Tool offset setting in deep hole drilling is not as simple as touching off an end mill. The drill is long, flexible, and guided by a bushing. The effective cutting position depends on drill overhang, bushing wear, and drill diameter tolerance. Correct offset setting is the difference between a good hole and a scrapped part.
Offset Classification
| Offset Type | What It Adjusts | Units | Typical Range | Effect |
|---|
| Tool length offset (Z-axis) | Drilling depth | mm | 0 to ± 100 mm | Controls final hole depth |
| Tool diameter offset (X-axis) | Hole diameter | mm | 0 to ± 0.50 mm | Controls hole size |
| Tool wear offset | Compensates for drill wear | mm | 0 to ± 0.10 mm | Maintains diameter over tool life |
| Radius compensation (BTA) | Cutter geometry compensation | mm | 0 to ± 0.50 mm | Adjusts for regrind geometry change |
| Entry position offset | Drill entry point | mm | 0 to ± 0.50 mm | Aligns drill with bushing |
Offset Direction
| Machine Configuration | Length Offset Direction | Diameter Offset Direction |
|---|
| Spindle moves (Z-axis) | Positive = drill moves toward workpiece | Positive = larger hole |
| Table moves (Z-axis) | Positive = table moves toward drill | Positive = larger hole |
| BTA with rotating workpiece | Positive = drill advances | Positive = larger hole |
Offset Measurement Methods
Touch-Off Method (Gun Drilling)
| Step | Action | Detail |
|---|
| 1 | Mount drill in spindle or holder | Tighten to specified torque |
| 2 | Jog drill to just above reference surface | Bushing face or workpiece surface |
| 3 | Place feeler gauge (0.10 mm) between drill and surface | Protects both surfaces |
| 4 | Jog drill down until feeler gauge is lightly pinched | Consistent contact pressure |
| 5 | Set Z-axis offset | CNC: measure or offset page entry |
| 6 | Subtract feeler gauge thickness | If feeler = 0.10 mm, subtract 0.10 mm |
Presetter Method
| Step | Action | Detail |
|---|
| 1 | Mount drill in presetter | Use appropriate adapter |
| 2 | Measure drill length | From gauge line to drill tip |
| 3 | Measure drill diameter | At drill tip (cutting edges) |
| 4 | Record offset values | L (length) and D (diameter) |
| 5 | Enter offsets into CNC | Per machine control format |
| 6 | Load drill into machine | Tool changer or manual mount |
Reference Part Method
| Step | Action | Detail |
|---|
| 1 | Drill test hole in reference part | Use known-good parameters |
| 2 | Measure hole depth | Compare to commanded position |
| 3 | Calculate depth offset | Measured depth − commanded depth = offset |
| 4 | Enter offset correction | Adjust tool length offset |
| 5 | Measure hole diameter | Compare to target |
| 6 | Enter diameter offset | Adjust tool diameter offset |
Gun Drilling Offset Setting
Length Offset
| Parameter | Typical Setting | Adjustment Basis |
|---|
| Drill overhang (tip to bushing face) | 3–10 mm | Reduced for small diameters, increased for clearance |
| Reference surface | Bushing face or workpiece face | Must be consistent for all setups |
| Thermal growth allowance | 0.01–0.05 mm per meter of drill length | Add to offset for longer drills |
| Bushing wear compensation | +0.005 mm per 100 holes (typical) | Monitor diameter trend |
Diameter Offset
| Factor | Effect on Diameter | Offset Adjustment |
|---|
| New drill (nominal diameter) | At nominal | Zero offset (baseline) |
| Drill regrind (smaller OD) | −0.01 to −0.03 mm | Add positive offset |
| Bushing wear | +0.005 to +0.020 mm | Reduce diameter offset |
| Coolant pressure change | ±0.002 to ±0.010 mm | Correct with offset |
| Material change | ±0.005 to ±0.020 mm | Re-establish baseline |
Gun Drill Offset Setting Procedure
| Step | Action | Check |
|---|
| 1 | Install drill in holder | Verify runout < 0.015 mm at tip |
| 2 | Measure drill length (gauge line to tip) | Record L measurement |
| 3 | Enter length offset | L = measured length − reference length |
| 4 | Measure drill diameter at cutting edges | Record D measurement |
| 5 | Enter diameter offset | D = (nominal − measured) + wear allowance |
| 6 | Jog drill to bushing face | Verify clearance |
| 7 | Set Z-zero at bushing face | Consistent reference for depth |
| 8 | Drill first hole | Use proven parameters |
| 9 | Measure hole depth | Compare to commanded |
| 10 | Adjust length offset if needed | Depth error = offset correction |
BTA Offset Setting
BTA Drill Head Offset Considerations
| Element | What Affects Offset | Setting Method |
|---|
| Drill head length | Regrind reduces length | Measure from gauge line to cutting edge |
| Cutter diameter | Regrind changes effective diameter | Measure cutter OD |
| Guide pad condition | Worn guides affect hole diameter | Measure guide pad OD |
| Drill tube length | Tube length variation | Measure overall assembly length |
