Microdrilling is characterized not just by small drills but also a method for precise rotation of the microdrill and a special drilling cycle. In addition, the walls of a microdrilled hole are among the smoothest surfaces produced by conventional processes. This is largely due to the special drilling cycle called a peck cycle. The smallest microdrills are of the spade type. The drills do not have helical flutes as do conventional drills and this makes chip removal from the hole more difficult. Drills with a diameter of 50 micrometers and larger can be made as twist drills. Drills smaller than this are exclusively of the spade type because of the difficulty in fabricating a twist drill of this size.
There are several important geometric characteristics of spade-type microdrills. First, the point of the drill is not a point at all. Even on conventional twist drills, the end is not truly pointed. Instead, the end of the microdrill consists of a cutting edge (called the chisel edge) made by two intersecting planes which also define the two primary cutting edges of the drill. The chisel edge removes material primarily by extrusion and cutting at high negative rake angle. The specific cutting energy along the chisel edge is relatively large compared to the drill's primary cutting edges. The chisel edge also adds to the drilling complexity because of the lack of a point. As the rotating drill first contacts the work piece (remember the drill has a very small structural rigidity) anything on the surface, including microroughness and material slope, will cause the drill to walk on the surface as it is trying to begin removing material. Walking is characterized by an eccentric motion of the drill as it turns perhaps coupled with a non-time- varying bending of the drill about its longitudinal axis. Depending on the feed per revolution of the drill during hole start-up, the drill may begin drilling at a slant with the drill deflected like an end-loaded cantilever beam (which it is with superimposed column loading). If permitted to continue, the drill will quickly break. If the drill is strong enough to survive the large stress imposed in it due to drilling at a slant, the resulting hole will be slanted rather than normal to the work surface.
A second consequence of the chisel edge is its relatively long length compared to the drill diameter. This results in a relatively high thrust force along the drill axis. While the sloped cutting edges are increasing the diameter of the hole machined by the chisel edge, the specific cutting energy along the cutting edge is normally lower than at the chisel edge. The result is a large thrust force compared to the diameter of the drill. Again, this size effect works against the microdrilling process similar to the size effect in micromilling.
Microdrills are typically made of either cobalt steel or micrograin tungsten carbide. The steel drills are less expensive and easier to grind but are not as hard or strong as the tungsten carbide drills. The drill point angle is based on the material to be drilled. The normal point angle is 118 degrees and 135 degrees is used for hard materials. The larger included point angle provides more strength at the drill point.
A microdrilling spindle uses a vee-block bearing arrangement. The drill is mounted in the mandrel and is fabricated integral with the mandrel. The mandrel rides against four convex diamond surfaces which are the only points of contact. So long as the drill was ground with the mandrel supported in a similar manner, the drill will be concentric about an axis. That axis may not coincide with the mandrel axis but that is not significant as long as the offset is not sufficient to cause excessive vibration, and it normally is not. A small pulley is fastened to the drill mandrel and a drive belt passes around the pulley and drives the drill from an external motor. The belt tension is the only force holding the mandrel against the diamond pads and a slight upward component of the belt tension is used to retract the drill. The upper end of the mandrel rides against a ceramic material which provides the drill thrust force. This disk may also rest against a force sensor to measure drill thrust force which is often used to indicate the extent of drill wear.
Microdrills must be used in a peck cycle wherein the drill is repeatedly withdrawn and reinserted into the hole being drilled. This is necessary to help clear chips from within the hole. A thin cutting fluid is also recommended to aid in chip clearing. The fluid should be moving, as in an air-oil mist rather than stagnant. Stagnant fluid will allow chips to reenter the hole along with the drill. The effect of not using a fluid is clearly shown. The hole has more, large chips, on the order of 5 micrometers in size, and the drilling thrust force under such a condition is typically higher than if the chips are cleared from the hole. With no fluid to help clear chips, two hole is packed with chip debris and the axial force on the drill is typically several times (2-3) higher than with fluid. In very soft materials, complete removal of the drill from the hole each peck cycle can cause a slight taper near the hole entrance. This can be avoided by incomplete removal of the drill. For softer materials, chip removal is not as severe a problem since the machining forces for such materials is normally lower than for hard materials but chips left in the hole can cause the drill to wander from an axial path and can result in a drilled hole with a center which does not lie along a line.
The recommended speeds and feeds for microdrilling are as varied as the materials which can be drilled. Microdrilling is not generally a high speed process since dwelling of the drill at the bottom of the hole can cause hardening of the work piece leading to increased drilling forces. For most metals, typical spindle speeds are in the 2000 to 4000 rpm range and feeds are in the range of a micrometer per revolution, or so. Care must be taken when drilling plastics to avoid melting of the material which can lead to adhesion of the plastic to the drill. This can cause drill breakage or poor sidewall smoothness.
The applicability of microdrilling as a complementary process with features produced by lithography and electroplating has been investigated. A cross section of a copper microgear made by lithography is shown. The average roughness of the hub wall is 0.4 micrometers. A microdrilled hole in the same material gave a roughness of 0.15 micrometers over a much longer bore length. Microdrilling can also be used to augment lithography for mesoscopic (millimeter and larger) sized components. Often parallelism of deep holes is of concern. To determine typical values for parallelism of microdrilled holes, glass fibers were inserted into a number of holes drilled with a very slow starting sequence. This is necessary to ensure the drill does not walk on the surface of the part and that the hole axis aligns with the undeflected axis of rotation of the drill. Holes with a length-to-diameter ratio of 8 were drilled at 4000 rpm. The three-dimensional misalignment of the inserted fibers was measured to be 0.08 degrees (1.5 milliradians), which included skewing of the fiber in the hole due to oversize of the hole which was estimated to be 0.5 micrometers.
Microdrilling has one major disadvantage because of the drill geometry. Because of the drill point, a flat-bottomed hole can not be produced. If one is attempting to produce cylindrical cavities in a micromold, there must be a relatively thick plating base under the mold material, or the structural substrate of the mold could act as the plating base. To fully develop the diameter of the hole, projected onto a plane perpendicular to the drilling direction, requires the drill point to extend 30% of the drill diameter beyond the depth of the fully developed hole. For holes in the 100 micrometer region, requires a thick plating base to be deposited. One method for creating flat-bottomed blind holes is to use an end milling tool instead of a drill. The disadvantage to this procedure is that drills have a typical L/D ratio of 4 to 14 while end mills typicall have a ratio of only 1.5 to 3.