What are Cutting Speeds?
In manufacturing, cutting speeds are worked out by combining two essential parameters - speed and feed. In a nutshell, speed is the rotation of the workpiece or tool while feed is how fast you feed your cutting tool into the material. Alongside depth of cut and radial engagement, you get data used to calculate your cutting tool’s material removal rate (MRR).
How important are cutting speeds for tool performance?
Vital. The engineers at Exactaform often find that cutting speeds are one of, if not the biggest, areas of mistake during cutting. Cutting speeds affect all machining factors, and in turn, they’re affected by these same factors. Too slow or too fast, you’ll find tool life and performance are compromised. Part quality, surface finish, and cycle times also depend on correctly calculated cutting speeds.
Factors Affecting Cutting Speeds
The material being cut
The material being cut has a massive impact on cutting speeds. Different metals and composites have unique properties, and even variations of the same material can vary hugely. These properties need to be taken into account when working out cutting speeds.
In particular, three key material properties affect cutting speeds:
- Hardness: Soft metals, like aluminium and brass, generate less friction and can be cut at higher speeds. In contrast, harder metals, like stainless steel and titanium, might require slower speeds to ensure tool wear and heat remain at acceptable levels.
- Ductility: Slower cutting speeds are needed to prevent ductile metals like copper from deforming and jeopardising surface finish.
- Surface finish: The required surface finish of a component plays a big role in defining cutting speeds. Quite often, reduced cutting speeds are used to create a higher-quality finish by reducing vibration.
Type of cutting tool
Much like your workpiece material, cutting tool type has a significant impact on cutting speeds.
- PCD tools: PCD (Polycrystalline Diamond) tools can cut non-ferrous metals much faster than carbide. Plus, for face milling, PCD tools can hold more teeth, boosting productivity. More teeth = more feed, and so PCD = more speed. Plus, these tools are tough and resistant to wear. This elevated durability means they can take the punishment of higher cutting speeds while remaining precise.
- Carbide: Carbide tools can’t operate at the same speeds as PCD tools, but they can still operate at high speeds across a wide range of metals. Carbide tools generally balance good cutting speeds and durability.
Tool wear and tool life
There’s always a ‘sweet spot’ to be found with cutting speeds and tool wear:
- Faster speeds generate heat and friction, wearing tools down faster (especially with harder metals like titanium).
- Slower speeds might extend tool life in some cases, but they can also tank throughput and productivity.
The key is to find the optimal balance between maintaining tool life while keeping productivity exactly where it needs to be.
Cutting Speeds and Tool Parameters
What is the relationship between cutting speed and feed rate?
As mentioned above, cutting speed is how fast the tool spins in relation to the workpiece, while feed rate is how fast the material is fed to the tool. In the UK and Europe, we use the metric system to calculate cutting speeds and feed rates, while the US uses imperial measurements.
Feed rate and cutting speed are measured in two different ways- Feed rate: measured in mm a minute, or mm per tooth. For example, in mm per minute, if you increase your RPM, your feed rate will remain the same, but if you increase RPM for mm per tooth, your feed rate will increase. This measurement is decided on a machine-tool user preference.
- Cutting speed: measured in RPM or metres per minute. RPM is a fixed rotation, while metres per minute is the distance travelled.
Exactaform’s advice: Calculate the feed rate in mm per tooth and the cutting speed in metres per minute.
Equations to understand the essential calculations
Converting cutting speed measurement units from metric to imperial
What is the optimal spindle speed for different cutting speeds?
They’re related. The higher your RPM, the higher the feed rate you’ll need. The higher the RPM, the more wear you’ll have on your tool. As such, there is no optimal speed; you’ll need to work it out on a case-by-case basis.
Effects of cutter diameter on cutting speeds
The bigger the diameter of the tool, the slower the RPM. Cutting diameter increases, for example, when drilling bigger holes. Alternatively, when face-milling, a bigger diameter tool will reduce cycle times. You could think of this like painting: a bigger brush with more surface area could reduce the time it takes to paint a fence, and you won’t need to go as fast as someone using a smaller brush.
Choosing the right cutting speeds for specific tools
Machinists need to carefully consider if a cutting speed is right for a tool, and for a business more generally. Choosing the right cutting speed depends on a multitude of factors, everything from part stability too material being cut all needs to be considered. One thing people tend not to consider is the number of parts you need to produce over a certain time. Preserving tool life is often a top priority, and so every tool will have a sweet spot between wear and performance, but this depends on your use case.
Effects of Cutting Speeds on Cutting Tool Performance
Material removal rate
Material removal rate, or MRR, is a great way to calculate how efficient a tool is. To calculate MRR, you’ll need to multiply feed rate by the Radial Depth of Cut (RDOC) and the Axial Depth of Cut (ADOC).
Calculation for MRR
MRR = (Width of Cut × Depth of Cut × Feed Rate × Spindle Speed)/1000
All engineers aim for a high material removal rate so they can reduce cycle times and boost productivity.
Chip load
Calculation for chip thickness
Chip thickness = feed per tooth x √radial engagement / cutter diameter
Radial cut and feed per tooth affect chip thickness. If you reduce the speed and keep the same feed per tooth, the chip thickness stays the same. This is only the case if you are feeding in mm/rev.
Feed rates also play a pivotal role in chip loads. The more aggressive your feed rate, the thicker your chip. While this can boost MRR, a high feed rate can affect swarf evacuation, tool life, and surface finish.
Finding the right chip load through correct cutting speeds and feed rates is essential. Chips that are too thin or thick can cause issues with part quality and tool life.
Cutting Speeds for Steel
As an example to show the importance of cutting speeds, and how much they can vary, let’s look at an example from one of the most commonly machined materials on the planet: stainless steel.
Stainless steel represents a wide range of materials and compositions. As a general rule of thumb, your cutting tools have to be sharp because the material is sticky.
Cutting speeds for two different types of stainless steel
Free-cutting stainless steel
- Grade 303 austenitic Steel
- Cutting speed bracket - 120-250 m/min
- Cutting feed bracket – 0.1-0.25 mm/tooth
Hard-cutting stainless steel
- Grade 2205 duplex steel
- Cutting speed bracket – 70-130 m/min
- Cutting feed bracket 0.08-0.2 mm/tooth
This example illustrates just how much cutting speeds and feeds can vary, even across the same category of metal.
There’s no one-size-fits-all approach to cutting speeds. Each part, material, and machine tool needs to be taken into account to find the right speed and feed to get the best from your tools.
Exactaform Exactaset Product
Exactaform are known for their exceptional PCD tooling, and the Exacta-Set is one our greatest designs.
For intense cutting speeds and impressive MRR, the Exacta-Set PCD face mill is a great choice. It’s both efficient and performance-focused: changeable cartridges allow for re-tipping, and the height of the teeth can also be adjusted to maintain alignment and accuracy.
If you work with non-ferrous metals, the Exacta-Set promises superior performance to carbide, with higher cutting speeds and more teeth.
Conclusion and final tips
As mentioned before, cutting speeds affect and are affected by all other facets of machining. Getting them right is essential for performance and efficiency.
If you think your cutting speeds and feeds might be off, tool wear is your best indicator of a problem. The type and severity of wear tell you how your cutting data stacks up.
Tool balancing is also a factor worth mentioning. A well-balanced tool plays an important role in maximising spindle speed. Thankfully, Exactaform thoroughly checks the balance of all their tools before they’re sent out for delivery.
