Slot Milling: Definition, Diagram, Working, Types, Pros and Cons [PDF]

What do you mean by Slot Milling?

Slot milling is like using a fancy drill with lots of sharp edges to make neat cuts in a piece of material. You can make exact slots, holes, or channels in things like metal or wood. It’s super handy because it’s used in making all sorts of stuff, from machines to furniture.

To do slot milling, you need special tools called slot milling cutters. They’re like the superheroes of cutting tools!

It’s important to know different slot milling methods, what they’re good for, and what might not work so well. This helps pick the best way to cut stuff depending on what you’re making.

Types of Slot Milling and Cutters

Following are the main types of slot milling cutter types:

#1 Face Milling

Face milling cutters are like the go-to tools when you need to make straight grooves, especially in slotting jobs. They’re flat and have lots of sharp teeth all around.

face milling

When you use one, it digs into the material and makes nice, straight grooves. But here’s the catch: they’re best at making straight grooves. They’re not so great for making curvy or complicated shapes.

So, while face milling cutters are super handy for making straight grooves, they’re not the best choice for fancy or tricky jobs. But hey, they still get the job done when you need those straight lines!

#2 End Milling

End milling is when you use a regular endmill cutter or a shell mill to cut slots. This tool is pretty cool because it can make all sorts of slots: closed ones, wiggly ones, and even ones that go deep.

end milling

When you’re using it, it works kind of like face milling because it cuts both the face and the side of the material. But here’s the thing: while face milling makes things smooth, end milling makes closed grooves on both ends.

Since endmills come in different sizes and materials, machinists like using them for slot cutting more than other methods.

#3 Side Milling

This cutter is awesome for milling because it can really dig into the sides of a piece of metal and remove a bunch of material. It’s got teeth all over the sides and around the edges.

side milling

It’s made just for cutting keyways and slots, and it comes in different styles: some with teeth that are staggered, some that lock together, some that are flat, and others that are half-cut. Each style is made for different kinds of slots, depths, and jobs you might need to do.

Side cutters are super consistent and get a lot of work done, especially when you’re making long, deep slots.

#4 Gang Milling

Gang milling cutters are like a bunch of groove cutters all attached to one stick. They’re really good at making slots with precision and without costing too much. Using lots of cutters at once saves a ton of time and makes slot cutting faster.

gang milling

The cool thing about them is that they can make slots with all sorts of designs in just one go. But here’s the catch: because they work so efficiently, they create a lot of force when cutting.

To use gang milling cutters, the machine and the stick they’re attached to have to be super strong. So, it’s important to set them up properly to handle all that force and make sure they work their best when making slots.

#5 Woodruff Key Slotting

Woodruff key slot cutters are round cutters with a sturdy stick for steady cutting. They’re usually made of a tough material called HSS M2 grade and are used for making keyways in shafts.

woodruff key slotting

For businesses that need really precise and small slots, the Woodruff-Style tiny-width cutters are perfect. They look like flat cutters and are great for making tiny slots because of their curved sides.

These cutters make slots that are kind of like half circles, so the bottom of the slot can be either square or rounded. To use them, you just position the cutter on the material and adjust how deep it goes to fit the standard sizes of Woodruff keys.

#6 T-slot Milling

Making slots in the shape of a “T” is called T-slot cutting, and it’s a pretty common thing in industrial machines, especially for making machine beds. To do this, you use a special cutter called a T-slot milling cutter.

T-slot milling

This tool is really handy when you need slots that go deep into the material because it can do lots of different machining jobs. It’s great at cutting exactly where you need it to along the spinning axis, which lets you put those T-shaped grooves exactly where you want them.

Since it’s so good at making slots that stick out above the material’s surface, the T-slot cutter is a must-have tool for lots of different machining jobs.

Tips and Precautions for Slot Milling

To get the best results when cutting slots, you need to be careful and skilled. Here are some tips to help you improve your slot milling performance:

#1 Instead of cutting straight in, try starting with a ramp down. This helps the cutter enter smoothly without causing too much stress on the tool. A ramp angle of 45 degrees or more is usually good to prevent any damage to the machine or tool. You can even use steeper angles for deeper slots or tougher materials.

