When a rotary broached form comes out oversize, twisted, or bell-mouthed, the broach usually gets the blame. In practice, the pilot hole is the more likely culprit. The wobbling action described in our complete guide to rotary broaching generates the form with remarkably low cutting forces—but only if the round hole it enters is the right diameter, round, straight, and chamfered correctly. Get the pilot wrong and no amount of speed and feed tuning will save the part.
This guide consolidates the pilot hole rules published by rotary broaching tool and holder manufacturers into one working reference: why the pre-hole size decides success or failure, the +3% hex and +10% square sizing rules, worked calculations you can follow for any form, a size chart for common hexes and squares, chamfer angle and diameter specs, pilot hole quality requirements, thin-wall corrections, and a first-cut checklist. Where a number comes from a manufacturer’s published guidance, we say so; where it is geometry or arithmetic, we show the math.
Why the Pilot Hole Makes or Breaks the Form
A rotary broach does not remove the whole polygon cross section. It shears the corners into a round hole, one small scallop at a time, while the flat sections of the form are meant to be already “there”—defined by the wall of the pilot hole. The pilot diameter therefore decides how much material each corner of the broach must displace, and that single number drives the radial cutting force, the torque reaction through the holder bearings, and the accuracy of the finished form.
Too small, and the broach stops shearing and starts extruding. If the pilot is at or below the across-flats dimension, the broach’s flats are cutting material, not just its corners. Cutting forces spike, the tool can twist in the hole or skid instead of synchronizing, the holder bearings take loads they were never designed for, and in stainless or titanium the rubbing surface work-hardens and destroys the cutting edges. The typical outcomes are a scrapped hole, a chipped broach, or both.
Too large, and the corners never fully form. The polygon’s corners are the deepest material to reach: they sit beyond the pilot hole wall at radius R, and the broach has to grow the form out to that radius. Oversize the pilot and there is nothing left for the flats to define at the thinnest points—worse, on thin-walled parts the wall can spring away and leave the form undersize. Somewhere between “pinched” and “hollow” is the window manufacturers aim for, and it is narrower than most machinists expect.
The Core Sizing Rule: +3% for Hex, +10% for Square
The general shop-floor version of the rule is simply “drill the pilot slightly larger than the across-flats dimension.” Rotary broaching manufacturers publish the precise version. Polygon, a long-established rotary broach maker, recommends sizing the pilot hole at the across-flats dimension plus 3% for hexagonal forms and plus 10% for square forms—numbers the company carries on both its pre-drill guidance and its online pilot hole calculator. Slater, another broach manufacturer, states the floor: the pilot must be at least as large as the across-flats dimension. Treat +3%/+10% as the target and across-flats as the absolute minimum below which you never go.
Why does a square need a proportionally bigger hole than a hex? It is pure corner geometry, and the math is worth internalizing because it explains most of the sizing rules you will encounter:
- For a hexagon with across-flats AF, the corners sit at a radius of AF / (2 × cos 30°) ≈ 0.577 × AF from the center.
- For a square with across-flats AF (the side length), the corners sit at a radius of AF × √2 / 2 ≈ 0.707 × AF.
- With a pilot at +3% (radius 0.515 × AF), each hex corner must cut only about 0.062 × AF of radial depth. With a pilot at +10% (radius 0.55 × AF), each square corner still must cut about 0.157 × AF—roughly two and a half times more stock per corner, spread over a wider 90° corner instead of a 60° one.
In other words, even with its more generous +10% allowance, a square broach works harder per corner than a hex does. That is why manufacturers give squares the larger percentage, and why square forms are the ones that punish an undersized pilot most visibly—with pressure marks, twisting, and rapid edge wear.
Worked Examples and the Pilot Hole Size Chart
The calculation itself takes seconds: pilot diameter = across-flats × 1.03 (hex) or across-flats × 1.10 (square). Two worked examples show the whole method, including the one refinement that matters on imperial hexes.
