Annular cutter

An annular cutter (also called as core drill, core cutter, broach cutter, Trepanning drill or cup-type cutter) is a hollow, multiple cutting edges cutting tool used to make/drill holes in ferrous and non-ferrous metals. An annular cutter cuts only an annular groove at the periphery of the hole and leaves a solid core or slug at the hole center.^[1] Annular cutter is a substitute for slower, more expensive and less efficient Twist drills and Hole saws. The function of Annular Cutters is more or less same as a hole saw but differs in geometry and material. Annular cutters are faster, easier and are more accurate than the conventional twist drills or drill bits.

An annular cutter or core drill removes the material from the metal to create cylindrical holes in a circular cross section. An annular cutter, unlike drill bits, cuts only on the periphery of the hole to be made. Annular cutters use almost the same method as concrete core drills, but are for metal drilling. An annular cutter is a multipoint cutting tool and is made from material like high speed steel and tungsten carbide tips.

Annular cutters are available in different sizes generally starting from diameter 12 mm (1/2’’) to 200 mm (7 7/8’’) and larger if required. The length of the annular cutter can be achieved depending upon the thickness of the material to be drilled. In the market the lengths 30 mm, 55 mm, 75 and 110 mm annular cutters are popularly available. These sizes though vary from manufacturer to manufacturer. The annular cutter’s material should be harder than the material to be drilled. Therefore there are various types of material used to manufacture annular cutters, for example different types HSS and TCT material. Annular cutters are also called as core drills because, after drilling hole, the drilled material is ejected from the cutter in the form of a cylindrical blank, this blank is called a core.

The first annular cutter was invented by Mr. Dough Hougen which he patented in 1973.^[2] The High Speed Steel annular cutter was invented in 1983 by Michael Beharry ^[3] and the tungsten carbide tipped annular cutter was invented in 1985 by Mr. Mikiya Toshio and Mr. Shoji Michihiro.^[4]

Though people use annular cutters on conventional drilling machines like radial drilling machines or pillar drilling machine, it is not recommended because these machines have very high RPM which creates high tool breakage risk for annular cutters as they are hollow unlike solid twist drills. It is always recommended to use an annular cutter or core drill with a magnetic core drilling machine (also called as magnetic drill, magnetic base drilling machine, etc.) as these machines are slower. A magnetic drilling machine is ideally made for using annular cutters.

Different Shanks of Annular cutters:

Weldon shank: "Weldon shank" annular cutters have 2 flats and this is the traditional method adopted by many of the popular magnetic drill manufacturers. "weldon shank" annular cutters fix in the chuck by tightening Grub screws in the two flat areas.

Universal-shank: "Universal shank" annular cutters has one flat and three quick connect grooves. This is also a popular method for using annular cutters into mag drills quickly, without any strain of tightening grub screws. mostly "universal shank cutters" can be used in "weldon -shank machines, where as weldon-shank annular cutters can't use in "one-touch" machines without the help of an extra adapter. while using "universal shank cutters" into "weldon shank" machines, the tightening area (flat area) is only one, which is also not 100% sufficient to tighten the both screws.

History

During the early 70s, Mr. Dough Hougen invented the Rotabroach annular cutters. During those days twist drill bits were so popular that people were not in mood to accept a new idea of cutting holes with annular cutters. Hougen took the annular cutter to many drill bit manufacture but he got a dispappointing response, but when he took the annular cutters to the fabricators and construction steelworker they were very well interested in this concept because during those period the construction steelworker and fabricators used a ''portable'' magnetic drilling machines which was at least 100 lbs. (45 Kg.) and required 2 workers to lift it to the construction site. These magnetic drills were then called a ''widow maker'' because the drill bit bound off the drill breaking free and throwing the worker off the structure.^[5]

The annular cutter made by Hougen were comparatively safe, the annular cutter won't break free, the workers liked the cutters but the annular cutter could not solve the heavy weight problem of magnetic drilling machine. Hougen later reinvented the magnetic drilling machine making it just 28 lbs (12.70 Kg.). The reinvention of the magnetic drilling machine with its portability also gave birth to high volume sales of annular cutters.^[5]

Benefits

Time saving

As an annular cutter cuts only on the periphery of the hole to be made, it has to remove much less material as compared to conventional drill bits which cut and remove the complete material of the hole. Annular cutters are considered to be 3 to 4 times faster than conventional drill bits. Also while using a twist drill bit or metal hole saw one has to do pre-drilling and step-drilling operations for bigger diameter holes, whereas an annular cutter requires no pre-drilling and step-drilling. It is a one-shot drilling operation.

