Principle Characteristics of Deep Hole Drilling and Types of Processing Systems
Introduction to Deep Hole Processing :Deep-hole machining (hole depth to hole diameter ratio > 5) has a very important position in machining field, accounting for over 40% of the machined hole amount. As science and technology develop, high-strength, high-hardness, high -value and harder-to-machine new deep-hole parts emerge successively; requirements for machining depth, precision and efficiency are promoted continuously for work pieces, making deep-hole machining the key process and difficult point in machining process. For process system rigidity, chip removal, cooling and lubrication issues, it’s harder and harder for to fulfill the requirements for precision, efficiency, and material in deep-hole machining or even cannot meet the requirements by traditional machining method. Therefore, a specific drilling technique is needed for supporting such deep-hole machining.

Definition of Deep Hole : Deep hole processing falls into deep hole and shallow hole types: The generally specified ratio of hole depth L to hole diameter d is greater than 5, namely hole L/d ﹥5 deep hole; hole L/d ﹤5 is shallow hole and that L/d ﹥100 super deep hole.

Deep Hole Drilling :

 A number of machine tool installation methods can be adopted for deep-hole drilling: 

1) work piece rotating with cutter feeding movement; 

2) fixed work piece with cutter rotating and feeding; 

3) work piece rotating with cutter reverse rotating and feeding.

 The specific installation method depends on work piece characteristics and the hole to be machined.

Two most common used deep-hole drilling cutters, gun drill and BTA single-tube drill, are shown as below:
High Material Removal Rate : The common features(feature) of deep hole drilling machine is the great amount of material removal rate and consistency of processing quality.

 Normally a deep hole can be drilled through at one time without retraction of tool during drilling. Besides very accurate hole size, straightness, surface roughness can also be achieved; moreover, various deep holes such as cross holes, blank holes and inclined holes can be processed. 

Again, it can reach high conformity of processing quality. Tool, machine and related equipment should meet the strict requirement during the whole process of drilling.
Deep-hole machining is under an enclosing or semi-enclosing state, therefore, the cutter’s work cannot be observed directly; by now whether the process goes smoothly can just be determined through external factors such as experience, listening sound, observing cutting chips and machine tool load, pressure gauge, touching vibration etc.
Cutting heat is not easy to be dissipated, 80% of which is brought away by the cutting chips during the process, while this figure is just 40% for deep-hole cutting. Heat by the cutter accounts for a big share for cutting heat, it is slow to dissipate and easily overheat; cutting edge temperature can reach 600℃, for which effective forced cooling method must be adopted.
It’s hard to remove cutting chips. It’s possible that drill bit bursts for that blockage resulted from that the hole is deep and the chips go a long way. Therefore, cutting chip length and shape shall be controlled; forced chip removal must be adopted.
The processing system is poor in rigidity, the drilling is likely to deflect for vibration resulted from limitation by hole diameter size, big major axis of the hole, thin and long drilling stem, poor rigidity; therefore supporting and guiding get extremely important, which is more obvious in gun drill machine tool.

Commen Methods in Deep Hole Processing

Presently direct material deep-hole machining systems in common use classified by cutter system mainly include:

1)Gun drilling system, 

2)BTA single-tube drill system 

3)Ejector drilling system. 

They represent the advanced, efficient hole processing technology. So, good precision effect can be achieved just through one-time feeding, and the machined hole position is accurate and precisely sized, having high straightness and Coaxiality as well as very good surface smoothness and repeatability; various deep holes can be machined easily.

Special deep holes such as cross holes, inclined holes, blind holes and flat-base blind holes can be solved very well.

In deep-hole drilling, combination of any cutter structure, cutting fluid pressure and flow aims for removing cutting chips from the hole in a better way. Viewed from machining principle and practice, any cutter system can drill out holes of excellent surface quality, small tolerance and good concentricity. Three common deep-hole drilling systems are introduced as follows
1. Gun Drilling System

Gun drilling system adopts internal cooling and external chip removal method; cutting fluid reaches the drill bit through hollow drilling stem for cooling and lubrication as well as removes cutting chips from drill bit and the V groove outside the drill stem.

