A deep hole is rarely the finished feature. More often, it is a bore that must accept a bolt head, seal a valve face, or seat a bearing — requiring a counterbore or spot face at the opening, at the bottom, or even on the rear face. These secondary operations must be integrated into the deep hole process, not treated as afterthoughts.
Definitions and Differences
Operation Comparison
| Operation | Purpose | Characteristic |
|---|
| Counterboring | Enlarging a portion of a bore to create a flat-bottomed cylindrical recess | Flat bottom, parallel sides for recessing fastener heads |
| Spot facing | Machining a shallow flat surface around a hole opening | Minimal material removal, creates true seating surface |
| Countersinking | Creating a conical recess for flat-head screws | Angled (typically 82° or 90°), not flat-bottomed |
| Back spot facing | Machining a flat surface on the rear face of a hole | Requires tool that passes through hole and deploys |
When Each Is Used
| Feature | Typical Application |
|---|
| Counterbore (front) | Recessed bolt heads, cap screws, hydraulic port faces |
| Counterbore (deep, stepped) | Bearing seats, seal grooves, multi-diameter bores |
| Spot face (front) | Casting or forging irregularities — providing flat seating |
| Back spot face (rear) | Valve seat faces, mating surfaces on the non-accessible side |
| Back counterbore | Through-hole with recessed back face |
BTA Counterboring Heads
Types and Diameter Ranges
| Type | Diameter Range | Operation | Typical Application |
|---|
| Push counterboring head | Ø44–402 mm | Tool pushes through existing bore | Enlarging bores from the machine side |
| Pull counterboring head | Ø40–254 mm | Tool pulls back through bore | Precision finishing, back-side work |
| Ejector counterboring head | Ø25–300 mm | Dual-tube system | Retrofitted conventional machines |
| Trepanning head | Ø55–610 mm | Annular cut, core recovery | Large-diameter counterbores |
Design Features
| Feature | Function |
|---|
| Indexable carbide inserts | Replaceable cutting edges, grade flexibility |
| Guide pads | Support cutting forces, maintain concentricity |
| Coolant-through design | High-pressure coolant (up to 280 psi / 20 bar) for chip evacuation |
| Thread connection | 1-start or 4-start threads for BTA/ejector system compatibility |
| Pilot/guide section | Aligns with existing bore to maintain concentricity |
Push vs Pull Counterboring
| Aspect | Push | Pull |
|---|
| Direction | Tool feeds forward through bore | Tool retracts backward through bore |
| Chip evacuation | Forward, ahead of tool | Backward, behind tool |
| Concentricity | Good — tool follows existing bore | Excellent — tension pulls tool straight |
| Preferred use | General counterboring | Precision/straightness-critical operations |
Tip: Pull counterboring produces better straightness because the tool is under tension (pulled straight) rather than compression (which can cause the tool to follow the path of least resistance). If the application permits, specify pull counterboring for bores requiring H9 or better tolerance after counterboring.
Back spot facing solves a difficult problem: machining the rear face of a hole without repositioning the workpiece. The tool passes through the hole, deploys a cutting blade, and machines the rear face on the retract stroke.
