Flex CNC Drilling Distribution Guide

• 1
  What flex CNC drilling distribution changes about a hole
• 2
  Feeds, speeds, and peck depth on flexible drilling systems
• 3
  Tool holding and runout: where hole accuracy is actually lost
• 4
  When flex CNC drilling distribution is the wrong route
• 5
  What to verify in a flex CNC drilling distribution partner
• 6
  Which drilling route fits which hole

A drilling cycle carries three loads at once: thrust pushing the tool down the axis, torque turning it, and radial load bending it sideways. A conventional drill press resists all three with one rigid frame, so the drill simply follows the spindle. A flex CNC drilling distribution setup splits those jobs. The spindle supplies rotation and feed; the positioning system carries the workpiece or the head; the tool holder has to absorb what is left.

That split is the whole point. When the machine can move the part under a stable spindle, hole location no longer depends on how far the operator can swing an arm. It depends on encoder resolution, thermal drift, and how well the fixture repeats. On our 5-axis centers the rotary table is Ø400 mm, which sets the practical work envelope for parts that need holes on several faces.

The tradeoff is stiffness. Any flexible axis is a spring in the loop. A long drill hanging out of a small holder will deflect before the machine ever reports an error. So the distribution layout decides which errors are machine errors and which are simply tool deflection that no controller can correct.

One number matters more than the brochure. Hole position tolerance and hole size tolerance are different budgets. Position comes from the machine; size comes from the drill, the pilot, and the material. Mixing them in one tolerance callout is the most common mistake we see on incoming drawings.

• 1
  Position vs sizeMachine accuracy sets location; the drill sets diameter and roundness
• 2
  Flexible axis = springEvery moving axis adds compliance to the loop
• 3
  Fixture repeats firstA good machine cannot fix a loose clamp

On softer aluminum, 6061-T6 runs comfortably at 2,500–4,000 rpm with 0.10–0.20 mm per revolution feed using HSS or carbide drills. The flexible axis rarely limits you here. The limit is chip evacuation. If chips pack into the flutes, torque climbs fast and the drill walks.

Titanium TC4 (Ti-6Al-4V) is the opposite case. Surface speed drops to roughly 25–40 m/min and feed to 0.05–0.10 mm per rev. Heat stays in the cut instead of leaving with the chip, so peck depth should stay shallow, typically one diameter or less. Deep pecks on titanium work-harden the hole wall and the next pass rubs instead of cuts.

Stainless 316L sits between the two. Feed around 0.08–0.15 mm per rev with generous coolant. 17-4PH in the H900 condition is harder and will push you toward carbide and shorter overhang.

Peck cycles matter more on a flexible machine than on a dedicated drill press. Every retract and re-entry is a chance for the tool to re-center slightly differently. On hole depths over 3× diameter, use a pilot or spot drill and keep the peck depth consistent rather than varying it by feel.

• 1
  Aluminum 6061-T62,500–4,000 rpm, 0.10–0.20 mm/rev, watch chip packing
• 2
  Titanium TC425–40 m/min, 0.05–0.10 mm/rev, peck ≤1× diameter
• 3
  Stainless 316L0.08–0.15 mm/rev with flood coolant

Runout is the quiet killer on drilled holes. A drill with 0.05 mm of indicated runout will cut oversize by roughly twice that figure. On a Ø6 mm hole with a ±0.05 mm size tolerance, runout alone can eat the whole band. Check runout at the tool tip, not at the holder taper.

Holder choice follows hole depth. For depths up to 3× diameter, a good collet chuck is enough. Beyond 4× diameter, switch to a hydraulic or shrink-fit holder, or reduce the drill to a stub length. Every millimeter of overhang multiplies deflection roughly with the cube of the length, so short tools pay off fast.

Through-spindle coolant changes the picture on deep holes. It clears chips from the bottom instead of pushing them back up the flutes. On 316L and titanium, internal coolant often decides whether a hole is repeatable at all.

Spot drilling is not optional on curved or angled entry surfaces. Without it, the drill tip skates before it bites, and the hole center moves. On a 5-axis setup the entry angle can be normalized by tilting the part, which is often the better fix than adding a spot operation.

• 1
  Check runout at the tipOversize typically runs about twice the indicated runout
• 2
  Shorten overhangDeflection scales roughly with length cubed
• 3
  Spot curved entriesOr tilt the part on a 5-axis table

Flexible drilling wins on hole patterns spread over large parts, on moderate quantities, and on features that need repositioning rather than a dedicated fixture. It loses when the geometry demands something a rotating drill cannot produce.

Holes with tight concentricity to an existing bore, cross-holes that intersect at a controlled depth, and holes on a curved surface at a shallow angle are usually better machined by milling or by interpolation with an end mill. An interpolated hole is slower but it has no drill walk and no chisel edge.

Very small holes push in the other direction. Below roughly Ø1 mm, peck drilling with a micro drill on a flexible machine is fragile; a dedicated high-speed spindle or EDM often gives better process capability. The same is true for holes deeper than 10× diameter in stainless or titanium.

Material hardness sets a ceiling too. Hardened tool steel above about 45 HRC belongs on EDM or on a grinding route, not on a drill that will rub and burn.

Quantity matters as well. One prototype hole pattern is a programming job. A 10,000-part run with six holes each may justify a dedicated fixture and a shorter cycle; the flexible route is not automatically cheaper at volume.

• 1
  Better milledCross-holes with controlled intersection depth
• 2
  Better interpolatedHoles needing tight concentricity to a bore
• 3
  Better on EDMMicro holes and hardened steel above ~45 HRC

A distributor that only stocks tooling cannot tell you why your hole drifted. Ask what happens after the sale. Do they run test cuts, or do they hand you a catalog and a speed chart?

Process capability evidence matters more than a spec sheet. Ask for a first-article report with actual measured hole positions and diameters, not a nominal drawing. We run 100% inspection before shipment, with raw material check, in-process monitoring, and final inspection, and we issue reports on request.

Machine mix tells you what the shop can actually quote. A partner with only three-axis mills will outsource anything needing holes on five faces. Our floor has 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers, with a maximum processing size of 4,000 mm.

Certifications set the paperwork floor for regulated work. ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022 cover quality, automotive, medical devices, and information security respectively. If your program needs one of these, ask before the RFQ, not after.

• 1
  Ask for measured dataFirst-article reports with real hole positions
• 2
  Check machine mixFive-face holes need 5-axis capacity in-house
• 3
  Match certificationsIATF 16949 for automotive, ISO 13485 for medical