We learned in the previous article that stainless steel is not exactly hard as such, but tough, work-hardening and generates a lot of heat when cut. And that it is these things that make drilling it so difficult.
But the practical question remains: how could you still drill it? And how to do it as efficiently as possible?
In this article, we will go through the best drilling technique for stainless steel and the choices you should make regarding bits, speed and feeds, and procedure. The main points of this article can be summarized as follows: to drill the common austenitic stainless steels efficiently,
- Prefer drill bits with good hot-hardness such as HSS-Co or carbide
- Use very low speed (RPM) to limit tool temperatures
- Use a high feedrate to cut under the work-hardened layer
- Prefer a drill press whenever possible
- Apply oil as cutting fluid
These points and all the options you have are explored in detail below.
Before we start, I should note that there are many different types, alloys, and delivery conditions of stainless steel, which vary widely in their machining characteristics. This article focuses on drilling the most common austenitic 300-series alloys such as the popular AISI/SAE 304 and 316L in annealed or lightly cold-worked state.
If you are interested in drilling stainless steel of other types or conditions, see the section on hardened stainless steel in my follow-up applications article.
Which drill bits to use for stainless steel?
The first thing to choose for drilling stainless steel is the drill bit. This selection can be broken down to three areas: material, type and coating. We will next look at each of these in turn. For those in a hurry, the key points are summarized in the table below.
|Drill bit |
As discussed earlier, the main challenges that stainless steel places on the drill bit material are high cutting pressures and tool temperatures. The key characteristics for the drill bit material are therefore:
- Hardness and hot-hardness: the drill bit material should be hard at both at room and at higher temperatures to resist deformation, fracture and wear
- Toughness: the drill bits should at least moderately tough to resist imperfect cutting conditions or impacts, which are hard to avoid particularly with handheld drilling
- Price: each drill bit has a limited life and should not be too expensive to replace
Based on these considerations, I rank the common drill bit materials for stainless steel as follows:
Drill bits made of cobalt-alloyed high speed steel (HSS-Co) are overall the best choice for stainless steel. They are noticeably harder than standard non-cobalt HSS bits, and can withstand the cutting pressures better.
The better hot-hardness of cobalt-HSS bits is particularly significant as it allows you to cut faster and worry a bit less of keeping the tool temperature to a minimum. Cobalt-HSS bits retain a safe hardness level of 55 HRC up to a temperature of around 1000 F (550 C) instead of the 800 F (425 C) typical of non-cobalt bits. (Bayer&Becherer, 1989)
Cobalt-based HSS bits are also often preferred to the more advanced tungsten carbide bits in drilling stainless steel. While cobalt-HSS bits fall below carbide in hardness and hot-hardness, they have a definite edge over carbide in toughness. Thanks to this, they are less prone to breakage in case of jams and other issues that are all too common with stainless steel. Although more expensive than plain HSS, cobalt-HSS bits are also much more affordable than carbide ones.
Solid carbide or properly tungsten carbide (WC) drill bits are a good second choice for stainless steel. The main advantage of carbide bits is that they are much harder than any type of high-speed steel, and retain their hardness up to much higher temperatures. They allow higher cutting speeds, more productivity, and in most cases longer life. In the odd manual, non-CNC drilling jobs, carbide bits are quite useful as you do not have to worry about speeds, feeds and lubrication quite as much as with HSS bits.
The biggest drawbacks of carbide bits are the low toughness and the high price. Carbide bits are more brittle than most HSS types, and all too easily fractured in case of jamming, intermittent cutting and chatter. Per bit, carbide is often more than five times the price of cobalt-HSS, and the bits are therefore often sold individually rather than in sets. Because of the price and lower availability, carbide drill bits see relatively little use outside professional CNC machine shops.
Carbide bits also allow you to cut rarer high-strength stainless steels much better than other bit types.
Non-cobalt high-speed steel (HSS) bits are the most common general-purpose drill bit type. Against general belief, plain HSS can be used in drilling austenitic stainless steels (e.g. AISI/SAE 304 and 316L). Although certainly not the best choice from a performance point of view, plain HSS bits cut just fine with proper technique. Further, they are cheap, ubiquitous, and may often be the only set you have at hand.
The main challenge in using non-cobalt HSS bits with stainless steel is their low hot-hardness. They are well hard enough for cutting the tough, work-hardening stainless steels at room temperature, but as the cutting edge temperature rises above 800 F (425 C) their endurance starts to be questionable. (Bayer&Becherer, 1989)
When drilling stainless steel with plain HSS bits, it is important to keep the cutting edge temperature down. Use very low speed, a high feed and lubricate the cut with oil.
