How to pierce holes on the tilted surface: bits for guide holes and counters
A drill that enters a sloped face walks off center, grabs, and snaps. This guide shows the shop-floor method: flatten a small seat with a two-edge square end mill, then run the guide hole and the counter with bits ground for that job. Read it to pick the right sequence, feeds and bit geometry before the first cut.
In this article
- 1
- 2
- 3
- 4
- 5
- 6
- 7
Key takeaways
Why a drill walks on a tilted surface
A standard 118° twist drill touches a sloped face at one point. The chisel edge lands on the high side, the lip grabs, and the point slides downhill. Deflection of 0.2 to 0.5 mm is common before the drill finds any material to bite. On a 15° slope with a 6 mm drill, that is enough to put the hole outside a ±0.05 mm position tolerance.
The same geometry causes the grab. Only one cutting lip is engaged at the moment of entry, so the axial force pushes the drill sideways instead of into the work. The drill bends, then springs back when the second lip engages. You hear a chirp, and the hole entrance is bell-mouthed.
Harder materials make it worse. In 17-4PH or 4140 at 30 HRC, the drill cannot self-center at all, and the tip chips before the full diameter is in the cut. Aluminium hides the problem because the drill cuts fast enough to look acceptable, then the counter sinks off-axis.
- 1Slope angleAbove 5° from normal, spot the seat first.
- 2Surface conditionCast skin and scale make the drill slide further.
- 3Drill lengthLonger flutes bend more under the same side load.
Which drilling bits suit the guide hole and the counter
The guide hole is cut with a two-edge square end mill, not a drill. Its flat front face and relieved center cut a shallow square seat, typically 0.5 to 1.0 mm deep, with the bottom normal to the hole axis. The two-edge design gives you a true flat bottom instead of the concave center a four-edge cutter leaves.
For the guide hole itself, use a stub-length twist drill, 135° point, split point if the material is stainless or alloy steel. The split point cuts at the center, so it needs less axial force to start. Run the guide hole about 1× the drill diameter deep before switching to the final drill.
The counter bit is where most shops get it wrong. The included angle should sit between 170° and 190°, and the web thickness at the point should be 0.20 to 0.40 times the drill diameter. A center depression of about 0.01 × Ø keeps the point from skidding on the seat you just cut.
Flute geometry matters for chip flow. A groove opening of 75° to 85° clears chips quickly but leaves loose chips that can be dragged back into the cut. A narrower opening bends chips against the flute wall. Pick the wide opening for tilted entries, and use through-coolant or peck cycles to flush.
- 1Spot seatTwo-edge square end mill, 0.5–1.0 mm deep.
- 2Guide holeStub twist drill, 135° split point, 1×Ø deep.
- 3Counter170–190° included angle, web 0.20–0.40 × Ø.
Feeds, speeds and depth control for each operation
Seat cutting with a 6 mm two-edge square end mill in 6061 runs at about 6,000 rpm and 600 mm/min. In 304 stainless, drop to 1,800 rpm and 120 mm/min. Take the full 0.5 to 1.0 mm depth in one pass. Two passes usually mean the second pass finds an off-center seat.
For the guide hole, the surface speed depends on the drill material. HSS in aluminium runs 30 to 60 m/min. Carbide in 4140 runs 60 to 90 m/min. Feed per revolution for a 6 mm drill sits between 0.08 and 0.15 mm/rev in steel, up to 0.25 mm/rev in aluminium.
Peck depth controls chip evacuation on the tilted entry. Use 1×Ø pecks for the first three pecks, then move to 2×Ø. If the drill squeals on the first peck, reduce feed by 30% rather than speed. Speed changes alone do not stop the walk.
The counter runs at roughly 60% of the guide hole speed and half the feed. Spot the counter with a 90° chamfer tool if the print allows, then finish with the 170–190° bit. Measure the seat diameter after the first part; do not trust the tool nominal.
- 1Seat passOne pass, 0.5–1.0 mm deep, no spring pass.
- 2First pecks1×Ø until the full diameter is engaged.
- 3Counter60% speed, 50% feed of the guide hole.
How the material changes the method
Aluminium 6061 and 7075 tolerate a steeper entry. The drill cuts before it can slide far, so a 10° slope sometimes works without a seat. Do not skip the seat on 7075 if the position tolerance is under ±0.05 mm. The higher strength means higher side load when the drill grabs.
Stainless 304 and 316 work-harden in the first 0.1 mm. A drill that rubs instead of cutting hardens the seat surface, and the next pass chips the edge. Use a split-point carbide drill, keep the feed up, and never let the drill dwell on the seat.
Titanium Ti-6Al-4V and Inconel need lower surface speed and higher feed per revolution. Ti-6Al-4V runs at 25 to 40 m/min with 0.08 to 0.12 mm/rev. Inconel runs at 15 to 25 m/min. Both need through-coolant and rigid setups, because any deflection turns into a broken drill.