Example 1: 1/4 in. hex. On nominal across-flats of 0.250 in., the rule gives 0.250 × 1.03 = 0.2575 in. Note, however, that a “1/4 in.” hex broach usually cuts slightly over nominal—a typical 1/4 in. hex broach has an actual cutting size near 0.2525 in. across flats. Running the rule on the actual cutting size: 0.2525 × 1.03 ≈ 0.260 in. A 6.60 mm drill (0.2598 in.) or a letter G drill (0.2610 in.) lands right on it. The lesson: whenever your broach certificate or catalog page states an actual cutting size different from nominal, apply the percentage to that number.
Example 2: 1/4 in. square. With across-flats equal to the 0.250 in. side length, the rule gives 0.250 × 1.10 = 0.275 in. A 7.00 mm drill (0.2756 in.) is an almost exact match. Metric forms work identically: a 6 mm hex needs 6 × 1.03 = 6.18 mm, and a 10 mm square needs 10 × 1.10 = 11.0 mm.
| Hex form (across flats) | Rule | Calculated pilot | Nearest standard drill |
|---|---|---|---|
| 1/8 in. (0.1250) | +3% | 0.1288 in. | #30 (0.1285) or 3.30 mm (0.1299) |
| 5/32 in. (0.15625) | +3% | 0.1609 in. | #20 (0.1610) |
| 3/16 in. (0.1875) | +3% | 0.1931 in. | 4.90 mm (0.1929) or #10 (0.1935) |
| 1/4 in. (0.2500) | +3% | 0.2575 in. / 0.2601 in.* | 6.60 mm (0.2598) or G (0.2610) |
| 5/16 in. (0.3125) | +3% | 0.3219 in. | 8.20 mm (0.3228) |
| 3/8 in. (0.3750) | +3% | 0.3863 in. | 9.80 mm (0.3858) |
| 1/2 in. (0.5000) | +3% | 0.5150 in. | 13.10 mm (0.5157) |
| Square form (across flats) | Rule | Calculated pilot | Nearest standard drill |
|---|---|---|---|
| 1/8 in. (0.1250) | +10% | 0.1375 in. | 3.50 mm (0.1378) |
| 3/16 in. (0.1875) | +10% | 0.2063 in. | 5.20 mm (0.2047) or #4 (0.2090) |
| 1/4 in. (0.2500) | +10% | 0.2750 in. | 7.00 mm (0.2756) |
| 5/16 in. (0.3125) | +10% | 0.3438 in. | 8.70 mm (0.3425) |
| 3/8 in. (0.3750) | +10% | 0.4125 in. | 10.50 mm (0.4134) |
Two practical notes on using the chart. First, these are starting values from the published +3%/+10% guidance—always cross-check your specific broach against its manufacturer’s data sheet, especially on double hex and double square forms. Second, drilling is not your only option for making the pilot: when the calculated size falls awkwardly between standard drills, or when tolerance stack-up matters, bore or ream the pilot to the exact calculated diameter. A reamed pilot also pays for itself in form accuracy, as the next section explains.
Chamfer Specifications: Angle and Diameter
The second half of pilot hole preparation is the lead chamfer at the hole mouth. Guidance published for gear and spline manufacturing calls for a 60° to 90° chamfer at the hole entry, with the chamfer diameter slightly larger than the largest dimension of the broach. For a polygon broach, that largest dimension is not the across-flats—it is the across-corners: about 1.155 × across-flats for a hex and 1.414 × across-flats for a square. For a 1/4 in. hex, the across-corners work out to 0.250 × 1.155 ≈ 0.289 in., so a chamfer opening in the 0.30 in. range satisfies the rule.
The chamfer does three jobs at once. It presents the broach’s leading corner a conical funnel so the tool enters the hole self-centered instead of digging into a sharp edge. It gives the displaced corner material somewhere to flow and shear, which suppresses the burr that would otherwise roll over the hole mouth. And it protects the first few thousandths of the form, which is exactly where a chamferless entry tends to produce a bell-mouthed, oversize start.
