Clearing
The space is a manufacturing method of machining technology , in which a broach to a broaching machine is drawn along a workpiece (exteriors) or is pulled through an existing bore (indoor). The tool has several cutting edges, which are arranged one behind the other and each staggered by the chip thickness . The infeed is therefore integrated in the tool. Since the geometry of the cutting wedge is known, the machining process with a geometrically defined cutting edge , which also includes turning , milling or drilling , is one of the methods .
Typically, the tool is pulled through a hole in the workpiece, whereby the tool cross-section is reproduced in the workpiece. Broaching can, however, also be carried out with moving workpieces or to create external surfaces. Typical shapes are grooves, general profiles and internal teeth. Broaching is a very productive and accurate process. With just a single stroke, even complicated shapes can be produced in finished part quality, which are difficult or impossible to produce using conventional processes such as turning, drilling or milling. But the broaching tools are relatively expensive and are only suitable for a certain shape, so the flexibility is low. Broaching is therefore mostly used for the production of complex shapes in large numbers in automotive and mechanical engineering. The tools usually consist of coated or uncoated high-speed steel and are reground if necessary; Carbide or cutting ceramics are only used in rare cases : for example in hard broaching, a variant of hard machining of materials with a hardness of over 50–60 HRC . The cutting speeds are in the range from 1 m / min to 30 m / min, with high-speed broaching also up to 129 m / min. Broaching allows ISO tolerances from IT8 to IT7 and roughness from 1.6 to 25 µm.
Definition according to DIN 8589
Broaching is defined as follows in DIN 8589, which is often cited in the specialist literature: Broaching is cutting with a multi-tooth tool with a straight, helical or circular cutting movement. The feed movement is replaced by the staggering of the cutting teeth of the tool.
Range of workpieces
With the spaces, complicated internal profiles can be created as sets of teeth on gears or racks (also helical), hub keyways , splined hubs , and other grooves, holes on connecting rods or forks , sockets , the functional surfaces of open-end wrench and ring gears or cylinder lock cores . Crankshafts can also be manufactured using the special turning broaching process in combination with turning .
Range of materials
Broaching is suitable for a large number of materials. However, the strength should be above 400 to 500 N / mm², otherwise the chips will be too long and below 1200 to 900 N / mm² in order to keep the load on the cutting edges low. A high cutting edge load goes hand in hand with increased wear on the tools .
Since the feed is integrated in the tool during broaching, the planned material must be taken into account when designing the tools and thus when designing the process. In addition to the machinability in general, the chip formation and the achievable surface quality are particularly important .
Steels
Case-hardened and heat -treated steel can be machined well in the standard, normalized condition, provided there is a uniform structure of pearlite and ferrite , with a medium grain size. Steels with high strength, i.e. a higher carbon content of around 0.6%, can sometimes be machined more economically in the soft-annealed condition.
Free-cutting steels generally have good machinability due to their sulfur content and can be broached easily. The tool wear is less with them than with case-hardening and heat-treatable steels, and the achievable surface quality and tool life are higher. However, sulfur has an adverse effect on the heat treatment. In cases where it cannot be used as an alloying element, additions of lead have proven useful.
Irregularities in the material have a negative effect. Satisfactory results are not achievable in particular with lines made of ferrite in the reaming direction. Solidifications on the surface, which can result from previous processing, for example during cold extrusion , are unfavorable. Cold rolling or hammering, on the other hand, hardly causes any problems. The processing of such workpieces by broaching is even common.
cast iron
Cast iron with lamellar graphite and ductile iron can be processed well in case a fine and uniform graphite is present distribution and the casting skin in front of the spaces is removed, for example by shot peening . Larger accumulations of cementite - the main constituent of cast iron - and slag inclusions lead to high local stresses on the tools, which result in poor surfaces.
Non-ferrous metals
Most light metals and their alloys as well as some bronzes can be cleared well. The surface quality is usually much better than that of steel.
Broaching process
The clearing can be divided according to several criteria. The classification according to DIN 8589, which is frequently used in machining practice and specialist literature, is based on the shape created in plan, round, screw, profile and shape spaces. These are each subdivided into interior and exterior spaces. In practice, however, a distinction is usually made directly between indoor and outdoor spaces, as these require different tools and machines. There are also the special processes of turning broaching and chain broaching, as well as hard machining, which is usually carried out as dry machining without cooling lubricant , and high-speed broaching with cutting speeds of up to 120 m / min.
