Indexable Drilling with Korloy KTD: Grade Selection and Parameter Optimization

Indexable drilling has become the dominant technology for hole making in medium-to-large diameter ranges. Unlike solid carbide drills, indexable tools allow users to replace only the cutting insert while retaining the steel body, driving down cost-per-hole in high-volume production. Korloy’s KTD indexable drill series offers a practical balance of penetration rate, hole quality and tool economy for steel, stainless steel and cast iron applications. This guide explains the grade selection logic and recommended parameters that help shops get the most from their KTD investment.

Why Indexable Drilling Matters in Modern CNC Production

Indexable drills cover diameters typically from 12 mm to over 60 mm, filling the gap between solid carbide and spade or brazed tools. The key advantage is modularity: the drill body lasts for years, while inserts are replaced or indexed as they wear. For job shops running mixed material batches, this flexibility reduces tooling inventory and setup time.

However, indexable drilling also introduces challenges. Two or three inserts must share cutting forces evenly, chip evacuation is constrained by a narrow flute, and entry/exit conditions can cause burr formation or insert chipping. Choosing the right insert grade and geometry, combined with disciplined parameter selection, is what separates a stable 5×D hole from a scrapped part.

Korloy KTD Series Overview

The Korloy KTD range is designed for general-purpose indexable drilling across ISO P, M and K material groups. Bodies are available in 2×D, 3×D, 4×D and 5×D length-to-diameter ratios. A central insert typically handles the high-speed cutting near the drill center, while peripheral inserts manage the outer diameter and wall finish. This division of labor means that peripheral inserts often use a tougher grade to resist notch wear, while central inserts can run a harder, more wear-resistant grade.

KTD bodies feature internal coolant channels as standard. Through-tool coolant is not optional for indexable drilling; it is required to evacuate chips from the hole and to cool the insert corners where heat concentrates. Korloy recommends a minimum coolant pressure of 20 bar for 3×D holes and 30–50 bar for 4×D and 5×D applications.

Grade Selection by Material Group

Korloy’s PC-series grades map cleanly onto the material groups encountered in indexable drilling. The table below summarizes the most common pairings.

For mixed production where the same KTD body must switch between steel and stainless, PC9530 can serve as a compromise grade. It sacrifices some wear resistance in steel compared with PC5300, but its resistance to built-up edge reduces the risk of corner chipping when switching materials without a full tool change.

Cutting Parameter Recommendations

The following parameters assume a stable setup with through-spindle coolant, rigid tool holding and workpiece clamping that minimizes vibration. Always reduce speeds by 15–20 percent when entering or exiting inclined surfaces, cast skins or cross-holes.

Feed rate should be selected based on hole diameter and machine thrust capability. Larger diameters allow higher feeds because the cutting force is distributed across a longer edge. If the machine spindle load exceeds 80 percent during a test cut, reduce feed by 10 percent rather than dropping speed. Reducing speed excessively accelerates built-up edge in stainless and promotes crater wear in steel.

Chip Evacuation and Hole Quality

In indexable drilling, chip packing is the primary cause of insert fracture and poor hole straightness. Korloy KTD bodies use straight flutes with polished surfaces to encourage chip flow, but the user must still manage coolant delivery. A minimum coolant flow rate of 15 liters per minute is recommended for 20-mm diameter tools at 3×D.

For 4×D and 5×D holes, consider a peck cycle even with through-tool coolant. A peck depth of 1.5×D to 2×D allows chips to clear before the next plunge. This modest reduction in cycle time is preferable to a broken insert and a scrapped component.

Hole tolerance and surface finish depend heavily on peripheral insert condition. If the hole is oversize or exhibits radial scoring, inspect the peripheral insert for notch wear first. In cast iron, a worn peripheral insert often shows a distinct notch at the depth-of-cut line. Replacing only the peripheral insert while retaining the central insert is a valid strategy to balance tool life and cost.

Practical Setup Tips for Longer Tool Life

First, check runout at the insert tips. Mount a dial indicator on the machine table and rotate the spindle by hand. Total indicator runout should be below 0.02 mm. Excessive runout loads one insert disproportionately and causes early corner chipping.

Second, use a torque wrench for insert screws. Over-tightening deforms the insert seat and creates micro-cracks in the carbide substrate. Korloy specifies screw torque values in the product catalog; for most KTD sizes, 2.5 to 4.0 Nm is typical.

Third, match the drill point angle to the workpiece entry surface. KTD bodies are typically 140 degrees. When drilling into curved surfaces or forgings with irregular entry, reduce feed to 50 percent for the first millimeter of penetration to prevent insert impact damage.

When to Choose KTD Over Solid Carbide or Exchangeable-Head Drills

Solid carbide drills excel in small diameters, tight tolerances and high-speed applications. Exchangeable-head drills offer fast tip changes but require a proprietary interface. KTD indexable drills occupy the middle ground: lower cost per edge than exchangeable heads, and capable of diameters where solid carbide becomes prohibitively expensive.

For batch sizes above 50 holes in diameters from 16 mm to 40 mm, KTD is usually the most economical choice. Below 20 holes, the setup time may not justify the tool change. Above 50 mm, custom brazed or modular boring tools sometimes become more competitive depending on tolerance requirements.

Conclusion

Indexable drilling is a production workhorse, but its performance depends on disciplined grade selection and parameter control. Korloy KTD drills, paired with the correct PC-series grade and adequate coolant pressure, deliver predictable hole quality across steel, stainless steel and cast iron. By treating central and peripheral inserts as distinct wear zones, shops can extend tool life and reduce cost-per-hole without sacrificing process security. For applications outside the standard ranges, Korloy’s application engineering team can recommend custom grades and geometries based on workpiece material certificates and machine capability data.