#2 Make sure you control the chips. Slots can trap the chips inside, which can mess up your work and damage your tools. It’s important to cut the slot in multiple passes so there’s enough space for the chips to escape. This also allows you to remove more material in each pass.

#3 Keep the spindle loaded. Make sure that at least one sharp part of the cutter is touching the material you’re cutting. This keeps everything running smoothly and helps you get rid of as much material as you need to while keeping the slot nice and even.

#4 Use down milling whenever possible. It’s more stable and better for chip removal compared to back milling. Just make sure your system can handle the extra load from down milling.

#5 Use larger cutter diameters for more stability, especially when cutting deep slots. Bigger tools are stronger and less likely to break, especially if they’re made of hard materials. However, it can be tricky to find the right-sized cutter for small but deep slots.

#6 Adjust your cutting feed for better results. Too slow and you’ll waste time, too fast and you might run into overheating issues. Finding the right balance is key to efficient slot milling.

Slot Milling Toolpath Methods

By planning out how the tool moves, machine experts make machining work better. For slot milling jobs, they’ve figured out some ways to do it smarter, which makes the parts and machines better.

#1 Conventional Toolpath

The simplest way to cut slots is called conventional milling. This method just makes straight cuts along the slot, and it’s easy to set up and gets a lot done quickly. Plus, it can work with all sorts of cutting tools.

When you’re cutting really deep slots, you might notice a lot of shaking. Also, if the material is tough, this method doesn’t work as smoothly. Since the tool stays in the material, it’s under a lot of pressure, especially with deep cuts, which can be risky. Plus, it tends to get really hot really fast.

#2 Trochoidal Toolpath

Trochoidal milling is a special way of moving the tool in a curved pattern. It’s great for slot milling because it keeps everything steady, lets the cutting edges cool down properly, and makes sure there’s enough space for the chips to be removed easily.

Because it doesn’t push too hard on the material from the sides, trochoidal milling is handy for cutting tough stuff. But, it does need more complex programming and you might have to choose from more tools to get it just right.

#3 Pluging Toolpath

Plunging is like drilling with a milling cutter to make all the cuts straight down into the slot. It’s not the fastest way to do it, but it’s really steady because there aren’t any sideways forces causing shaking, overheating, or bending of the tool.

So, when you need to be careful with the tool or you’re dealing with deep slots, plunging is the way to go. Plunging doesn’t give the smoothest finish, especially on the sides of the slot. So, after plunging, you’ll still need to use another method to get the surface just right.

Pros and Cons of Slot Machine


  • Slotting machines are great for making deeper pockets or slots compared to end milling.
  • They simplify the creation of both inner and outer profiles, eliminating the need for frequent spindle realignment.
  • Slot milling makes machining easier by removing the need for constant spindle adjustments, improving accuracy, and streamlining production.
  • Slot milling cutters are perfect for working with solid-jaw mandrels, guide bars, and flat metal items.
  • These cutters allow for precise shaping and slot design by efficiently removing material as they rotate.
  • They’re flexible and useful for a variety of metalworking tasks, providing precision and effectiveness in making complex components.


  • Slot milling can cause high vibrations, especially when cutting complex slots or tough materials.
  • Deep cuts in slot milling may be less secure due to the significant radial forces generated during continuous cutting.
  • The cutting process can quickly heat up the tool and workpiece, potentially reducing the tool’s lifespan and affecting material quality.

Slot Milling Applications

  • Slot milling is commonly used to create keyways, pockets, and slots in various workpieces.
  • These slots are essential for the basic functionality or assembly of products across many industries.
  • It enables the accurate production of mechanical elements such as gears and pulleys.
  • Slot milling allows for the creation of dimensionally precise shapes that may be difficult or impossible to achieve with other methods.


Slot milling has many important uses in industries, making it a valuable way to cut materials. It simplifies manufacturing and allows for the creation of complicated shapes.

That’s all for now on “Slot Milling”. If you have any questions or concerns about this article, feel free to ask in the comments. And if you found it helpful, don’t forget to share it with your friends.

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