Classification according to DIN 8589
All machining processes are defined and classified in DIN 8589. All procedures have a serial number there. In the evacuation process, this always begins with the sequence 3.2.5. This stands for the third main group ( cutting ), the second group ( cutting with a geometrically defined cutting edge ) and the fifth process (broaching). The further subdivision takes place according to the generated shape. The order number 3.2.5.4 is not used because the machining processes are divided according to a uniform scheme. The fourth process variant is intended for hobbing feed movement as in hobbing or Wälzhobeln . Since there is no feed movement when broaching, the order number is omitted.
Plan rooms
The plan clearing bears the order number 3.2.5.1 and is used to produce flat surfaces that can be inside or outside. These include grooves, the parting surfaces on cylinder blocks in engines, and the contact surface of screws on crankshaft bearing caps.
Round rooms
The round broaching with the order number 3.2.5.2 is only used for the interior broaching of round cross-sections. It is occasionally used as a combined round broaching and subsequent profile broaching with a single tool, for example in the production of gears. Round inner surfaces can usually be produced more economically by drilling or turning . Rubbing is used for high quality .
Screw broaching
The screw broaching with the number 3.2.5.3 is used to produce screw-shaped shapes, for example for helical gears. A rotating movement is superimposed on the straight cutting movement. Both can be done either from the workpiece or the tool.
Profile broaching
The profile broaching with the number 3.2.5.5 is the most frequently used method for generating any profiles that are produced with a profiled tool. Applications are the production of internal profiles, such as splines , hexagonal edge, inner squares and polygons, internal gears for automatic transmissions, sliding sleeves , spline hub profile or splined shaft profile. If the profiles are not rotationally symmetrical, the tool can run sideways, which leads to poor positional accuracy.
External profiles are also made. The most common are steering racks, steering nuts or half-bores of crankshaft bearing caps. It is also used for so-called "Christmas tree" profiles with which turbine blades are anchored to their shaft. External profile broaching is also suitable for closed profiles such as spur gears , which is referred to as tube or pot broaching . The tool consists of a hollow cylinder with inwardly directed cutting edges through which the workpiece is pressed. There are also variants with a moving tool. Broaching inner profiles with a collar is called clearing blind holes , but this is rarely used.
Shape broaching
The shape broaching has the order number 3.2.5.6 and is used with controlled circular cutting movement to create any shape. A distinction is made between swivel broaching with stationary workpiece and rotating tool and rotary broaching with rotating workpiece, similar to turning . A distinction is also made between simple turning broaching with straight tool movement and rotary turning broaching with rotating tools.
Turn broaching combines the advantages of turning as a continuous process with those of broaching with multi-edged tools. It was first used on an industrial scale in 1982 by American automobile manufacturers for machining crankshaft main bearings, but had been known for a long time. Because of the complex tools, it is only suitable for large-scale and mass production, but is very well suited for this because of the short cycle times. The dimensional and shape accuracy are basically good; Deviations from the roundness are due to the kinematics of the process principle and cannot be avoided, but are not particularly high and are between five and ten micrometers. The roughness is around R t = 6–8 µm and R a = 0.5–0.7 µm. If the tool movement is linear, the feed direction angle changes during machining. The tool rake and clearance angle are therefore no longer approximately identical to the effective rake and clearance angle. In addition, the chip thickness changes during tooth meshing, similar to milling . When broaching, the cutting edges are usually made of hard metal or cutting ceramics . They are used as indexable inserts in the tools, which can therefore also be adapted to different shapes.
Interiors
For interior broaching, the broaching tool is first inserted into the pre-drilled hole in the workpiece and gripped from the other side before the actual work movement begins. The broach with its many cutting edges is pulled or pushed through the workpiece and creates the contour of the broach in the opening in the workpiece.
Outside spaces, chain spaces
If the tool is guided along the outside of the workpiece during the working stroke, we speak of so-called external spaces . Here, a pre-machined outer contour on the workpiece, z. B. the jaw opening of a forged wrench finished. Because of the large cutting and displacement forces, the material to be processed must be rigidly clamped and supported. When broaching outside, you can also let workpieces pass continuously with a stationary tool. This procedure is called chain broaching.
Wet, dry, hard and high-speed rooms
Broaching is normally used with cooling lubricants to improve the removal of the chips and to avoid the generation of heat through lubrication. Oils are mostly used because of the normally low cutting speeds between 1 m / min and 30 m / min. The temperatures are then around 200 to 600 ° C, so that high-speed steel can be used as a cutting material. With particularly powerful machines, up to 120 m / min can be achieved. The speeds are generally limited because the tools have to be accelerated and then braked again. At the beginning of the 20th century, however, the higher machine costs for the high speeds were accepted, as this also increases productivity.
