If you’ve ever stood in a machine shop watching a twist drill punch a hole in a metal workpiece, you know how easy it is for that “straight” hole to veer off at a sloppy angle. As someone who’s spent 12 years selling and servicing twist drills (and troubleshooting bad hole geometries with shop owners across three continents), I’ve lost count of how many times a simple, avoidable mistake led to a hole that was too out of perpendicular for its intended job—whether that’s a valve stem fitting, a truck chassis bolt, or a custom aerospace bracket. A hole that’s even a degree or two off can throw off assembly line fits, weaken structural joints, or lead to material failure down the line. Over the years, I’ve honed a set of practical, science-backed steps that work whether you’re using a $50 hand drill or a $50,000 CNC mill. Let’s break them down, no stuffy engineering jargon—just what I’ve seen fix bad holes time and again. Twist Drill
First, the foundation starts with the twist drill itself, and this is where my work as a twist drill supplier comes in. Most shops grab any drill off the shelf, but not all twist drills are made to maintain perpendicularity. Let’s talk about the core geometry here: the two cutting lips on a twist drill have to be exactly equal in length and angled 118 degrees (the standard point angle) to each other. If one lip is longer than the other, it will cut more material, which makes the drill “pull” to one side as it enters the workpiece—this is called drill wander, and it’s the single biggest cause of non-perpendicular holes. I can’t tell you how many customers come to me with drill bits that have one lip worn down 0.2mm more than the other, and they swear it’s a machine problem. It’s not—it’s a bad drill. When you’re choosing a twist drill for perpendicular holes, look for ones with precision-ground flanks, not just sharpened by hand. Hand-sharpened drills almost always have uneven lips, because it’s impossible to hold two 15-degree flank angles perfectly steady by eye. Also, pick the right drill for the material. A general-purpose twist drill works for mild steel, but if you’re drilling hardened stainless steel, you need a drill with a split point (not a standard chisel point) to reduce the center contact area. The chisel point on a standard drill creates a lot of friction and makes the drill wobble as it starts. A split point (the ones that look like a tiny notch at the drill’s tip) cuts that friction, and it helps the drill bite straight in before it can veer. I always recommend customers test a new drill on a scrap piece of the same material they’re working with: mark a 100mm line, drill the hole, and measure the angle with a digital protractor. If it’s more than 0.5 degrees off, that drill’s not up to the job.
Next up, the setup of the workpiece and the machine. A drill is only as accurate as what it’s mounted in. If your workpiece is sitting on a wobbly workbench or a mill table with a nicked surface, that’s going to throw everything off. Even a 0.1mm gap under one end of the workpiece can tilt it just enough as the drill pushes in to make the hole angle wrong. The fix here is simple: always clamp your workpiece to a rigid, flat surface. For hand drills, use a drill guide—this is a small metal or plastic block with a precisely angled hole that holds the drill steady. I’ve seen guys skip drill guides to save time, but they end up reworking 10 parts because the holes are off. For CNC or manual mills, make sure the workholding vise or fixture is aligned with the machine’s axis. Most shops forget to check this regularly. A quick test: use a dial indicator to touch off the vise’s jaw along the X and Y axes. If the indicator moves more than 0.02mm as you slide it along the jaw, the vise is twisted, and every hole you drill will be off. Another big setup mistake: not using a center punch. Before you drill, make a small indentation where you want the hole. This indentation gives the drill a starting point so it doesn’t skid across the surface at an angle. A lot of drillers skip this because they think the drill’s point will bite right in, but even a sharp drill can slip on a smooth metal surface at the start. That initial slip is often the root of a hole that’s 2 degrees off—once the drill is moving, it’s hard to correct that angle. If you’re drilling thin material, add a backup plate of the same material behind it. This prevents the workpiece from bending as the drill pushes through, which can also tilt the hole.
Then, there’s the drill feed speed and pressure, and this is where most new operators go wrong. I’ve seen someone crank the feed lever as fast as it will go, thinking it’s just like pushing a pencil through paper. Wrong. Feed rate is how much the drill advances with each revolution, and it has to match the drill size, material, and machine. If you feed too fast, the drill can’t cut the material cleanly—instead of shearing small chips, it’s pushing and gouging, which makes the drill bend slightly as it goes. That bend translates to an angled hole. If you feed too slow, the drill overheats, and the edges of the cutting lips wear unevenly, which brings us back to that drill wander problem we talked about earlier. For example, a 10mm twist drill drilling mild steel at 1000 RPM should have a feed rate of about 0.2mm per revolution. I tell my customers to practice on scrap: drill a hole, measure its angle, adjust the feed rate, and repeat until you get consistent results. For hand drills, don’t use your body weight to push the drill—let the drill’s own weight and the machine’s (or your arm’s) steady pressure do the work. Pushing too hard can make the drill flex, and that’s a guaranteed way to get a non-perpendicular hole. Also, don’t withdraw the drill too quickly when you’re clearing chips. Chips can get packed in the flutes, which adds resistance and makes the drill twist. Pull it out slowly, and if you’re drilling deep holes (more than 3 times the drill’s diameter), use a pecking cycle—retract the drill every 5mm to clear chips. I’ve seen shops ruin a $100 part because they skipped pecking and had chips jammed in the drill.
