion control systems. On the other hand, in relation to CNC control they have certain inherit weaknesses which have to be addressed, these include: rapid drops in torque with increased speed, a low torque to inertia ratio (loads cannot be accelerated very rapidly), low output power for motor size and weight, susceptibility to resonances, and in most cases the lack of a feed-back loop.

Addressing the shortcomings of stepper motors can be a tricky proposition. As an example, the preferred method of compensating for the fact that a stepper motor's torque drops rapidly with speed, is the chopper drive. A chopper drive loads the coil on each polarity change with a high voltage. The higher the voltage, the more quickly a coil loads, which enables the motor to maintain a reasonable level of torque. But the higher the motor speed, the more the positive effect of the chopper erodes. Additionally, the more you increase voltage in a stepper motor, the stiffer (jerkier) it runs, and the more it vibrates. So, while you’re correcting one problem you add to another. Adding insult to injury, stepper motors produce more noise and vibration and run rougher than servomotors even before the application of a chopper drive.

Another example of addressing the shortcomings of stepper motors is the lure of micro-stepping, which is the preferred method of compensating for the fact that stepper motors, even with of the best of control circuitry, run noticeably rougher than servos. In the micro-step mode, a motor's natural step angle can be divided into much smaller angles. For example, a standard 1.8 degree motor has 200 steps per revolution. If the motor is micro-stepped with a 'divide-by-10', then each micro-step would move the motor 0.18 degrees and there would be 2,000 steps per revolution. Typically, micro-step modes range from divide-by-10 to divide-by-256 (51,200 steps per revolution for a 1.8 degree motor). This increase in resolution helps substantially in lessening a stepper motor’s jerky, ratcheting response. Which sounds great, but there’s a catch. The real compromise is that as you increase the number of micro-steps per full step, the incremental torque per micro-step drops off drastically. Resolution and smoothness increase, but performance suffers.

The vast majority of stepper systems run in an open loop configuration, so position is known simply by keeping track of the input step pulses. The problem with open loop is once you lose a step, you never get it back, there’s no corrective action, and the errors just accumulate. The most common cause of lost steps is a phenomena referred to as resonance (vibrations). It is the bane of both full step and micro-stepping motors. It causes a sudden loss or drop in torque at certain speeds which can result in missed steps or loss of synchronism. The issue of resonance is most commonly addressed by mechanical or electrical rate damping. Rate dampers are the equivalent of shock absorbers on a car. With rate dampers the 'bounce' is suppressed to a single cycle. This scheme does reduce resonance, but since the resonance phenomena comes from a stepper motor’s basic construction, it is not possible to entirely eliminate it.

Another approach to addressing stepper resonance and the associated loss of position is the closed loop stepper system. A subset of traditional stepper motion, it features a feedback device or one of various indirect parameter-sensing methods to 'closes the loop' to verify position, control missed steps, detect motor stalling, and enable greater usable torque output. As closed loop steppers commonly approach cost parity with DC servos, they have received a very flat acceptance in CNC application.

Servo motors are always used in closed loop configuration. The servo controller directs the operations of the servo motors by sending position and velocity command signals to an amplifier, which drives the servo motors. An integral feedback device (encoder) sends back to the controller the actual position and velocity of each servo motor. The controller in turn corrects any discrepancies between desired and actual position/velocity.

DC servo dynamic performance advantages over stepper systems are a result of: high output power relative to motor size and weight, reserve power 2-3 times continuous power, high torque to inertia ratio, reserve torque of 5-10 times rated torque (meaning they can rapidly accelerate loads), resonance and vibration free operation. The closed loop nature of DC servo design means that the current position is always known. Acceleration and deceleration in a stepping motor system must be limited to make certain that the holding torque of the motor is never exceeded. Otherwise steps and position would be lost. A DC servo motor does not have that same limitation. The torque available for acceleration and deceleration will be higher. This higher torque generally means better performance. On most part programs, cutting speeds are defined by the material and cutting tool used, not by a machine’s cutting speed limitation. A key difference between machines is how fast the machine can accelerate to that cutting speed and how quickly it can come to a stop. These factors can substantially impact cycle time, many times more than top speed.

AC servo motors, also referred as brushless DC servo motors offer the greatest performance edge in CNC control. This motor is constructed differently from the DC servomotor. In an AC servomotor, the rotor is a permanent magnet and the stator is wire wound. Because there are no wires in the rotor, there is no need for brushes. Commutation is performed electronically by the servo drive rather than being performed by the brushes and commutator. First, because there are no brushes to arc, the maximum limit to the power you can feed to the motor is only limited to the power required to melt the wires in the stator. As you can imagine, this is substantially greater than the power you can feed through the rotating brushes of a DC servo motor. The result is that in the same frame size, an AC servomotor will provide power and performance well beyond DC servo and stepper motors. However, it is worth noting, that a machine design (its foundation and drive train) must be able to take advantage of these higher performance drives. It makes little sense to pay more for drives that do not provide additional machine performance.

Gantry Height

Increased gantry height and longer Z axis travel must be approached by any machine designer with care. While greater “Z” clearance does provide the ability to process taller parts, it can be at significant cost. As a gantry gets taller and a Z axis slide gets longer, if not appropriately supported by structural elements, accuracy and stability can diminish swiftly.

For this reason alone it is best to keep the Z axis and gantry height as low as possible, but there is also another reason. As the Z axis becomes longer (see graphic directly below) it is increasingly difficult to obtain consistent accuracy at the bottom of the stroke, and getting the long axis to travel in a perfect straight line is only the first problem. The “Z” axis is generally attached to the cross axis on the gantry, and if not appropriately supported by structural elements, not only does the Z axis moving on the gantry tend to vibrate and pitch with small alignment errors, but the entire gantry as it moves along the base can also vibrate and pitch.

If you absolutely need extra “Z” axis clearance, you can expect to either pay more for an appropriately structured system, slow machine execution, or live with a poorer quality cut. As a general rule, buy only the Z axis travel and gantry height you need. Getting a taller gantry in case you might need it in the future can be very costly for current production.