How to drill Stainless steel
Equipment you will need; suitable drill bit, cutting fluid, eye protection, ear defenders, heavy duty tape, marker pen or 3 corner pyramid punch, sturdy clamp, and felt or plastic to protect workpiece in the clamp.
Begin by clearing your workspace to ensure there is nothing that may catch in the drill when you start working. Make sure you have everything to hand so you can concentrate on getting the job done without interruption. This is a good time to set up your drill and ensure all cables are in good condition.
The correct PPE is of paramount importance when drilling; chips and swarf are sharp and travel at speed so make sure your eyes are well protected. If you wear prescription glasses make sure you wear additional goggles designed to wear over the top as regular glasses will not provide adequate protection. Gloves are not recommended when drilling as they pose a risk to becoming entangled in the drill; the extreme forces, rotation, and speed encountered when drilling can easily break a finger or wrist. Ear defenders are recommended to protect your hearing, exposure to loud machining noise can permanently damage your hearing and contribute to developing tinnitus. You can browse our full range of PPE products here.
- Mark the position of the hole with a marker pen, or if preferred, tap a small indent with a 3-corner pyramid punch. If you are concerned about the swarf damaging the surrounding area as it furls out of the drill bit you can use heavy-duty tape around the drill mark as protection.
- If the metal to be drilled is less than 3mm thick it may be possible to use a single bit to achieve the desired size hole, however, if the metal is thicker it is recommended to start with a bit half the size of the desired hole diameter for an initial hole and then step up to the final size drill for a second drilling.
- Firmly clamp the workpiece into position ensuring it is secure. If the drill bit grabs during operation, or when the drill exits on the blind side, it can spin the workpiece which can cause bad cuts, broken bones and damage to equipment.
- If drilling with hand tools, drop a liberal amount of cutting fluid/ lubricant onto the marked metal. If you are using a coolant delivery system, set that up as per the manufacturer’s guidelines. Coolant can be sprayed, dripped, or flooded, but it is important to use a liberal amount and that there is good contact between the fluid and the tool interface. Using cutting fluid will help to clear the swarf away from the drill bit to reduce the risk of becoming friction welded and reduces work hardening. We recommend the use of CT-90 metal cutting and tapping fluid which is available to buy here.
- You are now ready to start drilling. The table below shows suggested speed and feed rates for drilling different grades of Stainless Steel.
|Grade of Stainless Steel||Vickers Hardness||Surface Metres per Minute||Feed mm per rev/ drill diameter mm|
- When drilling stainless steel, one of the best indicators of whether the speed, pressure and feed rates are correct is to watch the swarf; the swarf should cleanly exit the hole and be helical in shape and short in length. Stainless swarf should resemble the original colour of the stock metal or have a yellow tinge to it. If it is darker or not helical, back the drill out, apply more coolant and check your machine settings. Then simply try again.
- Once the hole has been made make sure you do not touch the bit or the hole as they will be hot enough to cause a burn. Care should be taken when wiping the coolant off the metal as the swarf held in the coolant may scratch the surface.
These steps should have you drilling through stainless steel like a professional in no time, however, below is more information to give you a much deeper knowledge of ways to drill stainless steel to get the very best results every time.
Choosing the right drill bit.
There are many different types of drill bit and reamers suitable for drilling stainless steel. Choosing the correct type for a particular project is paramount to achieving the best results.
- HSS or High-Speed-Steel bits are designed for use on stainless steel and can be used for hand and machine drilling. It is recommended to use an HSS bit that is TiN tipped as the Titanium Nitride reduces friction which in turn reduces work hardening caused by excessive heat and minimises flank and crater wear.
- Cobalt drill bits and reamers are specifically engineered to provide heavy-duty performance on high tensile metals; these are manufactured from a steel alloy with between 5% to 8 % cobalt content. The 5% cobalt alloy is designated as M35 grade and the 8% alloy is graded as M42. Cobalt increases the strength of the parent alloy and significantly increases its heat resistant properties; these are two very strong considerations when drilling stainless steel as heat resistance to friction created during operation will reduce the work hardening rate, and the additional strength will protect against bit breakage and flank and crater wear. Flank wear is when the part of the bit in contact with the workpiece wears away, crater wear is when the metal from the drill bit becomes diffused into the swarf.
Our comprehensive range of TiN tipped and cobalt bits can be seen here.
Once the choice of material has been decided on it is important to consider the right size bit for the project. Paying attention to the geometry of the bit will greatly improve the quality of the finished hole and make the whole process much easier.
