A drill bit less than 3.175 mm in diameter is commonly referred to as a micro drill. In order for micro-drills to be highly efficient in use, a number of factors must be considered: factors such as the drill itself, processing parameters, hole depth, integrity of the installation, and the structure of the workpiece. It is necessary to have a scientific and innovative spirit to deal with these factors that are mutually influential and sensitive to the drilling process.

Mark Megal, marketing manager at Guhring (USA), said: "On many occasions, you have to use the micro-drills to do it."

Although tool manufacturers have done a lot of development in micro-drilling materials and geometric parameters, and don't need everything to be tested from scratch, it is still not a simple matter to control many factors in the drilling process. jobs.

The aspect ratio of the micro drill is significantly increased

It is well known that the greater the ratio of the length to the diameter of the drill bit, the greater the tendency to bend. By reducing the aspect ratio, the deflection force can be reduced, thereby avoiding breakage of the drill bit and an increase in the aperture error. Deeper holes require a larger aspect ratio of the drill bit. Usually the hole depth is more than 3 times the diameter is the "deep hole", and the depth of the micro drill hole generally exceeds this limit.

For example, a drill having a diameter of 3.175 mm has a hole with a hole depth of 31.75 mm and a length to diameter ratio of 10:1; and a drill having a diameter of 0.508 mm has a hole with a hole depth of 25.4 mm and a length to diameter ratio of 50:1. Therefore, as the diameter of the drill bit decreases and the brittleness increases, the deflection becomes a source of many problems. To control the brittleness of the drill bit, a trade-off is made between the hardness and toughness of the tool base.

In general, high-speed steel drill bits allow a certain degree of deflection and can withstand the corresponding bending force, but the high-speed steel has such elastic deformation ability and low hardness, which also reduces the wear resistance, thereby limiting the tool's life.

Cemented carbide has high rigidity and high hardness, so it can make tool life longer and machining accuracy higher.

Joe Kueter, manager of drilling production at MAFord Manufacturing, points out that the high wear resistance of cemented carbides results in three times the speed of micro-drilling and the life expectancy of high-speed steel. At the same time, the high rigidity of cemented carbide Helps to properly position and maintain the size of the hole.

However, cemented carbide is not omnipotent, and its high rigidity makes it easy to crack.

Peter Jones, a field sales engineer at Guhring, points out that using M35 cobalt high-speed steel as a micro drill can achieve a good compromise between cemented carbide and ordinary high-speed steel (M2, M7). He said: "The heat generated in the hole during cutting, coupled with the rolling of the tool, makes the cutting edge dull and draws out the channel, eventually leading to tool damage. The higher cobalt content makes the heat resistance of the M35 Increase and keep the blade sharp for a long time."

In addition, carbide drills require careful installation and use, and precise concentricity is especially important because lateral loads caused by different cores can cause the drill to crack.

Larry Brenner, Senior Production Manager, Milling and Drilling Division, Mitsubishi Metals (USA), recommends that micro-drills should be used on machines where the drill bit rotates (such as machining centers). He points out that the spindle of the machining center gives the correct center of the drill. Line positioning, while the eccentricity of the workpiece on the lathe can cause the bit to flex. Therefore, if micro-drills are used on a lathe, each factor that affects the concentricity must be adjusted in advance, especially for carbide drills, which cannot accommodate bending deformation.

If you are using a micro-drill on a lathe, it is best to re-cutter the tool turret mounting hole and use an adjustable boring tool holder to adjust the concentricity of the drill bit and the workpiece to an optimum state.

Brenner further pointed out that the beating of the tool holder should be minimized. For this purpose, the heat shrinkable tool holder should be preferred, followed by the hydraulic tool holder. The maximum runout value at the end face of the tool holder sleeve is required to be in the range of 0.005 to 0.0076 mm.

Eliminate initial centering error

When any bit is working, it is important to start a few turns. Because the bit is subjected to eccentricity when starting cutting. In addition, the irregular shape of the surface of the workpiece can cause lateral sliding, causing the tool to bend, break, or at least increase the deviation of the hole.

For drills with a diameter of 3 mm or less, Mitsubishi recommends using a rigid centering drill to drill an initial hole with a depth of 1 to 2 times. The apex angle of the centering drill should be equal to or greater than the microdrill apex angle of the final bore. If the apex angle of the centering drill is small, then when the micro drill is cut in, the two cutting edges contact the workpiece first, which is easy to cause chipping.

If a centering drill is not used, a method can be employed in which the feed amount at which the micro drill starts to cut is much lower than the subsequent normal feed. For example, the diameter of the drill is 1.613mm, the depth of the hole is 12.7mm, the normal feed is specified as 0.0508mm/r, and the feed with 0.0127mm/r is started to advance by 0.254mm. It can also be advanced until the land starts to contact the workpiece, and then it is converted to Normal feed. This approach also prevents the drill bit from slipping.

