Slot Milling: Process, Tooling and Best Practices

Mechanical engineering tolerances are essential for the performance of devices and machines. The concept of tolerances ensures that parts not only fit as expected, but also perform to their optimum levels. When the variation is either too much or too little, damage could occur, even to catastrophic levels.

When it comes to the mating of two engineering pieces in machining, the concepts of press fit and slip fit are often in the conversation. What is the difference between the two types of fit? That’s what this article is about. Understanding the difference will help you ensure that your parts perform as expected. Join us as we briefly expound on the two concepts.

What Is Slot Milling?

slot milling

Slot milling is the process of creating slots or channels in a part using a special rotating tool. The slots or channels can have a rounded or rectangular profile, but this is based on the shape of the end mill. Slot milling is a popular method in the manufacturing world.

You can achieve all manner of shapes with slot milling. The resulting groove may be opened or closed. Usually, this process is performed using an end mill. The process starts with the formation of a hole using the normal drill.

End mills are most popular for slot milling because they can withstand the high cutting forces. The end mills are also versatile, with a variety of materials and sizes available. The end mill designs can also be specialized to prevent chatter challenges, thus ensuring a high quality finish.

But there are other slot milling techniques. One such technique is side milling. Using a tool with cutting edges on the periphery, you can create slots on the work piece. The method has some level of precision and stability.

Types of Slot Milling Cutters

You can choose from among many slot milling cutters. Your guidance in this selection would be the cutter’s size and shape. The width of the tool would also be necessary to consider because it determines the width of the slots. That said, here are popular types of slot milling cutters:

Face Milling Cutter

face milling cutter

Machinists are fond of the face milling cutter for its capability to maintain a plane of motion with the work piece. The biggest plus for this cutting tool though is the adjustable depth and width. However, there is drawback of being restricted to linear groove milling.

Side Milling Cutter

Side Milling Cutter

The design of the side milling cutter makes it suitable for a wide range of cutting techniques, including slot milling. It is capable of removing matter from the sides of a workpiece. That’s because it can be equipped with cutting teeth on all sides.

When you want to perform slot milling for keyways or otherwise, there are so many options with the side milling cutters. Specifically, the market has numerous configurations of the cutter.

These range from staggered teeth to those with planes. Ask the supplier or your engineer for the specific capabilities of these configurations. But the bottom line is that the side milling cutter is very effective in making many forms of slots.

Gang Milling Cutter

Gang Milling Cutter

Gang milling cutters feature a singe arbor that carries groove cutters. This is the cutting tool of choice for users who are looking to increase productivity. It engages each of the groove cutters simultaneously to create slots. The slots could be of the same size and shape, or they could be varied.

As you can guess, the gang milling cutter generates significant cutting forces. The arbor carrying the cutters must be sturdy enough to withstand these forces. The same applies to the cutting tool, because you need to have a long tool life.

At this juncture, we bring the aspect of machine tool setup. The way you set up the slot milling machines matters a lot. Experienced machinists know how to setup the machines and the best tools to use for optimal slot milling results.

Woodruff Key Slotting Cutter

For the creation of slots for keyways in shafts, you need a woodruff key slotting cutter. This type of cutter creates a semi-circular slot in the shaft. The bottom of the slot can be square or rounded. There are standard dimensions for woodruff keys. These can be achieved with the proper cutter positioning.

T-Slot Cutter

T-Slot Cutter

This unique cutter generates lateral slots in the work piece. It so designed that it forms slots that form an inverted T-shape. Since this cutter is placed along the rotational axis, you can place it precisely where you want to create the deep slots.

The efficiency and accuracy of the T-slot cutter is the biggest advantage it brings to slot milling projects.

Slot Milling Processes

There are numerous slot milling techniques. Many shapes and sizes of slots that can be made in a work piece, including key and slot joint.

Let us review some of the most popular slot milling processes:

End Milling

As you would expect, the end milling type of slot milling uses the standard end milling cutter. It is most suited for forming non uniform slots in a work piece. These slots could be closed slots, non-linear grooving, or those with varying depths.

End mills for slot milling cutter is a popular practice among machinists because there are various materials and shapes to experiment with. End mills are renowned for their capability to withstand high cutting forces because of their suitable designs. With a little specialization, these cutting tools can minimize chatter and vibration even further, thereby producing a smoother surface finish in slotting operations.