In addition to the bit material, you also have different drill bit types to choose from:
- Twist drill bits
- Step drill bits
- Hole saws
Each of the bit types has a its own hole size and depth range, materials selection and price. The properties of the drill bit types and materials are discussed in the following sections.
Twist drill bits are the “standard” drill bits; they are probably the first thing that comes to your mind for drilling metal. Twist drill bits are the most common and the cheapest bit type, and available most materials and coatings.
Twist drills are usually the best choice for small holes under ½” in diameter into stainless steel. They are also the only choice for deep holes, the other common bit types being restricted to short holes only.
However, as the diameter increases above ½”, the twist bits soon get large, heavy and expensive, and will not fit into most drill chucks. For this reason, they are not very cost-efficient for large and short holes, like those into stainless steel sheet.
Twist bits vary in their tip geometry and ease of starting into stainless steel. As a general rule of thumb, be prepared to drill a pilot hole for all twist bits larger than 1/4”.
Step drill bits are a flexible alternative for drilling both small- and large-diameter holes into stainless steel sheet, sheet products and thin-walled tubes and profiles. They offer a wide range of common hole sizes in a single bit, extending well beyond the typical twist drill sizes. Models run from smallest hole size of 3/16” to a largest sizes of ½”, 7/8” or 1-3/8” and have capacity for up to 1/8”, 3/16” or 1/4” thick sheet.
Step drills are almost exclusively made of plain non-cobalt high-speed steel (HSS), and vary mainly in their coating and geometry. As stainless steel is quite tough on the HSS, you should prefer titanium nitride (TiN) or black oxide coated models to help reduce the cutting temperature. Limiting the speeds and using cutting fluid are highly recommended.
Step drills vary in their ease of starting. While some premium models may have a high-performance tip, many versions may require you to drill a pilot hole with a small HSS or HSS-Co twist drill bit.
Hole saws can be used to drill the largest of holes into stainless steel, which are outside the range of either twist drills or step drills. These saws are available up to at least 6” diameter and 2” of drilling capacity, although the full range and depth is rarely needed with stainless steel.
Hole saws are mostly available in two constructions: plain carbon steel and bi-metal. The bi-metal versions combine a carbon steel body with high-speed steel teeth, and are preferred with stainless steel due to the improved hot-hardness. Low cutting speeds and use of cutting fluid are still recommended to keep the cutting temperature down.
Some manufacturers have also introduced hole saws with carbide teeth, which, if available, are an excellent choice for stainless steel. As with twist drills, carbide allows for higher speeds and longer tool life, even without cutting fluid.
Compared to twist and step bits, hole saws are less susceptible to jamming during breakthrough. There is a still some risk of the saw binding, and you should always use the drill clutch to avoid possible kickback.
Apart from the bit material, coatings on the drill bit body may improve its performance with stainless steel. Coatings help by lowering frictional heat generation and cutting temperatures, making it easier for the drill bit to retain its hardness. They also help to reduce chip adhesion onto the tool and the development of a built-up edge.
Titanium nitride (TiN)
Titanium nitride (TiN) is a common drill bit coating that performs very well with stainless steel. Recognizable by its golden tint, this coating is very hard and durable, and excellent in reducing friction and chip adhesion. There are some indications that TiN-coated HSS would clearly outperform solid carbide in tool life in machining stainless 304 alloy. (Bayer&Becherer, 1989)
TiN-coated high-speed steel drill bits are sometimes wrongly advertised as “titanium bits”. This is a misconception, as the drill body material contains no titanium, and the thin titanium nitride coat is actually a ceramic compound with properties very different from those of metallic titanium.
A titanium nitride coating can be applied on all HSS, HSS-Co and carbide bits. Both twist and step drill bits are commonly available in a titanium nitride coated versions. For step drills, TiN is in fact probably the most common delivery condition.
Black oxide is probably the most common coating for HSS twist drill bits, and brings some advantage over uncoated bits with stainless steel. This coating, really an iron oxide (Fe3O4) relatively close to the red iron oxide (Fe2O3) we know as rust, is easily identified from the matte black appearance it gives to the drill bit surfaces. Just like titanium nitride, a black oxide coating will help to reduce friction, temperature and wear on the bit. It is generally considered slightly worse than a TiN coating from a performance point of view, although data is hard to come by.
In addition to twist drills, a black oxide coating is common also on step drills, and may be seen on some hole saws. However, the coating can be applied only on HSS and HSS-Co bits, and not on carbide.
