Cutting Tool Selection Guide

Deep-Hole Drilling: How to Choose a U-Drill or Crown Drill by Diameter, Depth and Material

How distributors choose U-drills and crown drills for deep holes — D12.5–45 mm, up to 15×D, with a real 151 mm hydraulic-pump drilling test.

Deep-Hole Drilling: How to Choose a U-Drill or Crown Drill by Diameter, Depth and Material

When a solid carbide drill stops being the right tool

A solid carbide drill is the right answer for most holes up to about 5×D. Past that — larger diameters, deeper holes, or batch work where a worn edge should be swapped rather than the whole tool reground — a distributor should be talking about indexable drilling: U-drills for single-pass holes in the D12.5–45 mm range, and crown drills where the depth-to-diameter ratio climbs toward 15×D. This guide follows that split. For shallow and medium holes, see our solid carbide drill selection guide.

U-drills: one pass, replaceable inserts, D12.5–45 mm

CutStars' U-drill range covers diameters from 12.5 mm to 45 mm, using SOMT / SPMT / WCMT inserts and holders. Three details from the product table matter most in a quote:
  • Coolant is delivered precisely to the tip, for better cooling at the cutting edge.
  • The holder hardness is over HRC 52, for wear resistance in the body that stays in service while inserts are changed.
  • The spiral flute gives 30% more chip-removal space, for smooth chip evacuation — the usual failure point in a deep hole is not the cutting edge, it is chips packing in the flute. A U-drill does not need a pilot hole in most cases, and when an insert wears, the insert is replaced rather than the whole drill being reground. For a distributor, that is the stocking argument: hold bodies and a range of insert grades, not a separate solid drill for every diameter.

Crown drills: up to 15×D, and regrindable

For deeper work, CutStars' crown drill (U-drill family) is specified for deep-hole drilling in steel, stainless steel and aluminium, for energy equipment, heavy machinery, shipbuilding, mining machinery and deep-cavity mould work. The sales table gives the figures a buyer will ask for first:
  • Maximum depth ratio up to 15×D.
  • Machining efficiency typically two to three times that of a standard U-drill.
  • The drill can be reground two to three times, with reground tool life reaching about 80% of a new tool's life. That last point is a cost-per-hole argument, not just a performance one: a crown drill that survives two or three regrinds at 80% life changes the unit cost of a deep hole in a way a single-use solid drill cannot match.

Deep-hole inserts and guide bars: the parts buyers forget to ask about

Deep-hole drilling also needs the consumables around the drill. CutStars lists high-efficiency deep-hole drilling inserts and guide bars for P, M, K and S materials, described as stable and high-efficiency across those groups, with a complete specification range available on request. A distributor quoting a deep-hole job should ask for the insert and guide-bar specification together with the drill body — quoting the drill alone leaves the customer to source the wear parts elsewhere.

A real deep-hole test: 151 mm in a hydraulic pump housing

This is a single-tool result, not a head-to-head comparison, but it shows what the deep-hole range is asked to do in practice. In a CutStars test on a hydraulic pump housing (QT500-7 ductile iron, Doosan vertical machining centre, internal cooling at 80 kg pump pressure), a D5 solid carbide drill (D5-165, 215 mm long) drilled a 151 mm-deep oil hole at 80 m/min and 0.15 mm/rev, finishing 900 pieces per tool — 136 metres of total drilled length. The depth alone is the point: at 151 mm, chip evacuation and coolant delivery decide the result, which is exactly why the U-drill and crown-drill features above (coolant to the tip, larger chip space, replaceable heads) are specified the way they are. For a solid-drill test result on another job, see our success case: solid carbide drill raises wheel-hub drilling efficiency by 28%.

A short selection checklist for a quote

Ask the customer for these five facts and the deep-hole tool choice narrows quickly:
  1. Hole diameter and depth — and the depth ratio (e.g. D20 at 150 mm is 7.5×D, which already points away from a standard solid drill).
  1. Workpiece material and hardness (steel, stainless, aluminium, cast iron — P / M / K / S).
  1. Through hole or blind hole, and whether the hole position or straightness is critical.
  1. Batch size — high-volume repeat work favours a replaceable-insert or regrindable crown drill over a solid drill.
  1. Machine and coolant: internal (through) coolant is specified for the deep-hole range; confirm the pump pressure available. With those five, CutStars can confirm the U-drill or crown-drill body, the insert grade and the guide-bar specification.

What to ask CutStars for

Distributors and importers can request the U-drill / crown-drill catalogue and parameter table, the deep-hole insert and guide-bar specification list, and application guidance for the specific hole. Enquiries are accepted by WhatsApp, the website form and email (info@cutstars.com).