The Basics Of Computer Numerical Control（2）
Key concept number four: You must Understand the forms of compensation
All types of CNC machine tools require some form/s of compensation. Though applied for different reasons on different machine types, all forms of compensation allow the CNC user to allow for unpredictable conditions related to tooling as the program is developed. Before discussing how compensation applies to CNC usage, let's look at compensation in general terms.
Compensation is used in many facets of everyday life. The airplane pilot must compensate for wind velocity and direction as a heading is set. The race car driver must compensate for weather and track conditions as a turn is negotiated. A bowler must compensate for the spin of the bowling ball as the ball rolls down the alley. A marksman firing a rifle must compensate for the distance to the target. The marksman analogy is amazingly similar to what happens with many forms of compensation on CNC equipment, so let's discuss it further.
Say a marksman is standing 50 yards from a target. By one means or another, the marksman would adjust the sight on the rifle to allow for the 50 yard distance. The marksman would make the necessary adjustment, but until the first shot is fired, the marksman will not know for sure whether the initial sight adjustment was perfectly correct. Once the first shot is fired and the marksman can see the resulting hole location, the sight may have to be fine tuned to adjust for minor imperfections with the initial adjustment.
In similar fashion, the CNC user will be faced with several situations when it will be impossible to predict the result of certain tooling related problems. So one form or another of compensation will have to be used to handle the problem. But just as the marksman may have to fine tune after the initial shot, so may the CNC user have to fine tune the initial compensation entry. More on how and why in a little while.
What are offsets?
All forms of compensation work with offsets. You can think of CNC offsets as like memories on an electronic calculator. If your calculator has memories, you know you can store a constant value into each memory for use during a calculation. This keeps you from having to enter the number over and over again with redundant calculations.
Like the memories of an electronic calculator, offsets in the CNC control are storage locations into which numerical values can be placed. Just as the value within the memory of a calculator has no meaning until referenced by its user within a calculation, neither does the value within an offset of the CNC control have any meaning until it is referenced by a CNC program.
From the marksman analogy, you can think of the values stored in CNC offsets as like the amount of adjustment required on the sight of the rifle necessary to compensate for the distance to the target. Keep in mind that the rifle only needed adjustment for one purpose, to adjust for the distance to the target. With most CNC machine tools, there is a need for at least one offset per tool.
Reasons for tool offsets
Offsets can be used for several purposes depending on the style of machine tool and type of compensation being used. Here are some of the more common applications for offsets.
To specify tool each tool's length
For machining center applications, it would be very difficult for the programmer to predict the precise length of each tool used in the program. For this reason, the feature tool length compensation allows the programmer to ignore each tool's length as the program is written. At the time of setup, the setup person measures the length of each tool and inputs the tool length value into the corresponding offset.
To specify the radius of the cutting tool
When milling on the periphery of the cutter (contour milling), it can be cumbersome and difficult for the programmer to program the cutter's path based on the size of the milling cutter being used. Also, if the cutter size must change (possibly due to re-sharpening), it would be infeasible to change the program based on the new cutter size. For this reason, the feature cutter radius compensation allows the programmer to ignore the cutter size as the program is written. The setup person inputs the size of each milling cutter into its corresponding tool offset. In similar fashion, turning centers have a feature called tool nose radius compensation. With this feature, an offset is used to specify the radius of the very tip of the turning or boring tool.
To assign program zero
Machining centers that have fixture offsets (also called coordinate system shifting) allow the user to specify the position of the program zero point within offsets, keeping the assignment of program zero separate from the program. In similar fashion many turning centers allow the assignment of program zero with offsets (this feature is commonly called geometry offsets).
To allow sizing on turning centers
Tool offsets are used on all turning centers to allow the operator to hold size with tools used within their programs. This allows the operator to adjust for imperfections with tool placement during setup. It also allows the operator to adjust the tool's movements to allow for tool wear during each tool's life.