| Tool | Activation Method | Min Hole Diameter | Max Counterbore Dia | Max Speed (Cutting) |
|---|
| Heule SOLO | Centrifugal ≥ 1,600 rpm | 4.5 mm | 1.95× hole dia. | 900 rpm |
| Heule SOLO L | Centrifugal ≥ 1,600 rpm | 25 mm | 1.95× hole dia. | 900 rpm |
| Heule SOLO2 | Blade extends at rest | 4.5 mm | 2.0× hole dia. | — |
| Heule BSF | Coolant ≥ 20 bar or air ≥ 5 bar | 6.5 mm | 2.3× hole dia. | Normal |
| NOGA UBack | Hydraulic coolant ≥ 6 bar | 8 mm | 2.7× hole dia. | Per chart |
Activation Methods
| Method | How It Works | Advantages | Limitations |
|---|
| Centrifugal force | Spindle speed above threshold extends blades by centrifugal weight; speed reduction retracts them | No external actuator needed | Requires speed change cycle |
| Coolant pressure | Coolant pressure (≥ 20 bar) drives piston to extend blades; pressure release retracts | Reliable, controllable | Requires through-spindle coolant at minimum pressure |
| Compressed air | Air pressure (≥ 5 bar) activates blade extension | Clean, no coolant required | Limited torque capacity |
| Manual | Mechanical collar locks/unlocks blade | Simple, reliable | Requires operator access, slower cycle |
Procedure: Centrifugal Back Spot Facing
- Drill through-hole to required diameter
- Position tool past the rear face of the workpiece
- Increase spindle speed above activation threshold (e.g., ≥ 1,600 rpm for Heule SOLO)
- Reduce spindle speed to cutting range (e.g., ≤ 900 rpm)
- Retract tool (Z+ direction) at cutting feed to machine the spot face
- Increase spindle speed again to retract blades
- Withdraw tool forward through the hole
Procedure: Coolant-Actuated Back Spot Facing
- Drill through-hole to required diameter
- Position tool past the rear face
- Activate coolant at or above minimum pressure (e.g., 20 bar for Heule BSF)
- Retract tool at cutting feed to machine the spot face
- Stop coolant flow to retract blade
- Withdraw tool forward through the hole
Warning: Back spot facing tools require clearance for the deployed blade. Before programming, verify that the tool can pass through the hole in the retracted state and that there is sufficient clearance on the back side for the blade to deploy and cut. Insufficient back-side clearance is the most common cause of tool collision in back spot facing operations.
Standard Counterboring Procedure
| Feature | Guideline |
|---|
| Pilot diameter | Match existing bore diameter within ±0.025 mm |
| Cutter diameter | Per required counterbore diameter (standard or custom) |
| Tool material | HSS for general use, carbide for production/hard materials |
| Coolant | Through-coolant preferred; external flood minimum |
Sequence
- Drill or bore the through-hole to final diameter
- Select counterboring tool with pilot matching the bore diameter
- Insert pilot into bore — alignment by pilot, not by machine positioning
- Set depth stop for counterbore depth
- Feed at recommended speed and feed for the material
- Dwell 1–2 revolutions at full depth to clean the bottom face
- Retract at cutting feed (do not rapid out)
Depth Control
| Method | Accuracy | Best For |
|---|
| CNC programming (Z-stop) | ±0.05 mm | CNC machining centres |
| Mechanical depth stop | ±0.10 mm | Manual machines, drill presses |
| Visual (mark on tool) | ±0.20 mm | Quick setups, non-critical depths |
Speeds and Feeds
Counterboring Cutting Speeds
Counterboring speeds are approximately 2/3 of drilling speeds for the same material:
| Material | HSS Speed (m/min) | HSS Speed (SFM) | Carbide Speed (m/min) | Carbide Speed (SFM) |
|---|
| Low-carbon steel | 25–45 | 80–150 | 60–120 | 200–390 |
| Alloy steel (annealed) | 20–40 | 65–130 | 50–120 | 165–390 |
| Alloy steel (hardened) | 15–25 | 50–80 | 40–80 | 130–260 |
