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Parabolic Flute & Deep-Hole Drills

Parabolic flute drills have a wide, open flute shape that moves chips out of deep holes, and deep-hole drills include long carbide coolant-through drills and gun drills. This sheet compares the flute forms and explains when each deep-hole option fits.

Ref CT-0011

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Chip removal limits how deep a twist drill can go before it packs and breaks. A standard flute narrows toward the center of the drill. A parabolic flute has a wider, more open shape and usually a thicker web. The open flute carries chips out with less packing, and the thick web keeps the drill stiff. Parabolic drills can often drill deeper with fewer pecks than standard drills. For deeper holes, coolant-through carbide drills push coolant through holes in the drill body to flush chips. Gun drills are single-flute drills with a V-shaped chip channel and internal coolant, used for very deep, straight holes on machines built for them.

How to identify

Look at the flute in cross section or at the end view. A parabolic flute looks wide and rounded, with thick land walls. Catalogs list "parabolic" or "PFX" style names. Coolant-through drills have small holes at the point. Gun drills have one straight flute, a carbide tip and a long tubular shank with a driver. Depth capability is listed in the catalog as a multiple of diameter, such as 5xD or larger.

Deep-hole options

Drill type	Flute and coolant	Best for	Watch for
Standard jobber or taper length	Standard helix, external coolant	Holes up to a few diameters deep	Must peck often in deep holes
Parabolic flute HSS or cobalt	Wide open flute, thick web	Deeper holes in steel, aluminum, plastic	Needs rigid setup
Extra-long with parabolic flute	Long flute	Deep holes in manual and CNC work	Start with a pilot hole
Coolant-through carbide	Internal coolant channels	High-production deep holes on CNC	Needs through-spindle coolant pressure
Gun drill	Single flute, internal coolant	Very deep straight holes	Needs a gun drilling machine or setup and a guide bushing

Key points

Use a pilot hole or a shorter starting drill for long drills. It keeps the hole straight.
Peck depth and peck frequency come from the drill maker for each material and depth.
Parabolic flutes work best with steady feed. Light feed makes thin chips that can still pack.
Coolant-through drills need adequate coolant pressure and flow. The drill maker lists what is needed.
Speeds and feeds are lower for long drills than for short ones of the same size. See the drill feed rate chart and SFM to RPM chart.

Common mistakes

Using a standard flute drill for a deep hole without pecking.
Running a coolant-through drill without through-spindle coolant.
Starting a long parabolic drill on a flat surface without a pilot.
Choosing a gun drill for a machine that cannot support it.

Governing standards: No single standard is listed for this sheet. Manufacturer catalogs for flute form, depth capability and cutting data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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