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33. CNC Part Program Verification

Programs prepared for any kind of CNC machine should be cautiously verified.Though there are exceptions to this rule, manually prepared programs are moreprone to having mistakes than CAM system generated programs. However, even the

best CAM system generated CNC programs could still include disastrous problems.

Syntax Mistakes-These are "silly" mistakes on the programmer's part that cause theprogram to be unacceptable to the control.

Motion Mistakes-This kind of mistake is usually harder to find and correct.

Setup Mistakes-Even a perfectly prepared program will behave poorly if setupmistakes are made.

Cutting Condition Mistakes-Though the program's motions may be correct, the

operator must be on guard for cutting condition problems. Feeds and speeds mustbe properly applied. While machining the first workpiece with any program, theoperator must be very cautious, watching for possible machining problems.

The basic procedures that will help insure safety during the verification of a CNC program arediscussed below

1.1 Machine Dry Run

Prior to letting a program cause motion, it is wise to let the control check the programfor syntax mistakes. With Machine Lock and Dry Run turned on, the operator canrest assured that the axes of the machine will not move. When the program isexecuted, the control will scan the program for basic mistakes. If the controldetermines a problem, it will go into alarm state. While there could still be seriousproblems if the control completes the program, the operator can be rest assured thatat least the program is acceptable to the control. "Free Flowing" Dry Run

Once the Machine Lock Dry Run can be executed without generating alarms, theoperator is ready to let the program generate motion. The Free Flowing Dry Run will

allow the operator to see the motion the program will generate, and also allow theoperator to control how fast motion will be. With Dry Run in the on condition, a multiposition switch (usually Feed Rate Override or Jog Feed Rate) acts like a rheostat,allowing the operator to manipulate how fast axis motion will be. If the operatorsenses a problem, Feed Hold is pressed. With no part loaded into the setup, theoperator can allow the motion generated by the program to take place and will beable to tell if the basic motions are correct. The first time the Free Flowing Dry Run isexecuted for a program, the operator will be most concerned with dangeroussituations like interference with obstructions and spindle direction. For this reason, itmay be necessary to repeat this procedure several times before the operatorbecomes comfortable with the cycle.

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1.2 On-line program verification

Any time spent between production runs verifying CNC programs must beconsidered as part of setup time - and anything that can be done to reduce on-lineprogram verification time effectively reduces setup time. Since new programs willcontain more potential for mistakes than proven programs, companies that performlittle repeat business should be highly interested in minimizing the time spentverifying the CNC program. One way to achieve this goal is to move the task ofprogram verification off line. This means performing as much of the programverification procedures as is possible while the machine is still running workpieces ina previous production run. Following are several ways to verify CNC programs.

The CNC machine makes a poor verification tool. If the quality of the CNC programis unknown before it is loaded into the machine, the setup person must be very

careful at every step of the verification process. A series of dry runs must beperformed just to confirm that the basic motions of the program will not causeinterference. And since the machine is down between production runs, the entiretask of program verification must be done on line.

More CNC controls are coming with graphic capabilities that allow you to plot aprogram's movement's right on the display screen. While this is an excellent feature,and one we would recommend you purchase if it is available, keep in mind that manycontrols do not allow a tool path to be shown for one program while the machine isrunning another program. In such a case, program verification is still an on-line task.If the tool path display exposes mistakes in the program, corrections must be made

during setup, meaning corrections must also be done on line. This can waste a greatdeal of precious machine time.

Almost all current computer-aided manufacturing (CAM) systems allow some form oftool path display aimed at helping a programmer locate motion mistakes in aprogram. More and more software companies are developing attractively pricedproducts aimed at helping their users verify CNC programs off line. Severalcompanies offer PC based tool path display capabilities, similar to those found inmany current CNC controls.

34. CNC Part Program generation from CAD ModelGeneration of part program for machining a complex shapes is very difficult withoutuse of available CAD/CAM software packages. Use of this leads to decrease in timerequired for part programming and increases the accuracy of programming.

Software packages like PRO/engineer, ideas, MasterCAM allows the user to createmachining tool paths. Within the package you can create milling and turning (lathe)machining simulations. After creating a series of these simulations, the user canmachine the 3D model created and at a later stage transfer the information to a NC

lathe or miller.

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Generation of part program requires some basic information like final geometry,workpiece geometry, orientation of principle axis and location of origin on the rawmaterial geometry, machining parameters, tool geometry, machine which is to be

used etc. Other than this some information like which type of operation has to bedone, volume which is to be removed, etc also has to be given.