Skip to content 
Machining processes such as CNC milling are central to the modern manufacturing industry, offering reliable and flexible production techniques with unique capabilities. Slot machining is one of the most versatile and widely used CNC milling processes in machining grooves with limitless shapes and sizes.
This process uses various slotting tools to make precise pockets, grooves, or slots in different components, meeting strict quality standards and tight tolerances. Understanding the slot milling process is advisable to fully exploit its vast benefits.
This guide provides a detailed introduction to slot milling technology and its techniques. Read on to learn more about best practices for optimal slot milling!
What Is Slot Milling?
Slot milling is a widely used technology in making channels or slots in a workpiece. It involves using a special rotating cutting tool, including a slot cuter or end mill, to remove material from the workpiece, forming preferred slots of different shapes and sizes.
Generally, slot milling is the best method for making slot format features such as keyways or grooves in workpieces. Grooves made by this milling process can be closed or opened with an extensive option for shapes. A slot cutter has a cylindrical face that is suitable for side cutting. On the other hand, an end mill performs plunge cutting and a degree of side cutting to create blind slots that start by drilling.
Slot Milling Techniques
Slot milling is one of the several milling operations with endless possibilities for making slots of varying sizes and shapes in a workpiece. This section gives an in-depth discussion of the numerous slot milling techniques:
End Milling
End milling is a versatile technique that involves using a standard shell mill or end mill cutter to make grooves in a workpiece. It is well-suited for making non-linear slot profiles, closed slots, and slots with different depths.
Machinists often perform slot milling operations with end mills because of their different sizes and materials. These end mills can make deep cuts and withstand high cutting forces. Besides, specialized end mill designs can mitigate chatter vibration during milling, ensuring improved surface finish of the milled surface.
The end milling technique is similar to face milling, where the edges of the milling cutter perform the cutting. Besides, an end mill makes a closed slot, while a face milling cutter flattens and smoothens the surface of workpieces.
Side Milling
Side milling is a type of peripheral milling that uses the periphery of the milling cutter to machine the side of a workpiece. Its stability and precision render it a widely embraced technique for creating clots. The side milling tools can withstand cutting forces and vibrations.
Moreover, a horizontal milling machine can make precise and accurate slots in a workpiece with stiff arbor support and long tool overhangs. Side milling is effective in machining flat or contoured surfaces on the side of machined parts.
T-Slot Milling
T-slot milling is a specialized cutting method for making ‘T’-shaped slots in workpieces. T-slot cutting applies to industrial machinery, especially for machining beds. The T-slotting process uses a special T-slot milling cutter. You must perform a regular groove machining operation to prepare space for the shank of the T-slot milling cutter. However, this initial process is only necessary when machining closed slots. In contrast, a T-slot milling cutter can cut directly for an open slot.
Woodruff Key Slotting
Another common slot milling technique is machining keyways or grooves to accommodate Woodruff keys. These keys usually have a semi-circle shape. Product designers and machinists often use key and slot joints in locking load-sharing or rotating components for efficient load transfer. For instance, you can fix gears onto the shaft with a key and slot joint.
Gang Milling
The gang milling technique uses multiple cutters mounted on the same arbor to machine various features on a workpiece simultaneously in a single run. As such, this process ensures accuracy and increases productivity by lowering the time for setup and handling. However, the arbor and machine tool must be sufficiently rigid to handle the excessive cutting forces involved in this process.
Gang milling is often ideal for handling the mass production of products with multiple similar features. The setup cost and complexity render it unsuitable for one-off or low-volume production.
Best Practices for Optimal Slot Milling
It is essential to consider specific parameters for higher accuracy, better surface finish, and material removal rate in slot milling. Here are some of the best practices for optimal slot milling operations:
Ramp Down Entry
The slot milling cutter must make a smooth entry to avoid overloading the tool. It would be best to employ a ramp-down motion instead of redial entry to prevent fully engaging the tool simultaneously, resulting in excessive vibrations and adversely affecting the tool. Moreover, a ramp angle of at least 45° is enough to prevent shocking the machine or cutting tool.
Importance of Chip Evacuation
There is often less space for cut chips formed during slot milling to escape since the slots are enclosed features, usually with one open end. If stuck in the cutting zone, these cut chips can deteriorate parts’ tolerances, surface quality, and tool life. As a result, it is necessary to maintain proper chip evacuation in slot machining.
However, machining experts recommend cutting the slot in at least two passes. It helps to create adequate space for chip evacuation in the second and subsequent passes, which you can program with high material removal rates.
