Cnc machining nm09/2
Tool nose radius compensation
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- Navigate this page:
- Point two
- Point three
- 4.24.1 When do I use it
- 4.24.2 Tool nose radius compensation commands
- 4.24.3 A method of remembering whether to use G41 or G42
- 4.25 How to call up and use tool nose radius compensation
- 4.25.1 The rules
4.24 Tool nose radius compensationPoint oneIf a lathe tool was ground so that angles formed a sharp point then tool nose radius compensation would not have to be considered as this point of the tool would always locate at the programmed point. Referring to the figure below, you will see that if you programmed a tool movement of say X50, Z100 then the point of the tool would be located at this program point. 
Figure 4.57 To have a tool without a nose radius will produce an often undesirable surface finish, with the added problem of the tool point frequently burning away. To overcome these problems a tool nose radius o/between 0.4 and 0.8 is normally applied. Point twoWhen considering a tool with nose radius as shown below, it is important to note that during the setting of the tool offsets, the tool contact point ‘a’ located in the datum in the Z plane while the contact point ‘b’ located the datum in the X plane. The intersection point of a tangent line drawn through points ‘a’ and ‘b’ forms the tool program point. This means that when a tool position of say, X5, Z100 is ordered, the tool program point would locate at these coordinates thus putting the two contact points (a and b) in their correct X and Z axis positions. 
Figure 4.58 Point threeConsidering a tool with a nose radius. If the tool movements were all X and Z values at 90° to each other then no special considerations regarding nose radius is required. However, where a taper or an arc is to be cut, tool nose radius compensations must be applied otherwise an oversize or undersized taper or arc will be cut. The example illustrated below shows how, even though the tool program point follows the finished size geometry of the workpiece, the taper cut will be oversize. 
Figure 4.59 4.24.1 When do I use it?Because tool nose radii only affects tapers and arcs, radius compensation need only be applied on the finishing cuts of programs in which arcs and tapers are required. 4.24.2 How do I apply it?The application of tool nose radius can be delivered by a number of ways including the use of reference charts or by trigonometry calculations. At this level of training the method which will be used is a programming command to which a Machine Control Unit (MCU) equipped with the appropriate ‘G’ code function will respond by calculating and automatically applying tool nose radius compensation until cancelled by another ‘G’ function command. 4.24.2 Tool nose radius compensation commandsG41 —The command used to apply tool nose radius compensation to the left of the line of travel. G42— The command used to apply tool nose radius compensation to the right of the line of travel. G40 — Cancel all nose radius compensation.
Ask yourself: ‘Which way must I move so that the cutting edge of the tool will move along the edge of the job? ‘ In the example shown below, the direction of motion required was to the right of the direction of feed and so a G42 command will be required.
In the example below, the direction of motion was to the left of the direction of feed and so a G41 command will be required. 
Figure 4.62 4.25 How to call up and use tool nose radius compensationThe program block in which the mode changes to G41, G42 or G40 is called the start up block. During this block, the MCU will respond to the selected command by calculating and then physically adjusting the position of the centre of the tool nose radius and the distances required in both the X and Z axis. This adjustment will position the radius portion of the tool's cutting edge at a tangent to the required curve or taper as shown below. Once applied, the tool noise radius compensation selected will remain in force until cancelled by the G40 command. 
Figure 4.63 When tool nose radius compensation is called up, the command, whether it is G41 or G42 must be called up during a motion in either linear feed rate or rapid traverse. The preferred method is to call up tool nose radius compensation on a feed rate motion line. Referring to the figure below, the tool is driven in rapid traverse from the point XI00, Z100 to the point X80 Z100. As this motion is simply in the X axis no compensation is required. As this job involves the machining of a taper the finishing cut must incorporate tool nose radius compensation. This is done by including the G42 command on the feed rate traverse to the point X70 Z90, as shown in the diagram. It is during this motion that the required compensation is set.
The program movements for this job would be written as follows: G00 X80 Z100 G42 G01 X70 Z90 4.25.1 The rulesAll non motion ‘G’ commands must be written before any ‘G’ command which refers motion control. This means that a G41 nose radius compensation command must always proceed any G01 X. Z. command. Eg. G41G01X20Z50. Once radius compensation has been set, do not interrupt the read ahead function with non motion commands as this may cause the MCU to miss applying the required compensation for one or more blocks. For example: G41 G01 X50 Z100 Z50 G00 X60 Z100 G96 S200 FO.2 M09 G01 X44 Z60 In this example the block G96 S2000 F0.2 M09 contains non motion commands to which the MCU will try and apply nose radius compensation. Because no radius compensation is required for such a block no slide position adjustment calculation is made for this and the next block which is a taper. The result will be an incorrect taper.
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