How to Correctly Set Cutting Speed and Feed Rate?

 

feed rate vs cutting speed

Practical Experience Sharing on Cutting Speed and Feed Rate ​​in Machining

In machining, cutting speed and feed rate are the most frequently mentioned yet misused parameters. They directly determine machining efficiency, surface quality, and tool life. This article is not just theoretical, but a highly practical guide. It covers the difference between cutting speed and feed rate, starting parameter selection, feed calculation for multi-edge tools, the impact of coatings and cutting edges on parameters, adjustment strategies for machine tool rigidity and spindle limitations, and how to adjust parameters when encountering noise, chatter, and tool breakage.

1. The Difference Between Cutting Speed and Feed Rate

Cutting speed and feed rate are two of the most core cutting parameters in machining. Many beginners confuse them, but they actually have distinct meanings, influencing factors, and importance. The following analysis will explore these from three perspectives.

First, let's look at the definitions.

Cutting speed refers to the linear velocity of the tool's cutting edge relative to the workpiece surface. It is typically expressed in meters per minute or feet per minute and is directly related to the spindle speed and tool diameter.

Feed rate, on the other hand, indicates the speed at which the tool advances in the feed direction. It is usually expressed in millimeters per minute or inches per minute. Sometimes, this is converted to feed per tooth (mm/tooth), particularly for multi-flute tools, for easier calculation and control.

In other words, cutting speed determines the circumferential distance the tool travels per unit time, while feed rate determines the depth and speed of each cut of the tool's cutting edge into the material.

The formulas for their calculations are as follows:

Parameters	Formula
Cutting Speed	$V_c=\frac{\pi \times D\times n}{1,000}$
Spindle Speed	$n=V_c÷\pi ÷D\times 1,000$
Feed	$V_f=n\times V_z=Z$
Feed per Tooth	$f_z=\frac{V_f}{n\times Z}$
$V_c$ = Cutting Speed (m/min)
$\pi =3.14(\text{The Circular Constant})$
$D$ = Diameter (mm)
$V_f$ = Feed (mm/min)
$f_z$ = Feed per Tooth (mm/tooth)
$Z$ = Number of Flutes

Second, let's look at the influencing factors.

Cutting speed is primarily governed by material type, tool grade and coating, cooling conditions, and the machine tool's spindle limitations. For example, when machining stainless steel, the cutting speed should be much lower than when machining aluminum alloys, as otherwise it can lead to tool overheating and wear.

Feed rate, on the other hand, is more significantly affected by tool diameter, number of flutes, workpiece clamping rigidity, and chip removal conditions. If the feed rate is too low, the tool will rub against the workpiece surface instead of cutting, increasing wear. If it is too high, it can cause excessive instantaneous load on the tool, leading to chipping.

Finally, let's consider their importance.

The two parameters play different roles in productivity and machining quality. Cutting speed primarily affects tool life and surface quality. Excessively high speeds can significantly shorten tool life, while too low speeds can reduce machining efficiency.

The feed rate is directly related to material removal rate and cutting process stability. An appropriate feed rate ensures moderate chip thickness, resulting in a smooth cutting sound and a good surface finish.

In actual production, experienced operators often first set the spindle speed based on the cutting speed range provided by the tool manufacturer, then fine-tune the feed rate based on machine rigidity and actual cutting conditions.

After clarifying the conceptual differences between cutting speed and feed rate, readers often have a more pressing question: how should these two parameters be correctly set and adjusted in real-world machining? Many beginners encounter problems like tool chipping, vibration, unusual cutting noise, and unsatisfactory surface finish during machining. These issues are essentially closely related to the selection of cutting speed and feed rate.

This article focuses more on sharing practical experience, but our website also provides a more comprehensive basic guide that explains the core concepts of cutting speeds and feed rates, including calculation formulas and influencing factors. For readers unfamiliar with these two parameters, we recommend first reading the basic guide and then applying the practical tips here. This will not only make it easier to understand but also facilitate practical application.

After reading the articles on this website, I believe you have a certain understanding of the core concepts of cutting speed and feed rate. If you still have questions or want to know more information, please continue reading!

Theoretical definitions are just the foundation; the real challenge lies in applying these parameters to specific machining conditions. Therefore, we will address the most common practical questions in machining. Starting from the questions themselves, combined with typical cases and empirical methods, we will help you find the most appropriate solutions and operational strategies for your on-site machining operations.

2. How should I set the "starting parameters" for cutting speed and feed rate? Where can I find reliable tables or recommended values?

The most reliable way to determine starting parameters for machining is to refer to the catalogs and technical data from tool manufacturers or insert suppliers. These data often provide recommended cutting speeds, feed per tooth, and appropriate depth of cut for different materials. These data are tested and verified, making them highly valuable references.

The general approach is to first select a recommended cutting speed range based on the workpiece material and tool type, convert it to spindle speed, and then calculate the linear feed rate based on the number of tool teeth and the recommended feed per tooth.

feed rate diagram

If manufacturer data is unavailable, you can use a common SFM comparison table as a starting point, but it's generally recommended to start at the lower end of the recommended values ​​and then gradually adjust based on machine rigidity and cooling conditions. Some manufacturers now offer mobile apps or online tools that can quickly generate parameters, making them convenient and practical.

3. How do you calculate feed for a multi-flute milling cutter? How do you choose the feed per tooth (FPT)?

Calculating feed for a multi-flute milling cutter is relatively straightforward: the actual feed rate (mm/min) is equal to the feed per tooth (FPT) multiplied by the number of teeth on the cutter, then multiplied by the spindle speed (RPM). The formula is: Feed = FPT × Number of Teeth × RPM. This means that as long as you know how much material each tooth of the cutter engages, you can deduce the overall feed rate.