Plastics like POM and PEEK cut clean but grab on the exit. Back the part with a sacrificial plate and reduce feed at breakthrough. A tilted exit burr on PEEK is hard to remove without marking the surface.
- 1AluminiumForgiving, but check position on 7075.
- 2StainlessNo dwell, high feed, split point.
- 3Titanium and InconelLow speed, rigid setup, through-coolant.
Step by step: piercing holes on the tilted surface
Sequence for a 6 mm hole on a 15° slope in 6061-T6
- 1Clamp and indicate the slopeSet the part so the tilted face is reachable in one setup. Indicate the actual angle; prints are often wrong by 1–2°. Confirm the hole axis is normal to the local face, not the global datum.
- 2Cut the seat with a two-edge square end millUse the smallest cutter that covers the drill diameter, 6 mm here. Run 6,000 rpm and 600 mm/min in aluminium. Depth 0.5–1.0 mm. Check with a pin that the seat bottom is flat and normal to the axis.
- 3Start the guide holeStub 135° split-point drill, 5.8 mm for a 6 mm finished hole. 1,500 rpm, 0.15 mm/rev. Peck at 1×Ø until the full diameter is below the seat. Stop if you hear chirping.
- 4Drill through or to depthSwitch to the full-length drill once the guide hole is established. 1,200 rpm, 0.18 mm/rev. Peck at 2×Ø. Add through-coolant if the depth exceeds 4×Ø.
- 5Chamfer or spot the counterIf the print calls for a flat-head screw, spot with a 90° chamfer tool to break the edge, then follow with the 170–190° counter bit at 60% speed.
- 6Cut the counter170–190° bit, web 0.20–0.40 × Ø. Feed at half the guide hole rate. Stop at depth, do not dwell. A dwell of 0.5 s is enough to rub the seat and burnish it off-center.
- 7Inspect the first partCheck position with a CMM or pin gauge, check seat depth with a depth mic, and check the counter angle with a dial indicator on a known pin. Adjust the next part, not the tool.
Bit geometry: which one to use and when
Match the bit to the operation, not to the drawer
| Operation | Bit geometry | Typical parameter | When it is wrong |
|---|---|---|---|
| Seat cut | Two-edge square end mill | 0.5–1.0 mm deep, one pass | Slope under 5°, no seat needed |
| Guide hole | 135° split point, stub | 1×Ø deep, 0.15 mm/rev | Deep holes over 5×Ø |
| Through drill | 135° point, jobber | 2×Ø pecks, 0.18 mm/rev | Cast skin still present |
| Counter | 170–190° included | Web 0.20–0.40 × Ø | Flat-head angle not 170–190° |
| Deburr | 90° chamfer tool | 0.2–0.3 mm edge break | Counter already breaks the edge |
The rule to remember
Never let a twist drill touch a tilted face. Cut a flat seat first, keep the guide hole at 1×Ø, and match the counter bit to 170–190° with a web of 0.20–0.40 × Ø. That sequence holds position on slopes past 5°, in aluminium, stainless and titanium alike.
Frequently asked questions
Can I spot the hole with a center drill instead of a square end mill?
A center drill has a 60° tip that sits on the slope the same way a twist drill does. It will walk. The only reliable spot is a flat-bottom seat cut with a two-edge square end mill.
If the slope is under 5° and the tolerance is loose, a stub center drill can work. For anything tighter, cut the seat.
How deep should the seat be?
Between 0.5 and 1.0 mm for most holes. The seat only needs to be wide enough to cover the drill diameter and deep enough that the drill lands on a full flat face.
Deeper seats waste time and can break into a thin wall. On a 3 mm wall, keep the seat at 0.5 mm.
Why does my counter come out off-center even after the guide hole?
The counter bit is following the guide hole, but the seat is not normal to the axis. Check the seat with a dial indicator before running the counter.
The other cause is a counter bit with too thick a web. Above 0.40 × Ø, the point cannot self-center and pushes to one side.
What included angle should the counter bit have?
170° to 190°. That range matches common flat-head and oval-head screws. Anything outside it will not seat the screw flush.
Check the print for the screw standard. Some aerospace fasteners use 100° or 82° counters, and those need a different bit.
Should I use coolant on a tilted entry?
Yes, especially in stainless, titanium and Inconel. Through-coolant helps clear chips from the first pecks, which is where the drill is most likely to grab.
In aluminium, flood coolant is enough. Mist coolant works if the peck depth stays at 1×Ø.
How do I check the hole after drilling?
Use a pin gauge for diameter, a CMM or optical comparator for position, and a depth mic for seat depth. Check the counter angle with a dial indicator on a pin of known diameter.
Inspect the first part after each tool change, not just at the end of the run.
Send us the tilted-face part
Upload the STEP file and tell us the slope angle. We return a quotation and a free DFM analysis within 12 hours, with 100% inspection before shipment.
12-hour quote100% inspectionNo minimum order quantity
More machining notes
We publish setup notes, tooling trials and inspection data from the factory floor.
