What is Aluminum Machining?
Machining is a subtractive manufacturing process, meaning it removes material from a workpiece to create the desired part or product. It is highly versatile, accommodating a wide range of metal and non-metal substrates. One of the most common materials used in machining operations is aluminum.
Due to its lighter material weight, lower material hardness, and greater formability, aluminum is ideal for use in machining and other manufacturing operations. Below we highlight some of the other advantages of using aluminum for machining applications, discuss how aluminum is used in various machining processes, and outline some of the typical machined parts made from aluminum.
An Overview of Aluminum Machining Processes
“Machining” is an umbrella term that encompasses a range of subtractive manufacturing processes e.g., milling, turning, drilling. There are also several types of machining technologies and techniques e.g., CNC machining, Swiss screw machining, vertical and horizontal milling, and electrical discharge machining (EDM). Below we outline how aluminum is handled in each of these machining methods.
- Computer numerical control (CNC) machining: The CNC machining process employs the use of computer software and CNC-compatible equipment to guide the movement and motion of machine tools across the surface of the workpiece. It allows for the production of highly precise and accurate aluminum CNC parts and products.
- Swiss screw machining: Swiss screw machining is ideal for the production of small—but highly precise cylindrical components made from aluminum, such as electronic or medical parts.
- Vertical/horizontal milling: Milling is a machining process that utilizes rotating cutting tools to remove excess material from the workpiece. Milling equipment can feature a vertical or horizontal configuration; vertical units are ideal for small quantities of simple aluminum parts, while horizontal units are more appropriate for large quantities of complex aluminum parts.
- Electronic discharge machining (EDM): EDM utilizes the electrical discharge generated between two electrodes to remove material from the workpiece. While it is generally used to process harder and more difficult-to-machine materials, it can be used on any electrically conductive material, including aluminum.
Why Can Aluminum Be Challenging?
If you’re making a bracket for Farmer Joe, it’s really not going to matter how efficiently you remove material. But if you’re making 10,000 brackets a week for Hustler Joe, you need to do a proper job.
The main challenge with machining aluminum effectively is simply getting the maximum material removal rates without blowing something up.
Aluminum can melt and fuse to the tool if there’s too much heat. So even though it cuts like butter, it won’t for long if the aluminum sticks to the tool and you end up doing friction stir welding instead of machining.
Beyond keeping friction to a minimum, chatter can be a beast when you’re pushing the machine. This is especially problematic when you’re trying to machine clean-looking pockets. Ok, enough whining. Let’s get into how you can kill it on the floor.
Cutting Tools for Aluminum
Under no circumstances should you ever use a general-purpose cutter for aluminum. Technically it’ll work, but aluminum is totally different from steel.
Here are a few aspects of tool selection that will help you get the most out of your machine.
Cutting Tool Material
Carbide: Hopefully that’s a no-brainer for you. Even in non-performance applications, carbide will exceed high-speed steel in value for both costs of the tool over service life and in surface finish.
Even still, there are a few good things to know about carbide that will help you match up the perfect tool for the job. Essentially, we just need to understand what we want out of a tool. Aluminum is soft cutting, meaning that the tool doesn’t undergo hard impact forces as it cuts.
What’s critical is maintaining a razor-sharp edge. For this reason, we would choose hardness over toughness for material characteristics. There are two main things that affect this property: carbide grain size and binder ratio.
For grain size, larger grain produces a harder material, whereas smaller grain produces a more impact-resistant, tough material. For aluminum, we want to maintain that edge sharpness, so we want a small grain size for maximum edge retention.
The other factor is the binder ratio. For carbide cutting tools, the binder is cobalt. This could have anywhere from 2%-20% cobalt content. Since cobalt is softer than the carbide grains, more cobalt means a tough tool, less cobalt means a harder tool. So, we’re basically just looking for a carbide cutter that has a large grain size and low cobalt content.
Feeds and Speeds
Lots of guys simply go with 1000 SFM to calculate their RPM. If you do this, you’re really not going any faster than everyone else.
To be honest, this is usually is what’s recommended for most cutters. 1000-1500 SFM is a totally normal speed to run your spindle at. With harmonic testing, though, you can hit 3x this speed. More on that later.
The feed rate is where a lot of guys chicken out. If you’re feeding a 1/2″ endmill at only 0.003″ per tooth, you’re just wasting time. For production, you want to push it at least at 1% of the cutter diameter per tooth. This means that 1/2″ endmill needs to be fed at least 0.005″ per tooth. With a stable setup and short tool, you might even be able to double that.
The only exception to this is when you’re working with small tools, like 1/8″ or less. Chip clearance can become an issue, which means that you’ll need to slow down for thinner chips.
Common Operations for Machining Aluminum
Here’s a list of the common stuff you’re going to be doing with aluminum, along with a few tips to help you along.
If you’re going to use a shell mill, definitely go with a super aggressive rake angle and polished inserts. Your finish will be amazing and you’ll be able to really push the RPM.
