Problem Description
Built-up edge in aluminum gun drilling shows up as a ragged, torn surface finish on the bore wall. I’ve pulled a 10 mm gun drill from a 7075-T6 block and found aluminum welded solid to the cutting edge, changing the effective geometry of the tool. The bore looked like it had been chewed out rather than cut, with surface finish readings jumping from 1.6 Ra to over 6.3 Ra.
BUE also changes hole diameter. I’ve measured bores running 0.05 mm oversized when BUE is present because the welded material forces the drill to cut wider. This pushes the part out of tolerance and adds rework time. On one job running 2024-T3 aluminum, I scrapped eight parts before I identified BUE as the root cause of the dimension drift.
Root Causes
Low cutting speed is the main trigger. Aluminum needs enough surface speed to generate proper chip flow. I run 6061-T6 at 180-200 m/min and 7075-T6 at 200-250 m/min. Below 150 m/min, the chip doesn’t slide off the rake face — it sticks and builds up layer by layer until the cutting edge is completely covered.
Different aluminum alloys have different BUE thresholds. The copper content in 2024 and the zinc content in 7075 affect how the chip behaves at the cutting interface. Alloys with higher alloy content tend to be less sticky but require higher speeds to shear cleanly.
| Material | Minimum Speed for No BUE | Recommended Speed | Feed Range | BUE Susceptibility |
|---|---|---|---|---|
| 6061-T6 | 150 m/min | 180-200 m/min | 0.03-0.06 mm/rev | Moderate |
| 7075-T6 | 170 m/min | 200-250 m/min | 0.03-0.05 mm/rev | Moderate |
| 2024-T3 | 160 m/min | 190-220 m/min | 0.03-0.06 mm/rev | High |
| 5052-H32 | 140 m/min | 160-190 m/min | 0.04-0.07 mm/rev | Low |
| 6063-T5 | 150 m/min | 180-210 m/min | 0.03-0.06 mm/rev | Moderate |
| AlSi10Mg (cast) | 130 m/min | 150-180 m/min | 0.04-0.07 mm/rev | Low |
Coolant lubricity is the second factor. Standard emulsion at 6-7% concentration doesn’t provide enough film strength for aluminum. The chip-to-tool friction is too high and material transfer starts instantly. I’ve tested coolant concentrations from 5% up to 15% and the BUE reduction curve flattens at about 10% — anything above that gives diminishing returns.
Incorrect geometry on the gun drill also contributes. I’ve found that a rake angle of 0 to +3 degrees works for aluminum, but if the regrind service sharpens to a negative rake, the cutting action changes from shearing to plowing. Plowing generates heat and heat causes BUE. I check every reground drill with a comparator before it goes into the machine.
Solutions
First, I bump cutting speed up to at least 180 m/min. On machines that can’t reach that RPM due to spindle limits, I reduce feed rate instead. Dropping from 0.04 mm/rev to 0.025 mm/rev reduces the load per edge and helps the chip break free. The combination of adequate speed and controlled feed gives me the best BUE prevention.
Second, I switch to DLC-coated gun drills. Diamond-like coating has a low coefficient of friction against aluminum — roughly 0.1 compared to 0.4 for uncoated carbide. I’ve seen DLC-coated drills run three times longer than uncoated before BUE appears. The coating prevents the aluminum from bonding to the carbide substrate, which is the root mechanism of BUE formation.
Third, I increase coolant concentration to 10-12%. The higher oil content improves lubricity at the cutting interface. I verify with a refractometer and adjust the mixture ratio before every aluminum job. I also use coolants with EP (extreme pressure) additives designed specifically for aluminum machining — these formulations provide better film strength than general-purpose coolants.
| Fix | Effect on BUE | Implementation Cost | Notes |
|---|---|---|---|
| Increase speed to 180+ m/min | Eliminates up to 70% of BUE | Free | Verify spindle can reach required RPM |
| DLC-coated drill | Reduces BUE rate by 90% | Higher tool cost (2-3x uncoated) | Pays back in tool life on runs over 100 parts |
| 10-12% coolant concentration | Improves lubricity, less friction | Slight consumable increase | Verify with refractometer weekly |
| EP additive coolant | Adds film strength layer | Moderate coolant cost | Best paired with DLC coating |
| Positive rake regrind (0-3 deg) | Ensures shearing, not plowing | Free (specify to regrind service) | Verify with comparator |
| Reduce feed below 0.03 mm/rev | Lowers load per edge | Free | Only when speed is limited |
Coolant Recommendations for Aluminum Gun Drilling
The coolant type matters as much as the concentration. Semi-synthetic coolants with EP additives outperform straight synthetics for aluminum because they provide better boundary lubrication. I use a semi-synthetic at 10% concentration for all aluminum jobs.
For gun drilling aluminum, the coolant has to do three things: lubricate the cutting zone, flush chips through the flute, and prevent the aluminum from welding to the tool. A general-purpose coolant that works for steel may not have the additive package needed for aluminum. I keep a separate coolant sump for aluminum work to avoid cross-contamination with the steel coolant.
Tool Coating Comparison for Aluminum
| Coating | Friction vs Aluminum | BUE Resistance | Relative Cost | Best For |
|---|---|---|---|---|
| Uncoated carbide | 0.40 | Poor | Baseline | Short runs, prototype work |
| TiN (titanium nitride) | 0.35 | Fair | 1.2x | General purpose, limited BUE benefit |
| TiAlN (titanium aluminum nitride) | 0.30 | Good | 1.5x | Higher speed aluminum drilling |
| DLC (diamond-like carbon) | 0.10 | Excellent | 2.5x | Production aluminum, BUE-prone alloys |
| CVD diamond | 0.05 | Excellent | 5x | High-volume, abrasive aluminum alloys |
Quality Inspection for BUE Detection
I inspect every aluminum part for BUE signs during the first-off inspection. I run my fingernail across the bore wall near the exit — if it catches on a rough patch, BUE has formed at some point during the cut. I also check the bore diameter at entry and exit with a bore gauge. A diameter difference of more than 0.02 mm between the two ends indicates that BUE changed the effective cutting size during the hole.
If I find BUE evidence, I pull the drill and inspect the cutting edge before the next hole. Catching BUE early prevents a scrap run and confirms the parameters are correct.
Prevention
I’ve standardized on DLC-coated drills for all aluminum jobs over 100 parts. The extra tool cost pays for itself in reduced scrap and rework. I also set a minimum spindle speed of 5,000 RPM for a 10 mm drill to ensure adequate surface speed. For smaller drills, I calculate the exact RPM needed to hit 180 m/min minimum surface speed.
For coolant, I maintain 10% concentration year-round on machines that run aluminum regularly. I check it weekly and top up with concentrate, not water. Diluted coolant invites BUE and shortens tool life. I also check the coolant pH monthly — if it drops below 8.5, I add a pH booster to maintain the emulsion stability.
For more on BUE causes across all materials, see my guide on built-up edge in gun drilling. I also cover BUE in aluminum from the application side with additional case studies. If you are seeing surface finish problems beyond BUE, check my deep hole drilling surface finish guide for a broader troubleshooting approach.
Key Takeaways
- BUE in aluminum is a speed problem first, a lubricity problem second — address speed before changing coolant
- Run 180-250 m/min surface speed depending on specific aluminum alloy
- DLC coating eliminates most BUE in production runs and pays back in tool life
- Keep coolant at 10-12% concentration with EP additives for aluminum work
- Use a refractometer to verify coolant concentration — don’t guess the mixture
- A drill that has already developed BUE needs replacement — cleaning won’t restore it
- Different aluminum alloys have different BUE thresholds; adjust parameters per alloy