In precision CNC machining, the choice between a flat-bottom drill and an end mill for machining flat-bottomed blind holes directly affects how much material is wasted when machining parts. When working with surfaces that are not smooth, starting holes, or getting a flat bottom in deep holes, picking the wrong tool can cause problems like the tool moving out of place, the hole's diameter changing, and even damage to the spindle.

Testing on the production line showed that using a special flat-bottom drill instead of a vertical end mill with a helical cut can cut the time for machining by 65%. This was tested with 4140 quenched and tempered steel (HRC 35–38). Also, the flatness of the hole bottom can be kept the same every time, with a variation of less than $0.008 mm. This means that there is no need for additional finishing operations.

This guide provides a detailed look at how these tools are designed, the settings used to cut different materials, and how they work in the real world. It also talks about complex scenarios, like deep-hole drilling in stainless steel. You will learn when to use flat-bottom drills and how to improve processes to make end mills more cost-effective.

Geometric Structure: Cutting Edge Distribution and Central Stress

The cutting edge of a flat-bottom drill is designed to be completely horizontal at a 180° angle. To deal with the problem of zero cutting speed at the center, a very small center point (point angle) is usually ground into the center, and its web thickness is greater than that of a standard drill.

This design makes the tool very stiff. When making countersunk holes, the force that pushes the drill bit down is spread evenly across the whole cross-edge. This makes sure the hole stays accurate.

On the other hand, the bottom edge of a flat end mill is designed to provide clearance during side milling. Its bottom edge is usually tilted 1° to 2° towards the centerline, creating a slightly concave shape.

When you drill with an end mill (direct vertical drilling), the material in the center is actually forced out rather than cut away. This creates a lot of friction, which can quickly cause the tool to wear out at the center and leave a small bump at the bottom of the hole.

Drilling Stability on Beveled and Cylindrical Surfaces、

When making holes at an angle on surfaces that are sloped at 15 to 45 degrees, regular drill bits often slip a lot down the slope. Flat-bottom drills, however, make contact with both the high and low points of the sloped surface via their 180° cutting edge, effectively counteracting more than 90% of the sideways component of the cutting force.

When machining these features, set the feed rate to 40% of the normal value. To get the best results, it's best to keep the feed rate very low, about 0.02 mm/rev, during the first phase of penetration.

Once the outer edge of the drill bit has fully gone through the material (usually about 2.0 mm deep), you can gradually speed up the drill. This step-feed logic prevents the cutting edge from becoming damaged by uneven stress distribution.

Comparison of Productivity Between Helical and Straight Drilling

Although end mills are favored for their versatility in the "mill vs. drill" debate, flat-bottom drills are more efficient in pure efficiency tests. When making a hole that is 2 times deeper than the drill bit, a flat-bottom drill moves 3.5 times faster than an end mill's spiral path.

While helical interpolation reduces axial force, it increases the load on the machine tool between the axes. For high-volume B2B orders, every 10-second reduction in cycle time can save tens of thousands of dollars.

If you're using a 10 mm flat-bottom drill to machine 6061 aluminum alloy, set the spindle speed to 4,500 RPM or higher. To get a very smooth surface, use a feed rate of 0.15 mm per revolution.

“Chip Control” in Deep-Hole Machining of Stainless Steel

Drilling flat-bottom holes in 316L or 304 stainless steel is difficult because these materials are sticky and get harder to cut when you try to drill them. If the tool stays at the bottom of the hole for more than 0.5 seconds, the material surface will harden quickly. This will cause the cutting edge to break right away during the next cutting operation.

When working with stainless steel, the flutes of flat-bottom drills must be polished. This process is called "flute polishing." This makes it easier for the chips to move around and prevents them from sticking to the tool body.

For stainless steel holes that are deeper than 3 times the diameter, it is recommended to use tools with nano-coatings (such as AlTiN or TiAlN). These coatings can withstand temperatures above $800°C and keep the cutting edge's chemicals stable.

Industry Insights: The Hidden Value of High-Pressure Internal Cooling Systems (TSC)

When making flat-bottomed blind holes, the biggest danger is "secondary cutting" of the chips. The bottom is flat, so the chips don't slide easily into the chip flutes like they do with regular drill bits.

A spindle-centered coolant system must be installed. It is recommended to maintain the internal coolant pressure between 30 bar and 70 bar.