How mandatory is it? Manufacturer guidance varies with the application, which tells you the honest answer: it depends. Lakeshore Carbide’s preparation notes call for a pilot hole with a lead chamfer as a requirement, while Tormach’s rotary broaching documentation treats the chamfer as optional but quality-improving. The working consensus for production work is to cut it whenever the part design allows—60° to 90°, opening just over the broach’s across-corners—and to skip it only when a functional edge at the hole mouth genuinely forbids it.
Pilot Hole Quality: Sharpness, Roundness, and Concentricity
Diameter is only half the specification. A rotary broach can only be as round, as straight, and as concentric as the hole it starts from, because the flats of the finished form inherit the pilot hole’s wall directly. Three quality factors decide whether you hold the 0.0005 in.-class accuracy the process is capable of, as covered in the accuracy section of our rotary broaching overview:
- Drill sharpness. A dull drill rubs before it cuts, work-hardens the bore surface (catastrophic in stainless), and leaves a torn, sized-everywhere wall. The broach’s flats then ride an inconsistent surface and the form wanders. Use a sharp, properly pointed drill, and spot or pilot-drill first so the drill starts true.
- Roundness and straightness. An oval or banana-shaped pilot shifts the form’s center and starves individual corners of stock. For tight work, ream or bore the pilot instead of trusting a drill—the wall becomes a reference surface, not a lottery.
- Concentricity with the turning axis. Drill and broach the pilot in the same setup as the OD turning wherever possible. A pilot hole that runs out to the part’s rotation axis produces a form that is geometrically perfect but eccentric to the part—invisible to a go/no-go gauge, obvious on assembly.
It is worth being clear about why the pilot is drilled round at all rather than approached some other way: the round hole is the fastest, cheapest way to remove the bulk of the stock, and the broach only finishes the corners. The trade-offs between making holes by drilling versus broaching—when each process is the right primary operation—are covered in our comparison of broaching vs. drilling.
Thin-Walled Parts: When a Bigger Pilot Backfires
Manufacturers often add a qualifier to the sizing rules: make the pilot as large as the part design allows. That qualifier exists for cutting-force reasons—a bigger pilot leaves less corner stock and lower pressure—and it is excellent advice right up until the wall around the form goes thin. Then it reverses on you.
The failure mode is springback. On a thin-walled section—a fitting nose, a tube adapter, a hollow shaft—the wall flexes away from the shearing corners instead of resisting them, then springs back inward after the broach passes. The finished form measures undersize on across-flats even though the broach itself is in tolerance. Because the broach “sizes” the form only against a rigid wall, no tool change will fix it; the correction is procedural:
- Drop the pilot toward the smaller end of acceptable (closer to the +3% target than to any extra margin) so the corners cut less aggressively.
- Reduce feed per revolution while the corners are forming—our speeds and feeds guide covers the thin-wall parameter corrections in detail.
- If the design allows, machine the thin wall feature after broaching, so the form is cut while the surrounding material is still rigid.
Rotary Pilot Holes vs. Linear Broaching Pre-Holes
Do not carry pilot hole rules between the two branches of broaching—they are sized by different logic. In rotary broaching, the pilot is defined as a percentage over the across-flats dimension of the form (+3%/+10%), because the broach only shears corners into an existing wall and the pilot’s job is to leave the right amount of corner stock.
In linear (conventional) broaching, the pre-hole is matched to the broach’s own diameter envelope: guidance such as Slater’s part-preparation notes states that the pre-hole must be at least the broach’s minor diameter—the smallest diameter of the broach shank that must pass through the hole—so the broach enters freely and is guided by a bushing rather than by the hole wall. The pre-hole there is a clearance and alignment feature; in rotary broaching it is an active part of the form-generation system. This distinction matters most when a shop runs both processes, for example rotary-broaching a hex on turned parts while linear-broaching keyways on the same shaft family. For the linear side of preparation—bushing fit, pre-hole tolerances, and face squareness—see our guide to internal broaching part preparation, and for tooling logistics, the keyway broaching set guide.
