Bringing security to small hole drilling

When cutting conditions are unfavourable, producing holes below 24mm diameter can prove tricky due to lack of process security, a factor that often leads to poor internal bore finish or out-of-tolerance holes. Unstable conditions can result from a previous crash where turret misalignment has not been corrected, clamping or indexing problems, vibration, incompatible cutting fluids, or perhaps when using older or less rigid machine tools.

To overcome these problems, a suitably designed insert-based drill can provide the solution. Customers can attain increased hole making security and enhanced precision when using a tool with strong inserts and a shot-peened drill body for improved fatigue resistance, such as the new CoroDrill? 881 from Sandvik Coromant.

Two cutting edges
Innovations such as this are a particularly good choice for low-to-medium feed applications, as well as lathe-based non-rotating applications. CoroDrill 881 features two cutting edges per insert (rather than four), as well as general purpose, high performance geometry to accommodate all material groups and therefore appeal to a wide range of industries.

Potential parts with holes in the 14-24mm diameter range that could benefit from this technology include: flanges and slewing bearings for the power generation sector; crankshafts, connecting rods and transmission parts for automotive manufacturers; flanges, reducers and collectors for the oil and gas industry; and housings and security parts for pump and valve manufacturers.

In most medium-sized machine shops, drilling accounts for the majority of all machine time. Because of the high metal removal rates possible with indexable insert drills, they are becoming increasingly popular. Insert-based drill technology combines the cutting advantages of carbide with the alignment forgiveness of HSS. With a large radial adjustment range, the body of these drills will better accommodate misalignment whereas solid carbide drills are less  forgiving.

Impressive performance
The performance of CoroDrill 881 has already been demonstrated in a number of cutting trials. For example, in tests on a stainless steel (CMC 05.21) flange component drilled using a 16 mm diameter drill to 17 mm depth on a vertical machining centre, CoroDrill 881 outperformed a comparable competitor product by 25 per cent (achieving 250 holes against 200) thanks to increased process security. Cutting data was: 150 m/min cutting speed; 2985rpm spindle speed; 0.07mm/rev feed; and a feed speed of 209 mm/min. The central insert grade was GC1144, while the peripheral insert grade choice was GC2044.

Using the same insert grade combination in another VMC application (ISO 50), CoroDrill 881 completed 72 holes in Inconel 825 (CMC 20.2) to 19mm diameter and 25mm depth, in comparison to just 56 using a competitor drill. Cutting data was: cutting speed of 90m/min; 1508rpm spindle speed; 0.07mm/rev feed; and a feed speed of 106mm/min.

Cutting data
Generally speaking, recommended cutting speeds relate to the drill's outside diameter. The cutting speed decreases to zero at the centre of the tool, so to avoid any potential of edge build-up on the inner inserts, very low cutting speeds should be avoided where possible. For this reason, feed forces are comparatively low, at least when compared with conventional twist drills. Feeds should be selected to ensure that the insert's chip control geometry produces short chips which can be flushed away easily with the coolant.

An abundant coolant supply of adequate pressure is very important. The function of the coolant is not only to aid in the removal of chips but also to help correctAdidas