Cutting Tool used in CNC

Cutting tools play a vital role in machining operations, and the dimensions of the workpiece depend predominantly on them. Various cutting tools are used in CNC machines to support different machining operations. Form cutters are specialized cutting tools commonly used in CNC machining. The following are standard cutting tools regularly used in CNC milling and turning operations. 

Milling tool with holder 

A milling tool is a multi-edge cutting tool with multiple cutting edges that remove material from a workpiece through relative movement between the rotating tool and the stationary workpiece. The picture below shows a standard milling tool used in CNC machining centers.

The milling tool is usually inserted into a tool holder and securely fastened using length adjustment screws to adjust the tool length. There are different types of milling tools, with the most commonly used ones being end mill cutters. End mill cutters can be classified into two types: center-cutting and non-center-cutting. Center-cutting end mill cutters have cutting edges at both the center point and the edges, and they are typically used for plunge milling operations. On the other hand, non-center-cutting end mill cutters have cutting edges only along the sides and are commonly used for side-milling operations. The pictures below illustrate the difference between these two types of end mill cutters. 

In face milling operations, a larger diameter cutting tool with replaceable carbide inserts is sometimes used to machine a large surface area of the workpiece in a shorter time. The following pictures show some regular end mill cutters employed in various CNC machining centers.

Rough End-Mill Cutter –  Removes a considerable amount of material during a machining cycle. The rough cutting-edge design reduces vibration during extensive material-cutting processes.

Ball End-Mill Cutter - Used for narrow slotting and contour milling operations.

Indexable End-Mill Cutter – Utilizes replaceable carbide inserts, saving money compared to solid end-mill cutters. These cutters are usually employed for rough milling operations.

Milling tool holder 

During milling operations on a CNC machine, the cutting tool is inserted into a specific type of tool holder and clamped inside the spindle. The selection of the tool holder depends on the clamping arrangements of the spindle. The most commonly used tool holders in CNC machining centers are the CAT Flange type and BT Flange type. Both types of holders have a special clamping arrangement called a 'pull stud' (seen in the picture) located on the rear side of the holder, which firmly clamps the holder within the spindle. Some CNC machines use HSK-type tool holders, which are hollow and comparatively lighter than CAT or BT-type holders. HSK-type tool holders are designed for high-speed machining, and their structure is distinct from CAT and BT types. Unlike CAT and BT holders, HSK holders do not require a pull stud for clamping the holder with the spindle. The following pictures show two different tool holders used in CNC machining centers.

Turning tool and Boring bar 

For turning operations, a single-point cutting tool with only one cutting edge is typically used to remove material from the workpiece. The material is sheared off by the rotating workpiece and the stationary single-point tool. Most CNC turning centers use replaceable carbide inserts with two cutting edges on opposite sides. When one side of the insert becomes dull due to machining, the opposite edge can be used, making it functional again. Different types of inserts are used in CNC turning operations, and their shape depends on the specific turning operations to be performed. The following picture shows a simple insert-type turning tool and its components.

A turning tool consists of two main elements: a tool holder and an insert. The tool holder is usually made of forged steel and securely holds the insert in place using a screw and clamp mechanism to prevent displacement due to vibrations generated during machining operations. Inserts are typically made of cemented carbide, ceramic, or cermet (a composite material of ceramic and metal). Two main types of inserts are employed for turning operations: coated and uncoated. Coated inserts, with compounds of tungsten and titanium, improve hardness and are used for machining hard materials like iron. Uncoated inserts are employed for machining soft materials like aluminum. The configuration of the cutting edge of a turning tool determines the types of turning operations that can be performed. The point angle of the cutting edge on a rough turning tool is always greater compared to a finish turning tool. As a result, a rough turning insert can withstand higher cutting loads, while a finish turning insert is used for light cutting load turning operations. Inserts are equipped with chip breakers to break chips into smaller pieces. The following pictures show two different inserts used in CNC turning operations.  

A boring bar is also a single-point cutting tool specifically designed for boring operations. Instead of a tool holder, a solid cylindrical bar is employed with insert-type boring tools. The insert is positioned at the end of the bar at a specific angle. The boring bar is mainly used for internal turning operations within a cylindrical workpiece. In addition to boring operations, it can be used to create non-standard drill holes or enlarge small drill holes. The following picture shows a typical boring bar. 

Drill, Tap, and Reamer

Drills are commonly used to create holes of specific diameters on workpieces. Different types of drills, such as twist drills, spot drills, center drills, and indexable drills, are used in the drilling process. Twist drills are the most commonly used drills and have a conical cutting point with one or more helical flutes to efficiently remove chips during drilling. Twist drills are usually coated with titanium nitride, making them more durable and less prone to damage. Spot drills are used to create small spot holes on workpieces to assist the movement of the twist drill. Center drills are used in turning operations to create a 60-degree center at the end of a workpiece, providing guidance and positioning for steady holding. Indexable drills are double-fluted tool holders fitted with two replaceable carbide inserts. The following pictures show different types of drills. 

A tap is a tool used to create threads with a specific pitch in a drilled hole. There are different types of taps, such as hand taps, spiral point taps, spiral flute taps, and form or roll taps. Taps are typically driven by hand and are sometimes used in CNC machines. A repetitive or cyclic synchronized rotation of the spindle or workpiece is necessary to obtain perfect threads on the workpiece.

