Thread Bottom Hole Diameter Empirical Formula Optimization

Thread Bottom Hole Diameter Empirical Formula Optimization

In thread cutting processes, the correct determination of the bottom hole diameter, depth, and chamfering at the hole entrance is crucial for achieving high-quality threads. This article introduces how to reasonably determine the bottom hole diameter and depth based on material characteristics and thread specifications, and provides detailed calculations for common thread types.

1. Determining the Bottom Hole Diameter

During thread cutting, the cutting edges of the tap primarily cut the material, but also exert pressure on the material, causing it to deform and flow toward the thread crest. Therefore, the diameter of the hole drilled before tapping should be larger than the internal diameter of the thread. The bottom hole diameter can be determined by referring to relevant handbooks or using the following empirical formulas:

• Brittle materials (such as cast iron, bronze, etc.):
  d0=d−1.1pd_0 = d - 1.1p
  Where dd is the thread outer diameter and pp is the pitch.

• Ductile materials (such as steel, brass, etc.):
  d0=d−pd_0 = d - p

2. Determining the Bottom Hole Depth

When tapping blind holes (non-through holes), since the tap cannot reach the bottom of the hole, the hole depth should be greater than the required thread depth. The depth of the blind hole is typically calculated using the following formula:

• Blind hole depth = Required thread depth + 0.7 × Thread outer diameter

For common threads, the simple calculation for the bottom hole diameter is:
Bottomholediameter=Threadouterdiameter×0.85Bottom hole diameter = Thread outer diameter \times 0.85
For example:

• M3 thread: Bottom hole diameter = 2.4mm

• M4 thread: Bottom hole diameter = 3.1mm

• M5 thread: Bottom hole diameter = 4.2mm

• M6 thread: Bottom hole diameter = 5.1mm

• M8 thread: Bottom hole diameter = 6.8mm

For metric threads, the bottom hole diameter can be calculated as:
Bottomholediameter=Threadouterdiameter−1.0825×PitchBottom hole diameter = Thread outer diameter - 1.0825 \times Pitch

For imperial threads, the bottom hole diameter can be calculated as:
Bottomholediameter=Threadouterdiameter−1.28×PitchBottom hole diameter = Thread outer diameter - 1.28 \times Pitch

3. Considering Thread Tolerance Grades

In actual machining, the thread tolerance grade also affects the bottom hole dimensions. For example, the bottom hole diameter for a standard metric thread can be calculated as the outer diameter minus the pitch, using the formula:

• Metric threads (MM type):
  Thread depth = 0.6495 × Pitch
  Internal thread diameter = Nominal diameter - 1.0825 × Pitch

For example, M20x2.5-6H/7g is a coarse metric thread with a nominal diameter of 20mm and a pitch of 2.5mm, with the internal thread tolerance grade of 6H.

4. Different Thread Types and Material Differences

Different types of threads (e.g., Unified Thread Standard (UN), pipe threads, etc.) have their own standards and calculation methods. For example, in the case of Unified Thread Standard (UNC):

• Unified Thread Standard (UNC):
  Thread depth = 0.6495 × 25.4Threadsperinch\frac{25.4}{Threads per inch}
  For example, for a 3/4-10 UNC thread, the outer diameter is 3/4 inch and the pitch is 2.54mm.

• Pipe Threads (e.g., NPT):
  Thread depth = 0.6403 × 25.4Threadsperinch\frac{25.4}{Threads per inch}

5. Bottom Hole Calculations for Turning Threads

In turning thread processing, the bottom hole diameter and depth, as well as other parameters, need to be optimized according to the thread type. For example, in the processing of a 3/4"-10UNC external thread, the calculation formulas are as follows:

• Pitch calculation:
  P=25.4Threadsperinch=25.410=2.54mmP = \frac{25.4}{Threads per inch} = \frac{25.4}{10} = 2.54 \text{mm}

• Outer diameter calculation:
  OuterdiameterD=34inch=25.4×34=19.05mmOuter diameter D = \frac{3}{4} \text{inch} = 25.4 \times \frac{3}{4} = 19.05 \text{mm}

• Speed calculation:
  Speed N=1000Vπ×D=1000×120π×19.05=2005rpmN = \frac{1000V}{\pi \times D} = \frac{1000 \times 120}{\pi \times 19.05} = 2005 \text{rpm}

Summary

The proper determination of the bottom hole diameter and depth is crucial for ensuring the quality of thread machining. By combining material characteristics, thread types, and tolerance grades, the above empirical formulas can effectively calculate the bottom hole dimensions. In addition, when turning threads, it is important to consider the effects of speed, tool materials, and machine structure to ensure machining accuracy. Mastering these empirical formulas not only improves processing efficiency but also reduces production costs, ensuring thread machining quality and reliability.