I was sitting in on interviews for a deep hole drilling position at our shop. We needed someone who could run our BTA machines — not just push the green button, but understand the process well enough to handle the tough jobs, the ones where the parameters aren’t in the manual and you have to figure it out as you go. The pay was good — twenty-eight an hour plus benefits for the right candidate — so we got a lot of applicants. The stack of resumes on the HR manager’s desk was three inches thick.

I’ve always believed that hiring is one of the most important things you can do in a shop. One bad hire can cost you months of productivity, scrap thousands of dollars in material, and drive your good operators crazy. One good hire can elevate the whole team. So I took these interviews seriously, even when the HR manager was rolling his eyes at how long I was taking.

The first few interviews were depressingly similar. Guys who’d run deep hole machines for years but couldn’t explain why they did what they did. They knew the feeds and speeds for common materials by heart, but ask them to troubleshoot a surface finish problem and they’d just shrug. “Change the feed rate, I guess.” Button pushers. Reliable ones, maybe, but not the person you want on a tricky job where the coolant pressure is dropping and you need to know whether it’s the filter, the pump, or a chip blockage.

I was getting frustrated. We’d interviewed eight people and none of them had shown the depth of understanding we needed. My boss was getting impatient. “Just hire the guy with the most experience,” he said. “They’ll learn.”

I didn’t want to settle. So I came up with a question. It wasn’t technical in the traditional sense. It was a scenario question, designed to see how the candidate thinks about the system rather than whether they memorized feeds and speeds.

I tested the question on my coworkers first. The maintenance guy said “clean the filter.” The production manager said “stop and check the tool.” The senior operator said “listen to the machine, it’ll tell you.” Nobody gave me the full answer I was looking for, but the variety of responses told me the question was good — it revealed how people think.

Here it is: “You’re drilling a 25mm hole through a 600mm block of 4340 steel. The part is fixtured, the guide bushing is set, the whip guide is in position, and the coolant pressure is where it should be at sixty bar. You hit the green button and start drilling. The first fifty millimeters go fine. Then you notice the coolant pressure starts climbing — from sixty bar to sixty-five, then to seventy. The chips start coming out smaller and darker than they were at the start. What do you do?”

The answer I got from most candidates was some variation of “I’d stop the machine and check the drill.” That’s not wrong, but it’s not insightful either. It’s the kind of answer you give when you don’t really understand the relationship between coolant pressure, flow rate, and chip formation in deep hole drilling. Stopping and checking the drill is always safe, but it wastes time when the drill is fine and the problem is elsewhere.

Then this one guy, mid-thirties, named Carlos, answered differently.

He said, “I’d check the coolant filter first.”

I sat up straighter.

“Here’s why,” he said. “Climbing coolant pressure with smaller, darker chips means the coolant is getting restricted somewhere between the pump and the cutting zone. The most common restriction in a deep hole drilling system is the coolant filter. Chips are building up on the filter element, reducing the flow area. The pump works harder to maintain flow, so the pressure rises. But the actual volume of coolant reaching the cutting zone drops. That’s why the chips are getting smaller and darker — less coolant flow means less cooling at the cutting edge, which means the chips are hotter and more deformed.”

He was spot on. Climbing pressure with degrading chip quality is almost always a filter clog, not a tool problem. A clogged filter restricts flow, which reduces cooling at the cutting zone, which causes the chips to change. Most operators see pressure climbing and think “more pressure means more coolant flow” — but they’re wrong. With a fixed-displacement pump, climbing pressure with no change in pump speed means flow restriction. It’s basic fluid dynamics.

I hired him on the spot. My boss was surprised I made the decision so fast, but I knew. Carlos had the kind of systems-level thinking you can’t teach.

Carlos worked with us for three years before he moved to a shop in Oklahoma to be closer to family. In that time, he taught me more about deep hole drilling than I’d learned in the ten years before. He had this way of thinking about the process in terms of physics fundamentals — flow rates, heat transfer, chip evacuation dynamics, material science — instead of just memorized parameters. He could look at a worn drill under a microscope and tell you not just that it was worn, but why it wore that way and what needed to change.

That interview question became my standard for hiring deep hole drillers. Here’s what I learned from it: a lot of people can run a deep hole machine, but very few understand it. The difference between a button pusher and a real deep hole driller is the ability to diagnose problems from symptoms. To look at a chip and know what it means. To look at a pressure gauge and know whether the numbers are telling you the truth or lying to you.

