Correct use of centre drills on lathes
A centre drill is used to create a centring point on workpieces. Our guide explains the different types of centre drills, what you should bear in mind when working with centre drills and what the difference is between spot drills and centre drills.
Design of centre drills
Centre drills are particularly short and have a stepped profile with an additional centre point at the very front. The diameter of this tip is also the nominal diameter of the centre drill.
The countersink angle for centre drills is 60°. Depending on the shape of the centre drill, this can also have an additional protective countersink to protect the centre hole from damage. Centre drills can also be used on both sides.
Use of centre drills
A centring drill is used to create a centring point on workpieces. This centring hole is useful for two purposes:
- For example, a centre drill can be used to insert a centre punch to support the workpiece over the tailstock of a lathe (Figures 1 to 3).
- A centre drill can also be used for pre-drilling. Due to the small dimensioned centre point of the drill, low radial forces and reaction forces occur through the chip when drilling a workpiece. This prevents the drill from "running", which significantly improves the positioning accuracy of the hole to be drilled. The actual hole can then be drilled with a twist drill, which is guided laterally from the start by the centring hole (Fig. 4).
Centre drill shapes
Depending on the required standard of a bore, centring drills are available in shapes A, B and R (Fig. 1). All shapes can be left or right cutting - only the tips differ. The countersink angle is 60° for all shapes.
Centre drills with a 120° protective countersink belong to shape B.
Shape R, on the other hand, has curved running surfaces, which reduces the contact surface to the workpiece and provides sufficient space for the cutting oil during drilling. Thanks to their good guidance for holes that do not run towards the centre point, form R centring drills are also suitable for centring holes in taper turning with tailstock adjustment.
Coatings
In addition to the typical HSS centre drills, there are also centre drills with special coatings that are used for machining materials of different hardnesses.
Possible coatings:
- TiN-coated (titanium, nitrogen)
- TiAlN-coated (titanium, aluminium, nitrogen)
You can find a brief overview of the different coatings for metal drills in our information sheet:
| TiN | TiAIN | |
| Components of the coating | Titanium, nitrogen | Titanium, aluminium, nitrogen |
|
Suitable for soft materials (aluminium, plastics, non-ferrous metals, brass, copper, bronze) |
+ | + |
| Suitable for hard materials (stainless steel, steel, titanium alloys) | + | ++ |
| Suitable for very hard materials (e.g. hardened, alloyed steel grades) | + | |
| Coating colour | gold | Black, anthracite |
Using centre drills correctly
Before a centring hole is drilled on the lathe, the surface of the workpiece should be straightened/machined by facing (Fig. 1). To increase the service life of the centre drill and minimise friction between the tool and the workpiece, always work with cutting oil when using centre drills.
Clamp the centre drill into the drill chuck. Then push the tailstock towards the workpiece. Ensure that the workpiece is clamped in the jaw chuck for as short a time as possible. Switch on the lathe and carry out a smooth and slow infeed movement (Fig. 2).
The centring hole should be deep enough for optimum subsequent machining (Fig. 3).
Correct speed
The correct speed depends on the following factors:
- Material of the workpiece (rule of thumb: the harder the material, the lower the speed)
- Diameter of the centre drill (rule of thumb: the larger the diameter, the lower the speed)
- Coating of the centre drill
Difference between spot drills and centre drills
In contrast to a centre drill, a pilot drill does not have a centre point. This means that the drilling paths are shorter, which is helpful for the production of many holes. Just like the centre drill, the spot drill is not suitable for producing a complete drill hole. With a spot drill, workpieces are drilled with pinpoint accuracy in order to subsequently enable drilling without "running".
Tip
Our centre drills
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