There are a variety of drill bits used in mechanical drilling.Drill bit types include solid carbide bits, replaceable bit heads, blade bits, and gun drills.

  The whole hard alloy drill bit can be subdivided into micro-drills, shallow hole drills, deep hole drills, etc., according to the diameter and hole depth. These drill bit combinations bring a wealth of choices to drilling processing and thus achieve the maximum range of application coverage.

  In this paper, we will talk about a very special application of drilling processing - Deep hole drilling. For drilling applications, the depth of the hole is defined as the multiple of the diameter of the hole, such as 3 times diameter, 5 times diameter, or 8 times diameter, etc., which belongs to the scope of shallow hole drilling. When the hole depth is greater than 8 times the diameter, it enters the scope of deep hole drilling. Generally speaking, 10-40 times the diameter is the application field of the whole hard deep hole drilling, and it is better to use a gun drill when it exceeds 40 times the diameter.

Deep hole drilling processing, for the design and application requirements of the drill bit are extremely high.
Application requirements, such as:

• 1. Tolerance of the pre-drilled hole diameter and depth, +0.02/0, the depth of the pre-drilled hole is 2 times the diameter.
• 2. The pressure of the cutting fluid should be sufficient, preferably greater than 20 bar.
• 3. The flow of the cutting fluid should be sufficient, especially for large holes or vertical machining.
• 4. Special drilling processes are required when interrupted holes, exit slopes, or long chips materials are involved.

Special consideration is required for the design of deep hole drills, such as:

• 1. Polishing in the slot to reduce the friction of chips and thus facilitate chip removal.
• 2. Special cutting edges for drilling materials, such as straight cutting edges providing safety, and bent cutting edges optimizing chip formation.
• 3. Special bars with better toughness.
• 4. Optimized core thickness and back cone, as well as flute design, are more suitable for deep hole applications.

Hard alloy deep hole drill

  Deep hole machining is very common in the field of mechanical processing and is widely used in industries such as automobiles, petroleum and natural gas, energy, shipbuilding, etc. Typical parts in these fields include diesel engine crankshafts, oil well valve bodies, ship engine connecting rods, wind turbine main shafts, etc. 

  Take the crankshaft of a diesel engine as an example, the most critical machining difficulty lies in the drilling depth and its consequences, the drilling depth is usually 10 times the diameter or more. Especially for steel crankshafts, the drilling depth is the most likely to have problems, because it is required to discharge smoothly and clean, and how to deal with the long spiral chips formed at the outlet when machining through holes is a more difficult problem. Moreover, chip removal is generally more difficult when machining deep holes.

  According to the difference of application field and workpiece material, people have developed various types of cutting fluid, including: pure oil, emulsion and micro-oil mist lubrication (MQL), here we want to emphasize MQL in particular, because this way is relatively friendly to the environment, not only to reduce the consumption of cutting fluid and reduce the cost of later maintenance, but also to reduce the harm to the environment and personnel. Any matter has its multifaceted nature, MQL sometimes causes too high cutting temperature, which will reduce the overall performance of the cutting tool, and we need to find a way to avoid this defect.

Tool design:

• 1. Special slot type for deep hole machining: Higher safety, taking into account the strength of the tool and the chip space.
• 2. Coatings dedicated to MQL cooling: higher wear resistance in case of higher processing temperatures.
• 3. Adopt the latest polishing process: Ensure that chips can still be discharged more smoothly in long drilling and deep situations.
• 4. Optimized edge treatment: effectively reduces edge wear and avoids abnormal chipping failure.

• Drill bit specifications: D8*120*L160*SD8
• Part name: Crankshaft
• Part material: steel P2.5
• Material hardness: HB269-330
• Cutting depth: 80mm
• Machine tool: horizontal machining center
• Main spindle interface: HSK50A
• Blade Handle: Hydraulic blade handle
• Cold: MQL