Brenner pointed out that another challenge in the use of micro-drills is to maximize the speed to maximize production potential, but in terms of maximum speed specifications, the drill tends to walk in front of the machine. Some machines operate at their maximum speed and still do not reach the optimal cutting speed for micro-drills. For example, a drill with a diameter of 1 mm has a cutting speed of 91.44 m/min and requires a spindle speed of 28000 r/min.

The hardness of the material being processed has a large influence on the initial recommended value for determining the micro-drill cutting speed and feed rate. For example, MAFord recommends that when machining 1018 mild steel (20HRC) with a solid carbide drill with a diameter of 1.32mm, the cutting speed is 91.44m/min and the feed is 0.038mm/r. However, when the drill is used to process plastics and synthetic materials, the cutting speed can reach 198.12 m/min and the feed rate can reach 0.127 mm/r. When processing difficult materials (such as nickel-based alloys, titanium alloys), the cutting speed is only 15.24 ~ 18.29m / min, the feed is only 0.0305mm / r.

Step drilling sequence

Typically, drilling micro-deep holes uses a step-by-step drilling sequence that periodically exits the drill bit to break the chips and prevent clogging. Step-by-step drilling also helps to prevent continuous extrusion at the bottom of the hole, which is especially important when processing cold-hardened materials.

Brenner pointed out that it is generally believed that step-by-step cutting has to completely withdraw the drill bit, but it is not. If interrupted feed (a few turns or a short time) is used, chip breaking can also be used. In addition, the complete exit of the drill bit also tends to create a flare and leave some of the chips in the hole, so it has to be re-cut. These situations are undesired.

Many problems often occur in the last 20% of the drilling depth. Brenner pointed out that this is because the hole is gradually deepened and the chip discharge is very difficult. The specific solution will vary depending on the condition of the workpiece and material. The application engineer should determine the step-by-step cutting plan on a case-by-case basis.

When it comes to micro-drilling of circuit boards, although the micro-drills designed to process ductile materials are very similar from the material and diameter of the drills, the cutting geometry of the two is quite different.

Kueter of MAFord pointed out that although the PCB drill can be used to process harder materials after careful installation and commissioning, Ford generally does not do this, preferring to carefully prepare special drills for tough workpiece materials. An important direction is to minimize the groove length to increase the strength of the drill bit. Kueter also stated that the user requested to drill a 25.4mm deep hole, but we do not need to provide a bit length of 25.4mm, and generally provide a bit with a slot length of 9.525mm or 12.7mm.

Kueter pointed out that some circuit board drills are made of so-called "stepped handles." For example, a drill with a diameter of 0.1524 mm has a drill hole depth of 1.524 mm and a full length of the groove of 1.524 mm, but the drill part has a diameter. It is not directly connected from the tail of the trough to the shank with a diameter of 3.175 mm, but is transitioned through a 0.762 mm intermediate diameter. In this regard, Kueter believes that when drilling tough materials, the length of the drill should be as short as possible, so it is not advisable to add a transition diameter structure.

Kueter also pointed out that from the point of view of geometric parameters, circuit board drills usually use a larger helix angle, and the groove cross-section size is also thinner than other micro-drills. For micro-drills that process stainless steel and other difficult-to-machine materials, smaller helix angles and thicker groove cross-sectional dimensions are used. He also pointed out that in order to reduce the stress on the micro-drill, it is necessary to make an inverted cone - the diameter is reduced in the direction of the shank. The amount of inverted taper is generally 0.005 to 0.127 mm. Since the bit length is often less than 25.4 mm, the inverted cone is typically 0.0127 to 0.0254 mm per 25.4 mm length. Kueter emphasizes that as long as the borehole has depth, a reverse taper is required. In particular, for materials that are "retracted" in the processing of titanium alloys, the drill bit will be glued into the holes without proper back taper.

Kueter introduced a unique method for the user to overcome the “retraction” phenomenon in titanium alloy processing: the tool driller's radial runout at the drill tip is required to be at the upper limit of the tolerance, so that the amount of expansion during drilling is large and the workpiece is “retracted”. "Not to hold the drill bit."

Good internal cooling effect

Practice has proved that the use of internal cooling drills is very effective in improving the productivity of deep hole processing. Its advantages are not only to send the cutting fluid directly to the drill tip, but also to cool the effect, but also to play the role of forced chip removal and help chip breaking. When the hole depth is more than 3 times the diameter, the effect is more obvious when using the inner cooling drill bit, but so far, the inner cooling drill bit is often limited to the drill bit having a diameter of 3 mm or more.

