Broaching a Hex
Broaching a hex is a precision machining operation used to cut clean, accurate internal hexagonal sockets into blind or through-holes in metal and plastic workpieces. Essential for manufacturing hex-socket drive fasteners, medical orthopedic bone screws, aerospace aerospace linkages, and custom mechanical drive couplings, broaching transforms round pilot holes into crisp six-sided polygonal geometries. Machinists achieve internal hex shapes primarily through two methodologies: high-speed rotary (wobble) broaching on CNC lathes and mills, or linear push broaching on arbor and hydraulic presses.
Rotary (Wobble) Broaching vs. Linear Push Broaching
The premier method for cutting internal hexagons in modern CNC turning centers and milling machines is rotary broaching, historically known as wobble broaching. A rotary broach tool holder mounts the hexagonal cutting tool at a fixed one-degree angular offset relative to the rotating workpiece centerline. As the spinning part contacts the broach, the tool is driven in rotation while its cutting face wobbles in a subtle, synchronized orbital motion.
This wobble action exerts cutting force on only one tiny corner of the hexagon at any given microsecond, peeling micro-chips in a shearing motion akin to an oscillating chisel. Because cutting pressure is concentrated rather than spread across the entire hexagonal perimeter simultaneously, rotary broaching requires only a fraction of the thrust force needed for traditional punch pressing. A 3/8-inch hex can be formed on a standard CNC lathe in under three seconds with zero secondary setups.
Review this comprehensive engineering comparison contrasting rotary broaching against linear push broaching and CNC milling.
| Machining Methodology | Typical Cycle Time | Thrust Force Required | Blind Hole Capability | Best Production Environment |
|---|---|---|---|---|
| Rotary (Wobble) Broaching | 2 to 5 Seconds | Low to Moderate (1,000 - 3,000 lbs) | Yes (Requires chip relief drilled pocket) | High-volume CNC lathe/mill production runs |
| Linear Push Broaching | 15 to 45 Seconds | Very High (8,000 - 20,000+ lbs) | No (Through-holes only; chips push through) | Arbor press / hydraulic press toolroom jobs |
| CNC Thread Milling / Slotting | 45 to 90 Seconds | Minimal (Rotary cutting spindle) | Yes (Cleans chips with coolant flush) | Extremely hard alloys (>45 HRC) or huge hexes |
| Sink EDM (Electrical Discharge) | 10 to 30 Minutes | Zero mechanical cutting force | Yes (No chip relief pocket needed) | Hardened tool steels, tungsten carbide, exotic space alloys |
Rotary broaching on CNC lathes eliminates moving parts to secondary broaching machines, drastically lowering labor costs.
Pilot Hole Sizing, Chamfering, and Chip Relief Pockets
Flawless hex broaching is impossible without precise pilot hole preparation. The diameter of the pre-drilled pilot hole must be slightly larger than the flat-to-flat dimension of the desired hexagon—typically 1% to 3% larger across the flats. This minute over-sizing ensures that the broach cuts only the six sharp corners and flat sides, preventing material galling and reducing machine spindle thrust loads.
Furthermore, a generous lead-in chamfer (typically 90 to 120 degrees inclusive) is mandatory. The chamfer diameter must exceed the point-to-point (across-the-corners) dimension of the hex broach, guiding the cutting tool into the bore without chipped edges. In blind hole broaching, the pre-drilled pilot hole must be drilled significantly deeper than the final required hex depth—providing an empty subterranean cavity for severed metal chips to curl and pack away cleanly without bottoming out the tool.
