CNC Machine Tools

A wide range of tool types and configurations are available for CNC milling machines. Discussing every type, variation and use is beyond the scope of this article. This article introduces the most commonly used tools for prototype and short run production machining. Any tool supply catalog will list many others.

  • End mills (Flat, Ball, Bull and Chamfer)Face mill
  • Corner Rounding tools
  • Slot Tools
  • Spot-Center Drill
  • Twist Drill
  • Tap
  • Reamer
  • Counterbore

End Mills

Milling tools include flat, ball, bull nose and chamfer.

Figure 1: Mill Tool Nose Types

Flat nose mills are used for milling 2D contours and pockets. Ball nose mills are used for 3D milling. Bull nose end mills have a radius corner. They are used to create a fillet on the bottom of a wall. Because they are sturdier than an end mill they are also sometimes used for roughing operations. Chamfer mills have an angled nose used to create a chamfer or to de-burr parts.

Number of Flutes

Milling tools usually have either two or four cutting flutes. Two flute cutters provide more chip clearance when milling in close areas. Four flute mills are more rigid, can be fed faster, and are preferred when greater chip clearance is not required, such as when milling an outside contour.

Center-CuttingEnd Mills

Milling tools are either center cutting or non-center cutting. Center cutting mills can plunge straight down into material, while non-center cutting tools cannot.
Figure 2 below shows the cutting end view of a center cutting and non-center cutting end mill. Notice that the cutting edges of the center cutting end mill continues to the center of the tool. The center of the other has a small hole at the center.Non-center cutting end mills require a pilot hole, ramping or helical motion to plunge into material.

Figure 2: End View of Center and Non-Center Cutting End Mill

Face Mill

A face mill has cutting inserts that are replaced when worn. They are rigid, may have up to eight or more cutting edges, and can remove material quickly. They are often used for the first machining operation to quickly create a flat finished face on the part.

Figure 3: Face Mill

Corner Radius Tool

Corner radius (also called Corner Round) tools are used to place a fillet on the outside corner of a part.

Figure 4: Corner Round Tool

Slot Mill/Slotting Saw

Slot mills include side milling cutters,slitting saws, and Woodruff keyset cutters. Slitting saws and side milling cutters are installed on a special arbor. Woodruff cutters are single piece tools used for creating slots and undercuts that can be held in a standard tool holder.

Figure 5: Slot Tools

Hole-Making Tools

Center-Spot Drills

Center(spotting) drills are short and very rigid drills used to create a conic on the face of the part. Because they come to a sharp point and resist bending, they locate the hole precisely. The conic helps prevent the subsequent drill from wobbling and ensure the drill is located precisely and drills straight down.

Countersink drills are used to create the conical face for a machine screw. Combined spotting-countersinks are used to create a screw clearance hole and countersink in one operation.
There are many different sizes and tip angles of center, countersink, and combined drills. Be sure the tip angle of the countersink matches the included angle of the machine screw, and that the drill diameter is greater than the screw head diameter.

Figure 6: Countersink and Center Drill

Twist Drill

Twist drills are available in many diameters and lengths. Usually made of high speed steel, carbide, or cobalt, they may also be coated with titanium nitride (TiN) for longer life. The tip angle of most twist drills is 118 degrees.

Figure 7: Twist Drill


Cutting taps form threads by shearing material away. Form taps (roll taps) form the thread by forming the metal to shape. Form taps produce no chips and are used for soft materials including aluminum, copper, brass and plastics.

Figure 8: Taps

Bottoming taps are used to tap blind holes. Spiral point taps push the chip ahead and out the bottom of a through hole.
Taps require a hole drilled to the correct size to ensure the thread is formed properly. For example, a ¼-20 cutting tap requires drilling a .201 (#7) hole.
Most CNC Machines support rigid tapping, which means the tap can be held in a rigid holder. The tap is advanced at a feed rate that matches the thread lead into the hole. The spindle then stops, reverses, and backs out of the hole.
Machines without rigid tapping require special tapping attachments. Always refer to the manufacturers’ instructions as the speed, feed, and other machining parameters for tapping attachments may be different that those for rigid tapping.


Use reamers to create holes of precise shape and excellent surface finish. Reamed holes are usually accurate within .0002 inches diameter. For example, a reamer is used for holes used for ground pins and bushings.
Reamers require a specific size hole be drilled before use. Cutting speeds and feeds are also important. Remove too little or too much material and the hole will not be the correct size.

Figure 9: Reamer


A counterbore looks similar to a end mill with a pilot in the center. It is used to spot face holes, and the pilot ensures the spot face is centered on the hole.
Counterboring is not necessary when using a CNC machine. Rather, create a spot face using a pocket or circle mill tool path. This saves having to buy and stock counterbore tools and pilots, and the time required to load and set up the counterbore.