Problem Description
Built-up edge forms when workpiece material welds to the gun drill cutting edge. I see this most often on aluminum alloys and low-carbon steel like 1018. The welded material builds up layer by layer, changing the effective cutting geometry. A drill with BUE cuts differently with every pass, and the results are unpredictable.
The results show up immediately. Surface finish jumps from 1.6 Ra to 6.3 Ra or worse. Hole diameter drifts as the buildup changes the drill’s effective size. I’ve measured bores opening by 0.08 mm as BUE accumulated during a single cut. On critical jobs, that kind of variation means the hole is scrap before the drill reaches full depth.
BUE also affects tool life indirectly. As the welded material builds up, it increases the cutting forces on the drill. The extra load accelerates edge wear and can lead to chipping or complete tool failure. I’ve seen drills fail catastrophically because BUE overloaded the cutting edge at the moment the buildup broke loose.
BUE Identification by Visual Appearance
I inspect for BUE by looking at the cutting edge under 20-40x magnification. The appearance of the buildup tells me what material caused it and approximately how long it has been building up. Knowing the visual signature helps me diagnose the root cause faster.
| Material | BUE Appearance | Location on Tool | Typical Shape | How to Confirm |
|---|---|---|---|---|
| 1018 steel | Silver-gray, smeared layer | Rake face near cutting edge | Irregular, layered build-up | Scratch test — comes off as flakes |
| 12L14 steel | Dark gray, thin film | Rake face | Even coating, hard to see | Wipe with acetone — reveals shiny edge below |
| 6061 aluminum | Bright white, soft lump | Rake face and margin | Bulky, irregular | Feels soft, can be scraped with fingernail |
| 7075 aluminum | Light gray, brittle layer | Rake face | Smaller lumps than 6061 | Chips off in small pieces |
| 316 stainless | Dark streak, hardened | Rake face near nose | Thin, difficult to see | Bore finish looks torn, not smooth |
| Brass/copper | Gold-tinted smear | Cutting edge | Very thin, shiny | Usually visible as color change on edge |
Root Causes
Low cutting speed is the primary driver across all materials. Each workpiece material has a minimum speed below which the chip doesn’t shear cleanly. For 1018 steel, that’s about 80 m/min. For 6061 aluminum, it’s 150 m/min. Below those thresholds, friction at the chip-tool interface generates enough heat to weld the chip material to the carbide.
| Material | Speed Below Which BUE Forms | Recommended Speed | Recommended Feed |
|---|---|---|---|
| 1018 steel | 80 m/min | 100-130 m/min | 0.03-0.06 mm/rev |
| 12L14 steel | 60 m/min | 80-110 m/min | 0.03-0.06 mm/rev |
| 6061 aluminum | 150 m/min | 180-220 m/min | 0.03-0.06 mm/rev |
| 7075 aluminum | 170 m/min | 200-250 m/min | 0.03-0.05 mm/rev |
| 316 stainless steel | 50 m/min | 60-90 m/min | 0.02-0.04 mm/rev |
| 4140 steel (annealed) | 90 m/min | 100-140 m/min | 0.03-0.05 mm/rev |
Insufficient coolant lubricity is the second factor. Emulsion at 6% concentration has poor film strength. The boundary lubrication layer breaks down and metal-to-metal contact happens. I run 8-10% for steel and 10-12% for aluminum to maintain a robust lubricant film. For stainless steel, I use oil-based coolant because emulsion doesn’t provide enough lubricity even at high concentrations.
Drill geometry also matters. A gun drill with a negative rake angle or a dull edge is more prone to BUE. The cutting action shifts from shearing to plowing, which generates extra heat and promotes material adhesion. I specify a 0 to +3 degree rake angle for all my gun drill regrinds and reject any regrind that comes back with a negative rake.
Solutions
Raising cutting speed by 20-30% eliminates most BUE cases I’ve encountered. On a job running 1018 steel at 70 m/min, I increased spindle speed to achieve 110 m/min and the BUE disappeared entirely. Surface finish dropped from 6.3 Ra to 1.6 Ra. The fix took five minutes and cost nothing.