Titanium aluminum nitride (TiAlN)
Titanium aluminum nitride (TiAlN) is a rare high-end coating similar to TiN, but with even better performance. This coating has remained relatively rare on twist drills outside professional production, but has recently been introduced to step drills. TiAlN-coated bits work very well for stainless steel, with high price remaining as the only drawback.
Speeds and feeds
In general, stainless steel should be drilled with a slow speed and a high feed. To achieve this, you should set the rotation speed of your drill press as low as possible, and press the bit against the workpiece with substantial force. Let’s take these in turn:
The low cutting speed is essential for keeping the cutting edge temperature down. It is particularly important with plain HSS drill bits, whose hot-hardness leaves a lot to be desired. Carbide bits, on the other hand, are not equally critical in this respect.
A typical recommended cutting speed value for 300- and 400-series stainless steels with HSS tools is 40 surface feet per minute (SFM), or 12 m/min. This is a very low cutting speed; for comparison, the recommended speeds for mild steel are more than double and those for aluminum and brass more than ten times this value.
What does the 40 SFM mean in practice? The table below answers the question. To achieve this cutting speed in drilling, you will use a certain RPM on your drill, which depends on the drill diameter. The table indicates the RPMs for common drill sizes corresponding to cutting speeds around 40 SFM.
|Drill diameter||Spindle speed|
[rpm] @ 30…50 SFM
When drilling with a drill press, the RPM is usually easy to select and hold steady. You may have some issues with large bits on small benchtop machines, which do not often have the the gears to go below 500 or 700 rpm. Handheld drivers, on the other hand, typically have enough RPM range, but holding a low RPM stable is often difficult.
Note that the cutting speeds and RPMs here are rough guidelines, not exact values. The cutting speed recommended for stainless steel is in fact more often a range, such as 30–50 or 30–70 SFM, not a single value, and varies from source to source. Just get the RPM into the correct range, drill, and adjust as you see fit.
Also note that it is always OK to deviate downwards from the recommended speed. The speeds as you find them in machining handbooks have actually been determined to give a 45-minute tool life in continuous production use, and are a compromise between productivity and tool life. If you want to save your bits, run slow and cool.
While RPM and even cutting speed are probably familiar to many, the concept of feedrate in drilling is not so often encountered.
The feedrate, or just feed, means the speed at which your drill bit descends downwards into the material. Measured in inches or millimeters per revolution (ipr or mm/rev), the feedrate is easily understood as distance advanced downwards in one revolution of the spindle. In drilling, the feedrate alone determines the depth of cut, which is exactly one half of the feedrate with the common two-fluted drill bits.
Why do you need high feed with stainless?
Stainless steel should be drilled with a high feedrate, so that your drill bit cuts deep. Why is this? If stainless steel is so hard to drill, wouldn’t you want to be careful and cut light?
The truth is that with stainless steel, a high feedrate actually lowers the cutting pressures and makes the cut easier. This counter-intuitive result is a consequence of work-hardening: the cutting process hardens not only the chip that is released, but also a thin material layer on the workpiece surface.
Now, if your feedrate is low, the next bit flute must make its cut into the work-hardened layer left by the preceding flute. This results in a very high cutting pressure. If your feed is high, on the other hand, you may be able to cut under the hardened layer, where the material is softer.
Which feedrate is enough for stainless?
So how much feed should you exactly be using? What counts as a high feedrate in drilling stainless steel?
For a solid reference, we can look at the famous Machinery’s Handbook (Oberg, 2016), which gives a nominal recommended feedrate of 0.015 in/rev (0.38 mm/rev) for most stainless alloys in its tables. This particular rate is given for an 0.60” standard two-flute HSS bit when cutting at a low speed of 20…25 SFM. The Handbook also gives you a method to convert the number to other speeds and bit sizes.
To make this feedrate more concrete, we note that it translates to a depth of cut of around .008” or 8 thou (0.2 mm). This is a solid cut, and should be enough to get under the work-hardened layer.
How to control the feed?
Recommended feedrates are best used as rough guidelines, not hard and fast rules. The main reason for this is that you can rarely control the feedrate accurately in manual drilling; indeed, chances are that you have never even thought of feed in terms of inches per revolution.
With standard drill presses and handheld drivers, what you control is the axial force you put on the bit, not the advance per revolution. For a low feedrate, apply only a little force; to get a high feedrate, press hard.
