How Tool Diameter Influences Fillet Restrictions in Milling

When it comes to machining, understanding the limitations and capabilities of your tools is essential for achieving precision and efficiency.  One important aspect to consider is the vertical depth of the cut relative to the tool axis. Typically, machinists try to adhere to a limitation where the vertical depth of cut along the tool axis is less than 2 to 2.5 times the diameter of the tool. But why is this the case, and how does it impact the machining process? Let's dive into the details.

The Basics of Internal Fillets and Tool Diameter

An internal fillet is a rounded corner between two walls of a component when the angle between those walls is less than 180 degrees. Typically, fillets are intended to reduce stress concentration and improve the overall strength of a component.  Internal fillets are also necessary to accommodate the rotating tool when side milling as, intuitively, a round tool can not cut a sharp internal corner.  Understanding the limitations of tooling is important to designing components that can be produced with the best possible accuracy, surface finishes, and efficiency.   

Why Tooling Limitations in Milling Exist

In the intricate world of machining, understanding the limitations of tooling is crucial for machinists striving for precision and efficiency. Tooling limitations exist due to various fundamental factors inherent in the machining process. These factors—angle of engagement and vibration, internal fillet design considerations, tool wear and longevity, and accuracy—play a significant role in determining the performance and lifespan of cutting tools.

Angle of Engagement and Vibration

When side cutting with an end mill, the angle of engagement is anywhere between 180 degrees to near 0 degrees, where 180 degrees is the tool cutting a full slot the width of the diameter of the tool, and near 0 degrees is the tool just grazing the material.  To prevent excessive tool deflection, the optimum angle of engagement is generally kept well below 45 degrees, tthe smaller the angle of engagement, the less tool deflection. 

Machining internal features, such as a rectangular pocket, if the radius of the fillets in the corners of the pocket is exactly ½ of the diameter of the tool, then the angle of engagement suddenly jumps to 90 degrees or more, depending on the angle, as it reaches the corner.  This sudden increase in the angle of engagement gives a momentary increase in tool deflection, and at the same time, the motion of the machine stops to change direction.  This results in the tool rebounding, then vibrating, as the load is suddenly increased and immediately released resulting in a poor surface finish at best, or at worst, the fillet being overcut. 

When the depth the tool has to cut begins to be more than 2.5 times the diameter of the tool, this effect is amplified due to the rigidity of the tool decreasing with length.

Internal Fillet Design Considerations

When designing a component, the smallest internal fillet radii of a feature determine the largest possible tool that can cut that feature.  For instance, if a polygonal pocket has internal corner fillets of 1/8-inch, then the largest possible tool that can be used to machine this feature is ¼-inch.  Adhering to the rule of thumb on the maximum depth of cut for a 6mm tool would mean the pocket would ideally be no deeper than 5/8 inches.

Should the design requirements be for fillet radii that will require a tool to cut deeper than 2.5 times the diameter, or if the feature must accommodate a mating component that a fillet would interfere with, corner reliefs are a common design method to improve the machinability of a feature.  A properly designed corner relief can allow for the use of an appropriate diameter tool and/or accommodate a mating component while still reducing stress concentrations.

The most effective design consideration is to make the internal fillets just slightly larger than the radius of common tooling.  When the internal fillet radius is even slightly larger than the radius of the intended tool, the angle of engagement will increase and then taper off more gradually as the tool cuts the fillet, minimizing the vibration.  Here in the US, tooling most commonly still comes in imperial sizes, for instance, 1/4-inch diameter. 

Therefore, if the feature can accommodate an internal fillet size of 0.13 inches instead of an even 1/8-inch (0.125), this difference of 0.005 inches can improve results and increase efficiency rather dramatically.         

Tool Wear and Longevity

Using a tool with a diameter that is appropriately sized for the vertical depth of cut minimizes wear. Tools used for deeper cuts beyond the recommended ratio tend to wear out faster due to the increased stress and vibration, leading to more frequent replacements and higher costs.

Accuracy

Tools that cut deeper than 2.5 times their diameter will deflect away from the intended geometry and this deflection will affect accuracy and surface finish.   Maintaining the 2 to 2.5 times diameter to depth ratio will produce superior results with the best possible accuracy.

By adhering to best practices, such as maintaining optimal engagement angles and considering appropriate tool sizes for specific depths, machinists can significantly enhance their output quality and operational efficiency.

DMAC Machining Optimizes Fillet Machining for Precision and Quality

At DMAC Machining, we incorporate these principles into our machining processes to enhance the parts we deliver to our clients. Here's how we leverage these guidelines to ensure top-notch quality and precision.

1. Tool Selection: We carefully select tools that match the specific requirements of each project. By adhering to the 2 to 2.5 times diameter rule whenever possible, we ensure that our tools are capable of maintaining the required fillet radii and vertical depths, leading to superior surface finishes and improved accuracy.  We also constantly seek out the latest in cutting tool technology to help us achieve the best possible results.

2. Custom Toolpath Strategies: DMAC Machining employs the most advanced CAM software to plan the best strategies for cutting efficiency and accuracy. By using advanced simulation and planning software, we can predict and mitigate potential issues related to excessive depth cuts, ensuring that each fillet is machined to perfection.

3. State-of-the-Art Equipment: We have modern machine tools capable of leveraging the creative tool paths created in our software.  We also invest in the highest quality tools and cutting fluids specifically designed to handle the unique challenges of machining fillets.

4. Continuous Improvement: We constantly review and refine our processes to stay at the forefront of machining technology. By keeping up to date with the latest advancements in tool design, machining strategies and equipment, we ensure that our clients receive parts that meet the highest standards of precision and quality.

5. Customer Communication: We pride ourselves on building relationships with our customers.  When we see an opportunity for a customer to save cost and improve quality by suggesting a minor design edit like a fillet radius, we feel obligated to bring it to their attention.  Quite often, a short conversation can save a customer significant cost and improve the final product.  We consider this a win-win as our customers' success is our success.

Our dedication to these principles ensures that we deliver parts of the highest precision and quality to our clients.

Machining Based on USA Standard Fractional Sizing

Here at DMAC Machining, we would much rather work in the metric system. However, as the most commonly available tooling here in the US is still imperial fractional size, we have chosen to stay with the imperial fractional tooling for the time being.  

Imperial fractional sizing of endmills and other cutting tools ensures the availability and compatibility with a wide range of machining equipment and applications. Choosing fractional sizing ensures that tools are readily available and versatile for various machining tasks. Whether it's a 1/8-inch, 1/4-inch, or 1/2-inch endmill, using standard sizes helps in quick tool changes and easy replacement, keeping downtime to a minimum.  

Using standard tool sizes is more cost-effective. Standard sizes are more readily inventoried and available from distributors, making them currently more affordable than metric tooling. This cost saving is passed on to our clients, ensuring high-quality machining at competitive prices.

Achieving Optimal Results

Understanding how tool diameter influences fillet restrictions in milling is fundamental to the machining process. Adhering to the guideline of keeping the fillet radii and vertical depth of cut 2 to 2.5 times the tool diameter ensures precision, tool longevity, and high-quality surface finishes.

We integrate these principles into our workflow, enhancing the quality of our work and extending the life of our tools and equipment. Thanks to our meticulous attention to these critical machining guidelines and our use of standard fractional sizing, you can trust DMAC Machining to deliver parts that not only meet but exceed your expectations.