Coolant, Lubrication, Chip Removing, Guiding And Skewness In Deep Hole Drilling

     Under the normal pressure and temperature, the cooling cuttings filtration system can supply sufficient clean coolant to the cutting tool. Its main functions are: Cool the tools and the workpiece, reduce thermal deformation of tools and workpiece, and improve tool life to ensure the size accuracy and surface smoothness of the holes for higher tools durability; lubricate the tool surface to reduce friction and cutting deformation and reduce tool wear; remove the chip and absorb the vibration and noise resulting from the cutting movement and the friction; 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.