| Stainless steel (304) | 15–30 | 50–100 | 50–100 | 165–330 |
| Grey cast iron | 20–35 | 65–115 | 60–120 | 200–395 |
| Aluminium | 50–70 | 165–230 | 100–160 | 330–525 |
| Titanium alloy | 10–15 | 33–50 | 30–60 | 100–180 |
| Nickel superalloy | 10–15 | 33–50 | 25–40 | 80–130 |
Counterboring Feed Rates
| Counterbore Diameter | Feed (mm/rev) | Feed (IPR) |
|---|
| 6–10 mm | 0.05–0.10 | 0.002–0.004 |
| 10–16 mm | 0.08–0.15 | 0.003–0.006 |
| 16–25 mm | 0.10–0.20 | 0.004–0.008 |
| 25–40 mm | 0.12–0.25 | 0.005–0.010 |
| 40–65 mm | 0.15–0.30 | 0.006–0.012 |
Spot Facing Parameters
Spot facing typically uses the upper end of the counterboring speed range (lighter cut) with reduced feed:
| Operation | Feed Adjustment | Depth of Cut |
|---|
| Counterboring | Full feed | Full counterbore depth |
| Spot facing (front) | 50–70% of counterboring feed | 0.2–1.0 mm |
| Back spot facing | 30–50% of counterboring feed | 0.1–0.8 mm |
Counterbore Dimensions
Metric Socket Head Cap Screws (DIN 974-1)
| Screw Size | Counterbore Ø (mm) | Counterbore Depth (mm) | Close Fit Drill Ø (mm) |
|---|
| M3 | 6.5 | 3.0 | 3.4 |
| M4 | 8.25 | 4.0 | 4.4 |
| M5 | 9.75 | 5.0 | 5.4 |
| M6 | 11.25 | 6.0 | 6.4 |
| M8 | 14.25 | 8.0 | 8.4 |
| M10 | 17.25 | 10.0 | 10.5 |
| M12 | 19.25 | 12.0 | 12.5 |
| M16 | 25.50 | 16.0 | 16.5 |
| M20 | 31.50 | 20.0 | 20.5 |
| M24 | 37.50 | 24.0 | 24.5 |
Inch Socket Head Cap Screws (ASME B18.3)
| Screw Size | Counterbore Ø (in) | Countersink Ø (in) | Close Fit Drill |
|---|
| #10 | 3/8 (0.375) | 0.218 | #5 (0.206) |
| 1/4 | 7/16 (0.438) | 0.278 | 17/64 |
| 5/16 | 17/32 (0.531) | 0.346 | 21/64 |
| 3/8 | 5/8 (0.625) | 0.415 | 25/64 |
| 1/2 | 13/16 (0.812) | 0.552 | 33/64 |
| 5/8 | 1 (1.000) | 0.686 | 41/64 |
| 3/4 | 1-1/8 (1.125) | 0.821 | 49/64 |
Deep Hole Considerations
| Challenge | Consequence | Mitigation |
|---|
| Long tool overhang | Chatter, poor surface finish | Use largest possible pilot diameter; minimise overhang |
| Pilot misalignment | Eccentric counterbore | Ensure pilot-to-bore clearance ≤ 0.025 mm |
| Tool deflection | Tapered counterbore depth | Reduce feed, check tool rigidity |
Chip Evacuation
| Operation | Chip Type | Evacuation Method |
|---|
| Counterboring (through-hole) | Short, broken chips | Coolant pushes chips through the bore |
| Counterboring (blind) | Chips accumulate at bottom | Through-coolant required to flush chips up the flute |
| Spot facing | Fine chips, small volume | External coolant usually sufficient |
| Back spot facing | Fine chips | Chips fall away from tool; coolant clears the face |
Coolant Requirements
| Tool Type | Minimum Pressure | Flow Requirement |
|---|
| HSS counterbore | Flood coolant | Moderate |
| Carbide counterbore | 10–20 bar | Moderate-high |
| BTA counterboring head | 20–100 bar | Q = 4.5 × D (L/min) |
| Back spotfacer (centrifugal) | Flood or through-coolant | Moderate |
| Back spotfacer (coolant-activated) | 20 bar minimum | Through-spindle coolant required |
Spot Facing Design Guidelines
Diameter
| Fastener Type | Spot Face Diameter |
|---|
| Socket head cap screw | 1.5× head diameter |
| Hex head bolt | 1.5–2× head diameter |
| Washer face | Washer OD + 1 mm |
| Flange face | Flange OD + 1 mm |
Depth
| Workpiece Surface | Spot Face Depth |
|---|
| Machined surface | 0.2–0.5 mm |
| Cast surface | 0.5–1.5 mm |
| Forged surface | 0.5–1.5 mm |
| Plate (non-flat) | Up to 2.0 mm |
Tip: The minimum spot face depth should be sufficient to clean up the highest irregularity on the surface. For castings and forgings, inspect the surface condition before setting depth. A spot face that is too shallow will not achieve full contact; one that is too deep reduces the effective clamping length of the fastener.
