Take you into the world of gun drilling

1.Hole processing is generally divided into shallow processing and deep processing. There is no standard scope for this definition. According to the professional point of view of the industry to evaluate whether it belongs to deep hole processing, generally we define the ratio of depth to diameter as L/D>5, and general deep hole twist drills can still be used when the ratio of length to diameter is L/D=5-10. When L/D When ≥20, special deep hole cutters must be used. However, the greater the ratio of depth to diameter, the greater the difficulty of deep hole machining, and a series of problems such as iron filings discharge and coolant flow selection will also arise.

2.Gun drilling is characterized by good precision and low surface roughness in one pass. Gun drills are more suitable for processing small diameter holes. Usually, holes with a diameter of less than 10mm are more appropriate for gun drilling. Most advantageously, it allows iron filings to be discharged smoothly.

Summarized as follows:

The hole depth and diameter are relatively large, the drill pipe is slender and rigid, and it is prone to deflection and vibration during work. Hole precision and surface quality are also difficult to control.

There are many chips and the chip removal channel is long, the chip removal is not good, and it is easy to be blocked and broken.

The drill bit works in a nearly sealed state, and the time is long, the heat is not easy to radiate, and the drill bit is extremely easy to wear.

3.A gun drill is composed of a gun head, a gun shaft and a drill handle. The gun head is usually made of hard alloy material, the gun shaft is made of high-strength alloy tube, and the gun handle is the part that connects the drill bit with the machine tool. The gun drill has two main cutting edges and one auxiliary cutting edge. The one close to the core is called the inner edge, the other one is called the outer edge, and the intersection point is called the drill tip.

4.Choice of drill tip geometry. Gun drilling mainly depends on the outer edge angle φ, the inner edge angle φ and the eccentricity of the drill tip. Drill point angle 2φ[2φ=180°-(φouter + φinner)], generally speaking, harder materials require smaller drill point angles, and softer materials require larger drill point angles, regardless of the drill point Regardless of the angle, the selection of the outer edge angle and the inner edge angle must ensure that the radial separation of the drill bit during cutting and the main cutting force can reasonably act on the support area of the drill bit, so as to play the role of squeezing the surface of the hole. The intersection of the cutting edges is typically 1/4 diameter from the center of the drill. According to the force balance analysis of the cemented carbide gun drill, the radial cutting force of the outer edge is required to be equal to the radial cutting force of the inner edge, but it is difficult to guarantee it in the actual machining process. In order to avoid drilling eccentricity, only when the radial cutting force of the outer edge is greater than the radial cutting force of the inner edge can the resultant radial force always act on the surface to be processed. Therefore, when it is generally used to process cast iron and ordinary steel, the outer edge angle φ outer = 30, and φ inner = 20.

PS:
Due to the asymmetric shape of the gun drill structure, the center of mass deviates from the central axis, which brings certain difficulties to manufacturing, regrinding and quality inspection after regrinding, and also increases the cost. In addition, its structural rigidity and torsional strength are low (the torsional rigidity of a round drill pipe with the same diameter is 2.3 times that of a gun drill), which reduces the drilling speed used and the feed rate is small, but the chip removal is relatively smooth. The processing efficiency is low.

5.

6.Drill bushing

Since the drill center is off-center, gun drills are not self-centering, so a bushing or pilot hole is required for plunge machining. When using the drill sleeve, the appropriate size must be selected, and the gap between the drill sleeve and the gun drill head should be kept between 0.003mm-0.008mm.

The drill sleeve used must be wear-resistant and have high hardness; it is generally made of cemented carbide or high alloy tool steel, and its hardness is 63-65HRC; the manufacturing precision of the gun drill sleeve must be high, and the surface roughness of the inner hole is generally Ra1.6- 3.2, the coaxiality of the inner and outer diameters is 0.002mm, and the runout of the end face is 0.005mm.

During processing, the distance between the bottom surface of the drill sleeve and the surface of the workpiece generally does not exceed 0.5mm or the drill sleeve is close to the surface of the workpiece.

Coolant

When chips accumulate in the chip flute and cannot be removed in time, excessive torque will break the gun drill. Therefore, the cutting fluid flow, flow rate, and pressure should be strictly required.

The flow rate should meet 0.2-0.65 liters per second, and the flow rate should increase with the increase of the hole depth to ensure a greater flow rate of the cutting oil and achieve the purpose of smooth chip removal.

The flow rate should generally not be less than 5-8 times of the cutting speed, about 480-720m/min

The pressure should be 3.5-10Mpa, the smaller the processing hole, the higher the pressure. High pressure and low flow can be used when processing small diameter deep holes; low pressure and large flow can be used when processing large diameter deep holes.

Pure drilling oils with sulfur and chlorine are best. Sulfur helps reduce built-up edge and chlorine helps chip evacuation, both of which will prolong the life of the gun drill.

Compound coolant, matched by 5%-50% mineral oil, water and compound; it is a substitute for drilling oil; the emulsion is also a chemical solution that does not contain mineral oil. The coolant needs to maintain a concentration of 8%-10%, 10% is the best.

From pure oil to emulsion, the service life of the gun drill is reduced by 30%-50%.

Cooling temperatures should be kept below 120°F.



Cutting parameters

When using, the cutting amount should be selected correctly. In general, the cutting speed: V=1.1-1.65m/s, the feed rate: s=0.015-0.03mm/r, and the feed rate should be small at both ends and large in the middle. Reasonably select the cutting amount according to the workpiece material to control the degree of chip curling and obtain C-shaped chips that are beneficial to chips.

When processing high-strength material workpieces, the cutting speed V should be appropriately reduced. The size of the feed rate has a great influence on the formation of chips. Under the premise of ensuring chip breaking, a smaller feed rate can be used.