Once you understand why each tool has to be defined by its tool length separately, it should be easier to under- stand the way a particular tool is actually used.
Tool length setting can be achieved by at least three common methods:
n Preset method ... using an external setup equipment
n Touch off method ... individual tools measured
n Touch off method ... based on the longest tool
The following sections will describe each method in detail, including advantages and disadvantages of one method over another. Regardless of the actual setup pro- cess used at the machine, the program code will always be the same - the setup differences are limited to the ma- chine tool only, not to the programming method:
G43 Z25.0 H02 (M08)
is always common to all tool length setup methods.
at machine zero Gauge line
PART VISE TABLE
Z0
D A B C
Z2.0 = Target location
Total travel distance Distance-To-Go
TOOL LENGTH OFFSET 93
CNC Control Setup for Milling and Turning
For comparison, the major benefits - and the inevita- ble disadvantages - of each tool length setup method are summarized in the following table:
Even the brief descriptions should present good un- derstanding of the pros and cons of each method. In the following sections, you will find additional details, in- cluding several practical example.
Tool Assembly Fixture
Before the tool length can be measured (by using any method), it is important to secure the cutting tool into the tool holder, as necessary for machining. For this pur- pose, most machine tool manufacturers provide a special tool assembly fixture, that can be permanently mounted on a work bench, close to the CNC machine.
Mounting the tool holder in the machine spindle and then setting the cutting tool into it is a very poor practice that can severely damage the spindle. Always use the off-machine method, such as the tool assembly fixture.
Preset Tool Method
Preset tool length setting uses the actual tool length measurements found during the setup (A dimension - see page 92). As the name of the method suggests, 'preset tool' simply means that tool settings take place before ac- tual setup at the machine. This method of setup is called off-machine tool setup.
A special hardware (even software in some cases) is required to set the tool length off-machine.
Tool Presetter
The device that is used to set tool length off-machine is called the tool presetter. There are many designs, me- chanical and electronic, in a rather large price range.
Some presetters resemble standard tool height gage, oth- ers use various indicators. A typical, and relatively inex- pensive, tool presetter is shown in the following sketch:
Note that in the above table, the Off-machine setup is listed as both an advantage and a disadvantage. To a large extent, it depends on the overall management of company operations. Setting the tools and their offsets away from the machine significantly reduces lead time at the machine. That is certainly an advantage. On the other hand, it also requires another person to do the setup, plus a potentially high cost of the initial equip- ment, particularly for a large CNC machine shop. That is an obvious disadvantage. In order to obtain setup advan- tages only and minimize the disadvantages, a careful justification study may be necessary or even mandatory before any significant investment.
Method Advantages Disadvantages
Preset n Minimized setup time
n Off machine setup
n Possible high cost of equipment
n Off machine setup Touch off n Easy at the
machine
n Time consuming
n Setup costs higher
Longest tool
n Easy at the machine
n Improved setup time
n Faster method than Touch off, but still time consuming
Never assemble the tool in the machine spindle
TOOL ASSEMBLY FIXTURE
ELECTRONIC TOOL PRESETTER
Small and medium machine shops often use a home made solution. They use a relatively inexpensive stan- dard height gage to set the individual tool lengths. In this case the height gage is dedicated to a single purpose of setting tool lengths off-machine. Usually, a presetting fixture is an important part of this method, because it provides the necessary reference point that matches the reference point of the CNC machine.
Preset Method Application
To use this method for tool length offset, it is neces- sary to know dimensions A, C, and D, as shown in the last illustration (see page 92). In order to understand how this method works at the machine, it is important to understand how the CNC system calculates the total travel distance to reach the target position. This is the Distance-To-Go on many controls. For the example, consider the following known and unknown dimensions used in the tool length setup:
Although easy to interpret, round numbers have been used for the better understanding - keep in mind that in reality, the numbers will most likely have three (metric) or four (imperial) decimal place accuracy.
To calculate the Total travel, the CNC system will consider all relative offsets and their Z-axis setting:
n Externaloffset (also known as theCommonoffset)
n CurrentWorkoffset (typically G54 G59)
n CurrentTool lengthoffset (Geometry)
n CurrentTool lengthoffset (Wear if available)
n The Z axistargetlocation
Total travel is always the SUM of all these dimen- sions. For example, based on the illustration on page 92, the setup dimensions are as follows (for G43 command):
2Total Z axis motion Example for the PRESET method:
From all three common methods of setting the tool length offset, this is the only one that requires the dimen- sion D - the distance between the spindle gage line and the top of the machine table.