 This system is mainly for machining small-diameter (normally smaller than 35mm) deep holes, requiring high cutting fluid pressure, which is the most common deep-hole drilling method. Among GSM series machine tool types, ZK21\ZK21C\ZK21E\ZK21F\ZK21G are all gun drill machines.
The gun drilling system is also a gun drill machining system, suitable for machining medium and small lots, it can easily match common-use machine tools such as turning mill, boring mill, and machining center etc. The cutter uses gun drill and high-pressure aerial fog as cooling and chip-removal power.

2. Double-tube(ejector) Deep Hole Driling

Ejector drilling system is a deep-hole drilling system adopting external cooling and internal chip removing; cutting fluid enters from the upper oil-filling mouth of the coupler, most of the fluid goes forward into the ring space between the internal and external drill stems, reaching the cutter end for cooling and lubricating;

It pushes cutting chips into the internal chamber of the internal drill stem and then removes them out; a small amount of cutting fluid, by utilizing injection effect of fluid mechanics, is injected to the rear part of the internal drill stem at high speed from the upper crescent nozzle on the internal drill stem, forming a low-pressure area in the chamber of internal drill stem, sucking back the cutting fluid and chips generated in the cutting area, pushing cutting chips to the outside quickly under double forces of sucking and pushing.

 Compared with BTA system, this relatively indpedently system needs lower cutting fluid pressure and reduces the requirements for drilling system sealing. For the sake of internal tube, ejector drill has a limited min diameter scope normally no less than Ф18mm. Among GSM series machine types, ZK21S is a double-tube ejector drill machine.

3. BTA Deep Hole Drilling

BTA single-tube drilling system adopts external cooling and internal chip removing method; cutting fluid enters from between the external drill stem wall and machined work piece surface through oil disperser, reaching the cutter end for cooling and lubricating as well as pushing out cutting chips from inside the drill stem. 

Besides functioning as a guide, oil disperser can also provide a passage for feeding oil to cutting area; this system is widely used; however, as limited by the chip-removing space inside the drill stem bore, it is mainly used for deep-hole machining of diameter over Ф12mm. Compared with ejector drill, the high cutting fluid pressure makes single-tube drilling system more reliable, especially when drilling materials whose cutting chips are hard to break (such as low-carbon steel and stainless steel etc). Among GSM series machine types, ZK21D is BTA single-tube drill machine.

Cooling and Lubrication, Chip Removal, Guiding and Skewness in Deep Hole Processing

Under the normal pressure and temperature, the cooling cuttings filtration system can supply sufficient clean coolant to the cutting tool. Its main functions are:

1) Cool the tools and the workpiece, 

2)Reduce thermal deformation of tools and workpiece, and 

3)Improve tool life to ensure the size accuracy and surface smoothness of the holes for higher tools durability; 

4)Lubricate the tool surface to reduce friction and cutting deformation and reduce tool wear; 

5)Remove the chip and absorb the vibration and noise resulting from the cutting movement and the friction; 

6)Protect the machined surface and prevent the corrosion.

Chip Breaking
In drilling, the most keypoint is to obtain the satisfactory shape and size of chip break. Overlong overlarge chips will wind the tool and block the chip removal pipe. High energy consumption in excessive chip breaking makes tools excessive wear and generate(generates) much heat, adding to the work load of oil temperature cooling system.

 Workpiece material, tool grinding angle, chip breaker grooves, cutting parameters, selection of cutting liquid and other factors affect chip formation. So, length and shape of chip breaking shall be controlled. Besides, effective high-pressure chip removal must be conducted.

Cooling
Because processing is conducted under fully or partially closed state, cutting heat is not easy to be dissipated, 80% of which is brought away by the cutting chips during the process, while this figure is just 40% for deep-hole cutting.

 Heat by the cutter accounts for a big share for cutting heat, it is slow to dissipate and easily overheated; cutting edge temperature can reach 600°C, for which effective cooling method must be adopted.

Lubrication
The quality of hole surface machined by deep hole machining tool partly depends on the quality of cutting fluid. There are(is) special metal cutting oil and water based cutting fluid for deep-hole drilling on the market. 

This kind of cutting oil and cutting fluid containing EP additives shall be suitable for high temperature on the blade and high pressure on the support plate. It can guarantee normal lubrication and heat dissipation of the cutter. When the cutting temperature exceeds 55°C, and the tools and pumps are lack of proper lubricating, coolant will rapidly aging, the most suitable temperature is about 40°C.