Colin ELdon, National Sales Manager at CooL Jet Systems, said that the proper use of HPC (High Pressure Cooling) systems can greatly increase productivity. He reviewed a practical example of a user: a drill diameter of 1.397 mm, a hole depth of 13.335 mm, and a workpiece material of 302 stainless steel. In the past, conventional cooling (pressure is 4 atmospheres), cobalt high-speed steel drill bit, rotating speed of 1600r/min, single-piece working time of 42 seconds, and bit life of 175 pieces. Later, the new process of double-drill processing was adopted: Firstly, the MZE-type solid carbide centering drill of Mitsubishi Corporation was used, without cooling, the rotation speed was 6000r/min, the feed rate was 0.0254mm/r, and the centering hole depth was 2.54mm. The second step is to use Mitsubishi's MZS internal cooling micro-drill, with a speed of 9000 rpm, a feed rate of 0.0203 mm/r, a step-by-step cutting step of 1.397 mm, and a coolant pressure of 102 atm. The total machining time of the two drills was 16.5 seconds (60% of man-hour saving) and the tool life was increased to 875 pieces. Obtaining such a huge benefit, the cost is only 3.3% higher for each part tool.

According to Brenner of Mitsubishi, the company produces micro drills with a diameter of 1mm to 3mm and a cooling pressure of at least 68 atmospheres, which varies with the size of the two tiny cooling holes on the drill bit. The cooling hole has a minimum diameter of 0.1524 mm and is used for the smallest diameter drill bit. In order to ensure adequate coolant flow, sufficient pressure must be guaranteed. For large-size drills, the inner cooling hole diameter is 1.524mm, and the coolant flow rate is 16.4 liters/min at 68 atmospheres. At the same pressure, the coolant flow rate when drilling with the micro-drill is only 1.89 liters/ Minute.

Mitsubishi also recommends that the cooling system be capable of filtering out particles as small as 5 microns; the precision filter sleeve used should be sealed at 68 atmospheres, whether internal or external. It is recommended to use a water-soluble coolant with EP additives such as sulfur and chlorine. Since the viscosity of the oil is 8 to 10 times that of water, it is not suitable.

MAFord's latest line of internally cooled micro-drills (minimum diameter 1mm) adds core gain, which helps ensure bit strength. The inner cooling spiral hole runs through the drill body and can be placed close to the front or back of the groove. 

The company specializes in the development of small spiral angle internal cooling drills because it helps the chip to drain out of the hole. Kueter pointed out that the internal cooling drill should be able to greatly reduce the number of step cuttings, especially when processing cold work hardening materials such as 304 or 316 stainless steel.

Small holes bring big challenges

Starro Precision Products is a Swiss-based threading and other processing company. There is close cooperation with MAFord in the application of internal cooling micro-drills. Lee Dwyer, vice president of sales and manufacturing at the company, said: "It must be understood that the coolant and tool geometry you choose will have an effect."

What makes Starro unique is that tolerances are maintained at ±0.005mm on certain production processes. Dwyer pointed out that existing drilling data is often used for bit rotation, so Starro has to develop many micro-drill applications for spiral machines and machining centers. Dwyer pointed out that the centering is a decision, the machine must be in good condition, the spindle radial runout should be less than 0.0025mm; the main advantage of the internal cooling micro drill is to improve the tool life and cutting speed. Compared to carbide drills that do not use coolant, the tool life of internal cooling drills is increased by a factor of three and the cutting speed by 30%, depending on the material of the workpiece.

For long-term application of micro-drills, it is particularly important to optimize each element of the entire cutting system.

Tom Krueger, National Sales Manager for Kyocera Tycom's Industrial Micro-Tools Division, points out that for small batch production, low-cost standard tools can be used. However, for the mass production of specific products, the production workshop should analyze and optimize the entire process flow.

For a specific workpiece material, the use of a dedicated drill bit, drill tip geometry, groove length, helix angle, and the diameter and length of the shank provides the best results. If the machine tool using the drill bit is carefully analyzed, the productivity will be further improved.

Krueger cites an example: on a special machine tool, a stainless steel medical part is machined with a 0.0381 mm diameter drill bit. The workpiece and the tool rotate at 5000 rpm and rotate in opposite directions. Thanks to Kyocera Tycom's suggestion, improvements to the machining process, such as adjusting the machine's concentricity, resulted in a doubling of productivity.

Practice has proved that if you want to increase productivity, you have to spend time, money, and active work. For example, Starro has invested in equipment and production processes and has carried out research on a range of micro-drill applications. Dwyer, the company's vice president of sales, pointed out that without spending effort, there would be no gain.

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