Understanding M12 × 1.5 Bolt Specifications

The question asks for the appropriate blank size (diameter) required to prepare a bolt specified as M12 × 1.5. Let's break down what these specifications mean:

• M12: This indicates the bolt has a nominal major diameter of 12 mm. The 'M' stands for metric.
• × 1.5: This denotes the pitch of the thread, which is the distance between adjacent thread crests. A 1.5 mm pitch signifies a standard coarse thread for an M12 bolt.

Bolt Blank Size Determination

Preparing a bolt involves creating the threads on its shank. There are primarily two methods for thread formation:

• Thread Cutting: This involves removing material using a cutting tool (like a die or lathe tool). For thread cutting, the blank diameter is typically very close to the nominal major diameter (e.g., 12 mm for M12), possibly with a slight allowance for the cutting tool.
• Thread Rolling: This is a cold forming process where the thread is created by plastically deforming the material using hardened dies. This method is generally preferred for bolts due to higher strength and efficiency. For thread rolling, the blank diameter must be carefully controlled. It needs to be slightly smaller than the nominal diameter to allow the raised portions (crests) of the thread to form by displacing the material from the thread gaps.

Given the options provided, especially 11.85 mm (which is less than 12 mm), the question likely refers to the preparation for thread rolling.

Calculating Blank Diameter for M12 Bolt

For thread rolling, the blank diameter is a critical parameter. While exact values can vary slightly based on specific manufacturing standards (like ISO, DIN, or ANSI) and the desired thread tolerances, general guidelines exist.

• Nominal Diameter ($d$): 12 mm
• Pitch ($P$): 1.5 mm

The blank diameter for thread rolling an M12 bolt is typically specified in engineering handbooks or material standards. Common values recommended for M12 × 1.5 bolts intended for thread rolling usually fall slightly below the nominal diameter, often in the range of 11.80 mm to 11.90 mm.

The provided correct answer is 11.85 mm. This value fits precisely within the expected range for a blank diameter suitable for thread rolling an M12 × 1.5 bolt. It allows sufficient material to be displaced to form the full thread profile without compromising the bolt's core strength.

Step-by-Step Solution Breakdown

1. Identify Bolt Parameters: The bolt size is M12 × 1.5. This means the nominal major diameter is $d = 12$ mm and the thread pitch is $P = 1.5$ mm.
2. Determine Manufacturing Method: Bolt manufacturing commonly uses thread rolling.
3. Understand Thread Rolling Requirement: Thread rolling requires a blank diameter less than the nominal major diameter to facilitate material displacement for thread crest formation.
4. Reference Standard Practices: Standard engineering practice and relevant manufacturing standards dictate that the blank diameter for an M12 × 1.5 thread rolling should be approximately 11.8 mm to 11.9 mm.
5. Select Correct Option: The option 11.85 mm falls within the standard range and is the most appropriate choice for the blank size.

Therefore, the blank size (diameter) for preparing a bolt of M12 × 1.5 is 11.85 mm.