CYCLES
Lathe Cycles — To Use or Not to Use?
Conversion of Lathe Cycles
Fixed Cycles Repetition
Lathe Cycles G70–G72 — Part 1
Lathe Cycles G70-G72 — Part 2
Threading with G76 Cycle — Basics
Threading with G76 Cycle — Details
THE MATHEMATICS OF CNC
Focus on Numbers
When 1 Thou Equals 65 Dollars
Easing Up on Calculations
Feedrate Adjustment on Arcs
CNC GEOMETRY
Working in Planes
A Case for Polar Coordinates
Programming a Full Circle
WORKING IN THE FIELD
Are You a CAM Machinist?
CNC Programmer/Operator — Should One Person Be Both?
Using CAM Software in Small Shops
APPENDICES
Appendix 1 Interpreting a CNC Program
Appendix 2 Default Settings in Macros
Appendix 3 Create Your Own G-Cod
Appendix 4 Scaling Option
Appendix 5 Safety and CNC Programming
Appendix 6 Lathe Cycles G70-G72 — Part 2
INDEX
 | CNC Turning — Why Are There So Many Errors? January 2004, updated February 2013 |
Even in this era of CAD/CAM, many of the part programs that are available for two-axis CNC lathes are still developed manually. There is a good reason why. Purchasing a CAD/CAM system just for a lathe may not be the primary choice of many managers. Modern CNC lathes offer many time-saving features. For example, all centerline operations — such as drilling, reaming, and tapping — are simple point-to-point motions. Lengthy turning and boring cuts can be dramatically shortened by the use of very powerful machining cycles. Special cycles are also available for external and internal threading operations, even for machining castings and for simple grooving.
Yet, with all these programming tools at our disposal, many lathe programs suffer from a number of errors. It is not unusual to see frustrated CNC operators frantically changing programs at the machine, at a great cost to productivity. Why do so many lathe programs suffer this fate?
When I talk to programmers and operators, I get many opinions but seldom real answers. All it takes is a careful look at many of their lathe programs to see that the same type of error appears over and over again. Let’s look at them in groups, with some views on what can be done to prevent them:
Calculation Errors
For all their power and simplicity, no lathe cycle will provide calculations of the contour points. When an error is evaluated, typically the programmer either “guessed” or miscalculated. Brushing up on trigonometry is the first step toward improvement in this area. Of all the math subjects, trigonometry is relatively the highest knowledge programmer should possess. I know a few programmers who avoid this obstacle by asking their engineers to give them the contour points from Autocad or some other program.
Other types of calculation errors are accumulated ones caused by incorrect rounding. To avoid this problem, round only the final result — not the intermediate calculations. Many calculations can be confirmed by selecting a different mathematical approach.
Syntax Errors
Syntax errors often occur when the pesky letters O is used instead of the digit 0. These Os are the illegal characters that somehow find their way into the program. For example, the letter Y is not available on a two-axis CNC lathe. Fortunately, the control system will identify these errors.
Logical Errors
Watch very carefully for logical errors because the control system has no means to discover them. For example, a missing decimal point or a negative sign will give a totally new meaning to the programmed dimension. X1 is not the same as X1.0, and Z1.0 is not the same as Z-1.0.
Program Errors
Program errors that are fairly frequent are often those that can cause severe collisions. Making a tool change inside of a hole is one such error. Others include wrong tool or offset selection, excessive spindle speed or feedrate, selection of tool change position, and many others.
Offset Errors
This area is a fertile ground for many frustrations. In my experience, errors relating to the tool nose radius are at the top of this group. If a tool nose radius error is detected by the control system, the cause of this error is always the same — the programmed radius cannot fit into the area provided. This group also represents errors that may be the hardest to identify. Always check the offset settings. If the settings are correct, the program itself is at fault. This error often occurs when the tool nose radius is larger than an inside arc of the contour. It also occurs because of insufficient clearances. Because the most common tool nose radii are 1/64 (0.4 mm), 1/32 (0.8 mm) and 3/64 (1.2 mm), always provide a clearance that is at least twice the largest radius. Typically, 0.100 inches or 2.5 mm is sufficient as the minimum clearance. Keep in mind that this is per side, not on diameter.
Setup Errors
Setup errors are strictly the domain of the CNC lathe operator. Check all tools by running the program while the chuck is empty. Are there sufficient clearances? Can the tools index safely? Is the tailstock out of the way? Provide a sufficient grip of the workpiece in the chuck, then watch for the boring bar to exit from a hole safely. Running the first part in a dry run and single block modes may save the day.
Correcting Errors
This area deserves special mention. Many operators fail to identify the actual cause of an error. For example, if a drill does not provide the expected depth, is the programmed depth correct? Is the offset set correctly? Is it even possible that the drill was pushed slightly into the holder? Identifying the real cause of a problem is the first step towards its elimination.
I hope by reviewing some of the common errors found in many programs for CNC lathes that you can look at your own programs and see them in a much different light.
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