Twist drill

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Twist drill; from left to right: 8 mm drill bit for wood, metal and concrete as well as a center drill
Drill with inclined whistle notch clamping surface on the drill shaft

A twist drill (also twist drill or spiral flute drill ) is a drill . It has a conical head and almost always two cutting edges, each consisting of a main cutting edge, secondary cutting edge and cross cutting edge. The head is pointed with a typical point angle of 118 ° ( HSS drill) or 142 ° ( carbide drill). Since the cutting edge geometry is designed for machining metals, this type is also known as a metal drill . It can also be used to machine plastics and wood , but special drills are much better suited for this. Inside the twist drill there is the so-called core or core, the diameter of which at the tip corresponds to that of the cross cutting edge, but increases in the direction of the shaft in order to ensure sufficient stability. Since the guide chamfers, which are located on the circumference of the drill, rub against the inside of the bore and thus damage it, the outside diameter of the drill slowly decreases in the direction of the shank. The taper is about 0.02 mm to 0.08 mm per 100 mm length.

Overview

Giovanni Martignoni is considered to be the inventor of the helical or twist drill (1863) .

The eponymous coils or spirals are used on the one hand to remove the chips and on the other hand to supply cooling lubricant.

Twist drills are used with a length-to-diameter ratio of L / D up to 5. There are also special designs that go up to L / D = 200. In this area, however, special deep drilling tools are more suitable. The achievable dimensional accuracy, measured as an ISO tolerance , is around IT7 to IT10. They are also available in very small diameters down to well under a millimeter. There is no technical limit to the diameter. However, from a range of around 16 mm, drills with indexable inserts are cheaper and more productive.

Twist drills are very cheap and are suitable for drilling into solid material and guide themselves in the hole thanks to the ground guide chamfers on the sides of the shaft. If the cutting edges become blunt, they can be re-sharpened frequently and easily while maintaining the diameter. Special cuts are also possible, e.g. for step drilling in a single operation.

The disadvantage is that they tend to run on sloping and uneven workpieces. The large feed forces caused by the cross cutting edge and the limited cutting speed due to the sensitive cutting edges on the side of the head are also disadvantageous. The guide bevels rubbing against the inner walls of the bore damage the bore, which is why the quality that can be achieved is limited. With larger drilling depths, chips and cooling lubricant block each other.

Tool types

Twist drills are essentially divided into three types, which are standardized in DIN 1414-1 / 2: Type N for normal hard materials (normal chipping), such as B. steel, type H for hard, tough and brittle materials (short-chipping, such as high-strength steel , stone, magnesium and plastics) and type W for soft and tough materials, e.g. B. aluminum, copper and zinc (long-chipping). They differ in the size of the helix and often the tip angle.

Cutting edge geometry

Steel drill: point angle 118 °, helix angle 27 °
Drill tip of a steel drill with main cutting edge (HS) and cross cutting edge (QS)

There are several important angles on the twist drill: The so-called rake angle indicates the pitch of the helix. It also corresponds to the rake angle on the edge of the drill. However, the rake angle changes across the diameter. In the middle, where the main cutting edge merges into the cross cutting edge, it becomes strongly negative. The point angle corresponds to twice the tool setting angle and indicates the angle of the two main cutting edges . The size of the point angle and the rake angle depends on the material to be processed.

A large point angle will center the drill worse. It also increases the friction between the drill and the inner wall of the bore. The cutting edges become longer with a smaller point angle (with the same drill diameter). A small point angle ensures good centering and heat dissipation, but increases cutting edge wear.

Type N twist drills have a helix angle (also: helix angle) of usually 20 ° to 30 ° and a point angle of 118 °. For type H the helix angle is smaller (10 ° to 13 °) and the point angle 118 ° to 130 °, for type W larger (27 ° to 45 °), the point angle here is 130 °.

The tool diameter is greatest at the tip (nominal diameter) and decreases slightly towards the shaft in order to reduce the friction of the drill in the drill hole. This reduction in the tool diameter, referred to as taper, is approximately in the range from 0.02 mm to 0.08 mm in diameter over 100 mm in length.