The annular cutters have less teeth and less wear, therefore with an annular cutter holes can be made at higher or faster cutting speed and a faster feed rate of the drilling machine.^[6]

Energy savings

Because annular cutters do not have to drill/cut through the complete work piece, they require less thrust and thus less energy to drill through material.^[6]

More finishing

The hole made using an annular cutter is more finished, precise and accurate.^[6]

Easy

Due to the unique geometry of the annular cutters, making holes at awkward angles is also very easy.^[6]

Efficient cooling

The cooling of annular cutters is internal. The internal lubrication is provided through a hole with the help of a pilot pin. Internal lubrication makes the annular cutter faster and more efficient for deep hole drilling.^[6]

Best for light duty machines

As the annular cutters require very low RPM (from min 50 RPM), these cutters can be easily used with light duty drilling machines or portable magnetic base drilling machines.^[6]

Sizes

Annular cutters biggest advantage is the variety of sizes. Annular cutters start from 12 mm diameter up to 150 mm diameter and more if required. Various cutting depth (length) of annular cutters can be sourced depending upon the requirement.^[6]

Re-sharpening

HSS annular cutter can be easily re-sharpened on a grinding machine and can be re-used for drilling. Re-grinding can be done until the desired cutting length is available on the annular cutter.^[6]

Types of core drills

The types of core drills are categorized by the material used to manufacture them: Primarily a metal core drill is made from High Speed Steel and Tungsten Carbide Tips. So commercially the types of core drills are HSS core drills and TCT core drills.

High Speed Steel (HSS) core drills

HSS core drills are made from High Speed Steel material. These are most popular core drills or annular cutters in market. HSS core drills are cheaper for smaller diameter i.e 40 mm. The biggest advantage of HSS core drill is its ability to re-sharpen. Unlike TCT core drills, HSS core drills re-sharpened and used with good efficiency until the desirable length is remaining. HSS core drills have longer service life, high heat resistance: the charecteristics of HSS core drills is that drilling with these core drills can be done without the use of excessive force.^[7]

HSS Standard

The HSS Standard core drills are made from M2 steel. These core drills are one of the most popular core drills in western countries due to its cost and resharpening advantage. The Standard HSS core drills are used to drill Mild and Structural steel, Aluminium, Stainless steel of lower BHN., etc.^[7]

HSS Cobalt

The HSS Cobalt are core drills made from M42 Steel. These core drills have 8% Cobalt in HSS. These core drills are harder than the Standard HSS core drills which gives better performance to drill harder material. A high percentage of Cobalt in HSS makes it faster, stronger and durable. Cobalt HSS core drills are used to drill materials like Structural Steel, Stainless Steel, Cast iron, etc.^[7]

HSS with hard coating

The HSS core drills with Hard Coating are core drills with a coating on it, generally TiN Coating. This make the core drills more resistant to heat, longer tool life and even removal of chips. These core drills are gaining lot of popularity over the Standard HSS core drills these days. These core drills are used Structural steel, Stainless steel, etc.

Tungsten carbide tipped (TCT) core drills

TCT core drills are made from Tungsten Carbide material on their tips. Tungsten carbide is a very hard material and supposed to be one of the hardest materials made by man. TCT core drills are more expensive then HSS core drills but are faster and easy to make bigger holes. There are also different types in TCT core drills. TCT core drills are cheaper for bigger diameter i.e. above 41 mm. They are very efficient for cutting hard material. The chances of tip breakage in TCT core drill is more compared to HSS Core drills. TCT core drills has a disadvantage that it can not be reground. Once the TCT core drill is damaged it has to be thrown away, it can not be re used like the HSS core drills. It is always recommended by manufacturers to use TCT core drills only for larger diameter and for very hard material, while giving priority to HSS core drills because of its price and re-sharpening advantages.^[7]

TCT Standard

A Standard TCT core drill is a with tungsten carbide tips. These types of core drills are now available in vast range of diameter, for up to 150 mm. These are popularly used core drills on magnetic core drilling machines. These core drills are popularly used to drill materials like railway tracks, structural steel, stainless steel, cast iron, etc.^[7]