Side Milling

When you use the periphery of a cutting tool, you are said to be side milling. Thus, it is peripheral milling or some form of peripheral cutting. Side milling for slot milling is excellent at overcoming strong cutting forces.

If done on a horizontal milling machine, the accuracy of side milling process is high. Use side milling for creating slots with contours or even flat ones with great results.

T-Slot Milling

You can use the side milling cutter to create T-slots in a workpiece in a technique called T-slot milling. You need this technique in engineering works, especially if you handle have deal with machine beds. Before the T-slot milling cutter is applied for a closed slot, you need to perform the standard groove milling. For open slot milling, the T-slot milling cutter can be engaged directly.

Woodruff Key Slotting

Another slot milling technique emerges when you consider the use of a woodruff key. Woodruff keys are an ingenious method of engaging shafts to rotating components.

Though there are different shapes for these keys, the semi-circular ones are the most common. When machining a shape to accommodate the key, woodruff key slotting tools come in handy. You end up doing a type of slot milling called woodruff key slot milling.

Gang Milling

Well designed gang milling arbors that are sturdy enough can be used to create multiple slots on a work piece. This form of slot milling is popular where the machinist is looking to create multiple slots at once. Gang milling comes in handy as a mass productive method that significantly lowers the machine setup time.

Best Practices For Optimal Slot Milling

For optimal slot milling, the following tips can help:

Chip Removal

Slot milling has a big challenge in chip removal, particularly for deep slots. The slots are usually tight to the extent that chips can easily get stuck and interfere with surface quality and tolerances.

Therefore, the correct chip evacuation method is essential for the optimal results of the slot machining process. You want to keep the workspace as smooth as possible. That’s why you should consider installing chip conveyors to get rid of the chips. Proper chip control translates in higher productivity and profits.

Conversance with Slot Mill Controls

Modern slot milling machines can have numerous features and controls. You want to be familiar with these controls for an efficient slot milling operation. Tool positioning, feed rates, rotation speed, and other machining parameters should be top-notch.

A most important element of slot machining is the constant engagement of the spindle. It helps maintain quality surface finish and minimize vibrations. Without good control skills, one finds it difficult to apply such spindle tips.

Down-milling vs. Back-milling

Machinists will always have ideas about this. However, you want to use down milling technique for your slot milling operations. Just make sure that the slot milling cutter is stable enough to withstand the higher load. Up-milling method is undesirable because its generates high forces on the cutting edge. This can significantly reduce tool life.

Slot Milling Cutter Diameter

The size of the slot mill is part of the considerations when it comes to tooling. Improper tool diameter can lead to poorly machined surfaces. That’s particularly the case when dealing with deeper slots. The tool can even deflect to breaking point with deeper slots. So, choose a bigger diameter is the size of the slot allows.

For smaller slots, larger cutter diameters may not suffice. Its challenging milling such slots when they are deep. Expertise and optimization of available resources becomes the only option then.

Optimized Feed Rate

You can further optimize the slot cutter effectiveness for key and slot joints with balanced feed rate. The idea is to avoid too high and too low feed rates. In-between is preferable for thermal problems and lower productivity respectively.

Getting to the ideal rate takes a systematic effort. You may need to analyze several machining parameters alongside the cutting feed rate.

Ramp Down Technique

Tool entry into the work piece using the right method matters for face milling cutters, slot milling cutters and so on. It does not matter whether you are working with modern or traditional milling machines.

The ramp down motion has long been used to optimize milling operations. It is a combination of axial and radial forces on the cutting tool. Its biggest benefit is the prevention of shock loading. The ramp down technique therefore helps increase machine tool life.


Slot milling is a popular machining technique in the industry. The slot cutter is versatile and reliable, finding application in the creation of simple to intricate slots for a wide range of uses.

At Aria Manufacturing, we are leaders in slot milling expertise. We serve many clients across a broad range of industries. With our advanced machines, we are able to deliver world class slot milling operations, alongside other milling operations.

Get in touch with us today. We will inform you how we can progress from the original CAD design file and cutting program to the final product through consultation.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.