There are several factors which affects accuracy and quality when drilling a hole. Positioning, straightness, oversize, roundness and roughness are five of the most important properties to consider.
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Problems & solutions
Improving the straightness of deep holes
When carrying out drilling of deep holes, it is necessary to make a pilot hole using a stub drill. Once the stub drill has made an accurate pilot hole the longer drill can be used. This reduces the amount of drill wandering by 70％～80％ when compared to drilling without a pilot hole. To maintain straightness it is necessary to drill a pilot hole that has an L / D of 1～3.
Hole positioning accuracy
& measures for improvement
Hole positioning accuracy is the difference between the expected or planned position of a drilled hole and its actual position.
f the drill's entry into the workpiece is inaccurate, then the drill will not be able to drill accurately. In this case, the use of a drill with X-type web thinning or a stub drill is effective. Also a centre drill can be used for guidance or positioning of the drill by a guide bush can be carried out prior to machining.
Workpiece: JIS S50C 230HB
Cutting conditions: vc = 20m / min,
fr = 0.2mm / rev dl = 12mm, ld = 25mm (through hole)
Coolant: Wet.
Lip height & hole oversize
Oversize is the difference between the actual hole diameter and the drill diameter. An element affecting hole over size is non-uniformity of the two cutting edges that results from differing web thickness, eccentricity of the chisel edge and incorrect lip height. An increase in lip height has a proportional increase in hole oversize. Therefore, care
should be taken to ensure that the drill is correctly clamped and within the runout parameters. Additionally when regrinding is carried out it is necessary to maintain a lip height dis-crepancy of less than 0.02mm.
A general rule is that a drill oversize for a high-speed steel drill is around 1% of the drill diameter. Whereas for solid carbide drills it is usually between 20μm～40μm.
Workpiece: JIS SNCM439 310HB.
Cutting conditions: high-speed steel drill 10.5mm
vc = 18m / min, fr = 0.12mm / rev
ld = 32mm (through hole)
Coolant: Wet
Oversize in relation to materials
Oversize is also influenced by the tool material of the drill. The image shows the variation based on whether HSS, carbide, brazed, or indexable drills are used. When carrying out drilling it is essential to bear this in mind.
Settings and materials used in example above:
Workpiece: JIS S50C 180HB
1) HSS drill (Ø10mm): vc=20m/min,
fr＝0.2mm/rev.
2) Solid carbide drill (Ø10mm): vc=60m/min, fr=0.25mm/rev.
3) Brazed carbide drill (Ø20mm): vc=60m/min, fr=0.25mm/rev.
4) Indexable drill (Ø20): vc=100m/min, fr=0.08mm/rev.
The image shows a comparison of dia 10mm drill.
Improving roundness
At the point of entry the drill contact with the workpiece is only on the chisel edge. This can lead to the drill wandering. Drill wandering is a situation where the drill moves in a manner that produces a non-circular hole. To reduce this problem it is necessary to use a drill that has an X type thinning or reduce the drill revolutions and increase the feed rate.
Hole surface roughness & rifling
Elements affecting hole surface roughness include the balance of the cutting edge, chip disposal properties and welding of the margin. Due to problems with wandering, spiral marks called rifling can be scraped on the inside wall of the hole, thus reducing the quality of the surface finish. Methods to reduce this problem are the same as for roundness. However reducing the lip height and the eccentricity of the chisel edge are also important factors.
Workpiece: JIS S50C 180HB.
1) HSS drill (f10mm): vc=20m/min,
fr＝0.2mm/rev.
2) Solid carbide drill (f10mm): vc=60m/min, fr=0.25mm/rev.
3) Brazed carbide drill (f20mm): vc=60m/min, fr=0.25mm/rev
4) Indexable drill (f20): vc=100m/min, fr=0.08mm/rev
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