I still use that question. I’ve hired three more people based on their answers, and they’ve all worked out. And every time I train a new operator, I make sure they understand the coolant filter first. Not how to change it — that’s the easy part. I mean understand it. Understand that the relationship between pressure and flow in a deep hole coolant system is the key to diagnosing half the problems you’ll ever encounter. Because understanding the filter means you understand the whole system. If you don’t understand the filter, you don’t understand deep hole drilling.

The last time I talked to Carlos, he was running his own department at a shop in Tulsa, doing deep hole work for the oil and gas industry. Six BTA machines, a whole crew under him. I told him he was the best hire I ever made based on a single interview question.

“Not a question,” he said. “A pressure gauge reading.”

That interview changed how our whole shop interviews for skilled positions. We don’t just ask about experience anymore. We ask about scenarios. We present a problem and listen to how they think through it. A guy who says “stop the machine and check the drill” gets a different evaluation than a guy who says “check the coolant filter first and here’s why.” One knows the steps. The other understands the system.

I’ve used that filter question in probably twenty interviews over the years. The answers tell me more about a candidate in thirty seconds than their resume tells me in two pages. If they understand coolant flow and chip evacuation dynamics, they can learn anything else. If they don’t understand the basics of how the system works, no amount of experience with specific machines will make up for it.

Carlos still calls me every few months to catch up. Last time he told me he’d used my interview question to hire his own lead operator. “I changed it a little,” he said. “Instead of coolant filter, I ask about what they’d check if the surface finish degraded after the first hundred parts.”

“What’s the right answer?” I asked.

“Check the guide bushing for wear,” he said. “You taught me that.”

He was right.

That interview changed the way I think about hiring in general. It’s not about finding someone who knows the answers. It’s about finding someone who knows how to think through a problem. You can teach feeds and speeds to anyone. You can’t teach systems thinking. Either someone has that ability or they don’t, and the interview question reveals it.

Every time I hire someone now, I ask a version of that same filter question. And every time the candidate answers well, I think about Carlos standing in the conference room with his calm explanation of coolant flow dynamics. He didn’t just answer the question. He taught me something in the process. And that’s the kind of person you want on your team — someone who makes everyone around them better.

The answer to that question has never changed. It’s always the coolant filter. Climbing pressure with degrading chip quality means restricted flow. Every time. It’s not the drill. It’s not the material. It’s the filter. And the candidates who know that are the ones who understand deep hole drilling at a system level. They’re the ones you hire on the spot.

I think about that interview every time I train someone new. I start with the filter. Always the filter. Because if you understand the filter, you understand that deep hole drilling is a system, not a single operation. And that understanding is the difference between making holes and being a deep hole driller.

That filter question has become my legacy in a way. It’s the question I’ll be remembered for in our shop. And I’m fine with that. Because it’s a good question. It separates the thinkers from the followers. And in deep hole drilling, you need thinkers.

I still call Carlos every few months. He’s running a shop in Oklahoma now, with a crew of fifteen and a reputation for the hardest deep hole jobs in the state. “Still using the filter question?” I asked him recently. “Every time,” he said. “And I still get the same answers.”

I think about that interview whenever I’m struggling with a problem. I ask myself: “What would Carlos check first?” And the answer is always the simplest thing. The filter. The bushing. The coolant. The basics. The problems are almost never complicated. We just make them complicated by ignoring the simple stuff.

The coolant filter question has become part of our shop culture now. When something goes wrong, someone inevitably says, “Did you check the filter first?” It’s become a shorthand for “check the simple things before you blame the complex ones.” And it started with a single interview question.

I still think about Carlos’s answer sometimes when I’m troubleshooting a problem. Climbing pressure, degrading chips — check the filter. It’s never let me down. I’ve taught that to every operator I’ve trained. And now they teach it to their operators. It’s a chain that started with one good interview answer.

Carlos calls me every few months. Last time he told me he’d just hired a new guy who’d given the filter answer during the interview. “I knew he was the one,” Carlos said. “Reminded me of myself.” I told him that was the best compliment you can give someone in this trade.

I still have Carlos’s interview notes in my filing cabinet. I pull them out sometimes when I’m interviewing a new candidate, just to remind myself what a good answer sounds like. It’s not about the right words. It’s about the right thinking. Carlos had the right thinking. And that’s the only thing that matters in this trade. You can teach someone the words. You can’t teach them how to think.

That interview question has probably saved our shop more money than any single piece of equipment. A good hire is worth their weight in gold. And a bad one costs you twice that.