- How precise is your CNC Router 3 axes?
- Why do you install stepper motors and not brushless motors (servo motors)?
- What is the speed of the router?
1) WHY DO YOU INSTALL STEPPER MOTORS AND NOT SERVO MOTORS?There are certain mechanical considerations you must factor in when deciding whether you need stepper motors or whether you need to switch to servo motors (brushless motors) – or even to an intermediate solution with stepper motors and encoders. For a machine like the Next CNC Router, we chose Sanyo Denki, which are professional Japanese stepper motors that provide excellent performance in terms of torque and power. The chosen motors largely meet the performance requirements that a machine must have for our applications. In addition, the stepper motors have other advantages:
- High availability of stepper motors and drivers in the market
- Easy to configure and install even when replacing for repairs
- Cheaper than brushless motors
2) IS YOUR CN BASED ON AN OPEN-LOOP OR CLOSED-LOOP SYSTEM?Often we talk about open-loop automated systems and closed-loop systems, almost always implying that closed-loop systems, with feedback, are more reliable and precise and therefore the only solution. Indeed, there are applications where feedback is not needed and where the accuracy of other systems is more than acceptable. For example, most of us use the car to get from home to work, knowing that airplanes exist as well. Planes are much faster than cars but they don’t always offer the best solution and they wouldn’t necessarily get you to work any sooner – but would certainly be more expensive. With that principle in mind, we can work using an open loop system without feedback because the stepper motors do not require any feedback. With stepper motors, you know how to calculate how much an axis moves for each step so the position of an axis is always known – the control knows it at all times. In brushless motors, it is necessary to have feedback, through an encoder, because they are controlled by an analog voltage and it is not possible to deduce how much the axis is moving without reading data from an encoder.
3) DO THE STEPPER MOTORS IN YOUR CNC ROUTER LOSE STEPS?The so-called “loss of steps” and the resulting error in the position reference is a problem that has been overcome in modern stepper motors. Step loss can only occur if the motors are used incorrectly, for example, if they run at speeds that are too high, or lose torque, or if the mechanics connected to the motors are not suitable for the application. In a properly dimensioned CNC machine, loss of pitch is practically impossible. In stepper motors, torque is not constant but decreases as the speed with which they turn increases. There is, therefore, a speed threshold that must never be exceeded. With this principle in mind, the stepper motors continue to be a very good solution for low-speed applications. We find that they are perfect for use in stone processing machines. In the case of a Next Router, motors which can run up to 2700 rpm are used, but they are configured to run at a maximum speed of 500 rpm. This is to make sure that we are well below the threshold where we would encounter lost of steps and it guarantees that our Next Router will never lose steps when you are using it.
4) IS A CNC ROUTER WITH STEPPER MOTORS PRECISE?First, we must define what is meant by the terms precision and resolution.
- The precision in our case can be defined as the maximum difference between equal repeated movements in the axis.
- Whereas the resolution can be explained as the minimum movement that the axis can carry out.So the correct question would be the following one…
5) WHAT IS THE RESOLUTION OF A CNC ROUTER NEXT?The stepper motors installed in our CNC Router are controlled in micro steps. To complete a single rotation of the shaft (360 degrees) requires 3200 micro steps. This is the maximum number of micro steps to ensure full torque output from the motor. In datasheets of some imported motors (ie. from China), you might be quoted much higher step numbers. Although it’s true that the motors can achieve more micro steps in one rotation, once it goes beyond a certain limit the performance of the motor degrades substantially – so they become unusable. Returning to our case, using only 3200 micro steps means that the pitch angle of our motors is 0.11 degrees (360/3200). Obviously the smaller the pitch angle, the more resolute the engine is. Considering the diameter and the pitch of the screws installed in a Next Router we have the following resolutions:
- X axis has a resolution of 0.003 mm
- Y axis has a resolution of 0.003 mm
- Z axis has a resolution of 0.015 mmThat’s right, we are talking about resolutions in micrometers!
6) AND PRECISION?The precision depends on the backlash (play) of the mechanics used, and therefore on the maximum tolerances given by the “precision classes” chosen for: screws and recirculating ball nuts, recirculating pads, and their couplings. The mechanics must be chosen in such a way as to have an “acceptable” step deviation for the application. Acceptable means a tolerance less than the maximum precision you want to achieve in your applications. If in your applications you can make mistakes of a few millimeters, then having a deviation of step in the order of the millimeter could work well. Obviously, that’s not relevant in our case because the Next CNC Router can achieve tolerances in micrometers! In order to increase accuracy, in addition to selecting the appropriate accuracy classes, it is essential to use the correct preloads in recirculating sliding blocks and nuts. In the case of our CNC Machine, the choice of recirculating screws, precision classes, deviation of maximum acceptable pitch and the choice of preloads has led to a longer axis design (Y axis = 1400 mm of useful stroke). It means that if in a single movement all the components perform at their very worst, and if at the same time the movement happened on the whole axis of 1400 mm, there would be a maximum error of slightly less than 1/10 millimeter. Obviously, the backlashes of the mechanical components also depend on accelerations/decelerations and on the loads. In fact, using the machine with the right accelerations and the right loads, you will never reach the maximum stress limits for which the mechanics are designed. In our CNC Machine, we have installed an OSAI Controller that allows us to manage the accelerations with jerk ramp. Jerk is the derivative of acceleration, e.g. the variation of acceleration over time. The machine, therefore, does not move with constant acceleration but with variable acceleration allowing soft movements even when maximizing speeds. With this data and these considerations, we can safely say that the minimum precision of a Next CNC Router is much less than 1/10 of a millimeter.
7) WHAT IS THE SPEED OF YOUR CNC MACHINES?When we talk about machine speed we mean the axis feed speed. The speeds depend on the pitch and the length of the screws, the speed of the stepper motors and above all the relationship between the motors and the torque rpm. In our applications (marble working, of course) you do not need to have speeds that are too high because:
- The dimensions of the axes are not big
- The rapid movements, in terms of time, are trifling if related to theduration of the movements during the cut.
- The cutting speeds are very low if compared to those of other sectorssuch as plexiglass, paper, fabric, woodworking, etc.
CONCLUSIONThe dimensions of a CNC machine, together with all the design and component choices, are based on precision engineering. Case by case, various aspects of performance and technology are analyzed, allowing us to make the best choices. In the future, when we will build bigger machines, we will certainly also use brushless motors. We will write an article like this one, using a case study to explain why we have used brushless motors in bigger machines.