| Thread connection | Make-up affects overall length | Torque to specification before measuring |
BTA Offset Setting Procedure
| Step | Action | Detail |
|---|
| 1 | Assemble drill head to drill tube | Torque to specification |
| 2 | Measure overall length | From tool interface to cutting edge |
| 3 | Measure drill head diameter | Across guide pads (not cutters) |
| 4 | Enter length offset | Total assembly length − reference |
| 5 | Enter diameter offset | Based on target hole diameter |
| 6 | Install assembly in machine | Verify coolant collar alignment |
| 7 | Set Z-zero at bushing or workpiece face | Consistent reference |
| 8 | Drill test hole | Verify depth and diameter |
| 9 | Measure results and adjust | Fine-tune offsets |
Wear Compensation
| Tool Life Stage | Diameter Trend | Offset Adjustment | Frequency |
|---|
| New drill (first 10 holes) | At nominal | Baseline | — |
| Normal wear (10–80% of life) | −0.005 to −0.015 mm | Add +0.001 mm per 10 holes | Every 10 holes |
| Accelerated wear (80–100% of life) | −0.015 to −0.030 mm | Add +0.002 mm per 5 holes | Every 5 holes |
| End of life | Rapid diameter drop | Replace drill | — |
Depth Offset Adjustment
| Factor | Depth Error | Offset Correction |
|---|
| Thermal growth (drill) | Hole shallower by 0.1–0.5 mm | Add negative offset (shorten) |
| Thermal growth (machine structure) | Depth variation | Warm up machine before setting |
| Bushing wear | Depth drift | Monitor and adjust |
| Drill regrind (shorter) | Hole shallower | Reduce length offset |
| Fixture wear | Depth variation | Inspect and replace fixture |
Verification Methods
Post-Setting Verification
| Check | Method | Acceptance | Frequency |
|---|
| Drill length offset | Measure gauge line to tip | Within 0.1 mm of expected | Each tool change |
| Drill diameter offset | Measure cutting diameter | Within 0.005 mm of expected | Each tool change |
| Z-zero position | Jog to reference surface | Touch off — within 0.02 mm | Each setup |
| First hole depth | Measure depth | Within tolerance | First article |
| First hole diameter | Bore gauge | Within tolerance | First article |
Offset Verification Log
| Tool ID | Date | Length Offset | Diameter Offset | Operator | First Hole Result |
|---|
| GD-20-1284 | 2026-06-06 | L = −0.15 mm | D = +0.010 mm | J. Smith | Depth OK, Dia OK |
| BTA-50-221 | 2026-06-06 | L = −0.22 mm | D = +0.015 mm | J. Smith | Depth OK, Dia OK |
| GD-20-1285 | 2026-06-06 | L = −0.14 mm | D = +0.008 mm | M. Chen | Depth OK, Dia OK |
FAQ
Mount the drill in the holder, jog it to just above the bushing face, place a feeler gauge between the drill tip and bushing face, jog down until the feeler is lightly pinched, set the Z offset to zero at that position, then subtract the feeler gauge thickness. The resulting offset accurately positions the drill tip relative to the bushing face — which is the critical reference for depth control.
How do I set diameter offset for deep hole drilling?
Enter the drill diameter offset in the CNC tool offset page. The offset should be based on the measured drill diameter compared to nominal. Set D = nominal − measured diameter. For example, for a 20.000 mm nominal drill measuring 19.990 mm, set D = +0.010 mm. Adjust after first article inspection based on actual hole diameter.
BTA drill heads have multiple cutters and guide pads — the offset affects not just depth and diameter but also how the drill head loads the guides. Incorrect offset can cause uneven guide pad wear, oversize or undersize holes, and straightness problems. BTA offset must account for the entire assembly (drill head + drill tube), not just the cutting tip.
Monitor hole diameter over the tool life. As the drill wears, the hole diameter decreases. Add a positive diameter offset incrementally to maintain the target diameter. Typical adjustment: +0.001 mm per 10 holes for normal wear. Accelerate the adjustment as the drill approaches end of life. Verify every 5–10 holes with a bore gauge and adjust the offset as needed.
The bushing face is the most common and reliable reference point for tool length offset. It is fixed relative to the workpiece, accessible for touch-off, and directly related to the drill entry position. The workpiece face is an alternative reference but may vary between parts. Never use the chuck or spindle face as the primary reference — the drill overhang varies with setup.
Tool offset setting is a precision operation that directly controls hole depth and diameter. Take the time to set offsets correctly at every setup, verify with a test cut, and adjust based on measurement results. A 0.01 mm offset error can produce an out-of-tolerance hole in deep hole drilling. This article reflects industry practice as of 2026.