The two main cutting edges ( HS , see picture) at the drill tip run parallel, creating a so-called cross cutting edge ( QS ). It is usually offset by 55 ° to the main cutting edges , is transverse to the drilling or feed direction and has a width of about one fifth of the drill diameter. The cross cutting edge QS does not cut - contrary to the name - but has a scraping effect and increases the required working pressure on the drilling tool (the feed force for the cross cutting edge QS is about a third of the total feed force).

When drilling into solid material, the QS cross-cutting edge carries the risk of so-called "bleeding", an undefined lateral shift in position - which greatly impairs or makes it impossible to produce a precisely positioned hole. To prevent this effect, it is advisable to make a center hole beforehand with a center drill or to use an NC tapping drill . The drill bit is laterally guided at its tip through such a drilling in the workpiece and thus held in position; this is done through the different design of the point angles, which only guide the twist drill at one point on the support surface. A centering or tapping hole produced for this purpose should have a diameter of at least the size of the cross-cutting edge QS, but better about a third of the final hole diameter. As an alternative to machine centering, the position accuracy of the subsequently drilled hole can also be improved by hand punching . When producing larger bores, additional pre-drilling with a smaller drill can also prevent or reduce running.

The cross - cutting edge QS can be made smaller using a special grinding technique , the so-called point thinning , in order to reduce the feed force and thus the drilling torque.

Bevel shapes

Twist drill made of carbide with internal cooling lubricant supply and special four-surface bevel.

There are numerous possible bevel shapes for the tips of the twist drills. The conical surface grinding is standard. There are also numerous others, some of which are also standardized.

  • Conical surface point : This is the standard point, which is easy to produce and is insensitive to high mechanical loads. The drill head then has the shape of a conical jacket . The length of the cross cutting edge is about 20% of the diameter and is therefore relatively large; therefore the centering is relatively poor.

According to DIN 1412, a distinction is made between some frequently used forms:

  • Form A: Is also a conical surface grind, but with a sharpened cross cutting edge, which only makes up 8–10% of the diameter. This improves the centering effect and reduces the feed forces. It is mainly used when machining high-strength materials and large drilling diameters. This shape is easy to grind and can withstand even greater loads.
  • Shape B: Like shape A, but with an additional corrected main cutting edge. This is ground in such a way that the rake angle has a constant value of about 0 to 10 ° over the entire radius, instead of the usual 30 ° in the area of ​​the cutting edge, which can lead to lifting or bulging in unstable workpieces. Form B is mainly used for machining manganese steel or thin sheet metal.
  • Form C: conical surface point with cross point. Here the cross cutting edge is removed and replaced by two new main cutting edges by grinding the rear part of the free surface . These have sharp cutting edges and no longer squeeze the material, but can cut it. This results in reduced feed forces and better centering. This cut is used for materials that are difficult to machine such as chrome-nickel steel . However, the newly created corners on the main cutting edges are sensitive to impacts and excessive feed.
  • Form D: conical surface grind with sharpened cross cutting edge and faceted cutting edges. Here the corner of the cutting edge is beveled on the outer edge of the drill head, reducing the point angle in the outer area. This means that the loads on the secondary cutting edge are distributed over a greater length, which results in reduced wear, especially in the case of hard and brittle materials such as cast iron , since particularly heavy loads are present when the drill penetrates the cast skin . In the case of long-chipping materials, this grind leads to the creation of several chips that cross each other and thus hinder removal.
  • Form E: This is a special point with a center point and a particularly large point angle of the main cutting edges, which is around 180 °. The length of the cross cutting edge is only 5% of the diameter. This bevel is used to cut small plates out of thin sheet metal without creating a burr on the back. The sharp cross cutting edges are used to center the drill. The main cutting edges touch the sheet metal surface with their entire length, which enables burr-free drilling. However, the edge corners wear out very quickly.
  • Form V: The four-faced bevel is not standardized. Instead of the cone jacket, the tip consists of four flat surfaces. It is mainly used for very small drills with a diameter of around 2 mm and for hard metal drills.
  • Spiral point bevel: Has an S-shaped cross cutting edge and thus facilitates centering. Since the number of tool holders is limited on CNC machine tools, this bevel is often used there. This means that the center drill can be omitted and space for another tool can be created. In addition, the processing time is reduced because the operation is omitted the Zentrierbohrens.