This is something that a lot of guys don’t do right. If you step over half the diameter of the cutter and half down, you’ll be making a mistake for two reasons:
The cutter can handle more. Go nearly full width. My go-to is 95% of the cutter flat. The reason for this is that the cutter will be buried in the corners anyway. This means that you’ll have to slow down the feed so that the tool doesn’t explode in the corners. If you go a full 100%, you might get papery wafers between the tool paths because of cutter and material deflection.
50% stepovers are horrible for harmonics when you’re roughing at a respectable rate. The impact of the tool entering the workpiece is at the worst possible spot, slapping in with every tooth. Even bumping to tool over to 65% stepover will result in a noticeable reduction of chatter.
Another tip is to use a cutter diameter that’s slightly smaller than the inside radius of the pocket. If you’re using a 1/2″ endmill to cut 1/4″ rad pockets, you’ll have the tendency of gouging the corners with chatter as the tool changes direction. At high speeds, tools don’t change direction instantly, which means that the tool unloads cutting pressure. This is what makes those chirping noises.
I’ll usually ask if I can resize those rads to 0.265″ for clean corners. This reduces the contact that the tool has with the part geometry. The machine is also able to handle that rounded turn at higher speeds. Just think of a car on a race track. If it’s a sharp corner, the car slows down. If there’s a larger radius, the machine doesn’t need to slow down.
This will pretty well eliminate that chirping in corners that makes your parts ugly.
For extremely deep slots, there are two options that work well for me: either use trochoidal milling to reduce cutter deflection and chatter, or use a stub flute endmill.
Personally, I prefer stub flutes, since the tool is significantly stronger, and you don’t get any wasted motion as the tool zips back and forth. Deep slotting is one of those applications where it’s often worthwhile to use a specialized tool.
For shallow slotting (4xD and under), no special considerations need to be made. Just giver.
Use. Sharp. Drills. Carbide drills aren’t always the answer; there’s not really any point in running an expensive carbide drill if you don’t have the spindle RPM or production volume to justify it.
Generally speaking, just use a 135-degree split point drill and you’ll be ok. If there’s a web on the tip of the drill, you’re putting a lot of unnecessary heat in the cut.
General-purpose taps technically work, but taps specifically for aluminum are significantly more reliable. They have a much more aggressive rake angle, which means cleaner cuts and less heat.
Also, don’t be a wimp with RPM. If you never go over 200 RPM on your machines, you’re just wasting time. Of course, some machines are just old and tired, and there’s too much backlash to cut any faster. Really, though, you’re not going to be competitive on these machines anyway. Point is, tapping aluminum is easy, don’t waste time on it.
How to Get Awesome Surface Finishes on Aluminum
High RPM. It’s not much of a secret. Crank it.
Using a finishing tool that’s razor-sharp, high helix, and with very aggressive rake angles will also help you out with getting a super shiny surface finish.
One thing worth mentioning though is that you don’t want to waste your time making the part prettier than it needs to be. Sometimes you’ll just want to make the customer happy and impress them, but remember that there’s a difference between shiny and a high Ra.
It’s really worth it to do your surface finish calculations so you can determine the max feed rate for your finish cuts. I’ll usually do the math, and then back it off about 10% of that to be safe. If you straddle that edge, you’ll be wrong half the time.
Advantages of Using Aluminum in Machining Operations
In addition to its excellent machinability, aluminum demonstrates many characteristics that make it suitable for use in machining operations, such as:
- High strength-to-weight ratio: Aluminum is both strong and lightweight, qualities that are critical for machined parts used in high-performance applications, such as those found in the aerospace and automotive industry.
- Corrosion resistance: Aluminum is available in several grades, which vary in the degree of corrosion resistance demonstrated. In machining operations, one of the most commonly employed grades—6061—offers excellent corrosion resistance.
- Electrical conductivity: Aluminum exhibits greater electrical conductivity (about 37.7 million siemens/meter at room temperature) than other commonly machined metals, such as carbon steel (7 million siemens/meter) and stainless steel (1.5 million siemens/meter). This quality makes machined aluminum parts suitable for use as electrical and electronic components.
- Surface finishing and anodization potential: Aluminum readily accommodates various surface treatment and finishing processes, such as painting, tinting, and anodization. This quality allows manufacturers to improve the functional and aesthetic properties of the machined aluminum part or product.
- Recyclability: Aluminum is highly recyclable, enabling manufacturers to reuse scrap material produced during machining operations and construction materials from finished products discarded at the end of their service lives.
- Affordability: Compared to other machining materials (e.g., brass, titanium, and PEEK), aluminum is cheaper without significantly sacrificing performance. Additionally, its machinability results in lower production costs, while its lighter weight results in lower transportation costs.
As indicated above, the machining process accommodates a variety of materials, ranging from metal and plastic to paper and wood. In addition to aluminum, some of the materials regularly employed in machining operations include other metals (e.g., steel and stainless steel) and thermoplastics. Compared to these materials, aluminum offers a number of advantages:
Compared to steel and stainless steel, aluminum demonstrates a much lighter material weight and better machinability.
Typical Machined Aluminum Parts
Industry professionals employ aluminum in machining operations to produce a variety of parts and products. These components find application in a wide range of industries, including:
- Electrical and electronic products
Typical product examples include dowel pins, EMI housings, front panels, lighting fixtures, medical devices, and spline shafts.