Problem Description
Feed marks are visible helical lines that follow the tool advance pattern on the bore surface. They look like thread crests running along the hole wall. I have measured feed marks that were 0.02 mm deep on a 15 mm bore — enough to fail a 1.6 Ra surface finish callout by a wide margin.
Scratches are different. They run axially along the bore and look like the surface was dragged across something rough. Scratches come from chip drag or recirculating debris. I have seen both problems on the same hole, and each needs a different approach. Understanding the distinction is the first step to fixing surface defects in gun drilled holes.
Feed Marks: Causes and Fixes
Feed marks occur when the feed per revolution is too high for the gun drill geometry. Every gun drill has a maximum feed rate based on the tip geometry and the corner radius. Exceed that limit and the drill leaves a ridged surface instead of a smooth bore.
I check the theoretical feed mark height using this formula: feed mark height equals feed rate squared divided by 8 times corner radius. For a 10 mm gun drill with a 0.4 mm corner radius running at 0.05 mm/rev, the theoretical mark height is 0.00078 mm — invisible. At 0.12 mm/rev, that jumps to 0.0045 mm — visible and problematic.
Feed Mark Causes and Parameter Adjustments
Here is a table of feed mark causes and the adjustments I use to eliminate them:
| Cause | Symptom | Adjustment | Expected Result |
|---|---|---|---|
| Excessive feed rate | Even helical marks at feed interval | Reduce feed by 10-20% | Marks disappear or reduce below 0.002 mm |
| Worn drill corner radius | Marks appear gradually over tool life | Replace or regrind drill | Smooth surface restored |
| Incorrect tip geometry | Marks present even at low feed | Verify geometry against manufacturer spec | Correct geometry eliminates marks |
| Built-up edge on cutting edge | Irregular marks with torn surface | Increase cutting speed by 15-20% | Reduced BUE, cleaner surface |
| Inadequate coolant lubrication | Marks with heat discoloration | Increase coolant pressure or change oil type | Better lubrication, cooler cut |
| Vibration from worn bushing | Patterned marks with varying depth | Replace guide bushing | Stable drill, consistent surface |
I reduce the feed rate in 10 percent increments until the marks disappear. The sweet spot is usually 20-30 percent below the absolute maximum feed the tool manufacturer lists. If the tool says 0.08 mm/rev max, I run 0.05-0.06 mm/rev for standard surface finish requirements. This compromise between productivity and quality works well for most jobs.
A worn drill corner radius also causes feed marks. As the tool wears, the corner radius increases and the theoretical finish degrades. If feed marks appear on a job that was running fine with the same feed rate, I inspect the drill corner radius. A radius that has worn from 0.4 mm to 0.6 mm will leave visible marks at the same feed. Replacing the drill or regrinding the corner restores the surface finish.
Parameter Adjustment for Eliminating Feed Marks
The cutting speed also influences feed mark formation. Higher speeds reduce built-up edge and produce a cleaner cut. I increase cutting speed by 15 percent when I see feed marks with signs of material smearing on the bore surface. The combination of reduced feed and increased speed is the most effective parameter adjustment for surface finish improvement.
Here are the recommended parameter ranges I use for different surface finish targets:
| Surface Finish Target | Feed Rate (10 mm drill) | Cutting Speed | Corner Radius | Expected Mark Height |
|---|---|---|---|---|
| Ra 0.8 or better | 0.03-0.04 mm/rev | 80-100 m/min | 0.4-0.5 mm | Below 0.001 mm |
| Ra 1.6 standard | 0.05-0.06 mm/rev | 70-90 m/min | 0.3-0.4 mm | 0.0008-0.002 mm |
| Ra 3.2 acceptable | 0.07-0.09 mm/rev | 60-80 m/min | 0.3-0.4 mm | 0.002-0.003 mm |
| Roughing only | 0.10-0.15 mm/rev | 50-70 m/min | 0.2-0.3 mm | 0.003-0.006 mm |
I always start on the conservative side and increase feed until the surface finish just meets the specification. Running at the lowest possible feed wastes cycle time without improving quality beyond what is needed.
