Drilling micro-size deep holes.
There are several methods used for producing very small holes in workpieces. These include punch and die, electrical discharge machining, laser, photo etching, and electroplating. Each has some benefit in its favor, but all have drawbacks as well. Of these, none is capable of producing micro-size holes of exceptional depth. When very small diameter, deep holes are required, only micro drilling can be considered. Only drilling can produce holes with good cylindricity, surface finish and at acceptable production rates.
Industrial products that require micro drilling of deep holes include automotive fuel injection nozzles, chemical and textile processing nozzles, medical needles, air bearings, watch and camera parts, optical fiber connectors, etc.
There is no clear definition for a small diameter, deep hole. Definitions differ according to working conditions, drill materials, workpiece features, cutting conditions, machine tool used, etc. However, in general, holes less than 1.00 mm dia drilled to a depth of more than ten times the drill diameter are considered small and deep. Seen in Figure 2 are some examples of workpieces with deep micro drilled holes.
Drilling very deep holes with very small diameters is considered one of the most difficult machining tasks. To be done successfully, several problems must be overcome. For example, cutting performance is influenced by abrasion of the drill tip, drill breakage, and chipping. Cutting accuracy is reflected by the ability to maintain perpendicularity of the drill in the workpiece, roundness of the hole, and surface finish of the wall of the hole.
Such problems do not happen individually, but often simultaneously, and are caused by various factors, including the machine tool, tool holder, cutting oil, cutting conditions, and the traits of the material being drilled. Because these factors are taking place inside the hole, accurate information cannot be obtained about what is taking place. Moreover, in drilling deep micro holes, these problems are critical.
If both are operated at the same rpm, the peripheral speed of a small diameter drill is slower than that of a larger diameter drill. It would seem, therefore, that the small diameter drill should be operated at a higher rpm. For example, compare a 1.00 mm dia drill with a 10.00 mm dia drill, where the drill speed of both is 1000 rpm. Using the common formula for finding the circumference of each, the peripheral speed of the 1.00 mm dia drill is 3.14 m/min, and for the 10.00 mm dia drill it is 31.4 m/min. In order to run the smaller drill at the same peripheral speed as the larger, the drill speed of the smaller would have to be increased ten times, or 10,000 rpm. Such a high drill operating speed would have an adverse effect on other factors such as the machine tool, tool holder, cutting oil, etc.
Another factor peculiar to micro drilling is short tool life caused by drill breakage. In small diameter drills, drill bending and breakage is caused by fatigue from torsion, and naturally will occur more often in smaller than in larger diameter drills. Also a problem with micro drills is expansion of the entrance hole caused by vibration and shaking of the drill. Poor discharge of cutting chips also causes problems in micro drilling.
Solving problems unique to micro drilling are important to the success of this type of operation. One solution is through the use of a drilling machine designed to anticipate adverse drilling conditions and take corrective action automatically.
One such machine is the Micro Hole drilling machine--invented by Kenji Machida--made by Machida Iron Works, Tokyo, Japan, and distributed by our company, Figure 1. This machine is designed specifically for drilling small diameter deep holes automatically. Its normal operating range handles drills from 0.1 mm dia to 4.0mm dia, but has drilled holes as small as 0.03mm dia with special tooling. The machine makes use of a patented drill-tip sensor/torque detector that reacts to resistance caused by torque or drill path deviation. The torque detector, with the torque reading set within the tensile strength limit of the drill, determines when sufficient resistance is met and causes the drill to retract before it can break, Figure 3. This retracting movement of the drill also allows chip build up to be released and permits cutting oil to flow to the cutting edges of the drill.
At the start of the drilling cycle, the control is set to advance the drill toward the workpiece at a fast-forward rate (adjustable by change gears) until reaching a point just before the workpiece surface to be drilled. The control then changes to drilling feed speed (infinitely adjustable by control knob) and starts the drill into the workpiece. Immediately upon receiving a signal that the torque limit has been reached, the drill retracts. Torque can be set at any desired amount, even for a 0.1 mm dia drill.
To reduce idle time, fast forward feed motion is then signaled again and the drill moves back into the previously drilled portion of the hole, avoiding collision with the bottom of the hole by means of the step point memory device. Normal feed speed is then resumed and drilling continues. Thereafter, whenever the set torque limit is reached the drill will again retract and return at fast speeds, resuming normal drill feed speed as it re-enters the workpiece.
The same sequence is followed if the drill should bend and fail to make a straight hole. In this case, torque will also increase, causing the drill to repeatedly retract and re-enter the work until it begins drilling in a straight path.
Unique micro drill features
As stated earlier, it would seem that micro drills should be operated at a high rpm in order to achieve a peripheral speed similar to larger, standard drills. However, experience by Machida has shown that this is not the case. On the contrary, the smaller the diameter of drill used, the slower the revolution needed. Also, the feed ratio of the drill per revolution is greater than for conventional drilling.
In micro drilling of deep holes, the drill tip shape is very important, and--as with drill speed--some criteria have been developed by Machida based on extensive experimentation. It is suggested that special attention be paid to the chisel edge and chisel point. Concerning the chisel edge, no thinning is required. For small, deep hole drilling, thicker material at the chisel edge is better.
Concerning the chisel point--the dead point where no drilling occurs--shape and location can make a big difference with regard to drill breakage and drilling conditions. As seen in Figure 4, with drills less than 2.00 mm dia, a shift of the center point by 5 percent causes it to act as a two-stage cutter, reducing cutting and frictional resistance. In deep holes, the tip actually guides the drill rather than the drill being guided by the wall of the hole.
PHOTO : 1. The Machida Micro-Hole is an automatic machine for drilling small, deep holes ranging
PHOTO : from 0.1 mm to 4.0 mm dia. Automatic operation is achieved by dialing in drill speed, feed
PHOTO : rate, and hole depth. Micro-size holes with depths from 10X to 150X their diameters can be
PHOTO : drilled.
PHOTO : 2. Examples of deep drilled micro size holes are shown compared to a dime (a). Left is a
PHOTO : 40 mm long steel rod with a through hole of 0.5 mm dia (depth is 80 X dia). Drilling time
PHOTO : was 15 min. In the center, is a 10 mm long stainless steel rod with a 0.2 mm dia through
PHOTO : hole (depth 50 X dia). At the right is a 9.5 mm stainless steel rod with 12 through holes
PHOTO : on a bolt circle. This piece is seen in end view (b) showing the 0.5 mm dia holes spaced
PHOTO : leaving a 0.007" wall thickness to the OD of the cylinder. Seen in (c) is a 0.5 mm thick
PHOTO : steel disc, with a 0.1 mm dia hole in the center, round to within 0.2 microns.
PHOTO : 3. Sequence of drilling operation on the Micro-Hole machine is shown in this series of
PHOTO : diagrams. The machine's critical torque detector signals the fast retraction movement of
PHOTO : the drill, avoiding drill breakage and aiding in chip removal and cutting oil feed.
PHOTO : 4. Micro-hole drills should have a wide face angle and the chisel edge offset about 5
PHOTO : percent of drill diameter, as seen in sketch.
Al Feifer Vice President Engineering United International Technologies Inc, Walnut Creek, CA.
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|Publication:||Tooling & Production|
|Date:||Oct 1, 1989|
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