Drilling vs Helical Interpolation for CNC Holes: When Each Wins on Time, Flexibility and Tooling

First check: does the hole match a standard drill size?

This one question decides most routings. Catalog drills come in standard fractional sizes such as 1/8", 1/4" and 3/8", in letter and number sizes, and in whole millimeters²⁴.

A hole at any of those diameters can be plunged in seconds with an off-the-shelf drill²⁴. An arbitrary dimension — the cited example is 0.492" — changes the picture.

You would either buy custom drill tooling and add operations, or mill the hole by helical interpolation at several times the drilling cycle time²⁴. When you review drawings for high-volume production, flag every non-standard hole diameter and ask whether it can be moved to the nearest standard size before you commit to either process.

(Designing holes to standard sizes is one of several routing choices this decision feeds.)

Cycle time per hole: what each process actually costs

Drilling wins the per-hole contest on standard sizes by a wide margin — seconds of plunge versus several times longer for an interpolated hole²⁴. That is the time penalty you accept when you keep an odd diameter and interpolate instead.

Interpolation is nonetheless a supported production method, not a workaround. One end mill manufacturer publishes cutting data for helical interpolation in P-group steels: cutting speeds of 410–580 SFM, feeds of 0.0022–0.0041 IPT, and a maximum depth of 1×D per pass²¹.

That depth cap matters for deep holes: an interpolated hole deeper than one cutter diameter is built up pass by pass, so cycle time grows with every extra revolution of the helix.

Ask your tooling vendor for the published interpolation data for your specific cutter and material, then estimate the pass count for your hole depth before quoting cycle time.

Tool cost: extra material removal and custom tooling both bite

Tool cost per hole has two drivers, and both push the same direction. First, a drill is a fixed-diameter tool; a non-standard hole forces either custom drill tooling plus extra operations, or interpolation²⁴.

Second, any extra material removal wears tools faster. In one cited stainless shaft example, added stock removal moved cutter replacement from every 300 parts to every 200, adding $0.80–$1.20 per unit depending on tool life and spindle power¹¹.

That figure is that example's illustration, not a benchmark, but the mechanism is general: more stock removed per hole means more tooling spend per hole. Manufacturers respond with wear-resistant drill features — coatings and internal coolant that reduce heat, two-margin lands for stability — and market end mills for life in helical interpolation²⁸.

No source here publishes drill prices, end-mill prices, or holes-per-edge tool life, so a per-hole tool-cost comparison cannot be computed; request those numbers when you RFQ.

Machine capability checks before you fix the routing

Capability can rule out the process you would otherwise choose. Verify these on the actual machine before committing:

• ✓Confirm the spindle supports through-spindle coolant if holes are deep — it is described as almost mandatory for deep holes and pockets that external nozzles cannot reach, and is only available if the machine supports it
• ✓Check that internal-coolant drills exist in your diameter: one tool family offers them in 3/8–1 in (10–25 mm)
• ✓Confirm machine rigidity and power cover the cutter size and depth of cut your strategy needs — low rigidity or power limits both

When interpolation pays for itself: the setup argument

The countervailing case for milling a hole is the part, not the hole. If the part already needs milling — cross-holes, milled flats, slots.

Moving it to a second machine for drilling adds setup time and alignment risk, whereas consolidating in one setup cut total cycle time by 30–50% on complex parts in the cited turning-center example²⁶. That figure comes from one vendor's complex-part example, so treat it as an indication, not a rule.

Diameter flexibility adds to this. One end mill carries a published diameter range (Ømin–Ømax) for helical interpolation, so a single tool covers many hole sizes²¹, while drills are fixed-diameter standard tools²⁴.

Where a part carries several odd hole diameters plus milled features, one interpolation tool in one setup competes with a set of special drills across two operations. The sources do not quantify the break-even — how many non-standard sizes justify one interpolation cutter versus multiple drills — so price both routings on your actual part.

Match the tool data to your material group

Tool makers segment their catalogs and cutting data by workpiece material group — P for steels, N for non-ferrous such as aluminum, S for titanium and heat-resistant alloys, H for hardened materials^14,19. Cutting data valid in one group does not transfer to another.

The interpolation figures quoted above apply to one specific end mill in P-group steels only²¹; hardened, titanium and aluminum applications each need their own validated data. The same discipline applies on the drill side: modular drill systems are also specified per material group and application²⁸.

Before you fix the routing, confirm the tooling vendor publishes speed, feed and depth data for your material group and your hole depth. And if they do not, ask them to prove out the parameters on sample parts.

Before you request a quote

• The evidence leaves this open — put it to the supplier: no source compares drilled vs interpolated holes on tolerance, IT grade, position accuracy, roundness, straightness or burrs — ask each supplier to state achieved hole accuracy for their proposed process on your part
• The evidence leaves this open — put it to the supplier: no quantitative cycle-time benchmark exists for a drilling canned cycle vs helical interpolation on the same hole diameter, depth and material — ask suppliers to quote per-hole cycle time both ways
• Request a written quote with price basis, MOQ, tooling, payment terms and production lead time.
• The evidence leaves this open — put it to the supplier: no deep-hole aspect-ratio limit comparing the two processes is established — confirm maximum depth capability per process for your hole depth
• The evidence leaves this open — put it to the supplier: no material-specific comparison (steel vs aluminum vs titanium vs hardened) of drilling vs interpolation exists — validate parameters on your material with the tooling vendor
• Request the complete specification sheet with units, test conditions and configuration assumptions.
• The evidence leaves this open — put it to the supplier: no hole-volume break-even threshold for switching between the processes is established — the 30–50% setup figure is one vendor's complex-part example, so test the switch on your own batch