In the mechanical manufacturing industry, the cylindrical hole with the hole depth more than 10 times of the hole diameter is generally called a deep hole. Deep holes can generally be divided into general deep holes, medium deep holes and special deep holes according to the ratio of hole depth to hole diameter (L/D).

(1) L/D=10-20, belonging to general deep hole. It is often processed with an extended fried dough twist drill on a drilling machine or lathe.

(2) L/D=20 ～ 30, belonging to medium deep hole. It is often machined on a lathe.

(3) L/D=30~100, belonging to special deep hole. The deep hole drill must be used for processing on the deep hole drilling machine or special equipment.

In the production activities of machining, we often encounter deep holes with a length diameter ratio (L/D) greater than 10. Deep hole processing mainly has the following characteristics:

(1) Due to the large proportion of hole depth and hole diameter, the drill pipe is thin and long with poor rigidity. Therefore, it is easy to incline when drilling, resulting in vibration, which makes it difficult to ensure the surface roughness and dimensional accuracy of the hole.

(2) The working environment of the drill bit is bad, the heat is not easy to be discharged, and the difficulty in chip removal aggravates the tool wear.

(3) In the process of deep hole processing, it is not allowed to directly observe the cutting condition of the tool, but only to judge whether the cutting process is normal by working experience to listen to the sound of cutting, look at the chips, touch the vibration and workpiece temperature, and watch the instrument (oil pressure gauge and electric meter).

How to select deep hole machining tools?

The conventional drilling method is used for deep hole processing of parts, which requires lengthening the fried dough twist drill and chip breaking processing. Not only is the processing precision low, the surface roughness poor, the processing efficiency low, the operator's labor intensity high, and the quality difficult to guarantee, but also it is very easy to cause chip blocking or breaking the drill bit, resulting in greater processing difficulties. Therefore, it is best to choose a special tool for deep hole processing. What are the deep hole cutting tools? There are many kinds of tools for deep hole processing, such as gun drill, spray and suction drill, nesting drill, carbide indexable deep hole drill, internal chip removal deep hole drill and sub dry deep hole processing system.

1. Gun drill

The single blade external chip removal deep hole drill was originally used for processing gun barrel and gun barrel in military industry, so it got its name. Later, it was collectively called gun drill. It is mainly used to process deep holes with a diameter range of 3~30mm and a length diameter ratio of more than 100. The economic accuracy of the machined holes is between IT8 and IT10. The surface roughness of the machined surface is Ra3.2~0.8um.

Gun drill is composed of cutting part (cutter head), cutter bar and driver. The material of the cutter head can be hard alloy or high-speed steel, which is mainly determined by the material and hardness of the workpiece. The cutter head and cutter bar are generally welded by high-frequency welding, and the cutter bar is usually rolled into a V-shaped chip discharge groove with a cold drawn seamless steel pipe.

The cutting fluid is pumped into the inner hole of the drill pipe by the pressure pump (the pressure is about 3~8MPa), and then flows through the cutting edge. When the cutting fluid flows out along the v-shaped section space between the tool and the hole wall of the part, the chip is taken away. Due to the poor rigidity of drill pipe, large feed rate cannot be adopted, so the production efficiency is low. In addition, because the chips are discharged through the V-shaped groove of the drill pipe, the machined hole wall is easy to be scratched by iron chips, which may affect the surface roughness of the inner hole.

2. BTA drilling deep hole with internal chip removal method

As the gun drill pipe is asymmetric, it has poor torsional rigidity and can only transmit limited torque. Therefore, gun drill is only applicable to machining parts with small diameter holes. In order to adapt to the processing requirements of more complex large diameter deep holes, the technology of internal chip removal deep hole drilling came into being. The internal chip removal deep hole drill is suitable for machining deep holes with a diameter of more than 20mm and a length diameter ratio of no more than 100, with a machining accuracy of IT7~IT10 and a surface roughness of Ra3.2~1.6um. The staggered internal chip removal deep hole drill (BTA) is a typical structure of the internal chip removal deep hole drill (BTA is named after the English abbreviation of the International Association of Deep Hole Drills).

When the hole diameter of the part to be machined is large, the cutting part of the BTA drill bit is composed of several separated blades, which are alternately welded on the tool body. When the machining aperture is small, the integral cutter head can be used. The cutting edge of the integral cutter head is divided into three sections along the radius, and the cutting edge is divided into outer section and middle section by steps. The length of the outer cutting edge is equal to the length of the inner cutting edge, and a chip breaking groove is opened on the inner and outer cutting edges.

The outer circle of drill pipe is generally 10% larger than the hole diameter to be machined. During cutting, the cutting fluid is injected into the oil feeder by the hydraulic pump, and then the oil flows to the cutting area in the cavity formed between the drill pipe and the machined hole. The cutting fluid flows out through the hole in the drill pipe and takes away the chips.

Comparison between BTA drill and gun drill

From the processing principle, it can be seen that compared with gun drill, BTA method uses round drill pipe, so it has good torque resistance and rigidity, and can use a larger feed rate for cutting. In addition, because the chips are discharged from the inner hole of the drill pipe, the machined surface will not be scratched.

The main disadvantage of BTA drilling method is that special machine tools must be used, and the machine tool must also be equipped with an oil chip separation device to separate and recycle the cutting fluid through gravity precipitation or electromagnetic separation. In addition, a high pressure zone is formed between the workpiece and the oil feeder during the cutting process, so a reliable seal must be formed between the workpiece and the oil feeder before drilling.