There are three most common threads per inch for a 1/4 tap, including 1/4-20, 1/4-28, and 1/4-32. Below are the recommended drill bit sizes for a 1/4 tap, depending on the type of material and threads per inch percentage you intend to drill.
|Standard Screw Size & Threads Per Inch||75% Thread for Aluminum, Brass, & Plastics – Drill Bit Size||50% Thread for Steel, Stainless, & Iron – Drill Bit Size|
|1/4-20||#7, (dec. eq. 0.201)||7/32, (dec. eq. 0.219)|
|1/4-28||#3, (dec. eq. 0.213)||#1, (dec. eq. 0.228)|
|1/4-32||7/32, (dec. eq. 0.219)||#1, (dec. eq. 0.228)|
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How to Choose the Right Drill Bit
To choose the right drill bit, we first need to dive into the following three fundamental aspects that separate one bit from another, including material, coating, and geometry.
1. Drill Bit Material
Let’s start with the three most common materials from which the drill bit is made.
High-Speed Steel: A high-speed steel bit is the most basic and least expensive drilling material for general use. In addition, it is very forgiving in both press and hand drilling operations. And it can be re-sharpened to extend its durability.
High-Speed Steel with Cobalt: A high-speed steel with cobalt material can hold up better than generic high-speed steel. The cobalt gives high speed more resistance to heat and wear. And these bits can still be quickly re-sharpened, similar to high-speed steel.
Carbide: Carbide drill bits are the most expensive but durable drilling material. There are different grades, with the most expensive drill bits usually offering excellent heat and chip resistance. Carbide also allows coolant to be added through the holes in the drill bit. These through-drill bits are mainly for deeper holes and difficult-to-drill materials. They are generally used with a high-pressure coolant that flows into the tool and flushes chips out much better, keeping the cutting zone cooler while providing additional lubrication to prevent wear.
Although all of these drills can cut a hole in most materials, carbide drill bits will outlast cobalt by ten to twenty times on a rigid CNC machine. In other words, if a cobalt bit cuts one hundred holes, the carbide bit will cut one or two thousand holes before it has to be re-sharpened. That said, a carbide bit can easily cost ten times as much as a cobalt bit, so the investment is much more significant.
Despite its high price, the cost per hole when using carbide is usually the lowest, as it can produce many more holes. In addition, since carbide can run three to five times faster, the decrease in cycle time to produce those holes goes directly to your bottom line.
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2. Drill Bit Coating
Let’s cover five drill bit coatings for proper drilling because this decision can influence performance, including bright finish, black oxide, titanium nitride, titanium carbonitride, and titanium aluminum nitride.
Bright Finish: Bright finish coating is the cheapest option and performs well in specific applications. For example, low-carbon steel and aluminum can be drilled with a bright coating bit, usually without problems.
Black Oxide: Black oxide coating offers an advantage over the bright finish. It has a little more oxidation and additional heat treatment that can provide up to 50% longer service life while decreasing tooling costs.
Titanium Nitride: Titanium nitride is the most common coating. It is an ideal entry-level coating for applications where less heat is transferred to the tool for cutting more rigid materials. Titanium nitride is also distinguished by its bright gold color.
Titanium Carbonitride: Titanium carbonitride coating is a step beyond titanium nitride. It provides a more rigid and better-wearing surface than titanium nitride and is usually bluish or purple.
Titanium Aluminum Nitride: Titanium aluminum nitride coating has a much higher surface temperature than titanium nitride and titanium carbonitride. The gray-colored coating is excellent for high-temperature materials. It is still a good choice for steel and stainless steel. However, due to the aluminum content, it is not a good choice for drilling aluminum.
In addition to these standard coatings, many manufacturers have proprietary coatings that offer high lubricity and a high surface temperature range. Prices vary widely, and you can find drills with high-end coatings at reasonable prices. We’ve already discussed drill bit materials and coatings, so let’s go through the third essential ingredient in choosing the right drill bit: geometry.
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3. Drill Bit Geometry
Geometry will play an equally important role in the performance of drill bits. Perhaps the most apparent aspect of bit geometry is bit length, but there are other aspects, including stub length, flute length, jobber length, point angle, 118-degree point, 135-140-degree point, helix angle, and self-centering point. Drill bits come in two standard lengths, the length of the screw machine, commonly referred to as the stub, and the jobber.
Stub Length: The most common choice when drilling on a CNC is stub-length drill bits because they are more rigid. However, there are all kinds of lengths available for specialized applications. As with any cutting tool, you should use the shortest bit length possible; the shorter the bit, the stiffer it will be. Just ensure sufficient flute length to get the chips out of the hole.
Flute Length: What is the flute length you need for the hole you are drilling? Ideally, when the drill bit is at the deepest point of the hole, the flute length above the hole is twice its diameter. This allows chip evacuation. If the diameter is less than two times, chips can accumulate inside the flutes, causing problems with surface finish, hole size, and straightness; they can even break the drill bit.
Jobber Length: Using a long flute-length drill bit with flutes all the way up is also undesirable if you only drill shallow holes. A long jobber bit will not be as rigid and give less precise positioning of the hole.
