Guide: How to Choose the Right End Mill

Introduction

By the time you reach the machining phase, a significant amount of engineering and programming effort has already been invested in a project, but crucial decisions still need to be made.

For CNC milling projects, selecting the appropriate end mill is essential for achieving the desired results, and this involves considering factors like workpiece material, cutting parameters, and tool dimensions. Evaluate these important considerations to choose the most suitable tool for your application.

Taking extra care to select the optimal end mill that will reduce cycle time, extend tool life, and improve the quality of your product.

Key Considerations when Selecting an End Mill

Workpiece Material 

Every material has unique mechanical properties that influence its behavior during machining. For example, plastics demand a different machining approach and tooling geometries than steel.

Understanding the material you are working with and its properties will significantly narrow your search for the optimal end mill, and selecting an end mill with geometries designed for these specific characteristics will improve tool performance and lifespan.

Flute Count

A crucial factor in choosing an end mill is deciding on the correct flute count, which is influenced by both the workpiece material and the specific application.

Material 

For non-ferrous materials, 2 or 3-flute tools are common. The 2-flute offers excellent chip clearance, while the 3-flute excels in finishing and high efficiency milling. Ferrous materials use 3 to 14-flutes, depending on the operation and material hardness.

Application

For plunge milling, use a drill end mill to alleviate strain on both the machine and tool; drill end mills have the right geometry for pulling material away from the workpiece without damaging the tool.

During roughing, a substantial amount of material needs to move through the tool's flute valleys in order to be cleared. Therefore, it’s wise to use a tool with fewer flutes and larger flute valleys. Typically, tools with 3, 4, or 5 flutes are employed for conventional roughing.

When slotting, opting for a 4-flute tool is ideal because the reduced flute count creates larger flute valleys, enhancing chip evacuation efficiency.

When driving into a corner, especially with finish cuts, use an end mill with a smaller cutting radius than the workpiece corner radius. Eliminating 90° cutter contact improves finishes.

For finishing a ferrous material, a high flute count is recommended for best results. Finishing end mills include anywhere from 5-to-14 flutes. The proper tool depends on how much material remains to be removed from a part.

Tool Dimensions 

Once you’ve determined the material, operations to be performed, and necessary number of flutes, you need to make sure your end mill has the appropriate dimensions for the task. Key factors to consider include the cutter diameter, cut length and reach, and tool profile.

Cutter Diameter

The cutter diameter sets the cutting width. An incorrect size can lead to incomplete work or non-compliant parts. Smaller diameters offer better clearance, while larger ones provide more rigidity for high-volume tasks.

Cut Length and Reach

Determine the cut length by the maximum contact length needed. Use the shortest tool to minimize overhang and chatter. For depths over 5 times the tool diameter, consider reduced shank options.

Tool Profile

The most common profile styles for end mills are square, corner radius, and ball.

Square profile - has flutes with sharp corners that are squared off at 90°

Corner radius profile - replaces the fragile sharp corner with a radius, adding strength and helping to prevent chipping while prolonging tool life.

Ball profile - features rounded flutes, forming a "ball nose" at the tip. This profile excels at 3D surfacing and creating fillets.

An end mill profile is often chosen by part requirements, such as square corners at the bottom of a feature, requiring a square end mill. 

When possible, opt for an end mill with the largest corner radius allowable by your part requirements. If square corners are absolutely required, consider roughing with a corner radius tool and finishing with the square profile tool.

Cutting Parameters

Cutting parameters are crucial factors that dictate the removal of material from your workpiece. These variables have a direct impact on machining efficiency, tool longevity, surface quality, and the precision of the part. The primary cutting parameters are:

• Cutting speed – the rate at which the cutting tool's edge travels in relation to the workpiece, usually measured in surface feet per minute (SFM) or meters per minute (m/min). This rate is influenced by the tool material, the workpiece material, and the desired finish.
• Spindle speed – the spindle's rotational speed, expressed in revolutions per minute (RPM), is determined by the cutting speed and the tool's diameter.
• Feed rate – the rate at which the cutting tool moves through the workpiece, usually measured in inches per minute (IPM) or millimeters per minute (mm/min)
• Feed per tooth – the rate at which the cutting tool moves through the workpiece, usually measured in inches per minute (IPM) or millimeters per minute (mm/min)
• Depth of cut – the depth the tool penetrates into the material in a single pass
• Material removal rate – the volume of material removed per unit of time
• Cutting force and power requirements - determined by material properties, tool geometry, and cutting parameters, influencing machine load and energy consumption

Tool manufacturer websites typically have helpful, technical resources on their website that can be a good starting point to learn more about a specific end mill’s cutting properties; you can also work with your tooling rep to find the best fit for your project.

Chip Evacuation

Chip evacuation involves clearing away the machined material from the cutting area to prevent tool damage, maintaining surface finish quality, and ensuring efficient machining. Effective chip evacuation is also vital for prolonging tool life, avoiding the recutting of chips, and preventing heat buildup, which can cause tool wear or failure.

Consider the tool's geometry, flute design, and features like chip breakers to select an end mill that addresses chip evacuation concerns in your machining process.

Coatings

End mills may be coated with different materials, like titanium nitride or diamond-like carbon. End mill coatings help you manage heat, reduce friction, add lubricity to the tool, and help remove chips from your workpiece. That being said, coatings ever so slightly round off the end mill’s cutting edge. If you don’t “need” a coating and tool sharpness is a priority, go for a tool without one.

When selecting a coating, consider the material being milled and the finish you need. Similar to researching an end mill’s cutting parameters, you can reference the tool manufacturer’s website to match end mill coatings to workpiece material.

Remember, taking the time to choose the optimal end mill rather than settling for any end mill that can do the job will not only preserve your tools and reduce both wear and damage, it will also improve the quality of your parts, cycle times, and overall machining efficiency.