A feature of a CNC machine commonly known as the 'Automatic Tool Changer' means that one tool can be ex- changed with another tool, using CNC program data or MDI input, rather than by any manual interference.
CNC lathes are not included in this category - any tool change of lathe tools is always automatic. When it comes to CNC mills and machining centers, there are some major differences - differences that will influence the program development, as well as the setup proce- dures at the control system. Tool changing on milling type machines can be categorized in three groups:
n Manual tool change no automation
n Fixed type of automatic tool changers
n Random type of automatic tool changers
When there is no tool changer available, does it mean the tool cannot be changed? Not at all. Take an typical example of a tool room oriented shop, perhaps a proto- type tool and die shop, or a similar shop where one or two parts are occasionally required. There are many ma- chine shops falling into this category - while they all want to be competitive - and productive - they do not face the same requirements as true production oriented companies. Their use of CNC equipment is not always a high priority; often, it is the desire for precision rather than the best cycle time that determines which work will be assigned to a CNC machine.
For production oriented companies - and they are in majority - an automated tool changer under CNC pro- gram control is the best solution.
Manual Tool Change
As the name suggests, there is no provision for storing tools required for a particular job right at the machine.
Practically, it means the tools are available in some kind of a portable tool cart, located nearby the machine itself during machining. When a certain tool is required, the CNC operator will temporarily stop the program, re- move the existing tool and replace it with the next tool in the machine spindle. When the new tool is in the spin- dle, pressing the Cycle Stop button will continue pro- cessing the program with this new tool.
The main purpose of ATC is to exchange the tool in the spindle with the tool programmed next
While this manual type of a tool changer may be per- fectly suited to certain individual types of CNC work, it would be too costly to implement it for production work.
From a practical viewpoint, consider a manual tool change as a specialty type of setup only.
For CNC machines using manual tool changer, the part program should have:
n NO tool T address
n NO tool change M06 function
n M00 (mandatory program stop) after each tool
If there is no ATC involved, using the tool call T-ad- dress and the tool change function M06 are irrelevant, therefore not required. On the other hand, the program must provide means to do the tool change manually. De- pending on the person who writes such a program, the M00 function - Program Stop - can be with or without a comment attached:
...N64 M00 ... or ...
...
N64 M00 (DRILL OUT - USE 15 MM END MILL) Attaching a simple comment always helps the CNC operator and makes a better program documentation.
Description of the tool also helps to prevent the possible mistake of using the wrong tool.
Tool Magazine
While a manual tool change may be quite useful in certain industries, having an automatically controlled tool change is necessary for production work. In this case, all tools used by the program must be located at the machine. The temporary machine based storage area for these tools is called tool magazine or tool carousel. The operative word here is 'temporary' - while you can keep frequently used tools in the magazine at all (or most) times you may choose not to.
Modern CNC machines have very fast tool change times, measured in two ways:
n Tool to Tool time
n Chip to Chip time
Tool-to-Tool time is the amount of time expired be- tween one tool in the spindle and the next tool in the spindle.
Chip-to-Chip time is the amount of time expired be- tween making the last cut with one tool and the next cut with the another tool.
Manufactures may sometimes advertise tool-to-tool time, because it is more attractive (meaning shorter) than the chip-to-chip time. As always, compare equivalent specifications.
Tool Magazine Capacity
Only so many tools can be loaded into the tool maga- zine. Depending on the machine type and size, the num- ber of tools that can be stored in the magazine varies from low 20-30 tools to the high of a few hundred tools.
Control system of the CNC machine will always sup- port all available tools, which means the number of off- sets available for tool length and cutter radius will always exceed the number of tools that can be stored in the magazine.
One important part of any tool magazine design is the location where the actual tool change will take place.
This position is called the 'tool change position', 'wait- ing position', or 'stand-by position'. Other descriptions may also be used, largely depending on the manufactur- ers and their preferences.
A typical 20-tool magazine is illustrated above. Note the numbered tabs - the are pocket or ‘pot’ numbers.