A reamer is used to prepare a hole with precise tolerance after a hole has been drilled. It removes a small amount of material from the inner surface of a drilled hole, resulting in a good surface finish. Reamers are mainly used to achieve the exact diameter, proper roundness, and excellent surface finish of a drilled hole. Like taps, reamers are also hand-driven tools and can be used with CNC machines. Different types of reamers, such as straight flute, helical flute, and taper, are used for specific purposes. The following pictures show a tap and a reamer.

Work-piece mounting

During machining operations, it is essential to securely fix and hold both rotating and stationary workpieces to prevent any slippage while cutting. Different types of workpiece mounting systems are used for various machining operations, and workpiece mounting plays a crucial role in achieving accurate positioning, solid support, and strong clamping. Manual workpiece mounting is typically done for conventional machines, while most CNC machines handle it automatically. In the case of a CNC machining center, workpiece mounting is finalized using different types of fixtures, while a chuck mechanism is employed for CNC turning centers. Clamping configurations for metal cutting CNC machines can be categorized into two groups based on the machining operation and the basic construction of the clamping arrangement.

1. Based on Machining Operations 

Milling Fixture - This type of fixture is used for CNC milling machines and may utilize clamping setups such as V-blocks, T-bolts, vices, jigs, etc., for workpiece mounting. However, it is always preferable to use a fixture with a unique clamping arrangement designed specifically for CNC machining centers, considering the nature of the workpiece. A tombstone fixture is commonly used with CNC horizontal machining centers, where multiple workpieces are clamped around it using special manual clamping arrangements. The tombstone fixture is securely held on a palette, which is then attached to the machine table. The clamping arrangements around the tombstone are typically fixed and tailored to the workpiece's dimensions. Various clamping devices such as clamps, straps, T-slot bolts, nuts, and jacks are employed for clamping the workpieces on a tombstone. Care should be taken to ensure that the chosen clamping devices do not obstruct the cutting path of the tool during machining operations. 

Turning Fixture - Workpiece mounting for turning operations differs from milling operations. In Turning, a centrifugal or rotational force is applied to the workpiece in addition to the cutting load. Hand-operated workpiece mounting for turning machines commonly involves the use of a three or four-jaw chuck, while automatic workpiece mounting is achieved using a quick-acting collet. A power chuck with three or four jaws is usually connected to the machine's spindle and holds cylindrical workpieces for turning operations. Power chucks are typically operated with hydraulic pressure, controlling the movements of the adjustable jaws. The adjustable jaws can be minutely adjusted towards or away from the spindle center, and it is necessary to adjust the clamping jaws according to the diameter of the workpiece. A cylinder piston is used to collectively shift the jaws towards the center of the spindle, applying hydraulic pressure to firmly clamp the workpiece inside the spindle. The cylinder piston moves in the opposite direction to release the workpiece from the spindle.

For CNC turning centers, automatic workpiece mounting is usually accomplished using a quick-acting collet. The collet is positioned inside the spindle and has a slight horizontal movement inward and outward of the spindle. As the collet moves outward, the gap between the metal pieces of the collet increases, and as it moves inward, the gap decreases. When a cylindrical workpiece is inserted into the collet and moved towards the spindle, the three separate metal pieces of the collet hold the workpiece rigidly from the outside. The outward movement of the collet unclamps or releases the workpiece from the spindle. Since the gap between the collet's metal pieces is very small, different-diameter collets are required to clamp workpieces of various diameters inside the spindle. This type of clamping system, often found in turning centers, is equipped with an auto-bar-feeding mechanism.

Grinding Fixture - Grinding operations can be categorized into surface and cylindrical grinding. For surface grinding operations, workpieces are usually clamped on a table with the help of a magnetic bed or chuck, which securely fastens the flat surface workpieces to the machine bed using a magnetic force. In cylindrical grinding operations, workpieces are clamped using a three or four-jaw chuck. Refer to the pictures for clamping arrangements used in surface and cylindrical grinding operations.

 

Fixture for Drilling, Shaping & Slotting - Drilling machines commonly use a vice for holding workpieces, and sometimes a jig fixture is also employed. A jig fixture not only holds the workpiece but also guides the cutting tool path. In some cases, drilling operations can also be performed using milling and turning clamping techniques on CNC machines. For shaping, planning, and slotting operations, workpieces are clamped directly on the machine bed using T-slots and supported by T-bolts. Refer to the pictures for vice and jig fixtures.

 

2. Based on Basic Fixture Construction

Workpiece fixing systems for metal-cutting CNC machines can be categorized based on their basic construction.

 

Plate Fixture - This mounting system involves fastening the workpieces to the machine bed using different types of locators, supports, and clamping arrangements. Refer to the picture showing a T-slot, T-bolt, and clamping arrangement system used to fix the workpiece directly to the machine bed. 

Angle plate fixture - This workpiece fixing system is similar to a plate fixture, but the mounting plates are used to clamp the workpieces perpendicular to the machine bed. Refer to the picture showing an angle plate fixture system. 

Vice jaw fixture - The vice jaw fixing system is a simple workpiece mounting arrangement occasionally found in CNC machines. The vice is selected based on the size of the workpiece. Refer to the picture of a vice jaw fixing system.

Indexing fixture - Different types of indexing fixtures are used to clamp machining workpieces using mechanisms such as three-jaw chucks, collets, etc. In each step or index movement, the workpiece's fixing arrangement is shifted or positioned to a predefined zone. Refer to the picture showing a three-jaw clamping type simple indexing fixture.

Multi-station fixture - This type of fixture allows multiple workpieces to be machined collectively in a single setup or each part to be machined sequentially. Sometimes, workpieces are positioned in a way that multiple machining processes can be carried out with a single setup. Refer to the picture showing a multi-station fixing system.