The use of this treatment will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher, more durable item. Alloys are heated above the critical transformation temperature for the material, then cooled rapidly enough to cause the soft initial material to transform to a much harder, stronger structure. Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material. Hardened materials are usually tempered or stress relieved to improve their dimensional stability and toughness.

140 degree Spot Drill

Material is heated to the suitable temperature for hardening, then cooled rapidly by immersing the hot part is water, oil or another suitable liquid to transform the material to a fully hardened structure. Parts which are quenched usually must be aged, tempered or stress relieved to achieve the proper toughness, final hardness and dimensional stability.

Spot drills are used to create a small indent prior to drilling with a twist type drill, the indent acts as a location point and guide for a larger diameter drill to follow minimising the risk of deflection. Spot drills differ slightly from centre drills as they are usually on a nominal shank and do not have the smaller pilot diameter that a centre drill has.

Steel parts often require a heat treatment to obtain improved mechanical properties, such as increasing increase hardness or strength. The hardening process consists of heating the components above the critical (normalizing) temperature, holding at this temperature for one hour per inch of thickness cooling at a rate fast enough to allow the material to transform to a much harder, stronger structure, and then tempering. Steel is essentially an alloy of iron and carbon; other steel alloys have other metal elements in solution. Heating the material above the critical temperature causes carbon and the other elements to go into solid solution. Quenching "freezes" the microstructure, inducing stresses. Parts are subsequently tempered to transform the microstructure, achieve the appropriate hardness and eliminate the stresses.

What is spot drilling

Excellent for Steel, Stainless Steel and Aluminium whilst also suitable for Hardened Steel up to HRc55, Titanium & Cast Iron.

In conclusion, Spot drills and centre drills are both useful tools for creating holes in metal. However, they have different purposes and advantages. Spot drills are more accurate and help to prevent the secondary drilling cycles from wandering. Centre drills are used to drill support holes in the workpiece for a lathe centre and create a starting point for a larger diameter hole. The best way to choose between a spot drill and a centre drill is to consider the purpose of the drill bit, the hole size you require and what machine you are working on.

This treatment follows a quenching or air cooling operation. Tempering is generally considered effective in relieving stresses induced by quenching in addition to lowering hardness to within a specified range, or meeting certain mechanical property requirements.

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Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material and final mechanical properties to be achieved. Hardened materials are tempered to improve their dimensional stability and toughness.

Tempering is done to develop the required combination of hardness, strength and toughness or to relieve the brittleness of fully hardened steels. Steels are never used in the as quenched condition. The combination of quenching and tempering is important to make tough parts.

Spot drill angle

A centre drill (which can be made from HSS or Carbide) can be used to drill a starter or guide hole in many different material types such as cast iron, aluminium, copper, steel, and stainless steel. Centre drills have a short and stubby design, this eliminates the risk of deflection making them the perfect tool for pre-drilling applications. When pre-drilling for a lathe centre, accuracy is key as the hole will ultimately keep the job aligned and stable during the machining process.

Before drilling a hole or preparing to use a live (revolving) or dead centre on a lathe, it’s common to use a spot drill or centre drill to ensure accuracy for the drill or centre. Spotting drills and centre drills may have similar characteristics, but they are intended for two different purposes.

For expert advice on choosing the right drill for your application, please contact our technical team on 01924 869 615 or email salescutwel.net.

Three large bearing sets being removed from Metlab180" diameter by 156" high carburizing furnace from the hardening temperature (1550°F) for subsequent quenching into agitated, hot oil. Parts were carburized to a case depth in excess of 0.200" ECD. Fixture and component weight is about 40,000 pounds.

Centre drills are mainly used to create a hole in the centre of a workpiece. This hole will act as a guide/support for a revolving or dead centre and in some case a starting point for the hole to be drilled with a larger drill diameter.

In this blog I will help to differentiate between the two and help you make the right choice when selecting the best tool for your application.

The spot drill is a good tool to have in your arsenal, its function is no longer limited to the drilling of pilot holes. In addition to this, it is possible to use them as a chamfer mill and various angles are available. The angles can be 60°, 90°, and 120° and can be used for a wide range of processes, including:

Centre drills are first choice for predrilling when using revolving or dead centres in the tailstock on manual or CNC lathe.

Spot drill size chart

Unlike centre drills, spot drills can be used for multiple applications such as spot drilling, chamfering, countersinking, V grooving and engraving. They are available in HSS or Carbide with different point angles, most commonly 90° or 120°.

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Tempering is the process of reheating the steel at a relatively low temperature leading to precipitation and spheroidization of the carbides present in the microstructure. The tempering temperature and times are generally controlled to produce the final properties required of the steel. The result is a component with the appropriate combination of hardness, strength and toughness for the intended application. Tempering is also effective in relieving the stresses induced by quenching.

Spottingdrill bit

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A centre drill has a slightly different geometry than a spot drill, they are shorter, with a tapered, or cone shaped end that is much smaller in diameter than the shaft of the drill. The smaller diameter is used as guide and creates clearance for the tip of lathe centre. A centre drill can also be used as a countersink to allow countersunk screws to sit flush with the surface.

As mentioned above, a spot drill has multiple uses, the main use is spotting prior to drilling, this process increases the accuracy of the secondary drill. If the drill being used is not self-centring, then spot drilling is a must as it will minimise the risk of drill wander. Spot drilling will improve tool life and improve surface roughness. It’s worth noting that self-centring drills do not require spot drilling.

Spot drill vs center drill

Centre drills come in two common types, Type A (DIN 333-A) this has 60-degree chamfer and Type B (DIN 333-B) that has two chamfers, a front 60 degree and secondary 120-degree chamfer. Centre drills are commonly available in Metric or BS (British Standard Imperial) sizes.

Material is heated up to the suitable temperature and then quenched in water or oil to harden to full hardness according to the kind of steels.