For CNC gundrilling, a common screening rule defines a deep hole as one whose depth exceeds ten times its diameter. SME’s Tool and Manufacturing Engineers Handbook treats deep-hole drilling as a distinct machining operation, while the ASM Handbook, Volume 16, covers the process-specific demands of machining.

For example, a 12 mm hole at 150 mm depth has a 12.5:1 ratio. That is a clear signal to assess a dedicated gundrilling setup—not an automatic machine specification.

The ratio is only a starting point. A 10:1 hole in soft aluminum may behave differently from one in stainless steel, where chip evacuation and heat control can be more demanding. Check the smallest diameter, deepest feature, material, tolerance, and expected production volume.

Then ask whether the machine can deliver stable coolant pressure, guide-bushing support, and enough travel for the full workpiece. Watch the chips. Their shape and consistency can reveal trouble early.

A long, slender hole can also drift because of setup error or an uneven entry face, not just machine limits. The 10:1 rule helps frame the decision, but actual trials on representative parts provide stronger evidence.

Real shop conditions are messy. Plan for that.

Choosing a CNC gundrilling machine starts with the workpiece, not the catalogue. Carbon steel, stainless steel, titanium, and heat-resistant alloys differ in hardness, heat flow, and chip behavior. These differences affect cutting force, tool life, and coolant needs. ASM Handbook, Volume 16, describes deep-hole drilling as a distinct machining process. A common screening benchmark is a hole depth-to-diameter ratio near 10:1. For example, a 12 mm hole reaching 240 mm deep has a 20:1 ratio. That calls for careful checks of guidance, rigidity, and chip evacuation. Small details matter.

Match the machine’s rated diameter and drilling depth to the actual part, including setup allowance. Check spindle torque at the required speed, not just maximum spindle power. A long, narrow hole may need stable feed control and high-pressure coolant delivery. A tough alloy may need more torque and reliable heat removal than a softer steel part.

Ask for the machine’s working range, coolant pressure and flow, and demonstrated accuracy at your target depth. Then compare those figures with your tooling supplier’s recommendations. A spreadsheet can still mislead if it ignores part clamping or coolant filtration. I have seen capacity look sufficient on paper, yet the setup left too little clearance. A trial cut helps. Start with the hardest material and deepest hole you expect to produce, then verify straightness, surface finish, and cycle time.

When choosing a CNC gundrilling machine in 2026, compare spindle speed, feed rate, and coolant pressure as a working set. Spindle speed should suit the drill diameter and workpiece material, not just reach an impressive maximum. A small-diameter drill in alloy steel may need different cutting conditions than a larger drill in aluminum. Feed rate affects both cycle time and chip formation. Too aggressive a setting can cause deflection, rough holes, or premature tool wear.

Numbers matter.

Check the machine’s rated feed range under load, then compare it with the hole depths and tolerances you actually need. Coolant pressure is only part of the picture: confirm flow rate, filtration, and delivery through the drill. Poor chip evacuation can leave long chips packed in a deep bore, even when the pressure figure looks adequate. Ask whether specifications refer to continuous operation or brief peak performance. A tidy spec sheet can still mislead.

I would leave room for trial cuts, since real setups rarely match ideal test conditions exactly. Record hole straightness, surface finish, and tool wear during those tests; these details help reveal whether the three specifications work together.