- TOOLS IN PRACTICE
- HOW TO CHOOSE THE RIGHT METAL DRILL BIT – PART 2
HOW TO CHOOSE THE RIGHT METAL DRILL BIT – PART 2
In the section "Metal Drill Bits – Basic Information", we briefly introduced the three main types of HSS drill bits, their appearance, and their applications. For those who would like to learn more about how they are manufactured, we have prepared this chapter.
Manufacturing Methods for Metal Drill Bits and Their Influence on Drill Bit Properties
There are two primary methods of manufacturing twist drill bits – roll forging and grinding.
Roll Forging
- Heated material (a rod or wire) passes between two or more rollers that shape it into the basic form of the drill bit. The rollers have precisely machined grooves that create the drill's spiral flutes.
- This manufacturing process is less precise but faster and more cost-effective than grinding.
- Drill bits produced in this way are less wear-resistant, less accurate, but more flexible.
- They are therefore ideal for DIY use and general drilling applications where extreme precision is not required.
- Roll-forged drill bits are usually steam oxidised, giving them their characteristic black finish while improving corrosion resistance.
Grinding
- The drill bit achieves its final shape through precision grinding of the base material, producing highly accurate spiral flutes.
- These drill bits are significantly more accurate and more wear-resistant than roll-forged drill bits.
- They are intended for professional use.
- Ground drill bits retain their natural metallic silver appearance, although they may also receive additional surface treatments.
Did you know that the abbreviations HSS-R and HSS-G indicate how the drill bit was manufactured?
R = Roll forged, G = Ground
How can you tell whether a drill bit is roll-forged or ground by looking at its flutes and drill point?
Roll-forged drill bits may have irregular or rougher flutes and a drill point without a split point.
Ground drill bits feature uniform, precisely machined flutes that are smooth and well-defined. The point of a high-quality drill bit often incorporates a split point, which improves drilling accuracy and efficiency.
How does the material affect drill bit quality?
- Metal drill bits are typically manufactured from High Speed Steel (HSS). The quality of HSS can vary considerably depending on its chemical composition and heat treatment.
- Roll-forged drill bits are generally produced from HSS with a lower content of alloying elements, increasing flexibility while reducing manufacturing costs.
- Ground drill bits are made from HSS containing higher concentrations of alloying elements (such as vanadium, molybdenum, tungsten or cobalt), which increases hardness and wear resistance but reduces flexibility and increases cost.
- High Speed Steel containing cobalt (HSS-Co) offers greater heat resistance and hardness, making these drill bits ideal for drilling very hard materials such as stainless steel and titanium. HSS-Co drill bits are manufactured exclusively by grinding.
Did you know that cobalt drill bits are not made of cobalt but of High Speed Steel alloyed with cobalt?
Both HSS-Co and HSS-E are High Speed Steel alloys containing cobalt.
Drill Bit Colour – What Does It Tell Us and How Important Is It?
All HSS drill bits are originally silver in colour. Their final colour is the result of the surface treatment applied after manufacturing, which improves corrosion and wear resistance while also making it easier to identify the different types of drill bits.
Black Finish
Black oxide treatment (surface oxidation) is primarily used on roll-forged drill bits (HSS or HSS-R). It improves their corrosion resistance. On very inexpensive roll-forged drill bits, however, the quality of this treatment may be inferior.
Silver Finish
A bright silver finish is typically found on ground drill bits (HSS-G). However, you may also encounter roll-forged drill bits with this finish.
Golden Brown Finish
The characteristic colour of cobalt drill bits (HSS-Co). This finish makes it easy to distinguish them from HSS-G ground drill bits.
Bright Gold Finish
"Titanium drill bits" with a bright gold finish are HSS or HSS-G drill bits coated with TiN (Titanium Nitride), which increases their service life and hardness. However, this coating can never match the performance of HSS-Co drill bits.
If you'd like to learn even more about metal drill bits, also read "Metal Drill Bits – Basic Information" and
"What to Watch Out For – The Difference Between Apparent and True Drill Bit Quality."