Scratches from Chip Drag
Chip drag scratches happen when the drill retracts and pulled chips scrape against the bore wall. This is most common in deep holes where the chips settle at the bottom after the cut stops. When the drill pulls out, those chips get dragged along the full length of the bore.
I prevent chip drag by clearing the bore before retraction. I add a dwell of 1-2 seconds at the bottom of the hole with the coolant running. This flushes any settled chips past the drill tip. I also increase coolant pressure to 70-80 bar during retraction to keep chips in suspension.
For holes over 300 mm deep, I use a peck cycle. The drill retracts 50 mm every 100 mm of depth, which breaks and flushes chips before they accumulate. The peck adds cycle time but eliminates chip drag scratches entirely.
Tool Condition Inspection Points
A worn or chipped drill produces surface defects that look like a combination of feed marks and scratches. If the cutting edge has a chip 0.2 mm wide, that chip leaves a corresponding groove in the bore wall for the entire cut length.
I inspect the cutting edge under 10x magnification when surface defects appear unexpectedly. Here are the specific inspection points I check:
| Inspection Point | What to Look For | Acceptable Condition | Action if Failed |
|---|---|---|---|
| Cutting edge condition | Chips, cracks, or rounded edge | No chips larger than 0.05 mm | Replace or regrind drill |
| Corner radius | Measured with optical comparator | Within 0.05 mm of spec | Regrind to spec |
| Guide pad condition | Wear or galling on pad surface | No galling, wear within 0.02 mm | Replace drill or regrind pads |
| Bushing ID | Measured with bore gauge | Within 0.03 mm of drill OD | Replace bushing |
| Coolant filter differential pressure | Pressure gauge reading | Below 1 bar differential | Index or replace filter media |
| Coolant tank cleanliness | Visual inspection | No settled chips in tank | Clean tank quarterly |
If the edge is chipped, I replace the drill and send the damaged one for repair or regrind. Continuing with a chipped edge wastes time and material because the defect is guaranteed to worsen.
Bushing condition also matters. A worn guide bushing allows the drill to vibrate, which leaves a patterned surface on the bore wall. I check bushing ID with a bore gauge. If it is more than 0.03 mm oversize, the bushing gets replaced.
Scratches from Coolant Recirculation
Recirculating chips in the coolant system cause random axial scratches. When the coolant filter is bypassing or clogged, chips that should be caught recirculate through the pump and back to the cutting zone. These chips get trapped between the drill body and the bore wall and score the surface.
I check the coolant filter condition whenever I see random scratches. If the filter pressure differential is above 1 bar, the filter media is clogged and may be bypassing. I index or replace the filter and run a flush cycle before the next part.
I also check the coolant tank. If I see chips settled at the bottom, the tank has not been cleaned recently enough. Chips in the tank get picked up by the pump during high-flow operation. I clean the tank quarterly as standard practice. For more on optimizing coolant systems, see my article on deep hole drilling coolant pressure management.
Key Takeaways
- Feed marks come from excessive feed rate or a worn drill corner radius — check both before adjusting parameters
- Use the formula to predict feed mark height and keep it below 0.002 mm for standard surface finish requirements
- Chip drag scratches are prevented with a dwell at depth before retraction and adequate coolant pressure
- Random axial scratches usually mean recirculating chips — check the coolant filter differential pressure
- A chipped cutting edge leaves a full-length groove in the bore wall that cannot be fixed by parameter adjustment
- Inspect the tool cutting edge under 10x magnification when surface defects appear unexpectedly
- Reduce feed by 10% increments until marks disappear, targeting 20-30% below the manufacturer maximum feed rate
- Clean the coolant tank quarterly to prevent chip recirculation through the high-pressure pump