Drill materials

HSS-E drill: especially suitable for machining stainless steel

Twist drills are made of high-speed steel (HSS; high-speed steel), simple ones of chrome-vanadium steels (CV steel). For extreme applications in tough metals, there are carbide drills . These can be designed as solid carbide drills or with a shank made of HSS or ordinary steel with soldered-in cutting edges made of carbide or a removable drill tip.

The hardness and wear resistance of these drills can be further enhanced by various coatings, e.g. B. of titanium aluminum nitrides (TiAlN → violet color, AlTiN → anthracite), titanium carbonitride (TiCN → brown-black color) or titanium nitride (TiN → golden color). Coated drills are characterized by high corrosion resistance , a long service life and significantly increased feed and cutting speeds . Furthermore, the coating can prevent sticking or even welding of the material to be machined, primarily iron-containing material, to the cutting edge and possibly make the drill suitable for dry machining in order to be able to dispense with the use of cooling lubricants . However, the use of cutting coatings does not make sense when machining wrought alloys or aluminum alloys that tend to stick . Polished chip spaces are more suitable for these materials . The increased purchase price is usually offset by the advantages mentioned. Coated drills are mostly used in CNC machining.

For processing hardened steel , manganese steel , chilled cast iron , fiber-reinforced composite materials or concrete , drills with inserted carbide cutting edges or solid carbide drills are used. On automatic machine tools , too , the solid carbide drill has largely replaced the HSS drill due to the significantly higher cutting speed and better surface quality .
Solid carbide drills can be distinguished from classic HSS drills by their slightly higher weight and darker metal color. They also often have a stepped shank so that the receiving surface fits into the collet of a machine tool, as well as two fine drill holes that run lengthways through the drill and are used to supply coolant. Depending on the hardness of the material to be machined, a solid carbide drill can have a point angle of up to 140 °. Finally, a label such as "K10 / F20" provides information on the type of hard metal used.

Web links

Commons : Drill  - Collection of Images

Individual evidence

  1. ↑ drilling. (PDF) HSS Forum, accessed November 25, 2014 .
  2. ↑ Heinz Tschätsch: Practice of machining technology. Process, tools, calculation. 11th edition, Springer Vieweg, Wiesbaden 2014, p. 107 f.
  3. ↑ a b Herbert Schönherr: Machining , Oldenbourg, 2002, p. 161.
  4. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 161.
  5. ↑ Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 326 f.
  6. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 136.
  7. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 136.
  8. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology . 11th edition. Springer Vieweg, Berlin / Heidelberg 2015, p. 320.
  9. ↑ Types of drill. Lexicon DIY store knowledge, accessed on October 18, 2015 .
  10. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 160 f.
  11. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 161
  12. ^ Fachkunde Metall, Verlag Europa-Lehrmittel, 57th edition 2013, ISBN 978-3808511572 , p. 141
  13. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology . 11th edition. Springer Vieweg, Berlin / Heidelberg 2015, p. 320.
  14. ↑ a b c d Herbert Schönherr: Machining , Oldenbourg, 2002, p. 162.
  15. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 138.
  16. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 162.
  17. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 138.
  18. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 162
  19. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 139.
  20. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 139.
  21. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 139 f.
  22. ↑ Eberhardt Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Zerspantechnik - processes, tools, technologies , Vieweg Teubner, 12th edition, 2008, p. 140.
  23. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 163.
  24. ↑ Information from the Techniker Forum , accessed in January 2016