Keep in mind one important statement:
Program block that activates the tool length offset, eg., G43 Z2.0 H02 M08 (ABS. POSITION = Z2.000)
Description Example
A Tool length measured off machine, using a tool presetter (always positive)
150.000
B UNKNOWN represents distance between the tool tip and part Z0 (always negative result)
?
C Distance measured from the table top to the part Z0 (always positive)
200.000
D As per machine specification always known and always negative
-750.000
External offset [ D ] 750.000
+
Work offset (Z axis setting) [ C ] 200.000
+
Tool lg. offset Hxx (Geometry) [ A ] 150.000
+
Tool lg. offset Hxx (Wear) [ A ] 0.000
+
Z axis target position [ program ] 2.000 [ Z2.0 ] TOTAL TRAVEL FOR Z AXIS . . . . 398.000 mm
Regardless of the tool length offset setup used, the internal calculation of the total Z-axis travel motion (Distance-To-Go) will always be the same machine zero
Measured from
PART VISE TABLE
Z0 D = -750.0 A=150.0 B = ? C=200.0
Total travel Z2.0Target location
H-offset
TOOL LENGTH OFFSET 95
CNC Control Setup for Milling and Turning
will require offset two setting in tool length offset regis- try. On the display screen, the various settings used for this example (offset memory Type B shown) will look something like this:
At the machine, some fine-tuning may often be neces- sary. As the EXT and G54 offsets are fixed, the only ad- justment for the actual tool length must be done in the offset registry 02. For offset memory Type A, the adjust- ment is done directly to the current amount. For offset memory Type B and Type C, the offset adjustment is done in the Wear column.
Offset Adjustment Examples
2Example 1 Tool is SHORTER than previously set After replacement, the tool is 0.5 mm shorter than the previous setting. In this case, the preset tool length is not 150.0 but only 149.5 mm. In order to reach the Z0 that has not changed, the total travel distance has to increase:
Type A offset Geometry: 149.500 Wear: N/A Types B/C offsets Geometry: 150.0 00 Wear: 0.500
In both cases, the total Z-travel will be 398.500 mm.
2Example 2 Tool is LONGER than previously set After replacement, the tool is 0.5 mm longer than the previous setting. In this case, the preset tool length is not 150.0 but only 150.5 mm. In order to reach the Z0 that has not changed, the total travel distance has to decrease:
Type A offset Geometry: 150.500 Wear: N/A Types B/C offsets Geometry: 150.0 00 Wear: 0.500
In both cases, the total Z-travel will be 397.500 mm.
For the following two methods of tool length offset setup, the same approach will be used for consistency.
Touch-Off Method
While the touch-off method of finding a tool length offset is quite simple to perform at the CNC machine, the procedure itself increases the non-productive time be- tween job setups, sometimes quite significantly. In spite of this main disadvantage, it remains a very common and popular method, particularly in small shops, where the pre-setting alternative is either not practical or econom- ical. The touch-off method is also a common setup method when only a small number of tools is needed for a particular job. Using the touch-off method of tool length setup can also be justified in those machine shops, where there is no large volume of parts to be machined.
These include small job shops (custom machine shops), and some tool and die shops where making only one or two parts is the main objective.
The description touch-off describes the most signifi- cant part of the tool length setup process. To understand how this process works, it is important to understand that the tool lowest point - the tool end tip - is the major com- mand point for all program coordinates along Z-axis.
This is no different for the preset method, but it is more important to understand it for the touch-off method.
From the original illustration on page 92, only a single dimension - the B dimension - has to be known, and find- ing this dimension is also the main objective of the touch-off method. The process is quite straightforward:
1 Mount a tool holder with the assembled cutting tool into the spindle
2 Move the spindle to the machine zero position (home) along the Z axis
3 On the display screen, set the current relative tool position to zero (Z0.000)
4 Move the tool tip close to Z0 position of the part 5 Use a thin shim or even a piece of paper as a feeler 6 Gently, touch the feeler, using the setup handle in
its smallest resolution (minimum increment) 7 Upon contact with the feeler, the tool length offset
will be established
8 Turn the handle off do not move it
9 Use automatic registry entry function OR enter the measured amount manually into the proper tool length offset register
10 For utmost precision, the amount used by the feeler can be further compensated
...
...
-700.000 X