For aluminum, I use DLC-coated drills. The diamond-like coating has a friction coefficient of about 0.1 against aluminum, compared to 0.4 for uncoated carbide. I’ve seen tool life double or triple after switching. The coating prevents aluminum from adhering to the carbide surface. For steel jobs, I use TiAlN-coated drills — DLC is not as effective on steel because the coating wears differently against ferrous materials.
Coolant adjustment is the third lever. I measure concentration with a refractometer and add concentrate to reach 10% for aluminum or 8% for steel. I also check that the coolant nozzles are aimed directly at the cutting zone — a misdirected nozzle starves the cutting edge of lubrication. I’ve solved BUE problems just by repositioning a coolant line.
| Solution | Time to Implement | Effectiveness | Best For |
|---|---|---|---|
| Increase speed 20-30% | Minutes | 80% success rate | All materials |
| DLC-coated drill | Tool change | 90% reduction on aluminum | Aluminum alloys |
| TiAlN-coated drill | Tool change | 70% reduction on steel | Steel, stainless |
| Adjust coolant concentration | Minutes | 60% success rate | All materials |
| Positive rake regrind | Next regrind cycle | 50% improvement | Materials prone to BUE |
| Oil-based coolant | System change | 85% reduction on stainless | Stainless, superalloys |
Coating Comparison for BUE Prevention
| Coating Type | Friction Coefficient | Max Operating Temp | BUE Resistance | Best Material Match |
|---|---|---|---|---|
| Uncoated carbide | 0.40 | 800°C | Poor | Short runs only |
| TiN (titanium nitride) | 0.35 | 600°C | Fair | General steel, low demand |
| TiAlN (titanium aluminum nitride) | 0.30 | 800°C | Good | Steel, stainless |
| AlCrN (aluminum chromium nitride) | 0.25 | 900°C | Good | High-temp alloys |
| DLC (diamond-like carbon) | 0.10 | 400°C | Excellent | Aluminum, non-ferrous |
| CVD diamond | 0.05 | 600°C | Excellent | Highly abrasive non-ferrous |
BUE Response Procedure
When I see BUE on a gun drill, I follow a four-step procedure:
Step 1 — Remove the drill and inspect the edge under magnification. Note the appearance, location, and extent of the buildup. This tells me whether it is speed-related, coolant-related, or geometry-related.
Step 2 — Check the actual cutting speed at the tool tip. If the machine is running below the minimum threshold for the material, increase spindle speed first.
Step 3 — Check coolant concentration with a refractometer. If it is below 8%, add concentrate. If the coolant is old or contaminated, change the sump.
Step 4 — If the BUE persists after speed and coolant adjustments, check the regrind quality. A dull or negatively raked edge needs to be re-ground before the drill goes back into service.
For more on BUE prevention specific to aluminum, see my aluminum BUE prevention guide. I also cover coolant additives and their role in BUE prevention for different material types.
Prevention
I track cutting speed as my primary BUE prevention measure. Each job has a minimum speed recorded in the setup sheet. If the machine can’t reach that speed due to spindle limits, I plan for alternative tooling like coated drills or different geometry.
I also inspect the cutting edge under 20x magnification after every regrind. A dull edge from a poor regrind invites BUE immediately. If the edge looks rounded rather than sharp, I send the drill back to the regrind service with a note about the required rake angle.
Key Takeaways
- Low cutting speed is the root cause in 80% of BUE cases across all materials
- Each material has a minimum speed — run 20-30% above that threshold for safety margin
- DLC coating eliminates most BUE on aluminum jobs; TiAlN works better for steel
- Keep coolant at 8-10% for steel, 10-12% for aluminum — verify with refractometer
- BUE has distinct visual signatures by material — learn to identify them under magnification
- A dull regrind guarantees BUE — inspect every edge before running
- Follow a systematic four-step response procedure when BUE appears