Filtration
Good filtration of cutting liquid depends on 2 essential conditions: 

First, the required quality of hole surface and support plate. 

Second, prevent high-pressure pump wear and damage. 

Common methods: Drum filter, batch filter, autofilter, magnetic filter, etc. Fine coolant filtration guarantees normal processing and improves life span of 10-20μm cutter.

Coolant and Supply
Cutting oil and cutting fluid containing EP additives, special for deep hole processing, shall be suitable for high temperature on the blade and high pressure on the support plate. 

During the processing of deep holes it is required to reasonably choose the deep hole cutting oil, which will have a decisive impact on the processing quality and productivity. Coolant system shall supply sufficient, clean cutting oil (liquid). 

Deep hole drill machine can apply different cooling pumps such as plunger pump, gear pump and screw pump to different processing cases. If coolant demand is large, it can connect 2 or more hydraulic pumps to obtain enough supply.

Coolant Tank
Coolant tank capacity is ten times the maximum displacement of cooling pump per minute to keep dust and debris deposition and heat dissipation. There is the gap-type chip container and copper mesh filter above the coolant compartment.

Guiding and Deflection Control in Deep Hole Processing
Hole deflection is an important index for deep-hole machining quality. In actual production, the requirements for parts machining is increasingly high with higher requirements for deflection. It is specially(especially) true for deeper holes. So, how to control hole deflection is essential.
1. First, the movement of deep-hole drilling shall be considered. In deep-hole machining, the best deflection control is achieved through relative rotation, also called work piece rotation; the cutter rotates reversely and feeds with twin rotation; then work piece rotating, with cutter feeding with single rotation. Compared with gun drill machining, the effect is affected by that the work piece is fixed, with the cutter single rotating and feeding.
2. The mounting precision of guide sleeve in deep-hole machining plays a significant role in hole deflection during deep-hole machining; it’s also the most sensitive factor. Ensure the coaxiality of the guide sleeve, work piece main shaft and drill stem box main shaft; the relationship between hole deflection and machining length in deep-hole machining is linear. For instance, if guide sleeve is 50mm long and axis of guide sleeve deviates 0.05mm from that of workpiece, then deviation of a 1,000mm deep machined hole may be more than 1mm.
3. The grinding angle of deep-hole drill bit also affects the deflection of the hole machined; it is recommended to refer to this manual and the related contents on the company’s website, consider different grinding angles of deep-hole drill bit for different materials and holes to be machined, with which to achieve good hole deflection.
4. The uniformity of the materials of the workpiece to be machined also heavily affects hole deflection. In the process, deep-hole drill bit will deflect to the softer part of the machined material; therefore, in machining of high requirements for deflection, heat treatment of material is very important; efforts shall be made to prepare the material texture to be uniform.
5. Reasonableness of the selected cutting parameters also affects hole deflection to some extent. In case production schedule is not affected, it needn’t apply big cutter feed, which may negatively affect hole deflection.

Fabrication and Operation of Guide Sleeve
The processing system is poor in rigidity, the drilling is likely to deflect for vibration resulted from limitation by hole diameter size, big major axis of the hole, thin and long drilling stem, poor rigidity; therefore supporting and guiding get extremely important, which is more obvious in gun drill machine tool.
Fabrication of guide bushing is an important factor for determining the quality of the drilled holes; it functions to guide at the very first stage when the drill bit enters the work piece, mainly made of high-alloy tool steel or kentanium, having hardness of HRC63 – HRC65. Guide sleeve diameter shall be the same with the nominal diameter of the drill bit, tolerance level is ISOH6, inner bore surface coarseness is Ra0.8um; inner bore and external circle coaxiality is 0.015mm; guide sleeve and drill stem main shaft coaxiality shall be 0.02mm.

Note

New guide sleeve shall be 0.005-0.01 mm bigger than the drill bit; guide sleeve shall be replaced in case that the gap between the guide sleeve and drill bit is beyond the range 0.015-0.02mm, too big gap will affect the hole’s surface quality and straightness as well as reduce the life span of the drill bit.