Ensure the correct rake angle on the screw tap

When it comes to threading, the process often carries a reputation for being somewhat tricky. Achieving flawless results seems challenging, and the tap itself can break quite easily. Does this sound familiar? If so, it might be time to dive deeper into the geometry of a tap. Understanding the underlying principles can help avoid common pitfalls.

Today, we’re going to start with the rake angle—sometimes called the cutting angle. This angle is one of the most crucial elements in threading, as it plays a major role in chip formation. Proper chip formation is key to successful threading. In this article, we’ll explore how the right rake angle can prevent chip-related issues and how you can optimize your threading efforts using your knowledge of the helix angle.

What Exactly Is the Helix Angle?

To fully grasp the implications of selecting the right rake angle for threading, let’s first cover some foundational concepts. It might get a bit technical, but bear with me—it will pay off later!

Locating the Rake Angle on a Tap

The first thing to clarify is where the rake angle is located. The rake angle is the angle between the chip surface and the tool reference plane, which is defined by the axis of rotation and the cutting point. To visualize this, imagine the tap from above, ideally halfway along the thread length. Find the center and draw an imaginary line from there to the outermost tip of one of the cutting edges. The angle between this line and the chip surface is the rake angle.

The Rake Angle Plays a Crucial Role in Chip Removal

The rake angle on a tap determines how the chip forms, its shape, and how effectively it can be removed. The size of the rake angle is critical for this process. Rake angles can be positive, negative, or even zero, but in threading, they typically fall within the positive range. Simply put, the rake angle dictates how sharp or dull a cutting edge is, with significant consequences for the overall threading process.

The Rake Angle Directly Impacts Cutting Performance

The rake angle is included in certain formulas used to calculate central forces during threading, particularly the cutting force. This is the force needed to deform and separate chips and overcome friction. The size of the rake angle directly impacts the cutting force: the larger the rake angle, the smaller the cutting force. This relationship also affects the cutting performance of the tap.

What Effects Does the Rake Angle Have on Threading?

Let’s take a closer look at what the rake angle actually does during threading. Both small and large rake angles come with their own set of advantages and disadvantages, requiring careful consideration based on the specific application. There’s no single “perfect” rake angle; instead, the goal is to find the best compromise for a given situation.

Larger rake angles ensure consistent chip formation and reduce cutting force. While the cutting edge becomes sharper, it also becomes more susceptible to damage. On the other hand, smaller rake angles increase cutting force and generate more friction, potentially leading to reduced surface quality on the workpiece and increased wear. However, cutting edges with smaller rake angles are more robust and produce shorter chips, which can be advantageous depending on the application.

Choosing the Right Rake Angle for Your Tap

Now you’re probably wondering what rake angle your tap should have. This largely depends on the material you’re working with. Generally, the rule is: the harder the material, the smaller the rake angle should be. Conversely, for softer materials, a larger rake angle is preferable. For instance, a rake angle of under 6° works well with brittle brass, while soft aluminum can handle rake angles exceeding 18°. Materials with similar properties are often grouped together and color-coded for clarity. Exact rake angles can vary depending on the intended application and may differ slightly between manufacturers.   

The good news is that reputable manufacturers like BAER spend considerable effort optimizing the rake angle for you. That’s why there are so many different types of taps available! When purchasing, ensure that your tap is suited to the material you intend to work with—this will likely ensure the rake angle is appropriate. If, for some reason, it isn’t, the rake angle can be adjusted afterward through grinding.

Watch Out for Confusion: Rake Angle vs. Side Rake Angle

Unfortunately, the rake angle isn’t the only important angle on a tap. There’s also the side rake angle, which is often referred to as the spiral angle due to its connection with the helical flutes. Due to their similar names, confusion can easily arise between these two angles.

Closely related to the rake angle are the wedge angle and relief angle. Together, these three angles always add up to 90°. The relief angle is the counterpart to the rake angle on the opposite side of the cutting edge, defined by the wedge angle.

Want to ensure the rake angle on your tap is perfectly suited to your application? Look for high-quality tools with precisely calculated cutting geometry in the BAER online shop. Check it out today!