Equipment MLS: Understanding CNC Machining and their Controls

Understanding CNC Machining and their Controls

The goal of any user beginning in CNC should be the understanding of the basic machining practices of operating the CNC machine tool. The four basic areas a programmer should concentrate on are:

Understand the machine’s basic components.
Know the machine’s directions of motion or axes.
Familiarize yourself with any machine equipped accessories.
Find out what programmable functions are included with the machine and learn how they are programmed.

This article helps you learn more about each CNC function as it applies to your CNC machine(s). It explains how and why it functions the way it does and shows you how to adapt with any style of CNC machine tool.

CNC FUNDAMENTALS

CNC equipment offers important benefits. Improved automation has dramatically cut down the need for a machine operator and CNC’s can run unattended during an entire machining cycle, freeing the operator for other tasks. CNC’s today are unbelievably accurate, especially in repeatability specifications. The flexibility of a CNC can run a variety of different programs and have work pieces leading to fast changeovers and set ups. Motion control is the basic function of any CNC machine. All forms of CNC equipment have 2 or more directions of motion called axes. There are 2 axis types: 1. Lineardriven along a straight path. 2. Rotary- driven along a circular path. On a linear axis , the number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis and it’s very precise. CNC machines allow motions to be actuated by servomotors instead of turning cranks and handwheels of the past.

COMMAND OF AXIS CONTROL How is axis motion commonly commanded in CNC machines?

The program “0” point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a common location. Be careful though, when in the absolute mode if a motion mistake is commanded in the program only one movement will be wrong. However, if a mistake is made during incremental movements, all motions from the point of origin will be wrong. All CNC controls allow axis motion to be commanded by the use of a coordinate system. The goal here is to utilize a system where the information is commanded simply and logically. There are essentially two systems used for CNC machines: 1. Rectangular or “Cartesian” coordinate system. 2. Polar coordinate system. The rectangular or “Cartesian” coordinate system is the one most commonly used in CNC. With the advent of numerical control, it became necessary to identify the directions of travel that were different from machining of the past. John Parsons, the father of numerical control, started using coordinates to define points on airfoil surfaces. The rectangular Cartesian coordinate system, developed centuries earlier, allows the location of any point on a flat surface or plane to be defined mathematically with reference to two lines (axes) in the same plane and perpendicular to each other. The point of intersection of these two axes can be given a “0” value for dimensioning purposes. Graphing is a very common application since it closely resembles what is needed to cause action motion on a CNC machine. This assumes the absolute mode of programming, where the end points for all motions will be specified from the program “0” point. There is another way of determining end points for axis motion. This is known as the incremental mode. Here, end points for motions are specified from the tool’s current position, not from program “0”.

OFFSETS

A better way of assigning program zero is through offset. Offsets in your CNC control are storage locations into which numerical values can be placed, keeping you from having to keep entering the number again and again with redundant calculations. Depending on the style of machine tool and type of compensation used, offsets can be used in several different ways: 1. Specifying the precise length of each tool used. 2. Specify the radius of the cutting tool. 3. Assign program zero and allow the operator to hold size with tools within the program. Machining center controllers call it fixture offsets. Turning center manufacturers refer to it as geometry offsets.

INTERPOLATION

It requires more than determining end points to effectively command motion on most CNC machines. CNC control manufacturers try to make it easy to make movement commands within the program. During linear interpolation, the control will automatically calculate a series of tiny single axis departures, keeping the tool as close to the programmed linear path as possible. It appears the machine is forming a perfectly straight line motion when it’s actually doing linear interpolation. Many applications for CNC machine tools require the machine to form circular motions. This motion type is used to generate radii during machining. Circular motion requires the programmer to specify the radius of the arc to be generated. There are also other types of interpolation. For example, during the thread milling process, the machine must move in a circular manner along the x and y axes while at the same time a third axis (Z) moves in a linear manner. This allows the helix of the thread to be properly machined. This process is called helical interpolation. For turning centers, polar coordinate interpolation can be used to mill contours around the periphery of the workpiece. It allows the programmer to flatten out the rotary axis, treating it as a linear axis in order to make motion commands.

There are three basic motion types:

Modal---they remain in effect until changed.
End point---this motion is specified in each motion command.
Rapid---this motion type is used to command motion at the machine’s fastest possible rate to minimize non-productive time during the machining cycle.

