SST Shop Talk
Motion Control - The Heart of CNC
Motion control can be applied in many categories such as robotics, CNC operated machine tools and Kinematics, wherein motion control in kinematics are usually simpler. It can be mainly used nowadays with packaging, textile, assembly industries, printing, and semiconductor production. The hardware of a motion controlled machine usually consists of drive systems, motors, a computer, a PLC or Programmable Logic Controller to run the programs, and an amplifier.
The basic design of a motion control system would include a motion controller to produce a set of points including closing a position, a drive or amplifier to convert the control signal of the motion controller into a high power electrical current, an actuator, one or more feedback sensors, and mechanical components to convert the motion of the actuators to the desired motion.
CNC machines use programmable commands to make inputting motion to the machine easier rather than using cranks or other conventional machine tools. Almost all CNC machine tools can have programmable motion type (whether it would be rapid, linear or circular), the amount of motion, the feedback rate, and the axes to move.
Motion control is the simplest function of any Computer Numerical Control (CNC) machine. It is precise, consistent, and automatic system of control. CNC equipments need two or more modes of direction to which they are called axes. There are two common axis types and they are called linear and rotary. The linear axis type of motion control is driven along a straight path while the rotary axis type is driven along a circular path.
The operator of the motion controlled machine counting the number of revolutions made on the handwheel, added the generations of the dial would accomplish accurate positioning. The drive motor of the machine would be rotated to a resulting amount, which would then drive the ball screw, which would cause the linear motion of the axis. The feedback device at the end of the ball screw would confirm its revolutions.
The same linear motion can be found on a table vise. When you rotate the vise crank, it would also rotate a lead screw, which would then be able to drive the movable jaw in the table vise. In comparison to a motor controlled CNC machine, the linear axis in it is extremely precise compared to that of a table vise. This is because the number of revolutions of the axis drive motor in the CNC machine accurately controls the amount of linear motion along the axis.
A CNC command programmed and executed within a control of a machine would tell the drive motor of the machine as to how many number of precise times it would rotate. This in turn would rotate the ball screw then the ball screw would drive the linear axis. After the process has started, a feedback device located at the end of the ball screw would confirm the programmed number of rotations that the machine would run has taken in effect.
How would axis motion be controlled?
Utilizing a form of coordinate system would make axis controlling a whole lot simpler and more logical to the CNC control. Two coordinate systems that are being used in CNC machines that have been popular are rectangular and polar coordinate system, to which the more popular of the two is the rectangular coordinate system.
Graphing is a common application for the rectangular coordinate system and is needed to cause movement in a CNC machine.
The first person is the CNC programmer. He is like the "playmaker". He will create the programs that the CNC machines are intended to execute. Since the programs are in the form of CNC codes fabricated like sentences, he should have mastered these codes because they work like a different language. The regular CNC machine can use up to 50 codes, so that's like learning 50 new words for the newbie.
Also, the programmer should have at least and engineering or machining degree. Remember that the CNC machine will only execute WHAT IT IS PROGRAMMED TO DO. If the program is wrong, the whole operation goes down the drain with it. Moreover he should also be flexible and have a fast turn-around because a CNC machine is often used to machine a huge selection of different work-pieces.
The third person that will need is the CNC technician. Although this may still be the programmer, it is more convenient to always have a technical expert on-call because in the long run, you may have more than one CNC machine and you may need to prioritize over the other in case both gets crippled at the same time. Just like the programmer, the technician should also be flexible and articulate. CNC offers a lot of complexity when it is running right, how much more if it is behaving badly?
So, if you are having job openings for positions that need to be handling a CNC machine, ask the applicants first to do a demo for you and make sure that during the demo, they know what they're doing. An exam may also do wonders too. If they have no experience with any kind of CNC machine, it is advisable that you encourage them to take short courses on CNC.
Eighty-hour courses are available online and hey, it is better than nothing. Experts even encourage employers to hire CNC machinists who have finished AND PASSED the National Occupational Competency Testing Institute (NOCTI) assessment just so they could be sure that their CNC machines will go to good hands. After all, a CNC machine is still an asset.
The second person that you need is the CNC operator. He will simply recheck the programs loaded to the machine and push the right buttons to get the work done. However, thinking that a CNC operator can have little or NO SKILL AT ALL is wrong. A CNC machine operator must have at least basic machining skills and he should have undergone some form of training to run a CNC machine.
These machines can produce very intricate motions, making it possible to make shapes that cannot be created on conventional machine tools. So, the operator should foresee this complexity and know how to cruise with it. The skills that an operator must have though, are lesser compared to the operators of conventional machine tools.