In addition, you can use specialized tooling such as milling cutters with chip breakers to produce very fine chips with controlled chip thickness. Hence, use coolant or lubricant flushing and compressed air to remove chips to create quality and precise grooves.
Maintaining Spindle Load
It is essential to keep the spindle loaded during the cutting cycle when making slots. You must ensure that at least one cutting tooth is always engaged with the workpiece at any moment. You can adjust this with the tool’s radial immersion with the machined part.
Experts advise that interrupted cutting affects the surface finish of the slot milled. It is also counterproductive because it removes less material per unit. It also puts repetitive impulse load on the system, causing vibrations and deteriorating machine life.
Choosing Down Milling
The debate between back and down milling also applies to the slot milling process. The general suggestion is to choose down milling whenever possible because of its better chip evacuation qualities and stability. However, it would be best to maintain a setup sturdy enough to withstand the loading pattern of the down-milling process.
Opting for Larger Cutter Diameters
The stability of the slot cutter is usually a major concern when machining deeper slots. Besides, long tool overhangs for deep slotting operations can deflect and even break the tool in extreme situations. Hence, opting for tools with a large diameter would be best due to their higher strength. Tools with more rigid materials are also viable solutions to such challenges.
However, using a larger cutting tool diameter can be challenging as most slots are narrow but deep. Therefore, opting for big tools is not an option. As such, using the available resources to achieve desired results in such cases would be best.
Balancing Feed Rates
Another core aspect of achieving an efficient slot milling operation is balancing the cutting feed rates. Low feed rates can lower productivity, while high feed rates can cause thermal issues. Consequently, it is essential to balance the feed rate to prevent these complications.
However, it would be best to study the cutting feed and other machining parameters extensively to finalize your cutting program.
Advanced Toolpath Techniques in Slot Milling
Modern machine tool experts have developed advanced toolpath solutions for slot milling through extensive research and experimentation to optimize machining performance during slot milling. Here are the typical toolpath techniques in slot milling:
Conventional Milling
This is the most straightforward toolpath technique for making slots, comprising straight cuts along the slot axis. The conventional milling technique is easy to program and quite productive. Besides, it is compatible with most milling tools.
However, it makes high vibrations when milling hard-to-cut materials and deep slots. Also, conventional milling faces high radial forces since the tool is always cut, generating rapid heat. Consequently, it is unsuitable for making deep cuts.
Trochoidal Milling
Trochoidal, circular, or dynamic milling is an advanced toolpath technique where the slot cutter follows trochoidal/circular toolpaths. It reduces tool wear through even distribution of cutting forces, improving tool lifespan. Also, it provides sufficient cooling time for the cutting edges in slot milling and enough space for efficient chip removal.
Its lower average radial cutting forces make it a well-suited technique for milling hard-to-cut materials. However, Trochoidal milling requires complex tool selection and complex programming.
Plunging Technique
The plunging technique is primarily used in cases where a straight entry or exit may not be impractical. It involves making a smooth vertical entry into the workpiece to begin the slotting operation. This method offers stability as the non-existent radial forces reduce tool vibrations, deflections, and heat build-up issues. As a result, this technique is better suited for setups where the slot cutter is vulnerable to the listed problems. It involves long tool overhangs and deep slot milling.
However, plunging doesn’t offer a suitable surface finish, especially on the walls of the slots. Hence, your slots may require finishing operations with other toolpath techniques after plunging.
Conclusion
Slot milling is a productive method with extensive applications in modern industry. It provides a limitless scope of manufacturing, allowing manufacturers and designers to create workpieces with complex slots of preferred width and length.
AT-Machining is a top rapid prototyping and manufacturing services expert. We take pride in our state-of-the-art CNC machine facility and team of experienced and skilled professionals. We pay attention to detail as we offer top-notch CNC milling services, including slot milling, face milling, and other CNC machining services. Trust us to deliver quality products irrespective of your design requirements. Submit your CAD design file now for an instant quote!
FAQs About Slot Milling
What Are the Common Materials Compatible with Slot Milling?
Machinable materials compatible with slot milling operations include plastics, metals (aluminum, iron, copper, steel, titanium), and wood.
What Are the Common Tools Used for Slotting Operations?
Machinists use different cutting tools for slot milling operations depending on the slot requirements. Typical cutters used in slot milling include T-slot, shell end mills, end mills, gang milling arbors, shoulder cutters, and Woodruff key slotting tools.
Are CNC machines Suitable for Slot Milling?
Both vertical and horizontal CNC milling machines can make slots in a workpiece. However, you may have to use other machines, such as sinker EDM or drilling machines, in certain situations.
