Ensure the correct rake angle on the screw tap

When it comes to thread cutting, many people assume it’s a complicated process that’s difficult to master. Achieving perfect results seems challenging, and the screw tap often ends up breaking. Does this scenario sound familiar? If yes, it might be time to dive deeper into the geometry of a screw tap. Most problems can be avoided once you understand the underlying causes.

Today, we’ll start by exploring the rake angle on the tap, also known as the cutting angle. The rake angle plays a crucial role in thread cutting, as it significantly influences chip formation. Chip formation is a key factor determining the success of any thread-cutting operation. In this article, we’ll uncover how the right rake angle can help prevent common issues and how understanding the helix angle can enhance your results.

What is the Helix Angle, Anyway?

Before we delve into the implications of selecting the right rake angle, let’s cover some basics. It might get a little technical, but bear with me—understanding these principles will make your work easier in the long run!

Locating the Rake Angle on the Screw Tap

The first step is identifying where the rake angle is located. The rake angle refers to 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 it, imagine the tap from above, approximately midway along the thread length. Find the center and draw an imaginary line from there to the tip of one of the cutting edges. The angle between this line and the chip surface is the rake angle.

The Rake Angle’s Role in Chip Evacuation

The rake angle on the tap dictates how the chip forms, its appearance, and how effectively it can be removed. The size of the rake angle is critical here. It can be positive, negative, or zero, but in thread cutting, it typically falls within the positive range. Simply put, the rake angle determines whether the cutting edge is sharp or blunt, with significant consequences for the cutting process.

The Rake Angle’s Impact on Cutting Performance

The rake angle plays a role in formulas used to calculate central forces during thread cutting, particularly the cutting force. This force is necessary for deforming and separating chips and overcoming friction. Larger rake angles reduce the cutting force, while smaller ones increase it. Therefore, the rake angle directly affects the cutting performance of the screw tap.

What Effect Does the Rake Angle Have on Thread Cutting?

Let’s break down what happens when you choose a particular rake angle in thread cutting. Both small and large rake angles have their pros and cons, and finding the ideal balance is essential. There isn’t a single perfect rake angle; instead, it’s about finding the best compromise for your specific application.

Larger rake angles ensure consistent chip formation and decrease cutting force. However, they make the cutting edge sharper but more prone to damage. On the other hand, a smaller rake angle increases cutting force and generates more friction, potentially reducing the surface quality of the workpiece and increasing wear. Yet, cutting edges with a smaller rake angle tend to be more robust and produce shorter chips, which could be advantageous depending on the job.

Choosing the Right Rake Angle for Your Screw Tap

Now you’re probably wondering what rake angle your tap should have. This largely depends on the material you’re working with. Generally speaking, the harder the material, the smaller the rake angle should be. For softer materials, a larger rake angle is preferable. For example, machining brittle brass might call for a rake angle below 6°, whereas soft aluminum can handle angles exceeding 18°. Different materials are grouped by their properties and color-coded for clarity. Exact rake angles vary based on the intended application and may differ slightly among manufacturers.

Here’s some good news: reputable manufacturers like BAER invest considerable effort in optimizing rake angles for you. That’s why there are so many types of screw taps available. When purchasing, ensure your tap is suitable for the material you intend to work with—this way, the rake angle is likely already appropriate. If not, 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 screw tap. There’s also the side rake angle, which is sometimes called the spiral angle due to its relation to the helical flutes. Due to their similar names, confusion can easily arise.

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

Want to ensure your screw tap’s rake angle is perfectly suited for your cutting task? Head over to the BAER online shop to find high-quality tools with meticulously calculated cutting geometry. Check it out today!