CNC Router Feeds and Speeds: How to Calculate the Right Feed Rate

Getting your CNC router feeds and speeds dialed in is one of the most important things you can do for cut quality and tool life. Too slow and you’re burning up your bits; too fast and you risk snapping them. The good news is that the calculation is straightforward once you understand the three variables involved.

The formula:

Feed Rate (IPM) = RPM × Chipload × Number of Flutes

That number is your feed rate in inches per minute, which is what you’ll enter into your CNC software. Let’s break down each variable.

Step 1 – Choosing the Right RPM

RPM is the speed at which your spindle rotates. A good rule of thumb: as tool diameter increases, slow the RPM down, and as diameter decreases, speed it up. For example, a 3/8″ bit runs well at around 16,000 RPM, while a 1/8″ bit can handle 24,000 RPM. You’ll also find our recommended RPM ranges on page 11 of our catalog.

Step 2 – Finding the Right Chipload

Chipload is the thickness of material each cutting edge removes per revolution — think of it as the “bite size” of your bit. It’s the most critical variable in the calculation.

Too small a chipload and your bit is rubbing instead of cutting, generating heat and turning your material to dust. Too large and you risk snapping the bit. LMT Onsrud has done extensive testing and publishes recommended chiploads for each tool series across different materials. We’ve condensed the most common ranges on page 11 of our catalog as well.

One quick check: if your CNC is producing fine powder instead of small chip flakes, your chipload is too low. Increase the feed rate until you see actual chips coming off.

Step 3 – Counting Your Flutes

The number of flutes is the number of cutting edges on your bit. Most of our tools are either 1- or 2-flute. You can count them by looking at the tip of the bit and counting the sharp points. Your invoice description will also note the flute count.

A Real-World Example (Cutting Aluminum)

Let’s put it all together. Say you’re cutting aluminum with the Onsrud 63-620 (a 1/4″, single-flute bit). Based on our catalog:

• RPM: 18,000
• Chipload range: .003 – .006
• Flutes: 1

Low-end feed rate: 18,000 × .003 × 1 = 54 IPM

High-end feed rate: 18,000 × .006 × 1 = 108 IPM

A good starting point is the middle of that range, roughly 81 IPM. Dial from there based on how the cut sounds and looks.

Troubleshooting Your Feed Rate

Even with a solid starting calculation, you may need to adjust. Here’s a quick guide:

• Getting chatter or vibration? Move toward the lower end of your chipload range to reduce the load on the tool.
• Material melting or burning? Move toward the higher end. More feed rate means bigger chips and less heat buildup.
• Producing dust instead of chips? Your chipload is too low. Increase the feed rate.
• Bit breaking? Your chipload may be too aggressive. Step down the feed rate and check that your RPM is appropriate for the tool diameter.

As always, we’re here to help. If you’re unsure where to start with a specific material or tool, give us a call and we’ll walk you through it.