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For comparison, the major benefits – and the inevita ble disadvantages – of each tool length setup method are summarized in the following table: 

Gauge line 

at machine zero 



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. 



. Minimized setup • Possible high cost time 

of equipment 

• Off-machine setup Off-machine setup 

Touch-off. Easy at the 


| Time consuming – Setup costs higher 



. Easy at the 


Improved setup 


Faster method than Touch-off, but still time consuming 

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: 

Total travel distance 


-Z2.0 = Target location 


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. 


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: 

• Preset method … using an external setup equipment 

Never assemble the tool in the machine spindle 

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. 

· Touch-off method … Individual tools measured 


Looking from the front of a vertical machining center will provide the best way to understand the relationship of various machine features. 

The above illustration shows a part, mounted in a vise, with ZO at the top. There are four dimensions A to D. I ‘A’ dimension … represents the tool length measured 

between gauge line a the tool tip ‘B’ dimension … represents the distance between the tool tip 

and part Zo 

I ‘C’ dimension … represents the part zo measured 

from the table surface I ‘D’ dimension … represents the distance between top surface 

of the table and the gauge line Only one dimension – D – is always known. It is a fixed dimension that is set by the machine manufacturer and is available from machine documentation or one time measurement. 

Which dimensions from the remaining three (A, B, C) have to be known depends on the method of offset setup. 

Touch-off method … based on the longest tool 

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. 



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 225.0 H02 (M08) 

is always common to all tool length setup methods. 

CNC Control Setup for Milling and Turning 



CNC Control Setup for Milling and Tuming TOOL LENGTH OFFSET 

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: 

For the following two methods of tool length offset setup, the same approach will be used for consistency. 

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): 

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. 







Tool length measured off-machine, using a tool presetter (always positive) 

z -700.000 

Z 200.000 

UNKNOWN – represents distance between the tool tip and part zo (always negative result) 






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: 





Distance measured from the table top to the part 20 (always positive) 

As per machine specification | – always known and always negative 






Total Z-axis motion – Example for the PRESET method: 

External offset [D] 


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. 

Measured from machine zero 

A=150.0 H-offset 

Work offset (Z-axis setting) [C] 


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 (20.000) 4 Move the tool tip close to zo 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 reglster 10 For utmost precision, the amount used by the feeler 

can be further compensated 

Tool Ig. offset Hxx (Geometry) [A] 

Target location 


Offset Adjustment Examples 

D = -750.0 

Total travel 

B = ? 

Tool lg. offset Hxx (Wear) [A] 




Example 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 Zo 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: 150.000 Wear: -0.500 



Z-axis target position [program] 





-398.000 mm 


In both cases, the total Z-travel will be -398.500 mm. 

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: 

– Example 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 Zo 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: 150.000 Wear: 0.500 

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: 

External offset (also known as the Common offset) Current Work offset (typically G54 – 659) Current Tool length offset (Geometry) Current Tool length offset (Wear – if available) The Z-axis target location 

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 

In both cases, the total Z-travel will be -397.500 mm, 

Program block that activates the tool length offset, eg., G43 22,0 HO2 MO8 (ABS. POSITION = 22.000)