KYOCERA SGS Precision Tools Group
KYOCERA SGS Precision Tools Group
Flat bottom, straight flute, or standard: which drill actually belongs in the spindle?
If you’ve ever chased a burr that shouldn’t be there, watched a drill walk across an angled surface, or heard that sickening “crack” as carbide snaps in a cross-hole, you already know the frustration.
Drilling is often treated as the simplest operation in the shop: pick a diameter, hit cycle start, and move on. But when a hole goes wrong, it does more than scrap a part. It breaks momentum, erodes confidence in the process, and turns what should be a predictable operation into something you babysit instead of trust.
Most of the time, the question of whether a standard, a straight flute, or a flat-bottom drill should have been used only comes up after something has already gone wrong. Scrap is on the table, the machine is idle, and now the fix costs more time than the hole ever should have.
The truth is simple: drill geometry makes or breaks hole quality. The right geometry takes stress off the tool, keeps the cut stable, and lets you focus on the job instead of the drill. When drilling becomes boring, predictable, repeatable, that’s when everything else in the process starts to fall into place.
In this article, we’re going to break down when a standard drill makes sense, when a straight flute drill is the better choice, and when a flat bottom drill is the fix that saves the job. We’ll also cover how geometry affects chip control, burr formation, and tool life, so you can make the right call before the drill ever touches the material and get better holes the first time.
A drill is more than just a sharp point. Geometry includes the point angle, chisel edge, margins, flute shape, and coating, all working together to control how the tool cuts, how chips form and evacuate, and how heat is managed. The chisel edge and web thickness play a major role in thrust force and heat generation, especially in tougher materials. A thicker web increases strength but also increases thrust and heat, which is why geometry choice becomes far more critical in alloys like titanium and Inconel.
In mild steel or aluminum, a standard drill usually gets the job done without much trouble. But once you move into tougher materials like Inconel, titanium, or rough castings, geometry mistakes show up fast. The wrong drill can chatter, drift off center, overheat, or chip, and once a hole goes bad, the next tool often cannot fix it.
Every drilling decision should consider three things:
The standard twist drill, typically with a 118-degree or 135-degree point, is the most common drilling tool in the shop. It self-centers well, runs fast, clears chips efficiently, and performs reliably in straight through holes in aluminum, mild steel, and cast iron, along with many other everyday drilling applications.
For stable setups and straight drilling, a standard drill is often the right tool.
The limitation shows up when conditions change. Drilling into angled bosses, curved surfaces, pipe walls, or cross-holes introduces interrupted engagement and uneven cutting. In those situations, the pointed tip can skate across the surface, lose center, or take an uneven bite as it breaks into a cavity or cross-hole. The result is hole drift, edge chipping, scrap, or time spent correcting a bore that should have been good from the start. And in the worst cases, the drill snaps and leaves broken carbide buried in the hole.
Straight flute drills sit between a standard twist drill and a flat bottom drill, and they exist for one main reason: control.
Unlike a standard drill with a helical flute, a straight flute drill has little to no helix angle. That reduced helix limits the tool’s tendency to pull itself into the cut, which becomes especially important in tougher materials or unstable conditions.
Straight flute drills are commonly used in materials like stainless steels, hardened steels, composites, and other tough or layered materials where excessive helix can create aggressive self-feeding, chatter, or edge chipping. In titanium specifically, straight flute drills are used far less often due to chip evacuation challenges and heat retention, and are typically reserved for very shallow holes or highly controlled applications. They are also well suited for abrasive or brittle materials such as cast iron and glass-filled composites, where a helical flute can accelerate edge wear or promote cracking at entry. By cutting more neutrally, the drill is less likely to pull itself into the cut, pull the part in the fixture, or pull the drill from the holder, which keeps the process predictable and easier to control.
Where straight flute drills really shine:
The tradeoff is chip evacuation. Without a helix to lift chips out of the hole, straight flute drills are not ideal for deep holes. But within their window, they offer excellent stability and edge life.
Flat-bottom drills are designed specifically to eliminate many of the failure modes that show up when standard drills reach their limits, and when even straight flute drills no longer offer enough support or geometry control for the application.
Instead of a pointed cone, a flat bottom drill has a flat cutting face with defined corner edges that slice immediately into the material. When the tool contacts an angled or curved surface, it bites instead of slipping, which dramatically reduces drift and walking.
Flat-bottom drills excel in applications such as:
Many flat-bottom drills also use double margins or three flutes for added support. A margin is the narrow land along the outside diameter of the drill that guides it in the hole and keeps it cutting straight. With double margins, that guiding surface is increased, which stabilizes the tool, improves positional accuracy, and helps it handle interrupted cuts better than a basic twist drill.
A good comparison is drilling titanium. Using titanium Ti-6Al-4V as an example helps illustrate how geometry directly affects cutting parameters and tool behavior across different drill styles.
In titanium, a straight flute drill often makes sense for shallow, controlled holes where you want to minimize tool pull-in and maintain edge stability, especially when hole depth is limited and chip evacuation demands are low.
With a standard two-flute coolant-through drill using a single margin design, typical starting parameters may be around 165 to 170 surface feet per minute with approximately 0.007 inch per revolution on a half-inch diameter tool. The single margin helps reduce friction, while through coolant is critical for flushing chips and controlling heat in this material.
But once the hole geometry becomes more complex, such as intersecting features, angled entry, or a need for a flat floor, a flat-bottom drill becomes the better choice.