MODES OF OPERATION

The most common mistake is having the mode switch in the wrong position. Even though you can see that the mode switch has several positions, there are really only three basic modes of operation:

Manual mode---the CNC machine acts like a standard or conventional machine. For example, a CNC machining center acts like a manual milling machine, and a CNC turning center acts like an engine lathe. In the manual mode, the operator is allowed to press buttons, turn hand wheels, and flip switches for the machine to function.
Manual Data Input mode (MDI)---This mode has two positions on the mode switch, the edit position & the MDI position. With the switch in the edit position, a program is entered or modified to the use of a word processor. With the switch in the MDI position, CNC commands are entered through the keyboard and display screen for execution. Both manual modes provide capabilities that can be done in a more automatic way and both involve entering data through the keyboard.
Program Operation mode---This mode has two switches, memory (auto) and tape. They are used to verify programs and run production. The memory (or auto) mode is used to execute programs from the control’s memory. The cursor can scroll through the program while executing, letting the programmer follow the commands being executed. Tape mode, used in earlier days, was the only way to activate programs but with the advent of computers, tape is virtually extinct. Many CNC controls no longer have the option to run tape.

OPERATING CNC'S

The operator should document the most important sequences so the machine can function in the desired manner. Learning how to operate a CNC machine is simply knowing when each procedure is required and following the basic sequence to attain the machine’s desired function. Sequences such as powering up, powering down, loading tools, setting offsets, and editing programs are among the things an operator will be doing on a regular basis and can be easily committed to memory. Being able to visualize machining operations that are to be performed during the execution of the program is what the programmer must do.

An operations handbook should be kept nearby for those procedures that are not as often used as the regular functions. They are the type of procedures that make it easy to operate your CNC, providing quick and easy reference material related to key operational procedures. These are the setup sequences for:

Measuring program “0” positions.
Tool length information.
Manual & manual data input sequences.
Program loading & saving sequences---for loading programs into memory.
Program editing & display sequences---for displaying a directory of programs in memory.
The program running sequences for verifying and running programs in production.

VERIFICATION AND SAFETY

There are three areas of safety you should concern yourself with when working with any machine tool:

Operator safety. The operator’s safety is stressed. Sometimes there’s a tendency to short-cut procedures to save time but it’s always recommended to verify procedures.
Machine tool safety. The operator must ensure that no damage to the machine occurs. When a CNC goes down for any period of time, the cost of repairing the machine is usually very little compared to the lost production time.
Workpiece safety. Making sure all work pieces are to size. The cost of rough stock varies dramatically based on the user’s application.

Typical mistakes made by the programmer and how you can diagnose these problems as they occur are:

Syntax mistakes are made when the control is not able to execute the command given to the machine and it’s usually made when a program is entered manually.
Motion mistakes are harder to diagnose and since they deal with motion they’re generally caused by incorrect coordinates. Even if a CNC program is perfectly written, there can still be errors made during the set-up calculations.
Measurements, like tool lengths and diameters, may have to be made regarding the setup and numbers entered into the control before the program is executed.
Omission mistakes are forgetful mistakes when a manual programmer leaves a decimal point out or forgets to put in the feed rate.

Another procedure used to verify programs has to do with the feed hold button. Always having a finger on the feed hold button is a good idea in case an unexpected rapid motion takes place. Also the machine lock and dry run switches allow the control to scan the program for syntax mistakes. During the program, the spindle will come on, the turret will index, and the control will appear to be actually running the program. A free flowing dry run is a check for motion problems. During this procedure, the operator will turn the machine lock switch to its lowest position and set the rapid override switch to its slowest motion rate. Sometimes this procedure may be repeated several times to make sure the motions are correct. Before running the first workpiece, the operator must execute the cycle once more with the dry run switch turned off. This allows the operator to see one thing that could not be seen during a free flowing dry run.

There are some procedures that can help ensure the tool will machine precisely to size:

Consider what the tool will be machining.
Adjust the tool offsets for the tool to machine to size and check to see what direction it’s leaving excess stock.
Allow the tool to completely machine the workpiece.
Measure what the tool has done, rerun the tool, and confirm measurements that the new offset values are correct.

Even though these safety and verification procedures seem like a great deal of work, it is easily justified when compared to the possible losses and consequences of down time.

This is one article in a series of How to Buy Metalworking Equipment. Each article showcases and explains a particular type of metalworking machine. They were originally published in the Metalworking Machinery Mailer published by the Tade Publishing Group.

Links to other articles in this series: