These adjustments will be placed in the work offset of Z-axis setting, which is normally zero:
The measurement method used here is the so called touch-off method. This is not the most efficient method, but one of the most common, particularly in small ma chine shops. The principles described here apply to any other method of tool length offset measurement.
For the example, three tool length offsets have been entered into the offset registry. Although the illustration shows a column for wear offset, some controls have only one column only for the geometry offset (as measured). OFFSET
Z-AXIS AND MULTIPART SETUP
So far, the examples shown in this chapter have fo cused on the top view of X-axis and Y-axis of the setup. Even if the same fixtures (such as vises) are of the same make, it is to be expected to have differences, however small, relating to the X and Y axes. This situation has been the focus of all previous sections in this chapter.
In actual multi-part setup, the Z-setting between indi vidual fixtures is also an important issue to be addressed.
Z 0.000 01 G54 || 02 G5503 G56 X-839.528 ||X-624.883||| X -412.067 Y-383.712 I-384567| -382.631 Z 0.000 Iz 0.060 Z -0.040 04 G5705 G58 | 06 659
0.000 1 X 0.000 X 0.000 Y 0.000 Y 0.000 Y 0.000 Z 0.000Z 0.000Z 0.000
EXTENDED WORK OFFSETS
In the text presented so far, there were many refer- ences to the common six work offsets in the range of G54 to G59. These are standard work offsets, available on just about every CNC machining center.
While the six work offsets are more than enough for the majority of work done in smaller machine shops, there are cases when more work offsets are needed, par ticularly in larger companies. One such case is when a dedicated set of a dozen or more CNC machines is used to complete a single part that requires many operations. Take, for example, an automobile engine. There are many operations required for full completion of this complex part, including many faces. Large manufactur ers will be more efficient if they design a full line of in dividual cells (CNC and other machines), with each cell dedicated to a certain machining operation.
Some of these operations will require no more that the six standard work offsets, while others will require more than six. This is where the optional extended set of work offsets comes in. It is optional, because you have to pay to have it installed.
Note that the G54 setting in the Z-axis is 0.000 for the left vise – this is where the part was measured and offsets entered into the tool length offset registry. For the mid dle and right parts, the control system will make all nec essary adjustments. Note – Even if the part stock is exactly the same height, the adjustments may be necessary for deviations between individual vises
When three parts are setup in three different vises, the tool offset may or may not be the same for all three parts. A good bet is that they will not be the same. What are the options to make the adjustments for the part height dif ferences?
The first answer might be more tool length offsets. In theory, that would solve the problem, but only in theory. Consider not only three parts but sixteen – for example, four parts on each face of a four-sided tombstone, com mon in horizontal machining. Not only that the setup would take a lot of time, you may actually run out of the available offsets.
Better solution is to use the Z-setting of each work off set. Look at the three parts (illustration is simplified for clarity) with some minor height variations.
Just as you cannot expect absolute precision along the X and Y axes for multiple part setup, you cannot ex pected it in the Z-axis either.
The Z-axis offset settings control the height of the part and the machined depths. Assuming the part program is correct and that it takes into consideration the setup yari ations, it still comes down to the actual and physical setup at the CNC machine, using offsets.
Even with all parts having the same thickness, some deviation of the Z-axis settings is to be expected. With a single part, it is only a matter of the tool length offset set ting. With multiple parts used in a single setup, each part has to be accurate, which means each part has to have a unique setting in the program. Tool length offset has been described in a separate chapter, starting on page 87.
The first application will be the same part in three vises, with minor deviation in height. For clarity, the fix ture (vise) is not shown, only the important dimensions.
Fanuc and similar control systems offer additional forty eight (48) work offsets. Combined with the six standard work offsets, the number of work offsets is staggering fifty four!
As expected, there must be some difference in the pro gram between the standard and the extended work offset range. Indeed, there is.
Using EXT offset
The two work offset adjustments shown above control each fixture separately. That is not always the case, as some adjustments apply to all fixtures equally. For ex ample – for some reason, you change parallels in all three vises and replaced them with new ones that are 3 mm narrower. All three parts are now lower by 3 mm.
What are the options?
One option is to change the tool length offset for each and every tool by 3 mm:
Standard vs. Extended Work Offset Range
In the CNC program, the standard range of six work offset is recognizable by the G-code:
· G54 to G59 is the standard work offset range
Tool Length Offsets
Tool length is normally measured for a single part; you measure one tool length for each tool on the same part. This does not change for multiple part setup. In the illustration, the part in the left vise was measured.
H01 H02 H03
Tool T01 / H01 … from -368.345 to -371.345
• Tool TO2 / HO2 … from -312.794 to -315.794
Tool T03 / H03 … from -357.820 to -360.820
The extended set uses G54.1 command, followed by the P-address:
G54.1 P1 to G54.1 P48
Example: 654 = the first standard work offset G54.1 P1 = the first extended work offset (above G59) Incidentally, G54.1 P.. is the same as G54 P.., etc.
In all three cases, the change was the same – 3 mm. Since the parts are now lower than before, the original distance measured will be 3 mm larger. There is nothing wrong with this method in mathematical terms. What is wrong is its inefficiency. For a large number of tools, to change all offsets is time consuming and prone to errors.
Although the illustration is not to scale, it clearly shows that the middle part is 60 microns higher and the right part is 40 microns lower.
Part located in the left vise
CNC Control Setup for Milling and Tuming MULTIPART SETUP
The solution is to use the EXT (External or Common offset. All you have to do is to put the amount of 3 mm and its directional adjustment (minus) into the EXT off set, which is part of the work offset settings:
Suppose that a certain part has five operations before it is fully completed. Design permitting, each operation requires a specific (progressive) setup. If the part is small enough to fit on the machine table, a total of five different fixtures will be required. The core of the pro gram will be to make connections between the fixtures by using various offsets. For setup like this, study this chapter carefully – the information is there – just make sure you apply it well.
For more details on the EXT work offset, see page 85.
This chapter has presented various setting options ap plied to locating several parts on the machine table at the same time. As expected, there are variations of these set tings, depending on the exact work situation. Also, there are applications of these methods that have not been mentioned. Make sure you understand the examples as presented – then you will be able to apply the knowledge to similar settings, those that are not described in this handbook.