What are the best twist drills for stainless steel?
14 May 2021
In this article we look at what drill characteristics are best for drilling stainless steels. What is the best way to drill stainless steel? And what are the best drill bits to use? First, we will take a quick look at stainless steel itself.
What is stainless steel?Stainless steel is an iron alloy which is so called due to the alloying elements of chromium, titanium or nickel. The Chromium element (Cr) in stainless steel creates an oxide layer on the materials surface which gives it its known stainless properties, permanently protecting the iron against the effects of water and oxidisation. The minimum quantity of Cr is stainless steel is around 11%.
The term ‘stainless’ is a generic term given to a group of corrosion resistant materials. There are over 100 different forms of stainless steel. To group them, they can be split into the following categories:
Martensitic stainless steels have an increased nickel and carbon content. Therefore, they can be quenched and tempered giving martensitic stainless increased toughness. Corrosion resistance is reduced when compared to austenitic and ferritic stainless steels, but a good level of resistance combined with the materials toughness make it ideal for an array of applications, such as mechanical valves.
Austenitic is the most widely used stainless steel and is commonly defined under the 300 series of stainless steels. Austenitic stainless has excellent corrosion and heat resistant properties. Chromium content, as mentioned earlier, being the structures most important property for corrosion resistance is usually between 16% and 30%. The increased nickel content improves the surface finish. Austenitic stainless steels are widely used in marine, food and environmentally sensitive applications.
Ferritic stainless steels are like mild steels in their workability, but with better corrosion resistance. That said, the corrosion resistance is less than that of austenitic. Ferritic steel is commonly used in cold rolled applications.
Duplex / Super Duplex: This stainless steel is a two phased structure, or a composite of austenitic and ferritic steels (austenitic-ferritic). This partnership creates a stainless far superior in both strength and corrosion resistance. Super Duplex can often be found in offshore oil and gas applications.
What drills are best for stainless steel?Point angle: When cutting stainless steel, the more of the cutting edge coming into contact with the material the better, therefore a larger cutting angle is preferred. 130° minimum point angle is recommended with the ideal being 135°
Helix angle: Drilling stainless steel best suits a smaller helix angle. A high helix is paramount to remove swarf quickly to prevent heat build-up, which is one of the biggest issues faced when drilling stainless.
Tool material: Stainless steels can be hard materials and therefore, for best results the drill should be manufactured from HSSE-Co with a minimum of 5% Co. For some stainless steels and high production environments, solid carbide should also be considered, although brittleness of carbide requires optimum conditions. These tool materials also reduce the heat build-up which would be associated with softer materials such as solely HSS.
Coating: A coating is not a necessity when cutting stainless steel, the emphasis really is on the tool material and moreover the point and helix type. However, in high production environments where greater speeds, feeds and temperatures are placed on the drill bit, coatings such as TiAlN will prolong the drills life, whilst extending repeatability.
Drills Performacut recommend for soft stainless steel
Drills Performacut recommend for hard stainless steel