Deep Hole Drilling Tech Blog

The parts came back from quality control with a bright red rejection tag attached. REJECTED — SURFACE FINISH NONCONFORMING. I picked up the tag and read the reason code: “Surface finish exceeds maximum allowable roughness per print specification. Ra 3.2 micrometers maximum required. Measured value per profilometer: Ra 4.1.” I looked at the parts in the red bin. They were a batch of hydraulic manifold blocks — 4140 steel, with a 16mm hole drilled 300mm deep using a BTA drill. I’d run them myself that morning, and I knew the surface finish was good. Not a mirror finish — BTA drilling doesn’t leave a polished bore — but perfectly adequate for the hydraulic application. The customer had been accepting similar finishes from us for years without a single complaint. ...

The job came in on a Wednesday afternoon. Aerospace customer, one of those jobs where the purchase order has more paperwork than the print does. The part was a Inconel 718 flange, about 150mm in diameter and 500mm long. It needed a 6mm through-hole drilled straight through the center. Tolerance on position was 0.1mm at depth, surface finish Ra 0.8. Standard stuff for that industry, except for the material. Inconel 718 is not forgiving. It work-hardens, it retains heat, and it eats cutting tools for breakfast. The recommended cutting speed for deep hole drilling in Inconel with carbide is around 40 meters per minute, which works out to about 2,100 RPM on a 6mm drill. The recommended feed is 0.02 mm/rev, which gives you a penetration rate of roughly 42mm per minute. At that rate, a 500mm hole should take about 12 minutes. ...

We had an order for three hundred hydraulic cylinder rods, 1045 steel, each needing a 10mm through- hole at 600mm depth. It was a repeat job, one we had run maybe five or six times over the past two years. Simple work, good margins, the kind of job that keeps the lights on. The first sign of trouble came when I walked into the tool crib to grab the 10mm gun drills we needed. The drawer labeled “10mm” contained eight drills, but three of them were 10.2mm, one was 9.8mm, and the rest were a mix of diameters that someone had thrown in there because the drawer was open. Of the actual 10mm drills, two had chipped carbide tips that should have been sent out for regrinding weeks ago, and one had a bent shank that you could only see if you rolled it across a flat surface. ...

Problem Description Foaming in high-pressure coolant systems is a persistent issue in deep hole drilling. I have watched a perfectly good 80-bar coolant system turn into a frothy mess that could not clear chips from a 12 mm hole. The air bubbles compress under the high pressure and expand at the nozzle, reducing the effective cooling and lubrication at the cutting zone. When the coolant aerates, chip evacuation is the first thing to fail. ...

I retrofitted automatic lubrication on a deep hole drilling machine that was having way wear problems. The operators were supposed to grease the ways every morning, but it was not happening consistently. Some ways were dry, some were over-greased, and the machine was losing accuracy over time. The automatic system solved all of those problems. Here is what I learned from the retrofit and from installing systems on three machines since then. ...

The Unique Demands of Agricultural Hydraulic Cylinders Agricultural equipment uses large hydraulic cylinders for tractors, combines, and loaders. A typical cylinder might have a 150mm bore through 2 meters of barrel. BTA drilling is the standard method for these large diameters because single-lip gun drills cannot handle the material removal rate required for production efficiency. I have set up BTA drilling cells for three different agricultural equipment manufacturers, and each one had slightly different requirements based on their cylinder designs. ...

Starting deep hole drilling does not require a $200,000 new machine. I have seen small shops enter this field with well under $100,000 and grow their business from there. The key is knowing which equipment to buy, what to look for in a used machine, and how to budget for the supporting tooling and training that make the machine productive. Three Entry-Level Paths I have helped shops take three different paths into deep hole drilling. Each path has different upfront costs, capability limits, and growth potential. The right choice depends on the shop’s existing equipment, budget, and target work. ...

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. ...

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. ...

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. ...