The following table details recommended pre-drill pilot hole dimensions, chamfers, and chip cavity depths for common hex socket sizes.
| Hex Socket Size (Across Flats) | Pilot Drill Diameter | Lead-In Chamfer Min. Diameter | Min. Chip Relief Depth (Blind) | Recommended Feed Rate |
|---|---|---|---|---|
| 1/8 Inch (0.1250") | 0.128" to 0.130" | 0.150" (45° angle) | Hex depth + 0.080" | 0.001 to 0.002 IPR @ 2,000 RPM |
| 3/16 Inch (0.1875") | 0.191" to 0.194" | 0.225" (45° angle) | Hex depth + 0.120" | 0.0015 to 0.0025 IPR @ 1,800 RPM |
| 1/4 Inch (0.2500") | 0.255" to 0.258" | 0.300" (45° angle) | Hex depth + 0.150" | 0.002 to 0.003 IPR @ 1,500 RPM |
| 3/8 Inch (0.3750") | 0.382" to 0.386" | 0.450" (45° angle) | Hex depth + 0.220" | 0.0025 to 0.004 IPR @ 1,200 RPM |
| 1/2 Inch (0.5000") | 0.510" to 0.515" | 0.600" (45° angle) | Hex depth + 0.300" | 0.003 to 0.005 IPR @ 900 RPM |
Flooding the cut with sulfur-based cutting oil or high-pressure water-soluble coolant evacuates chips and prolongs M2/PM4 tool steel life.
How to Rotary Broach a Hex on a CNC Lathe in 5 Steps
Follow this precision machining setup to program, tool, and broach internal hex sockets on a CNC lathe.
Drill Oversized Pilot Hole with Chip Relief Depth
Drill the pilot hole 0.003" to 0.006" larger than the flat dimension, drilling deep enough to leave a chip accumulation cavity.
Machine a Generous 90-to-120-Degree Chamfer
Turn a lead-in chamfer slightly larger than the across-the-corners dimension of the hex to guide the broach smoothly into the cut.
Mount Rotary Broach Holder in Tool Turret
Install the 1-degree offset rotary broach holder into the CNC turret, verifying spindle center-height alignment with a dial indicator.
Program Spindle RPM and Feed Rate
Spin the workpiece between 800 and 1,800 RPM, rapid the broach to 0.020" before the face, and feed continuously at 0.002 to 0.004 IPR.
Retract Rapidly and Blow Out Chips
Upon reaching the programmed depth, immediately rapid retract the tool out of the bore without stopping spindle rotation and blow out chips.
Frequently Asked Questions (8 Questions Answered)
Q1: What is rotary broaching?
Rotary broaching (wobble broaching) is a machining method where a cutting tool held at a 1-degree angle wobbles in synchronization with a rotating part to shear polygonal shapes with low thrust force.
Q2: Can you broach a hex into a blind hole?
Yes, rotary broaching can form hexagons in blind holes provided the pilot hole is drilled extra deep to leave an empty pocket where chips can safely accumulate.
Q3: Why must the pilot hole be larger than the hex flats?
Slightly oversizing the pilot hole by 0.002" to 0.006" allows the tool to cut only the corners and sides, drastically reducing cutting pressure and preventing tool breakage.
Q4: What happens if you do not chamfer before broaching?
Without a lead-in chamfer larger than the hex corners, the sharp broach tips will crash into the flat face, chipping cutting edges and causing severe misalignment.
Q5: Can you rotary broach stainless steel or titanium?
Yes, using premium powder-metal (PM4) or micro-grain carbide broaches with TiN or TiAlN coatings allows successful broaching of 304/316 stainless and Grade 5 titanium.
Q6: Do you stop the lathe spindle when rotary broaching?
No, the spindle must be rotating at normal cutting speed (800 to 2,000 RPM) during entry and must NOT be stopped while reversing out of the hole.
Q7: What is the difference between push broaching and rotary broaching?
Push broaching uses a long, progressive multi-toothed bar pushed linearly through a through-hole on a press, whereas rotary broaching uses a single-face tool on a lathe.
Q8: How long does a rotary broach tool last?
In mild steel and aluminum, a quality high-speed steel hex broach routinely cuts 5,000 to 15,000+ parts before requiring re-sharpening.
Final Thoughts & Key Takeaways
In conclusion, understanding broaching a hex provides essential clarity, practical strategies, and actionable advice. By incorporating these foundational insights, adhering to verified safety guidelines, and following structured best practices, you ensure reliable, long-term outcomes while preventing common mistakes. Stay informed, consult certified professionals when needed, and maintain consistent quality care.