TCT with hard coating

The TCT core drills with Hard Coating are core drills with a coating on it, generally TiN Coating. This make the core drills more resistant to heat, longer tool life and even removal of chips just like the advantage of HSS core drill. The biggest advantage of hard coated core drills is that these cutters are very good for cutting larger diameters on hard material. These core drills are popularly used to drill materials like corroded or weathered material, structural and stainless steel.^[7]

Drilling tubes and pipes

Annular cutters can also be best used to make holes in pipes and tubes. Annular cutters help make holes in pipe and tubes without dimpling and deburring. Unlike the conventional method of bringing the workpiece to machine, with the help of portable magnetic core drilling machines, you can now bring the machine to the workpiece. This makes the drilling process on tubes very easy.

To make a hole in tube or pipe easily an annular cutter should be used with a magnetic base drilling machine and a tube/pipe clamping device. The tube clamping device is a metallic device which is clamped on the tube with an adjustable chain. After clamping the device the magnetic drilling machine is placed on the device, which gets clamped to the device by switching on its magnet.

With the help of magnetic drilling machines' proper RPM the annular cutter makes a perfect hole in the tube.

Generally a HSS annular cutter is used to make holes in tubes and pipes, as the TCT annular cutters has more chances of cutting teeth breakage due to curved surfaces of tubes and pipes.

As annular cutters are hollow there is no dead zone resistance. The cutting takes place only on the periphery of the hole to be made. Annular cutters distribute load equally on the periphery of the hole with its multiple cutting edges, thus the required thrust and energy is also much less comparatively.^[8]

Comparison between HSS and TCT core drills

Cost

HSS core drills are cheaper than the TCT core drills for smaller diameters up to 40 mm, while TCT core drills are cheaper than HSS for diameters above 41 mm.

Re-sharpening

Once the HSS cutter loses its sharpness, it can be re-ground on a grinding machine and can be used again. While, re-grinding the TCT core drill is possible but very difficult and time consuming.

Defect

The chances of defect in TCT core drills is more because the tungsten carbide tips are glued on the core drill and when the core drill falls down or crashes on the material, the chances tip breakage is more. HSS cutter on the other hand are completely made out of one material.

Speed

TCT cutter biggest advantage is speed and ease in which it may cut harder materials. Especially for larger diameter TCT core drills are very successful. While HSS core drills are good when drilling not very hard material and for small diameter hole sizes.

Hardness

Hardness of TCT core drills is more than the HSS core drills.

Advantages of Core drills over Conventional twist drills

Less power / Less efforts

The conventional twist drill has only one cutting edge/point, while an annular cutter or core drill has multiple cutting edges which helps to drill with very less horsepower and efforts.^[9]

Less material to cut

The conventional twist drill being solid and having only cutting point has to cut or remove the complete material of the hole to be made. While with annular cutters or core drills having multiple edges and being hollow cuts only on the periphery of the hole to be made.

Less time needed

As core drill has to cut less material as compared to twist drill, the time required is also too less. A core drill is considered to be 3 to 4 times faster than twist drills.^[10]

Less money

A single core drill makes 5 to 10 times more holes then a twist drill, plus core drills can be again reground to make another 1500 holes, thus saving a lot of tool cost.^[11]

No pre-drilling or step-drilling

Unlike twist drills, core drills even for larger diameters do not require any pre-drilling or step-drilling.

Example: To make a 50 mm diameter hole with twist drill, one has to start making a pre-drilling of 10 mm, than 15 mm than 20 mm, ...up to 50 mm slowly increasing the size of holes. While with core drill one can drill directly 50 mm in one-shot.^[9][11]

Higher accuracy

Holes made by core drills have higher accuracy compared to twist drills.

Burr-free holes

One of the biggest advantage of core drills over twist drill is burr-free holes and smooth holes, resulting in less scrap and wastage of material.^[9][11]

Disadvantages of core drills

The biggest disadvantage of core drill or annular cutter is that it can not drill blind holes. A core drill due to its hollow geometry can only make through holes. Another disadvantage is in drilling a pile of many plates. There are some core drills with a different geometry which can be used for drilling piles of plates, but still it is not 100% successful. The cost of a core drill is high compared to twist drill, but the cost per hole made with core drill is less as compared to twist drills, this is because a single core drill makes 5 to 10 times more holes than a twist drill.^[9][11]

References