Coolant

I have run gun drilling with both oil-based and water-based coolants across hundreds of jobs. The choice between them is not about which is better in general — it is about what matters more for your specific work: tool life and finish, or cost and cleanliness. Here is what I have found. The Core Difference: Lubrication vs Cooling Oil lubricates. Water cools. That simple distinction drives every other difference. A gun drill cutting edge needs lubrication to reduce friction between the tool and the workpiece. Oil does this naturally. Water-based coolants need chemical additives to approach the same level of lubricity. ...

Titanium alloys – particularly Ti-6Al-4V (Grade 5) and Ti-6Al-4V ELI (Grade 23) – are widely used in aerospace, medical, and marine applications. They offer an excellent strength-to-weight ratio and corrosion resistance. Unfortunately, they also happen to be one of the most challenging materials to deep hole drill. I have spent years dialing in titanium drilling parameters for everything from aircraft landing gear components to medical implants. What follows is what I have found works consistently. ...

I got the call on a Tuesday afternoon. Tom from purchasing — I could tell by his voice he was already fed up. “These new gun drills you sent us are garbage,” he said. “We’re barely getting thirty holes per tool. The old ones did sixty easy. I want a refund.” Now, I’ve been around gun drills long enough to know that when someone blames the tool first, the problem is usually somewhere else. A gun drill is a simple thing — a carbide tip brazed to a steel tube with a coolant hole running through the center. When it fails, it’s almost always because something in the system made it fail. But you don’t say that to a customer on the first call. You listen, you apologize, and you offer to come take a look. The customer is never wrong in the first conversation. They’re just describing a symptom. ...

It was a Monday morning in January, and I was already behind before the spindle even turned. The weekend had been rough — my oldest kid had come down with a fever on Saturday, and my wife and I had been up half the night trading off shifts with a thermometer and a bowl of chicken soup. Sunday was a blur of laundry, grocery shopping, and trying to catch up on sleep that never came. I came into work on about four hours of sleep, running on coffee and determination. ...

It was a Saturday night in August, about 11 PM. The shop was running a skeleton crew, just three of us on the floor to keep the machines running through the weekend. I was running a UNISIG BTA system on a job drilling 18mm holes through 700mm of 4140. The job was going well, chips were looking good, and I was on track to finish the batch by Sunday afternoon. ...

What Built-Up Edge Looks Like in Practice Built-up edge in aluminum gun drilling happens when the material welds to the cutting edge instead of flowing away as a chip. The welded material changes the effective geometry of the drill and causes poor surface finish. When I pull a drill out after seeing BUE, the cutting edge has a matte silver buildup that looks like cold-welded aluminum. The bore surface shows scratch marks and torn metal rather than the clean burnished finish I expect from gun drilling. ...

I test coolant regularly because I learned the hard way what happens when you do not. A batch of hydraulic cylinders came out with stained bores because the coolant pH had dropped too low. The coolant had gone acidic and was etching the steel surface. The customer rejected the whole batch — 45 parts scrap at $80 each. That loss was $3,600 from a $50 testing kit I had on the shelf but was not using. I test religiously now. Here is exactly what I use and how often. ...

A batch of hydraulic cylinders came out with a strange discoloration on the bore surface. The dimensions were fine. The surface finish was fine. But the parts looked wrong. The customer rejected the whole batch. I checked the coolant and found the pH had dropped to 7.5. The coolant had gone acidic, which caused it to etch the steel surface. The discoloration was light surface corrosion. The fix was simple. I changed the coolant and added biocide to prevent bacterial growth. The pH returned to 9.5 and the next batch was fine. But the rejected batch was a total loss. The cost of those cylinders was several thousand dollars. ...

Coolant selection determines tool life, surface finish, and hole quality in deep hole drilling more than any other process parameter. The right coolant makes the difference between a 500-hole tool life and a 2000-hole tool life in the same material. Here is what I use and why. Coolant Types Overview Three coolant types cover almost all deep hole drilling applications. Each has trade-offs in performance, cost, and maintenance. Type Lubricity Cooling Cost per Liter Filtration Requirement Best Application Neat oil Excellent Good $3 - $8 10 - 20 μm Gun drilling, BTA, demanding finishes Emulsion (oil-water) Good Excellent $0.50 - $1.50 20 - 50 μm General deep hole drilling Synthetic Moderate Excellent $1 - $3 20 - 50 μm High-speed work, aluminum Oil-Based Coolants Oil-based coolants provide the best lubrication for gun drilling. I use neat cutting oil for all critical gun drilling and BTA jobs. The oil film between the guide pads and the bore wall reduces friction by 30-50% compared to water-based coolants. ...

Here are the cutting parameters I use as starting points for deep hole drilling. These are starting points — adjust based on the specific material, machine, and tooling. I’ve collected these numbers over years of setup work, and they save me a lot of trial and error. Cutting Speed Selection by Material Cutting speed is the first parameter I set. It directly affects tool life and hole quality. Run too fast and the tool edge breaks down. Run too slow and you risk work hardening and poor chip formation. ...