High-speed Machining Formulas and Definitions





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Have you ever wondered what high-speed machining is? Are you curious about high-speed machining formulas and definitions?

HSM, also known as trochoidal milling, is all about efficiency. It’s become popular over the last few years, with more and more shops adopting the practice. There have even been some advances in Machine Learning that deal directly with HSM and optimizing your tooling.

This article will explore some of the backgrounds of high-speed machining, how it compares to conventional machining, and touch on some high-speed machining techniques.

Keep reading if you’re curious about the process and how you can apply it to your next project. Then contact Glenn Metalcraft for your heavy metal spinning and other fabrication needs. 

What is High-speed Machining (HSM)?

At its heart, high-speed machining (HSM) is all about increasing material removal rates while decreasing cycle time and improving tool lifetime.

Companies tend to argue about the correct definition of HSM. Some say that the process requires quick milling passes. You may also find the very technical definition — machining at the resonant frequency of the machine. When you break it down, all this means is using spindle speeds that help to minimize chatter.

Even the “High-Speed” part of HSM is disputed. Although generally, HSM is understood to start at 18,000 rpm, various shops have shown that the process can work with slower spindles as well.

Other machinists argue that HSM should deal with high material removal rates only. The argument here is that the true aim of HSM is to give a surface finish that’s good enough after one pass. 

Remarkably, the initial reason HSM became a consideration was the discovery that increasing spindle speed reduces the heat in the cut once you reach a particular point. At the same time, there’s also a change in cutting forces which likely contributes to the lower heat of the cutting edge.

Let’s look at an example using aluminum. At around 300 SFM (surface feet per minute), the surface temperature is approximately 650c or ~1200f. The research shows that at 1000 SFM, you start seeing the same temperatures! Even more interesting, research also shows that there’s a possibility to run faster and get even lower temperatures.

close-up of dictionary entries for machinery, machine tool, machinist

A Few Definitions

You’ll commonly hear a couple of definitions when talking about HSM and you can find a few of them below.

Feeds and Speeds

This phrase will also sometimes show up as “Speeds and Feeds.” In the simplest definition, it refers to the cutting speed and the feed rate. These two terms are often paired up when we talk about CNC cycles, but you can also use them separately.

You’ll usually see this term used alongside calculations for high feed rates and high RPM operations. Feeds and Speeds calculators can help achieve excellent finishes on both sides of the tool’s cutting edge.

Check out the full section on Speeds and Feeds further down.


SFM stands for surface feet per minute. It’s a reference to how quickly cutting tools move across the surface of a piece in a CNC cycle. Obviously, the speed at which your tool head moves impacts the whole process.

Chip Load

The chip load, otherwise known as “feed per tooth,” is the thickness of the material that gets cut away by each tooth. The chip tends to take away some of the heat with it during the milling process.

We care about this, particularly because with the wrong calculations, you may end up clogging your tool teeth and increasing temperature.

Radial Depths

When talking about the radial depth of a cut (RDOC), machine shops mean the distance a tool steps over into the workpiece. This is also sometimes called Stepover and cut width.

Chip Thinning

Chip thinning generally happens when the RDOC varies. When we talk about chip thinning, we’re talking about the thickness of the chip. Chip thinning occurs when the Chip Load and RDOC are not equal.

An easy way to think of this is to remember that if each tooth is working on the surface at a ninety-degree angle, our cut is most efficient. At that point, the RDOC is at 50% of the cutter diameter.

Below 50% RDOC, the chip thickness changes, and the cut becomes less efficient. This decrease in efficiency can lead to tools wearing down faster than expected and poor quality finishes.

industrial professional inputting high-speed machining variables into a CNC machining panel

Milling Variables

Beyond the definitions we’ve looked at above, there are some variables used in calculations for CNC machines that you should be aware of. We’ll discuss the formulas themselves a little further down.

  • Speed – Surface Feet per Minute (SFM)
  • Feed – Inches per Minute (IPM)
  • Feed per Tooth (FPT)
  • Adjusted Feed per Tooth – Chip Thinning (AFPT)
  • Feed per Revolution (FPR)
  • Depth of Cut (DOC)
  • Width of Cut (WOC)
  • Tool Diameter (D)
  • # of Teeth in Cutter (Z)
  • Metal Removal Rate – Cubic Inches per Minute (MRR)

High-speed Machining Formulas and Definitions

So how do you calculate optimal speeds and feeds? There are two main equations to use. Read on for more information on each part of the equation and how you can get the information that goes into it.

The first formula looks like this:

Feed Rate = Spindle RPM x Number of Teeth x Chip Load

You’ll need to know the RPMs (revolutions per minute) of your spindle and if you’re unsure, check with your manufacturer. They will have spec sheets that deal with your particular model.

Once you have found all your variables, plug them into the formula to calculate your feed rate.

The second formula for calculating SFM looks like this:

SFM = RPM x Diameter x pi/12

Now that you’ve got your RPM, you can plug the values to this formula to find the SFM.

Finally, if you have your SFM, you can also work back to get your Spindle Speed:

RPM = 12 x SFM / 3.14 x Diameter

Or, if you prefer a more simplified version:

RPM = 3.8 x SFM / Diameter

CNC high-speed machining center cutting mold

Expert Machining Done Right

The benefits of high-speed machine tools are quite clear. Now, it’s time to put that knowledge into practice. With a little patience and experience, you’ll find that HSM improves milling efficiency substantially.

Although the high-speed machining formulas and definitions we’ve talked about are fairly standard, they can feel complex and overwhelming. Contact Glenn Metalcraft to discuss full-service metal manufacturing when you’re ready to explore HSM further.

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