Point Angle: This is probably another familiar aspect of drill bit geometry to most people. When drilling metal on a CNC machine, you generally choose between a 118-degree point and a more comprehensive 135- to 140-degree point.
118-Degree Point: It is the most common in general-purpose high-speed steel drills made for cutting mild steel, aluminum, and other soft metals, and it is what is typically found in standard jobber-length drill bits.
135 to 140-Degree Point: It is more typical for stub-length drill bits, CNC machining, and more rigid materials.
Helix Angle: The helix angle of the drill bit must be considered because it is crucial for proper chip separation. Typically the helix in the 30-degree range is used for general purpose drilling in most materials. These will work fine most of the time, and you won’t need to worry about other options. However, suppose your application requires some specialization. In that case, small helix angles below 30 and up to 10 degrees are often selected for more rigid steel and aluminum alloys where good chip evacuation, fracture toughness, and edge strength are essential. On the other hand, larger angles, up to 40 degrees or more, are often used to drill difficult-to-machine materials, such as stainless steel, where low torque and strength help cut these hard rubbery metals. And the last one for the geometry list is a self-centering point.
Self-Centering Point: This is found in many cobalt and carbide drill bits. This eliminates the need for a starter bit and aids in drilling in the true position. Standard high-speed steel bits are usually not self-centering, as it is slower and more expensive to sharpen them with this feature. As a result, they tend to walk or wobble when trying to cut on a flat surface. The more expensive cobalt and carbide drill bits are manufactured with this self-centering point, allowing them to start cutting very quickly.
This virtual self-centering means no need for a hole drilled in place. It’s another way these expensive drill bits can be more productive than other inexpensive bits since not spot-drilling every hole saves a lot of cycle time. We have covered the basic materials, coatings, and geometry. Now let’s get into some specific cutting conditions and application-related recommendations.
Drilling Cutting Conditions and Application Tips
As I mentioned earlier, manufacturers can drill holes in the drill bit so that coolant comes directly to the cutting edge in the hole. This keeps the cutting zone cool and lubricated and greatly aids chip evacuation.
Steel Drill Bits: Typically, steel drill bits without coolant in the tool can only drill two or three diameters deep before having to peck to remove chips and get more coolant into the cutting zone.
Carbide Drill Bits: Good carbide drill bits without tool coolant can drill up to five times the depth of the diameter in carbon steel and aluminum before having to peck. The problem with peck drilling is that most tool wear occurs when the drill bit enters the material. Once the bit is in the cut, wear rates are very low.
So the peck-drilling significantly increases tool wear because you restart the cut several times per hole. You will also spend extra time pecking each hole when the TSC drill bit could do it in a single pass. So for tools, more than five times the depth, particularly when drilling hard or difficult-to-work materials, TSC bits and through bits become necessary.
How to Drill Deep Holes
If you need to drill deep holes, for example, eight times the diameter or more, you will generally need a pilot hole to start drilling. Typically, this is done using a drill bit to cut the hole one and a half times in diameter. Then start the long drill with a twist at about 300-500 rpm and slowly feed it into the pilot hole. Once the primary diameter of the drill bit is in the pilot hole, you can increase the revolutions to full speed and finish drilling to full depth.
Cutting Conditions: When drilling through holes, pay special attention to the material and cutting conditions. Drill bit manufacturers recommend reducing the feed rate before the bit tip pierces the material to avoid chipping and reduce heat in the cut. We want to reduce heat because when the bit bottoms out before going through, the material is fragile, and there is no place for the heat to go.
In order for that last bit of material to harden, the material is heat-treated, and breaking through the heat-treated layer can shorten the life of the drill bit. A 50% reduction in feed rate in the last two millimeters before the bit tip bottoms out usually eliminates this problem.
How Often Should a Drill Bit Be Resharpened
In general, if the holes are within tolerance, i.e., if the wear and chipping are less than half a millimeter of 0.02″, the bit can continue to be used. After that, it is usually time to re-sharpen or regrind. Pay close attention to chipping at the margins of the bit, i.e., if the wear is uniform, it is OK to resharpen.
How to Decide How Much to Spen on Drill Bits
You will often buy drill bits and decide what to spend on a specific job. When choosing, think about whether it is for a short, one-time batch or if it is for a large, recurring job with thousands of parts. Carbide may not be the best investment if you have a short run and can’t spend extra time adjusting cutting parameters. So, high-speed steel or cobalt makes sense in this example.
Remember that starting a business can always start with less expensive bits. Then, if you end up making many of those same parts in the future, you can work with your tooling supplier to find the best tool for the job, whether it’s carbide or high-end cobalt bits.
Carbide: It is much more expensive than the others and less forgiving if used inaccurately.
High-Speed Steel with Cobalt: They are easy to resharpen but do not offer nearly the tool life of carbide tools. Carbides can also typically run much faster.
Coatings: If you are machining complex materials or need long-lasting tools, select high-end coatings.
Geometry: As for the geometry, I’m just touching on some aspects, but keep in mind the material and the cycle time specifications when deciding which way to go with each of these elements.