High-pressure coolant cools the tool and also uses pressure to clean it. It forces fine chips accumulated at the bottom of the hole into the spiral flutes, completely eliminating the risk of tool breakage.

If the machine tool doesn't have its own cooling system, you need to use "chip-breaking cycles" (peck drilling). It is recommended to set the drilling depth per pass to 0.5 × D, followed by a full retraction to clear the chips, using a high-flow external cooling spray.

Rigidity and Vibration Control in Cross-Hole Machining

In complicated parts like hydraulic valve blocks, holes are often made that go through existing oil passages or spaces. This type of cut is the ultimate test of a tool's rigidity.

The short shank design of flat-bottom drills allows them to maintain extremely high concentricity when passing through intersecting areas. On the other hand, when you use an end mill, the balance of forces is immediately upset as the cutting edge goes into the cavity.

This often results in significant burrs at the exit end and leaves noticeable marks on the inside of the bore. It is recommended to reduce the spindle speed by 20% when passing through intersecting areas until the tool re-enters solid material.

The Enemy of Precision: Radial Runout and Clamping Systems

Even if you select a high-quality flat-bottom drill, it will be useless if the clamping system does not work well. When working on precision machining projects, it's best to avoid using ER spring chucks. Instead, opt for hydraulic chucks or heat-shrink chucks.

For flat-bottom drills, the radial runout (the amount by which the drill deviates from a perfect circle shape) must be controlled within 0.005 mm. Flat-bottom drills cut in a symmetrical way. This means that if there is any unevenness, one cutting edge will bear over 70% of the load. This can cause asymmetrical chipping.

If you see spiral patterns that aren't regular at the bottom of a hole that has a flat bottom, or if the hole is wider than .02 mm, you should first check that the machine tool spindle and the chuck are aligned. You shouldn't adjust the cutting speed.

Reference Guide to Actual Machining Parameters for Different Materials

On-the-Job Tips for Extending Tool Life

Don't let the tool stay at the bottom of the hole for too long after it reaches the set depth. Flat-bottom drills have a large contact area, so the frictional heat generated by idling will quickly soften the cutting edge.

The correct procedure is: When you reach the set depth, keep the same spindle speed and retract at twice the rapid retraction speed. This allows the pressure pulse of the coolant to carry away the heat from the bottom of the hole right away.

Check the tool's relief angle often for wear. If a distinct shiny wear band appears on the relief angle, it indicates that the feed rate is too low and the tool is "extruding" (coming out of the mold) rather than "cutting." In this case, you should increase the feed per revolution by 15%.

Production Decisions: Building an Efficient Process Chain

When choosing between milling and drilling, it's important to balance how much you need to produce and how flexible you need to be.

If you're dealing with batch orders of thousands of pieces, investing in high-quality flat-bottom drills can save hundreds of hours of labor. If you're in the prototyping phase, using the helical cutting strategy of end mills can significantly reduce initial procurement costs.

No matter which approach you choose, the key to achieving a 99.9% yield rate is to always focus on controlling chips and achieving precise clamping.

Still struggling to machine flat-bottomed holes in stainless steel with standard end mills? Jimmy Tool’s high-performance flat-bottom drill series, optimized for chamfered holes and deep blind holes, can help you reduce cycle times by 60%.

If you’re dealing with complex intersecting holes or specialty materials, click the button below to send your part drawings to our technical support team. We’ll provide personalized tool selection recommendations based on your machine’s rigidity and the material’s hardness.

FAQ

Q: Can you use a regular end mill to drill a flat hole?

Yes, but drilling straight in like a drill bit is strictly prohibited. The center cutting edge of an end mill is not designed for cutting, and drilling straight down vertically will cause the center to fracture. We recommend using a ramp entry and strictly limiting the helix angle to 1.5° or less.

Q: Why is a flat bottom drill better for counterboring?

Flat-bottom drills offer higher positional accuracy when machining countersunk holes. Thanks to their specialized guide margins, they maintain perpendicularity even on uneven cast surfaces, whereas end mills are prone to developing slight taper in similar operations due to insufficient lateral rigidity.

Q: Mill vs Drill: Which gives a better surface finish at the bottom?

This depends on the machining path. When drilling directly, a dedicated flat-bottom drill produces a smoother bottom surface finish. However, if a vertical end mill is used for the final bottom circular finish—moving laterally by approximately 0.1 mm—a mirror-like Ra value can be achieved.