In that same titanium application, a flat-bottom drill often requires more conservative parameters, typically closer to 125 to 130 surface feet per minute and around 0.005 inch per revolution. This reduction helps manage the higher cutting forces, protect the corner edges, and maintain accuracy in interrupted conditions. The flat cutting face and added margin support help maintain position and reduce the risk of edge failure in those interrupted conditions.
Now that you’ve seen how different drill geometries behave in the same material, the next step is making sure the setup supports the tool you’ve chosen. Geometry alone won’t save a drill if stickout, runout, or approach strategy are working against it.
Pilot holes become important when drilling deep or machining tough alloys. In modern CNC drilling with solid carbide drills, pilot holes must be approached carefully. Most carbide drills are designed to cut to center, and a large pilot can remove the material needed to stabilize the cutting lips. When a pilot is used, it is often better limited to a shallow spot face rather than a large-diameter pilot hole.
When using a flat-bottom drill on a flat face, a light spot or shallow pilot can help prevent the corners from skating at entry.
Tool stickout should always be minimized. Excess overhang increases deflection, hurts position accuracy, and shortens tool life. A quality collet chuck is a minimum requirement. Hydraulic or shrink-fit holders provide even better support and reduced runout.
Keeping runout under one thousandth of an inch is critical if hole quality and tool life matter, because excessive runout creates uneven loading on the cutting edges and accelerates wear. That same uneven loading is amplified by repeated engagement and disengagement, such as aggressive pecking cycles, which load and unload the drill torsionally and can significantly shorten tool life. Whenever possible, the goal should be to get the drill into the cut, make the hole, and get it back out cleanly in one continuous motion.
Many drill failures trace back to chip packing or excessive heat.
Flat-bottom drills tend to create flatter chips that break more easily, improving both evacuation and exit-condition stability. However, flat-bottom drills often generate higher cutting forces and heat than standard point drills, which is why they are typically run at lower surface speeds to protect the corner edges and manage temperature. With flat-bottom drills specifically, if exit burrs begin to appear late in the tool’s life, especially when they were not present earlier, it is often a sign that the cutting edges are worn and the drill is nearing the end of its usable life. Fewer burrs at exit mean less secondary deburring work later.
For deeper holes, standard drills benefit from pecking cycles to keep flutes clear and edges cool. If pecking is required, staggering the peck depths, for example removing half the depth first and progressively shortening subsequent pecks, helps manage chip load and evacuation more effectively. Once depth exceeds five times the drill diameter, through-coolant designs or high-pressure coolant become extremely valuable.
For shallow to moderate depths in free-machining materials, flood coolant works fine as long as chips are not allowed to pack in the flutes.
Coatings are not cosmetic. They directly affect friction, heat, and tool life.
In aluminum, slick, low-friction coatings help fight built-up edge and chip welding. Uncoated carbide or TiB2-type coatings are commonly used here because they stay sharp, resist material adhesion, and allow chips to release cleanly at higher spindle speeds without sticking to the flutes.
For stainless steels, titanium, and Inconel, heat resistance becomes critical. Coatings based on TiAlN or AlTiN chemistries are commonly used because they improve wear resistance, reduce crater and flank wear, and protect the cutting edge as temperatures rise.
When running dry or with limited coolant, the right coating becomes even more important. Matching the coating to the material and cutting conditions extends tool life and reduces overall cost.
Every drill geometry has a window where it performs best, and problems start when you push a tool outside of that window.
Flat-bottom drills are powerful problem solvers, but they are not universal tools. Once depth exceeds roughly two to three times the diameter without through-tool coolant, chip evacuation becomes more difficult and cutting forces rise quickly. With high-pressure through-coolant, many modern flat-bottom drills are capable of drilling significantly deeper, often five times the diameter or more, depending on tool design and application.
Understanding where each drill excels, whether standard, straight flute, or flat-bottom, prevents pushing a tool beyond what it was designed to do.
Better drilling starts with better decisions before the tool ever touches the material.
Q: Can I interpolate a flat-bottom hole with an end mill instead of using a flat-bottom drill?
A: You can, but it is usually slower and less rigid. Flat-bottom drills are purpose-built for this work and often produce better positional accuracy with less cycle time.
Q: Why does my drill squeal even though my speeds and feeds look correct?
A: Squealing usually points to chip evacuation problems, excess runout, or the drill rubbing instead of cutting cleanly. Verifying holder condition, reducing stickout, and improving coolant delivery often resolves the issue faster than adjusting speeds and feeds.
Q: When should I switch from flood coolant to through-tool coolant?
A: Through-tool coolant becomes valuable as hole depth increases, materials get tougher, or chip evacuation becomes unreliable. It is especially helpful beyond five times the diameter or when drilling heat-resistant alloys.
Q: Is peck drilling bad for carbide drills?
A: Aggressive pecking can shorten tool life by repeatedly loading and unloading the drill. When pecking is necessary, staggered peck depths help reduce wear and improve chip control.
Q: Why do my holes measure oversized even though the drill is new?
A: Oversized holes are commonly caused by runout, excessive stickout, or uneven edge loading. Improving holder quality and reducing overhang often fixes the issue without changing the drill.
Q: Can straight flute drills be used in aluminum?
A: They can be used in specific cases, but chip evacuation is usually better with a helical flute. Straight flutes are chosen for control, not speed, and are rarely the first choice in aluminum.
Q: How do I know when a drill is nearing the end of its life?
A: Signs include rising cutting forces, increased heat, loss of size control, and exit burrs that were not present earlier. Catching these signs early helps avoid broken drills and scrap parts.
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