The Basics Of Computer Numerical Control
Key concept number two: Know your machine
A CNC user MUST understand the makeup of the CNC machine tool beingutilized. While this may sound like a basic statement, a CNC user must be ableto view the machine from two distinctly different perspectives. Here in keyconcept number two, we will be viewing the machine from a programmer'sperspective. Much later, in key concept number seven, we will look at themachine from an operator's viewpoint.
Basic machining practice - the key to success with any CNC machine
Many forms of CNC machines are designed to enhance or replace what iscurrently being done with more conventional machines. The first goal of any CNCbeginner should be to understand the basic machining practice that goes intousing the CNC machine tool. The more the beginning CNC user knows about basicmachining practice, the easier it will be to adapt to CNC.
Think of it this way. If you already know basic machining practice as itrelates to the CNC machine you will be working with, you already know what itis you want the machine to do. It will be a relatively simple matter oflearning how to tell the CNC machine what it is you want it to do (learning toprogram). This is why machinists make the best CNC programmers, operators, andsetup personnel. Machinists already know what it is the machine will be doing.It will be a relatively simple matter of adapting what they already know to theCNC machine.
For example, a beginner to CNC turning centers should understand the basicmachining practice related to turning operations like rough and finish turning,rough and finish boring, grooving, threading, and necking. Since this form ofCNC machine can perform multiple operations in a single program (as many CNCmachines can), the beginner should also know the basics of how to processworkpieces machined by turning so a sequence of machining operations can bedeveloped for workpieces to be machined.
This point cannot be overstressed. Trying to learn about a particular CNCmachine without understanding the basic machining practice related to themachine would be like trying to learn how to fly an airplane withoutunderstanding the basics of aerodynamics and flight. Just as a beginning pilotwill be in for a great number of problems without understanding aerodynamics,so is the beginning CNC user have difficulty learning how to utilize CNCequipment without an understanding of basic machining practice.
Learning about a new CNC machine - the key points
From a programmer's standpoint, as you begin to learn about any new CNCmachine, you should concentrate on four basic areas. First, you shouldunderstand the machine's most basic components. Second, you should becomecomfortable with your machine's directions of motion (axes). Third, you shouldbecome familiar with any accessories equipped with the machine. And fourth, youshould find out what programmable functions are included with the machine andlearn how they are programmed.
While you do not have to be a machine designer to work with CNC equipment,it is important to know how your CNC machine is constructed. Understanding yourmachine's construction will help you to gauge the limits of what is possiblewith your machine. Just as the race car driver should understand the basics ofsuspension systems, breaking systems, and the workings of internal combustionengines (among other things) in order to get the most out of a given car, somust the CNC programmer understand the basic workings of the CNC machine inorder to get the most from the CNC machine tool.
For a universal style slant bed turning center, for example, the programmershould know the most basic machine components, including bed, way system,headstock & spindle, turret construction, tailstock, and work holdingdevice. Information regarding the machine's construction including assemblydrawings is usually published right in the machine tool builder's manual. Asyou read the machine tool builder's manual, here are some of the machinecapacity and construction questions to which you should find answers.
- What is the machine's maximum RPM?
- How many spindle ranges does the machine have (and what are the cut-offpoints for each range?
- What is the spindle and axis drive motor horsepower?
- What is the maximum travel distance in each axis?
- How many tools can the machine hold?
- What way construction does the machine incorporate (usually square ways,dovetail, and/or linear bearing ways)?
- What is the machine's rapid rate (fastest traverse rate)?
- What is the machine's fastest cutting feedrate?
These are but a few of the questions you should be asking yourself as youbegin working with any new CNC machine. Truly, the more you know about yourmachine's capacity and construction, the easier it will be to get comfortablewith the machine.
Directions of motion (axes)
The CNC programmer MUST know the programmable motion directions (axes)available for the CNC machine tool. The axes names will vary from one machinetool type to the next. They are always referred to with a letter address.Common axis names are X, Y, Z, U, V, and W for linear axes and A, C, and C forrotary axes. However, the beginning programmer should confirm these axisdesignations and directions (plus and minus) in the machine tool builder'smanual since not all machine tool builders conform to the axis names we show.