To reduce the required cooling lubricant, special coatings can also be used, which consist of several layers of hard materials and soft materials containing lubricants. In principle, dry machining is also possible. This is particularly common for hard machining . This is understood to mean the machining of workpieces that have a hardness of over 60 HRC. The cutting materials used for this are at least hard metal ; tools with indexable inserts made from cutting ceramics are also used occasionally .
Broaching tools
Broaching tools usually consist of high-speed steel (HS), depending on the application, also HSS-E (with cobalt alloy) or HSS-PM (powder metal) or with a titanium nitride coating.
- Outside spaces: broach
- Interiors: broach
Broaching tools consist of a shaft, lead-in, toothing, guide piece and end piece. In the chip part of the toothing, the chip thickness h is created by staggering the cutting teeth . The roughing teeth take over most of the chip removal. The finishing teeth consist of at least three teeth. The last finishing tooth and the first tooth of the reserve create the finished dimension. Each tool has at least three reserve teeth that have the same profile and dimensions and smooth (calibrate) the material surface. They also serve as a reserve when re-sharpening and thus ensure that the expensive tool can be used longer. The cutting angles and the chip chambers depend on the length of the area to be broached and the machinability of the material. The clearance angle is very small so that the profile is retained when the tooth face is resharpened. Chip breaker grooves in the flanks prevent chips from penetrating between the flank sides and the areas that have already been cleared and tearing them open. The tooth pitch must be chosen so that two to six teeth cut at the same time, if possible. The more teeth are in mesh, the smoother the broaching process. However, the required broaching force also increases with the number of teeth. The cutting speed is 1–60 m / min. The chip thickness is 0.05 mm for roughing and 0.005 mm for finishing.
Achievable accuracies
When broaching, complete machining is usually carried out in a single stroke. The surface on the workpiece is created by the last finishing tooth of the tool. The teeth behind it are identical to this and serve as reserve teeth when the last finishing tooth is reground. The next replacement tooth automatically becomes the last finishing tooth. The achievable dimensional accuracies are IT7 to IT9 ( ISO tolerance ). The better the vibrations can be avoided or damped, the greater the surface quality. The achievable roughness ranges from R t = 1.6 to 25 µm. The vibrations are often caused by movements of the end of the tool, if the tool is very long and is only pulled through the workpiece. Pushed tools, on the other hand, tend to buckle. Every time another tooth of the tool reaches the workpiece, the cutting force increases. In the case of straight-toothed tools with an inclination angle of zero, it increases sharply and thus promotes vibrations. In helical tools, the cutting edges gradually penetrate the material and thus lead to lower vibrations. There are also special designs in which the tools are both pulled and pushed in order to reduce vibrations.
Cooling and lubrication
Cooling and lubrication increase the service life of the tools. Here find coolants oil- or water-based application. Also, minimum quantity lubrication is used, especially in tools made of HSS-PM, as well as those with TiAlN coating.
Others
Broaching offers advantages wherever contours cannot be created by rotating tools ( milling ) or rotating workpieces ( turning ). The process stands for reliable adherence to dimensional tolerances and a high surface quality , e.g. B. wrenches , gears or grooves .
It is particularly suitable for processing larger quantities, as complex profiles can also be produced in a very short time.
The production of broaches is quite complex, so the tools are correspondingly expensive and have long delivery times.
See also
- List of machining processes
- Chip formation
- Machinability
- High speed machining
- Energy conversion and heat during machining
Web links
Individual evidence
- ↑ Uwe Heisel, Fritz Klocke , Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 23 f.
- ↑ Herbert Schönherr: Machining Technology , Oldenbourg, 2002, pp 324-329.
- ↑ Berend Denkena, Hans Kurt Tönshoff: Spanen - basic , 3rd edition, Springer, Berlin 2011, p. 253.
- ^ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 468 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ^ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 468 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 468 f. in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 469 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 467 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 325.
- ^ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 468 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p.
- ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. In: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology , Springer, 9th edition, 2010, p. 308.
- ^ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 468 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 487 f.
- ↑ Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology , Springer, 9th edition, 2010, p. 308.
- ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, pp. 490–493.
- ↑ Heinz Tschätsch: Practice of machining technology. Process, tools, calculation. 11th edition, Springer Vieweg, Wiesbaden 2014, p. 204 f.
- ↑ Heinz Tschätsch: Practice of machining technology. Process, tools, calculation. 11th edition, Springer Vieweg, Wiesbaden 2014, p. 204 f.
- ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. In: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
- ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 484.
- ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 339.
- ↑ Berend Denkena, Hans Kurt Tönshoff: Spanen - basic , 3rd edition, Springer, Berlin 2011, pp. 235–260.
- ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 475 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.