Calculating cutting depth when threading

When machining threads on a lathe with a single-point cutting tool, the cutting depth (also called depth of cut) is the distance that the cutting tool penetrates into the material during each pass. It is important to control the cutting depth to ensure proper thread geometry and avoid excessive tool wear or damage.

To calculate the cutting depth for threading, you'll typically be dealing with thread profile geometry. Here’s how you can approach the calculation:

1. Understand the Thread Dimensions

Before calculating the cutting depth, you need to know the dimensions of the thread you want to cut. For a metric thread (e.g., M6), the dimensions will be given in terms of the pitch (distance between threads), major diameter (the outer diameter of the threaded part), and the thread profile (which is typically a 60° V-shape for ISO threads).

2. Calculate the Thread Depth

The thread depth is the radial distance from the major diameter (outer diameter) to the root of the thread (the bottom of the thread valley). This is one of the key parameters you need to determine the cutting depth.

• For ISO threads, the standard thread depth (also called the "height" of the thread) is typically about 0.5413 × pitch for a 60° thread profile.

• If the pitch is $�$, the depth of the thread $ℎ$ can be approximated by the formula:

  $$ℎ=0.5413\times �$$

3. Cutting Depth per Pass

The cutting depth per pass is the amount that the tool will cut into the material in each pass. Generally, for thread cutting on a lathe, you don't take the full thread depth in a single pass because this would create excessive cutting forces, leading to poor surface finish and potential tool damage.

Approach 1: For Fine Threads (smaller pitch)

• Typically, the cutting depth per pass is set to about 1/4 to 1/2 of the total thread depth. For example, if the total thread depth is 0.5 mm, a safe cutting depth per pass might be around 0.1 to 0.25 mm.

Approach 2: For Coarse Threads (larger pitch)

• For coarser threads, the depth of cut per pass may be slightly larger, but typically it still falls within the range of 0.25 to 1/2 the total depth.

4. Adjusting for Thread Pitch

The cutting depth for each pass should be adjusted based on the pitch and the tool geometry. For example:

• In general, a finer thread (smaller pitch) will require smaller cutting depths for smoother finish and better thread accuracy.
• For larger threads (with a larger pitch), you can typically take slightly deeper cuts in each pass, but still within a reasonable limit to avoid tool overload and ensure quality threads.

Example:

Let’s say you want to cut an M8 thread (metric thread) with a pitch of 1.25 mm on a lathe:

1. Calculate the total thread depth:

   $$ℎ=0.5413\times 1.25=0.6766\text{mm}$$

2. Determine the cutting depth per pass:

   • If we decide to take 1/4 of the total depth for each pass:

   $$\text{Cutting Depth}=\frac{0.6766}{4}=0.169\text{mm}$$

   So, you would make passes with a depth of approximately 0.169 mm per cut.

3. Repeat the process until the final thread depth is achieved, adjusting the tool position as necessary.

Important Considerations:

• Tool Geometry: The tool’s tip angle and shape affect the cutting depth. For example, in threading tools, the included angle is typically 60° (for ISO threads), so the cutting tool's contact area with the workpiece will be along that angle.

• Finish Pass: After the initial passes, you may want to take a very light finishing pass to achieve a precise thread profile and smooth surface finish.

• Lathe Settings: Ensure that the lathe feed rate and spindle speed are appropriately set for threading. The cutting depth should be adjusted accordingly based on the material and the tool’s cutting ability.

By following these steps, you can calculate and adjust the cutting depth when threading with a single-point cutting tool on a lathe to achieve accurate, well-formed threads.