Tool magazine storage is not a permanent depository of cutting tools
in the back TOOL READY POSITION
TOOL POCKETS
15 16 17 18
19
20
01 02 03 05 04 07 06 08 09
10
11 12
13 14
SPINDLE
MILLING TOOLS - SETUP 57
CNC Control Setup for Milling and Turning
Magazine Labeling
Cutting tools required for a certain job are stored in the tool magazine in individual stations, often called 'tool pots' - short for 'tool pockets'.
Each pot is labeled by a number, from 01 to the num- ber that represents the maximum number of tools the magazine can hold, 20 in the illustration. These labels are fixed, so pot 06, for example, will always be the same pot. When the magazine rotates, the pot labels ro- tate as well. Rotation is bi-directional, whichever is shorter.
In the illustration, the pot 15 is located at the tool ready position, where the tool will be taken out of the magazine and placed into the spindle.
How these pot labels are used practically during ma- chine setup is covered in the next subject. In order to un- derstand the following setup concepts, you have to understand what the part program is 'telling' you first.
Take the two examples as both being correct program entries, typically located at the program start:
2Version 1 next tool call T is not programmed:
N1 G21
N2 G17 G40 G80 T01 N3 M06
N4 G90 G54 G00 X.. Y.. S.. M03 ...
2Version 2 next tool call T is programmed:
N1 G21
N2 G17 G40 G80 T01 N3 M06
N4 G90 G54 G00 X.. Y.. S.. M03 T02 ...
Look carefully - what are the differences? There is only one subtle difference - in the Version 1 example, there is no T02 at the end of block N4. This is a signifi- cant omission. Providing there is only one T-address per tool programmed, this format identifies a certain type of ATC, called the Fixed Type.
Fixed Type ATC
The Version 1 above represents the fixed type auto- matic tool changer, if programmed correctly.
Fixed type of automatic tool changer refers to a ma- chine design, where the tool always returns to the same pocket it was located in before the tool change. That means the pot number is - in effect - the tool number.
If three tools are identified in the program as, for ex- ample, T01, T06, and T17, the tools have to be placed into pots 01, 06, and 17 respectively.
This format is sometimes used for the random type ATC as well (described next), but represents rather a shabby programming style. For the fixed type ATC:
The major disadvantage of the fixed type tool changer is time loss. It takes time to move the magazine from sta- tion 01 to station 06 and station 17. Experienced pro- grammers try to number the tools so they are close together in the magazine, but that is not always possible.
Even then, there still is a time loss.
A qualified CNC operator may change the tool num- bers at the control, if the situation warranties it.
Random Memory Type ATC
The Version 2 above represents the random type auto- matic tool changer, if programmed correctly.
Random type of automatic tool changer refers to a CNC machine design, where the tool always returns to the magazine pocket occupied by the previous tool. If programmed properly, the tool magazine will rotate to its new position simultaneously with actual machining activities that are taking place.
For the random memory type ATC:
The process is the program has three steps:
n Call the tool to be placed into the spindle N2
n Exchange the tool in the spindle with the new tool N3
n Call the tool for the next operation N4 These steps are represented by blocks N2, N3, and N4 of the Version 2 example.
Keep in mind that tool numbering in the program is not always orderly 01, 02, 03, etc. Some tools are used so often, that programmers use the same number for all jobs. The benefit of this method is that it speeds up the setup process at the machine.
The description ‘random memory type’ may be a bit confusing - it simply means that the computer - the con- trol system - keeps track of all tools. Of course, some in- put from the CNC operator is required, in the form of registering the tool numbers in memory, covered shortly.
T.. in the program means the CURRENT tool number
T.. in the program means the NEXT tool number
Tool Waiting
One of the features of modern CNC machining centers is the 'tool waiting'. During machining, the tool that is in the ready position of the magazine is called the ‘tool waiting’. The tool is waiting for the ATC, when the next M06 tool change is programmed. As there is no maga- zine rotation, the tool in the spindle will be replaced with the tool that is waiting in the ‘tool ready position’.
Most programmers understand this concept well:
The later section on tool repetition (page 62) covers this subject in more detail.