First-Cut Setup Checklist
Run every new rotary broaching job through the same sequence and most first-article failures disappear before they reach the part. Everything below assumes the tooling basics from our rotary broaching tools and holders guide—a trued holder with good bearings and a clean broach shank seat:
- Pilot hole diameter. Verify against the +3%/+10% calculation (or the chart above), applied to the broach’s actual cutting size. Confirm the hole with a pin gauge, not by trust in the drill size.
- Lead chamfer. 60°–90° at the hole mouth, opening just over the broach’s across-corners dimension.
- Alignment. Indicate the broach (or holder centerline) to the pilot hole axis; runout of more than a few thousandths of an inch will cut an oversize, bell-mouthed form.
- Entry parameters. Start at low RPM and light feed for the first fraction of engagement until the tool synchronizes with the hole, then ramp to working parameters.
- First cut. Cut one part to full depth and stop. Do not batch on faith.
- Inspect. Check the form with go/no-go plug gauges for the polygon, verify across-flats on the flats as well as corners, and confirm concentricity to the OD if the assembly cares.
- Record. Log the pilot size, chamfer, RPM, and feed that produced a passing part—that combination is your process, and it is the first thing to check when drift appears.
Frequently Asked Questions
Why does a rotary broach need to spin?
The spin is what turns a shaping punch into a cutting tool. Tilted about 1 degree off the hole axis, the broach touches the work on only one corner at a time; as it rotates, successive corners take small scalloping cuts and the form is sheared into existence. Stop the rotation and you lose the scalloping action entirely—the tool would try to displace the whole polygon cross section at once, radial forces would spike, and the result would be an extruded, wrecked hole rather than a cut form. On a lathe the spinning workpiece friction-drives the broach; on a mill the machine spindle drives it. Either way, rotation is the process.
How do you make a pilot hole for rotary broaching?
Drill a round hole sized to the form’s across-flats plus 3% (hex) or 10% (square), using the broach’s actual cutting size if it differs from nominal; ream or bore it when the calculated size falls between drills or tolerances are tight. Add a 60°–90° lead chamfer at the hole mouth, opening slightly larger than the broach’s across-corners. Produce the pilot in the same setup as the OD turning whenever possible so it stays concentric with the part’s rotation axis.
How deep can you rotary broach?
The working rule is roughly twice the diameter of the form—a 1/2 in. hex broach is comfortable cutting a hex about one inch deep. Beyond that, chip evacuation, tool deflection, and friction against the finished form walls all degrade accuracy and surface finish. Deeper forms call for special tooling arrangements or, far more often, linear broaching on a dedicated machine, where form depth is limited only by broach length.
How do you set up a rotary broach?
Mount the holder (lathe turret or machine spindle), verify the broach is seated clean and true, and align the broach axis to the pilot hole within a few thousandths of an inch. Set conservative entry speed and feed, start coolant, and feed the broach into the prepared, chamfered pilot hole until the tool synchronizes, then bring the process up to working parameters. Inspect the first part with go/no-go gauges before releasing the job to production—the checklist above walks the full sequence.
When the Pilot Hole Is Not the Problem
A correct pilot hole solves the most common rotary broaching failures—but it cannot move the process’s physical limits. Forms deeper than the 2×-diameter rule, large across-flats dimensions beyond holder capacity, hard or abrasive materials at production volumes, and multi-form parts where seconds per part decide the contract are all problems a better pilot hole will not touch. Those are the jobs that belong on a dedicated broaching machine, where a purpose-built linear broach cuts the complete form—hex, square, keyway, spline, or irregular profile—in a single stroke, with accuracy held over long engagement lengths and cycle times measured in seconds.
At broachingmach.com we design and build broaching machines and engineer custom broaches for exactly those deeper, larger, higher-volume internal form requirements. If your rotary broaching work is running into depth, size, or throughput limits, send us the part print and target volumes—our engineering team will quote the broaching solution that fits your production.