Web thickness. The web thickness should preferably be a minimum of 1/8 of the drill diameter. The web is the central shaft of the bit that the flutes project from. The web supports the drill as it penetrates the metal being drilled, if this is too thin the bit may snap.
Point angle. This refers to the angle of the point at the very tip of the bit. Hard metals, such as stainless steel, require a wider point angle than a bit used on less hard metals; selecting the correct size point angle will reduce wear and travel, result in a better hole shape, and reduce machine chatter.
Lip relief angle. The lip relief angle refers to the angle at the outer corner of the lip and is responsible for supporting the cutting edge in contact with the metal. This lip relief angle is determined by the angle of the point; a smaller point angle means more web is presented to the workpiece, so the bit would need a bigger lip angle to support the cutting edge. The size of the lip angle increases as the drill diameter decreases. If the drill tip does not have an adequate lip relief angle then the cut will be poor, even if the bit is sharp, and will ultimately result in excessive wear and binding during operation.
Length. The length of the bit governs how deep the hole can be drilled, however, the longer the bit is, the more flex it will have. If the bit flexes, the hole may be inaccurate and not on the correct axis. By selecting a bit of suitable length, the likelihood of deflection and breakage during operation will be reduced. Bits are available in a variety of lengths, the most popular length for the majority of metal drilling operations are referred to as ‘jobber’. The length of the flutes on these bits are between 9 and 14 times the diameter. These are considered a good all-rounder for most projects.
Helix angle. The helix angle refers to the angle of the flutes to the face of the metal being drilled. The correct helix angle is important to ensure swarf moves from the excavated hole cleanly as the greater the helix angle, the smaller the capacity of the flute zone.
Recommended drill bit dimensions for drilling stainless steel.
|Recommended range of point angle||Recommended range of helix angle||Recommended range of lip relief angle||Drill bit diameter||Optimum lip relief angle|
When drilling deep holes with a depth more than 3 times the bit diameter, the speed and feed rates need to be reduced to lessen work hardening, ensure swarf keeps clear, and reduce the possibility of bit breakage.
The recommended reductions are shown below.
|Hole depth to diameter||Speed reduction||Feed reduction|
|3||10 %||10 %|
|4||20 %||10 %|
|5||30 %||20 %|
|6||35-40 %||20 %|
Step drilling is often recommended for thicker profiles of stainless steel to help keep swarf clear, allow for good penetration of coolant into the hole and to reduce friction at the drill point. The full feed and speed rates must be maintained when backing out and re-entering to ensure smooth transitioning through the layer of work hardened steel that will develop as the cutting is underway.
Step drilling technique entails the first drilling depth to not exceed 3-4 times the diameter of the bit, then withdraw the drill, the second drilling cycle depth should be no more than an additional 2 times the diameter of the bit then withdraw the drill; subsequent cycles can add 1 more depth equal to the diameter of the bit.
Frequent backing out with the drill bit and minimising dwell will help to reduce hole wall roughness and drill breakage.
Reducing work hardening when drilling Stainless Steel
Stainless Steel will work harden relatively quickly, especially with the heat generated by drilling intensifying the process. Once Stainless steel becomes work hardened it becomes incredibly difficult to drill; the surface will glaze which will cause the bit to deviate and bounce resulting in blunting of the drill bit, surface damage, or even snapping the bit.
There are several ways to reduce the severity of work hardening to ensure the drill holes turn out perfect every time.
- Keeping the drill bit cool and lubricated will drastically improve the quality and integrity of the drilling operation. Using a proprietary cutting or cutting and tapping fluid reduces the friction created during drilling processes which lessens the severity of work hardening. Keeping the cut edge and the drill bit well lubricated also reduces cutting time, improves the surface finish, and prolongs the life of the drill bit. For best results, it is recommended to provide a continuous supply of coolant/ lubricant to the bit and workpiece throughout the drilling operation; if this is not possible, stopping and manually applying the fluid to both the bit and the cut edge will also work well.
- Work hardening can even occur when using a conical punch to mark where holes will be drilled, this will make drilling difficult in a small, localised area as the drill bit may slip against the walls of the indent. Using a 3-corner pyramid punch will reduce the risks or simply use a marker pen to reference drill placements.
- The stainless steel can be annealed before embarking on deep drilling or very small diameter holes as this will soften it to improve the machinability, therefore, helping to reduce the risk of severe work hardening.
Whether you are an experienced fabricator or an avid hobbyist, you should now be well equipped to tackle any stainless-steel drilling job that comes your way.