Drilling developments [18/08/2009] The introduction of the indexable insert drill was a technology breakthrough, but further recent developments improve on the original design, as WNT (UK) explains With the development of drills featuring replaceable, indexable, typically carbide, inserts – commonly referred to as U-drills – hole production became more productive. Other drills are solid material (high speed steel, carbide), or solid with brazed tips, and are of 'twist' design (helical flutes) or 'spade' (open sided, no helical flutes). U-drills deliver high surface/spindle speeds, married to relatively low feed per tooth. The result is very high penetration rate when compared with the alternative drilling technologies. They feature two inserts that, together, cut the full diameter, but only one is actually set to cut the diameter, the other is set further inwards and cuts on, or across, centre. There is an overlapping portion, too. Adrian Fitts, business development manager, WNT (UK), underlines the drilling challenge. "When rough turning, the depth of cut is typically 4-5 mm, but with drilling the depth of cut is equivalent to the drill diameter. Therefore, the volume of metal removed in the same period of time is much greater when drilling, especially with the advent of through-tool coolant." Indexable insert drills are much better suited to use on CNC lathes, as they are able to deflect and compensate for any slight misalignment between lathe turret and spindle centreline. With solid carbide drills, this would cause the drill to wear and, possibly, break. U-drills can also be offset from workpiece centre line so as to drill a hole larger than nominal and, for WNT, the insert set to cut nominal diameter can be moved inwards, so a hole under nominal can also be produced (the drill body is obviously also under nominal diameter). U-drills can also be used as a boring bar to finish the hole to size or create counterbores and chamfers. Issues of note While these indexable insert drills delivered many advantages, users had to be aware of certain issues. By moving away from the traditional drill point, the indexable insert drill had to 'self centre' and drill a hole in the correct position. This was achieved by using inserts based on a Trigon shape to the ISO designation WCMX. While this overcame the self-centring issue, the 'per edge' cost of the inserts was raised, as it only had three cutting edges per insert. In addition, due to the differential in cutting speeds between the centre insert and the outer (peripheral) insert, with surface speeds ranging from zero to over 400 m/min, different insert grades are required. Image: Picture, centre, the new Impact drill uses square inserts, giving an extra edge over traditional U-drills The drive to better this approach has led to the development of a new generation of indexable drills. "The latest generation of Impact indexable insert drills from WNT has been designed to be far more robust than their predecessors," says Fitts. "And, while they do not fully replace the earlier versions, they do allow much higher performance, particularly in the more difficult-to-machine materials. The overall goal is to reduce cycle times, and, in designing these Impact drills, WNT has introduced a step-change, in terms of metal removal for drilling." They offer a more rigid platform for the insert, with this rigidity key to achieving optimum performance, as it allows feed rates to be dramatically increased. For example, the typical operating feed rate of a conventional indexable insert drill is between 0.075 and 0.14 mm/rev; for Impact drills, this range is increased by almost 100 per cent to between a figure of 0.14 to 0.225 mm/rev. So, taking a 24 mm drill as an example, for older technology the penetration rate would be 318.5 mm/min. For the Impact drill, the figure is 583.8 mm/min; an increase of over 83 per cent. Metal removal rate improves by the same amount, from 144 cm3/min to 264 cm3/min. The new drills have moved away from the traditional Trigon-shaped insert to a square insert. This insert shape is more suitable for the higher feed rates that are now being achieved, and there are economies to be made as there are four cutting edges per insert. Additionally, the carbide grade is the same for both the centre and peripheral insert. The self-centring effect of the Trigon insert is replaced by the rigidity of the drill body, which ensures that the hole is in the correct position, irrespective of the materials surface condition. Also, to accommodate the higher feed rates ,the thickness of the insert has been increased to 5 mm on the larger drills. This increased insert strength and the development of new grades of carbide have only added to the performance of the drill. Square approach best Having square inserts brings with it further advantages, in that the hole produced has, within 3°, a flat bottom. This in itself is of no great significance, but the advantages become clear when you recognise the versatility that this feature brings. Because of the square approach to the workpiece, the Impact drill can now be used to drill holes on uneven surfaces, such as castings; inclined faces, through material where interrupted cutting occurs; plus convex surfaces. And, unlike drills with Trigon- style inserts, these drills can pass through stacks of components, as the insert shape does not create the disc of material that is common with the older drills, and which causes drill failure in such situations. Indexable insert drills also offer potential to reduce cycle times in operations other than pure hole making. For example, where a cavity has to be machined in a component, the traditional method would be to profile the shape with a milling cutter, feeding down a few millimetres every pass. While effective, it was very time consuming. The alternative presented with indexable insert drills, such as Impact from WNT (UK), is to 'chain drill' the cavity and then quickly finish profile the shape with a milling cutter. The time savings are huge. The process is relatively simple, and as long as the following procedure is adhered to, the drill will perform perfectly. The first hole (diagram, left) is drilled as normal from solid; this is then followed by hole two, again, a normal hole from solid; the third, central hole is then drilled. By following this procedure, the hole with the intermittent cut is always symmetric and is cut with minimal effect on the drill. Swarf clearance is also well managed by using this sequence. To complete the pocket, simply repeat until the bulk of the material has been removed before finishing the profile with an endmill or similar cutter. Box Item Benefits in brief * The new design does not replace earlier U-drill designs, but does offer benefits in difficult-to-machine materials; * Square insert offers an extra edge over typical 3-edge U-drill designs; * Can cut on centre without centring insert, so can machine into rough surfaces; * Both outer and inner inserts are the same, unlike traditional U-drills where they are different; * Penetration rates, compared to previous U-drills, can be as high as almost double, and these benefits can also be exploited in non-hole making operations, such as bulk removal of material First published in Engineering Apprentice, Summer 2009 Related Companies: WNT UK Ltd