As for the selection of FPT, it is a critical parameter that determines machining results. Generally speaking, the FPT is determined by three key factors: tool type, workpiece material, and tool diameter. Different tool designs have different tolerances. For example, the recommended FPT ranges for end mills, ball cutters, roughing tools, and finishing tools vary significantly. The toughness and hardness of different materials also directly affect the FPT. For example, when machining aluminum alloys, the FPT can generally be set higher, while it should be appropriately reduced for stainless steel or titanium alloys. The larger the tool diameter, the larger the FPT that can be selected.

4. How do tool coatings, edge treatments, and chipbreakers affect cutting speed and feed rate selection?

Tool coatings, edge treatments, and chipbreakers have an impact on cutting speed and feed rate selection, which is often overlooked in practice.

cutting speed diagram

Tool material and coating determine its ability to withstand high temperatures and high friction. For example, TiN is suitable for ordinary steels, TiAlN is suitable for high-speed cutting and high-temperature resistance, and TiCN is wear-resistant and anti-sticking. The better the coating performance, the higher the cutting speed that can be tolerated, and the feed per tooth can be appropriately increased.

Edge sharpness and grinding method can change cutting forces and chip flow. A sharp cutting edge offers low resistance, allowing for higher speeds and feeds. Rounded or ground cutting edges generate high cutting forces, requiring reduced speeds or feeds to prevent chattering and chipping.

Chipbreaker design controls chip breakage and evacuation. For highly cohesive or ductile materials, such as stainless steel and titanium alloys, an appropriate chipbreaker prevents chip entanglement and reduces load, allowing for higher speeds or feeds. Improper chip breaking increases cutting forces, potentially leading to chattering or tool breakage. Therefore, tool type and geometry should be appropriately selected based on the material and machining method.

5. How does machine tool rigidity or spindle limitations (maximum RPM/power) affect parameter selection?

Machine tool rigidity and spindle performance have a direct impact on cutting parameter selection. In practice, operators often focus solely on the tool and material, ignoring machine limitations. The spindle's maximum speed, power, and torque characteristics limit feed rates and cutting speeds, especially when cutting large diameter tools at high speeds. Insufficient torque can lead to spindle stall, insufficient cutting forces, reduced surface quality, or increased tool vibration.

The overall rigidity of the machine tool is also critical. The stability of the bed, spindle support, guideways, and worktable determines the level of machining vibration. If rigidity is insufficient, chattering, chip accumulation, and uneven surfaces can easily occur, even when machining at theoretical speeds and feeds.

Therefore, before setting speeds and feeds, consider the actual capabilities of the machine tool, consult the machine and toolholder specifications, and confirm the maximum RPM, power, and torque to ensure that overload is not present. During machining, start with conservative test cuts, observe the cutting conditions, and then adjust accordingly. This maximizes tool performance while ensuring machining stability and workpiece quality.

6. How should you adjust speeds or feeds when encountering "noise, chatter, and tool breakage"?

Cutting Abnormality Adjustment Guide: Practical Steps to Address Noise, Chatter, and Tool Breakage

Step 1: Observation and Judgment

If harsh sounds, vibration, or tool damage occur during cutting, do not rush to change all parameters. Carefully observe the chip morphology (whether it's too thin, too thick, hot, discolored, or powdered), listen for any acoustic characteristics (high-frequency chattering or low-frequency muffled sounds), and make a preliminary assessment based on the workpiece surface quality. Sound and chips are often the quickest "alerts."

Step 2: Boldly Adjust the Spindle Speed

Experience shows that small speed adjustments have limited effectiveness. Therefore, it's recommended to experiment directly with larger adjustments:

Increase the speed to 150%-200% to see if cutting becomes smoother. Higher speeds reduce the load on individual teeth and lower vibration amplitude.

If increasing the speed doesn't work, try reducing it to about 50% of the original speed. Lower speeds help avoid resonance and stabilize cutting forces.

Step 3: Check Rigidity and Fixtures

If problems persist after adjusting the speed, thoroughly check the rigidity:

Is the tool overhang too long? Minimize it as much as possible.

Check the fit of the toolholder to the spindle. High-precision toolholders or a strong clamping system are preferred.

Check the workpiece clamping position and whether the soft jaws or fixture are loose. Insufficient rigidity is the root cause of over 75% of tool chatter problems.

Step 4: Optimize Feed and Depth of Cut

Check if the feed per tooth is too low. Too little feed causes the tool edge to rub against the workpiece surface instead of cutting, resulting in high-pitched noise and high-frequency vibration.

Excessive feed per tooth or deep cutting depth overloads the tool, leading to chipping of the cutting edge. Adjust the tool gradually within a reasonable range to maintain a moderate cutting thickness.

Step 5: Leverage Tool and Process Data

Consult the tool supplier's catalog or application manual for recommended speeds, feeds, and depths of cut for different materials and tools.

Use the manufacturer's calculation software or mobile app to quickly obtain reference parameters suitable for your working conditions.

Pay attention to design features such as tool coatings, edge preparation, and tool nose angle, as these directly affect cutting stability.

Step 6: Comprehensive Verification and Experience Gaining

After adjusting the parameters and rigidity, observe the cutting performance again. If the sound is smooth, the chip shape is normal, and the workpiece surface is smooth, the adjustments have been effective. At this point, it's important to record the optimal parameters for this material and tool combination so that they can be quickly reused in subsequent machining operations.

7. Summary

In summary, cutting speed and feed rate settings aren't determined by a single factor. Instead, they require a step-by-step optimization process that considers tool type, machine performance, material properties, and more. I hope this article provides a clear reference path, helping you avoid detours in actual machining and quickly find a stable and efficient parameter combination.