The key point here is that achieving the recommended high feedrates with stainless steel requires a lot of axial force. To reach the optimal cutting conditions and get under the work-hardened layer, you will have to press down hard on the bit. With small bits, you have to be careful to avoid snapping the bit, and with large ones, you may struggle to reach the required forces by hand.
This said, if might be able to control the feedrate directly if you have a more advanced drill press with a power feed on the quill or a CNC milling machine with a powered Z axis.
Although it’s hard to control the feed beforehand, you can always check your feedrate and depth of cut afterwards by observing the swarf or chips produced:
- Optimal drilling of stainless steel with low speed and high feed usually creates a long continuous spiral swarf
- A low feed and cutting into the work-hardened layer most often results in short chips
- Measuring the swarf thickness will give you a rough estimate of the depth of cut; multiply the depth of cut by two, and you have the rough feedrate
Driving the drill
Option 1: Drill press
Stainless steel is best drilled using a drill press. Compared to handheld drivers, a drill press allows for
- a high axial force
- steady low RPMs
- high torque
- steady feed
While these factors may be useful in drilling other metals too, they are particularly important with stainless steel.
The most important property of a drill press in drilling stainless steel is low gears. Many smaller models may not allow spindle speeds below 500 or 700 RPM, and thus do not get you down to optimal cutting speeds with any bit larger than 1/4”. Look for a model with a lowest speed around 200 RPM – this will allow you to drill at optimal speeds with all standard-sized twist bits.
Option 2: Handheld drill driver
Stainless steel may also be drilled using handheld corded or cordless drill drivers, but with some more effort.
The biggest challenge in drilling stainless with a handheld driver is delivering the high axial force needed for the efficient high-feedrate cutting. Assume an ergonomic working position, and prepare to push down hard for a long time.
Maintaining a steady low RPM may also be tricky. Large-diameter bits have a tendency to bind in stainless, particularly with thin sheets. Try to hold the machine steady and use the drill clutch to avoid kickback. Importantly, resist the urge to compensate for lacking push by using a high RPM, as this will only ruin your drill bit.
While clearly suboptimal, using a handheld driver is the only option with large workpieces that are inconvenient to move or do not fit into the drill press – stainless sinks already fitted into the kitchen are a prime example. You may also have to resort to handheld tool if a drill press is not available.
Should I use cutting fluid with stainless steel?
It is highly recommended to use a cutting fluid or lubricant when drilling stainless steel. Cutting fluids help to lower the cutting temperature by reducing frictional heat generation and by carrying some of the heat away. In doing this, the fluid extends tool life, improves cutting precision and surface finish and helps to avoid chip welding and other problems.
Oils are the best cutting fluid for stainless steel in low-speed drilling with HSS tools. Apart from the many cutting fluid products you can find in the hardware store, most mineral and vegetable oils will work well too. Castor oil and WD-40 are two common choices.
The best way to deliver the oil to the tool is to drip it into the drill hole with an oil can. You should add more oil every now and then as the oil is continuously carried away by the drill bit and the chip flow. Depending on the oil type and your speeds and feeds, some of the oil may also form smoke as the drill tip or chip temperature exceeds the smoke point of the oil.
Higher-speed drilling of stainless steel with carbide tools may call for a water-oil emulsion to be used as a cutting fluid. These fluids are commonly delivered with a dedicated system in the drill press or machining center, and are not very convenient to use in manual drilling.
Now, let us recap the content and summarize it into a drilling procedure for austenitic stainless steel:
- Select the best bit for the task (see discussion and table above)
- Position the hole without punching a center, as this will harden the spot. If necessary, use a small pilot drill to start the hole accurately.
- Apply oil to the hole location as you start
- Use a low RPM and a high axial force on the drill bit
- Try to keep the feed steady and avoid interruptions; aim for a continuous spiral swarf
- Add more oil periodically
- Be careful near breakthrough when drilling handheld
In this article, we went through the general guidelines of how to drill austenitic stainless steels: the tools, speeds and feeds, and technique. These guidelines should help you to drill austenitic stainless steel efficiently in most applications.
Due to the variety of stainless steels and products, there are still many considerations particular to each application, which were not covered in this overview. In the next article, we will take a look at the different forms and alloys stainless steel comes in, and give more specific instructions for each – if you are interested in reading more, be sure to check the follow-up!
Bayer, A. M., Becherer, B. A. (1989) “High-Speed Tool Steels”. In: ASM, 1989: ASM Handbook, Vol 16: Machining
Oberg, E., Jones, F. D., Horton, H. L. & Ryffel, H. H. (2016). Machinery’s Handbook (30th Edition.). South Norwalk: Industrial Press, Incorporated.