It is well known that CNC machining can produce a variety of shapes on a workpiece, whether it is a complex contour, a cavity or a precision hole. One of the common shapes is a slot to accommodate a key, seal or other functional component.
What about slot milling operation? Is it as simple as traditional end milling or face milling? This article will take you through the slot milling process and explain the basic techniques for creating slots.
What is slot milling?
A slot is a narrow channel milled into a workpiece for different functional purposes. It can be used to install fasteners, guide mechanical parts or facilitate assembly.
Slot milling is a milling technique that uses a rotating cutting tool to machine a channel, groove or groove in a workpiece.
The shape and size of these slots will vary depending on the design requirements. They can be closed, straight (rectangular) or curved (circular) and can be single slots or pairs.
Types of slot milling techniques
Slot milling is the ultimate goal. Any slot design can be created by different slot milling tools and techniques. The choice of method depends on the desired slot characteristics and its intended use.
- End milling: End milling is performed using an end mill, which has cutting edges on both the end face and the outer diameter of the end mill. The end mill cuts into the workpiece along its axis and mills a slot width that matches the width of the tool. End mills are common and come in standard sizes and depths. Therefore, they are the first choice for machinists to make slots.Their biggest advantage is the versatility of their design, which can produce nonlinear paths, variable depths, and even closed slots (for example, pockets in mold cavities).
- Face milling: Face mills are mainly used to machine flat surfaces using peripheral teeth. However, they can also machine shallow linear slots in large flat workpieces.Unlike end mills, they excel at bulk material removal over large areas, such as roughing coolant passages in engine blocks. Their large cutting diameter ensures stability, but their accuracy is insufficient for narrow or deep slots.
- Group milling: Group milling is a group milling technique that mounts multiple tools on a single spindle to machine multiple slots simultaneously. Typically, a pair of tools is used to mill two parallel slots. Fin structures can also be machined by gang milling. Due to the high material removal rate, this technique is well suited for high volume production. Due to the high cutting forces, a rigid tool configuration is required to prevent vibration or misalignment.
Commonly used slot milling cutters
- T-slot milling cutters: T-slot milling cutters are specifically designed to create T-slot profiles and are often used in milling machine tables or workholding systems to hold fixtures. The process is a two-step process, first, a standard end mill cuts the vertical slot, then a T-slot milling cutter (with a horizontal cutting profile) machines the undercut to form the “T” shape.
- Woodruff key milling cutters: Woodruff key milling cutters are small, disc-shaped tools with teeth on the edges that are used to machine semicircular slots. These curved slots are used to mount woodruff keys, which are used to secure power transmission or load-bearing components. Gears are also secured to shafts by these slots.
What is the difference between a slot milling cutter and an end mill?
An end mill is a general-purpose tool used for profiling, contouring, and face milling. It can cut in multiple directions (axial and radial) and has a variety of shapes, such as flat nose, ball nose or radius.A slot mill is an end mill specifically designed for cutting slots and keyways. It usually has a straight edge and is optimized for full-width cutting.
Slot Milling Tool Paths
There are many different ways in which a slot mill or milling cutter enters the workpiece and performs the operation. There are three basic tool paths that are used in the industry. Each path has its own unique uses and advantages:
- Traditional: This is the typical way to perform milling on a workpiece. The tool enters from one side and cuts linearly along the slot axis. This tool path works with most common tools, is simpler to program, and is suitable for shallow slot depths.However, it is less suitable for deeper slots (such as those deeper than 3xD) because high vibration and radial forces can deflect the tool or damage the workpiece.
- Punching: Plunging operations are similar to drilling: the tool (slot drill) cuts axially along the workpiece. Although this method sacrifices surface finish, it excels at deep slotting. The benefit is that axial and radial forces on the tool are reduced, minimizing the risk of tool deformation. This makes plunge milling ideal for deep slots in hard materials such as titanium, where tool stability is critical.
- Trochoidal: In trochoidal slotting, the tool makes a special spiral or circular motion along the cutting path. This method allows a single tool to make slots wider than the tool diameter, with relatively low radial forces and better chip evacuation.It is ideal for machining hard materials such as stainless steel or Inconel. However, trochoidal paths are complex to program and often require advanced CAM software. In addition, a final finishing operation may be required to smooth out the spiral marks left by the tool path.
Best Practices for Slotting
Slotting may seem simple in theory, but machinists often face many challenges when slotting. To ensure a successful milling operation, we recommend the following best practices.
Optimize tool entry with ramp milling
Sudden entry into the workpiece can cause shock to the tool, causing it to break or deform. Ramp entry (gradually lowering the tool at an angle) is recommended to evenly distribute cutting forces. A 45° ramp entry angle is sufficient to prevent tool overloading. For deep slots or hard materials, an 180° axial entry similar to drilling can reduce radial forces and prevent tool overloading. This approach not only prolongs tool life, but also avoids the catastrophic consequences of tool breakage during the entry process.
Chip evacuation strategy
Chips stuck in the slot can re-enter the workpiece or clog the chip vent, affecting surface finish. One way to address this problem is through multi-pass milling. Remove material in layers (for example, roughing to 70% of the depth first, then finishing). This allows room for the chips to evacuate from the slot wall. Another option is to use an end mill with a serrated edge or variable helix angle to break the chips into manageable pieces. Use high-pressure coolant or compressed air to flush the chips, especially in closed slots.
Use larger diameter tools
Given the higher cutting forces and deeper slots, a larger diameter milling cutter is needed to prevent deflection caused by the long overhang. The larger the diameter, the greater the strength, which is ideal for this particular operation.
Up and Down Milling
Climb milling (cutters rotate in the same direction as feed) is the preferred method for slotting operations. Climb milling reduces tool wear and improves surface finish because cutting forces push the workpiece downward, minimizing vibration. This is useful for thin-walled parts or materials that are prone to burring.
Spindle Engagement
Interrupted cuts leave pits on the slot surface, resulting in poor surface quality. It also wastes time and does not speed up the process. To avoid this, make sure at least one tooth of the milling cutter is always in the workpiece.
How can Xtproto help?
Designing a product that requires linear slots to be milled? Or need a custom part with tight tolerances? Contact Xtproto for precision CNC milling services. With 3-, 4-, and 5-axis milling capabilities, we can easily handle even the most complex or delicate designs. Whether you need rapid prototyping or high-volume production, we can ensure the slot accuracy of your product (metal, plastic, or alloy). Get a quote now. Simply send your design to Xtproto’s backend and our experts will turn your vision into reality!