As discussed in key concept number one, whenever a programmer wishes tocommand movement in one or more axes, the letter address corresponding to themoving axes as well as the destination in each axis are specified. X3.5, forexample tells the machine to move the X axis to a position of 3.5 inches fromthe program zero point in X (assuming the absolute mode of programming is used.
The reference point for each axis
Most CNC machines utilize a very accurate position along each axis as astarting point or reference point for the axis. Some control manufacturers callthis position the zero return position. Others call it the grid zero position.Yet others call it the home position. Regardless of what it is called, thereference position is required by many controls to give the control an accuratepoint of reference. CNC controls that utilize a reference point for each axisrequire that the machine be manually sent to its reference point in each axisas part of the power up procedure. Once this is completed, the control will bein sync with the machine's position.
Accessories to the machine
The third area a beginning CNC user should address is related to otherpossible additions to the basic machine tool itself. Many CNC machine tools areequipped with accessories designed to enhance what the basic machine tool cando. Some of these accessories may be made and supported by the machine toolbuilder. These accessories should be well documented in the machine toolbuilder's manual. Other accessories may be made by an after-marketmanufacturer, in which case a separate manual may be involved.
Examples of CNC accessories include probing systems, tool length measuringdevices, post process gauging systems, automatic pallet changers, adaptivecontrol systems, bar feeders for turning centers, live tooling and C axis forturning centers, and automation systems. Truly, the list of potential accessorydevices goes on and on.
The programmer must also know what functions of the CNC machine areprogrammable (as well as the commands related to programmable functions). Withlow cost CNC equipment, often times many machine functions must be manuallyactivated. With some CNC milling machines, for example, about the onlyprogrammable function is axis motion. Just about everything else may have to beactivated by the operator. With this type of machine, the spindle speed anddirection, coolant and tool changes may have to be activated manually by theoperator.
With full blown CNC equipment, on the other hand, almost everything isprogrammable and the operator may only be required to load and removeworkpieces. Once the cycle is activated, the operator may be freed to do othercompany functions.
Reference the machine tool builder's manual to find out what functions ofyour machine are programmable. To give you some examples of how manyprogrammable functions are handled, here is a list a few of the most commonprogrammable functions along with their related programming words.
An "S" word is used to specify the spindle speed (in RPM formachining centers). An M03 is used to turn the spindle on in a clockwise(forward) manner. M04 turns the spindle on in a counter clockwise manner. M05turns the spindle off. Note that turning centers also have a feature calledconstant surface speed which allows spindle speed to also be specified insurface feet per minute (or meters per minute)
Automatic tool changer (machining center)
A "T" word is used to tell the machine which tool station is to beplaced in the spindle. On most machines, an M06 tells the machine to actuallymake the tool change. Tool change (on turning centers) A four digit"T" word is used to command tool changes on most turning centers. Thefirst two digits of the T word specify the turret station number and the secondtwo digits specify the offset number to be used with the tool. T0101, forexample specifies tool station number one with offset number one.
M08 is used to turn on flood coolant. If available M07 is used to turn onmist coolant. M09 turns off the coolant.
Automatic pallet changer
An M60 command is commonly used to make pallet changes.
Other programmable features to look into
An M60 command is commonly used to make pallet changes.
As stated, programmable functions will vary dramatically from one machine tothe next. The actual programming commands needed will also vary from builder tobuilder. Be sure to check the M codes list (miscellaneous functions) given inthe machine tool builder's manual to find out more about what other functionsmay be programmable on your particular machine. M codes are commonly used bythe machine tool builder to give the user programmable ON/OFF switches formachine functions. In any case, you must know what you have available foractivating within your CNC programs.
For turning centers, for example, you may find that the tailstock andtailstock quill is programmable. The chuck jaw open and close may beprogrammable. If the machine has more than one spindle range, commonly thespindle range selection is programmable. And if the machine has a bar feeder,it will be programmable. You may even find that your machine's chip conveyorcan be turned on and off through programmed commands. All of this, of course,is important information to the CNC programmer.