Edge Distance
| Condition | Minimum Edge Distance |
|---|
| From hole centre to part edge | 1.5× fastener diameter |
| Between adjacent spot faces | 2× spot face diameter |
| From spot face edge to part edge | 1.5 mm minimum |
FAQ
What is the difference between counterboring and spot facing?
Counterboring creates a deep, flat-bottomed cylindrical recess (typically for recessing a bolt head below the surface). Spot facing creates a shallow flat surface around a hole opening to provide a true seating area on an uneven workpiece. Spot facing removes minimal material; counterboring removes significant material.
Heule SOLO (centrifugal activation ≥ 1,600 rpm, cuts at ≤ 900 rpm), Heule BSF (coolant activation ≥ 20 bar), NOGA UBack (hydraulic activation ≥ 6 bar), and Granlund Neptune (spindle start/stop + coolant). All work by passing through the hole in a retracted state, deploying a blade on the back side, and cutting on the retract stroke.
What is the standard counterbore size for an M8 cap screw?
Per DIN 974-1: Ø14.25 mm counterbore diameter, 8.0 mm depth, with a close-fit through-hole of Ø8.4 mm. The counterbore head diameter provides 1–2 mm radial clearance around the screw head.
What cutting speed should I use for counterboring in alloy steel?
For HSS counterbores: 20–40 m/min (65–130 SFM). For carbide counterbores: 50–120 m/min (165–390 SFM). Counterboring speeds are approximately 2/3 of drilling speeds for the same material, because the larger tool diameter generates higher surface speed and the interrupted cutting action of indexable inserts requires reduced speed.
What feed rate is recommended for counterboring?
Feed rates range from 0.05 mm/rev (6 mm counterbore) to 0.30 mm/rev (65 mm counterbore) depending on diameter and material. Use the lower end for hard or stringy materials and the upper end for cast iron and aluminium.
How is a back spot facing operation programmed?
Position the tool past the rear face, activate the blade (by spindle speed increase, coolant pressure, or air), retract at cutting feed to machine the face, deactivate the blade, and withdraw forward through the hole. The cycle uses Z+ feed for cutting, which is the opposite of conventional drilling.
What are the dimensions for standard counterbores?
Metric counterbores follow DIN 974-1. Inch counterbores follow ASME B18.3. Standard dimensions are tabulated in this article for both systems. Always verify against the current standard for the fastener specification being used.
Can counterboring be done on a deep hole drilling machine?
Yes. BTA deep hole drilling machines can be equipped with counterboring heads (push or pull types) for enlarging existing bores. The same coolant and chip evacuation systems are used, making BTA counterboring highly efficient for deep holes requiring stepped diameters.
Coolant-activated back spotfacers require minimum 20 bar (300 psi) through-spindle coolant. Centrifugal-type tools do not require coolant for activation but still need coolant for lubrication and chip evacuation. Always verify the minimum activation pressure for the specific tool being used.
How deep should a spot face be?
Typically 0.2–1.0 mm for machined surfaces, up to 1.5 mm for castings and forgings. The minimum depth should be sufficient to clean up the highest surface irregularity. For back spot facing, shallower cuts (0.1–0.8 mm) are recommended due to reduced tool rigidity.
Conclusion
Counterboring and spot facing extend the functionality of deep hole drilling by adding stepped diameters, recessed fastener seats, and true seating surfaces to the basic through-hole or blind bore. BTA counterboring heads (push types up to Ø402 mm, pull types up to Ø254 mm) handle heavy-duty enlargement operations using existing deep hole drilling machines and coolant systems. Back spot facing tools — centrifugal-activated or coolant-activated — enable rear-face machining in a single setup, eliminating the need for workpiece repositioning. Standard counterbore dimensions are specified in DIN 974-1 and ASME B18.3, while speeds and feeds follow the 2/3 rule relative to drilling parameters for the same material. When integrated into the deep hole drilling process sequence, these secondary operations add capability without the cost of additional setups or dedicated machines.
