Programming of CNC Machines. Ken Evans
using compressed air to clean up around the Ways is that when chips are blasted away from the table, many are forced behind the guards, further worsening the micro-switch problem described above.
Proper selection of cutting tools and work holding methods are paramount to the success of any machining operation. The scope of this text is not intended to teach all of the necessary information regarding tooling. You must consult the appropriate tooling manuals for selection of tool holders and cutting tools that are relevant to the required operation. Least expensive is not necessarily best.
Sound machining principals require that the most rigid setup possible be used that does not allow large overhangs of tools or workpieces. Ignoring these basic principals can cause tool and workpiece deflection and vibration that will contribute to poor surface finish and, eventually, tool damage which also makes it difficult to maintain dimensional accuracy.
Just as with the rest of the machine tool, there are components used with the actual cutting tool that make it what it is. Obviously, the tool cutting edge is where the metal removal takes place. Without proper tool clamping, the cutting action may not produce the desired results. Therefore, it is very important to carefully select the most effective tool clamping method.
In the case of a simple operation of milling a contour on a part, we may select a collet or a positive locking (posi-lock) end mill holder for the end mill. The correct choice would depend on the actual features of the part to be machined. If the amount of metal to be removed is minimal, then a collet would probably suffice. But if a considerable amount of metal is to be removed (more than two thirds of the tool diameter on a single depth of cut pass), then the posi-lock end mill holder selection is important. The reason for selecting the posi-lock holder is that under heavy cuts, a collet may not be able to grip the tool tightly enough. This situation could allow the tool to spin within the collet while in cut, with the result of ruining the collet and possibly damaging the part being machined. There is a tendency for the tool to dive into the workpiece when the tool spins within the collet and damage to the part may occur. Note: Most High Speed Steel (HSS) end mills have a flat ground on them to facilitate the use of the posi-lock holder. This flat area allows for a set-screw to lock into it, creating a rigid and stable tool clamping method. The clamping method for drills could be either a collet or a drill chuck. A keyed drill chuck usually is used for heavier metal removal or larger holes, whereas the keyless- type drill chuck is suitable for small holes. Generally, in the case of larger drills, a collet will be necessary to hold the tool. When holes are to be drilled, remember to center-drill or spot-drill first, so that the tool does not have a tendency to wander off location. The center-drill may be held in the same manner as a drill.
In turning, the selection of the type of tool holder is determined by the finished part geometry and the part material. There are a variety of tool holder styles as well as insert shapes available to accomplish the desired part shape and size.
For more information on the proper selection of inserts and tool holders, refer to the Machinery’s Handbook section titled “Indexable Inserts”.
Another valuable resource for technical data regarding the selection of inserts and tool holding are the ordering catalogs from the tool and insert manufacturers.
Cutting tools are a very important aspect of machining. If the improper tool and/or tool clamping method is used the result will most likely be a poorly machined part. Always research and use the best tool and clamping method for a given operation. With the high speed and high performance of CNC machines, the proper selection process becomes increasingly important. The entire CNC machining process can be compromised by a lack of good tool planning and improper use.
There are many different types of machining operations performed on either Turning or Machining Centers. The tool is where the action is, so if improper selection takes place here, the whole machining sequence will be affected. Years of study have been dedicated to this subject and are documented within reference manuals and buyer’s guides. Using these references will be helpful to correctly choose a tool for a given operation.
Remember that in your selection process, you are searching for the optimum metal-cutting conditions. The best way to understand how to choose the proper conditions is by studying the available data such as: the machine capabilities; the specific type of operation; the proper cutting tool(s) and tool clamping method(s); the geometry of the part to be made; the workpiece and cutter material; and, the method of clamping the part.
It is important to utilize the most technologically advanced methods of metal removal available. Do not hesitate to research this new technology. For example, in recent years, there have been numerous cutting tool innovations that include indexable insert coatings such as: Titanium Nitride (TiN); Titanium Carbon Nitride (TiCN) applied through Chemical Vapor Deposition (CVD), or Physical Vapor Deposition (PVD), and insert materials such as: Ceramic, Cubic Boron Nitride (CBN) and Polycrystalline Diamond (PCD), to name a few. These advances have enabled increased cutting speeds and decreased tool wear providing for higher production throughput. Another tool clamping innovation is modular tooling. This is a standardization of tool holders to facilitate the quick change of tools decreasing setup time. Refer to the tool and insert ordering catalogs from the tool and insert manufacturers for more information on modular tooling.
Important information about the tool must be given to the machine control unit (MCU), for the machine to be able to use the tool efficiently. In other words, the MCU needs the tool identification number, the tool length offset (TLO), and the specific diameter of each tool. A TLO is a measurement given to the control unit to compensate for the tool length when movements are commanded. The cutter diameter compensation (CDC) offset is used by the control to compensate for the diameter of the tool, for end mill style tools, during commanded movements.
The tool number identifies where the tool is located within the storage magazine or turret and often is the order sequence in which it is used. Each is assigned a tool length offset number. This number correlates with the pocket or turret position number and is where the measured offset distance from the cutting tip to the spindle face, in the case of a milling machine, is stored. For example, Tool No. 1 will have TLO No. 1. Finally, when end milling is necessary the diameter of the tool is compensated for. In most cases, the programmer has taken the diameter of the tool into account. In other words, the programmed tool path is written with a specific tool size in mind. However, more commonly the part geometry is programmed in order to facilitate the use of different tool diameters for a specified operation. When using the part geometry rather than the toolpath centerline for a specific tool diameter, an additional offset is called from within the program called cutter diameter compensation (CDC).
CNC equipment enables more efficient machining by allowing the combination of several operations into a single setup. This combination of operations requires the use of multiple cutting tools. Automatic Tool Changers (ATC) are a standard feature on most CNC Machining Centers, while many CNC Knee-Mills still require manual installation of the tool. The illustrations in Figures 1 and 2 are two types of tool holders used on CNC machines; they have some distinct physical differences. Both of the holders are tapered. The one to the left has a single ring at the large end of the taper while the other has two rings. The tool holder that has only one ring is designed for machines that require manual tool changes. The tool with two rings is designed for machines that have Automatic Tool Changers. These rings act as a gripping surface for a tool changer.