Another often-overlooked step: drill lubrication and cooling. Twist drills don’t just cut metal—they generate a lot of heat at the cutting edges, and that heat makes the drill expand. If one side of the drill is hotter than the other, it expands more, which makes that lip longer and causes the drill to pull to one side. Lubricant cools the drill and reduces friction, which keeps both lips at the same temperature as they cut. For mild steel, use a general-purpose cutting oil. For aluminum, use a synthetic lubricant to prevent clogging. For stainless steel, use a heavy-duty lubricant because it’s more abrasive. I see a lot of shops running dry just to save a few cents on lubricant, but that’s a false economy. A drill that wears out twice as fast because it’s running dry will end up costing you more in bad parts and replacement drills. Also, make sure the lubricant is applied directly to the cutting edges, not just dripped on the surface. If you’re using a CNC, use a through-spindle coolant system that pumps coolant right to the tip of the drill. That’s the most effective way to keep both sides of the drill cool evenly.
Now, let’s talk about post-drill checks, because even if you follow all these steps, things can go wrong. You don’t need fancy lab equipment to check perpendicularity. A digital protractor is enough, or even a straightedge and a feeler gauge for basic checks. Hold the straightedge against the side of the hole and the flat surface of the workpiece, then use the feeler gauge to see how much space is between the straightedge and the wall of the hole. If the space is more than 0.1mm over a 10mm depth, the hole is off. For critical parts, you can use a bore gauge, but that’s only for high-precision applications. The key is to check every 10 to 20 parts, not just the first one. Drill bits wear over time, so even if you start with a perfect drill, after 50 holes, the lips will wear, and you’ll start getting non-perpendicular holes. I always remind customers to resharpen their drills when they notice a drop in performance, and to use a quality sharpening service that calibrates the point angle and lip length. Cheap sharpening will make the drill worse than a dull one.
Let me share a real example to make this concrete. Last year, a customer in Detroit called me panicking—they were drilling engine valve seats for a new line of semi-trucks, and 15% of the parts were being rejected because the holes were off by 1.2 degrees, which meant the valves wouldn’t seal. They were using a budget twist drill, hand-sharpened, and drilling at twice the recommended feed rate. We walked through their process together: first, they swapped to a precision-ground split point twist drill from our line, which had equal-length lips and a consistent 118-degree point angle. Next, we aligned their vise with their mill’s X and Y axes using a dial indicator, clamping the workpiece to a new, flat backup plate. We adjusted their feed rate to 0.2mm per revolution, and started using a cutting oil applied directly to the drill tip. After that, their rejection rate dropped to less than 0.5%—all because they fixed three small mistakes. That’s the thing about perpendicular holes: they’re not magic. They’re the result of small, consistent steps, starting with the right tool.
At the end of the day, ensuring a twist drill makes a perpendicular hole isn’t rocket science, but it does require attention to detail—no cutting corners on drill quality, setup, feed rate, or cooling. As a twist drill supplier, my job isn’t just to sell you a drill bit; it’s to help you get the results you need, whether that’s a perfect hole for a valve seat or a frame bolt. If you’re dealing with inconsistent hole perpendicularity, or if you just want to upgrade your twist drills to get better results, I’d be happy to walk through your process, answer any questions, or help you pick the right drill for your materials and applications.
Center Drills References
- Machinery’s Handbook, 31st Edition. Industrial Press Inc., 2020.
- ASTM Standard B943-15, Standard Practice for Twist Drill Drilling of Metals. ASTM International, 2015.
- Stephenson, David A., and John S. Agapiou. Metal Cutting: Theory and Practice, 3rd Edition. CRC Press, 2016.
- DeGarmo, E. Paul, J. T. Black, and R. A. Kohser. Materials and Processes in Manufacturing, 12th Edition. Wiley, 2011.
Small Craftsman (Shandong) Machine & Tools Co., Ltd.
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