Belt grinder

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Mobile belt sander, here used by a mobile sharpening service for resharpening of cutting tools, 2019

A belt grinding machine is a machine tool for belt grinding with grinding belts . The appropriate hand tool is a belt sander . In addition to the components that every machine tool contains - drive , frame and control - the support and deflection rollers or wheels are important. The grinding belts run around at least two rollers or wheels, one of which is driven. The pulleys or flat elements such as plates that press the sanding belts onto the workpieces can be used as support elements. The hardness , the circumference and the groove of the support elements have an influence on the metal removal rate and the achievable roughness .

In addition, there are various additional devices such as the counter-pressure rollers that press the workpieces against the support elements, further belt deflection rollers, a cooling lubricant system with pumps , lines and nozzles , operator terminals and cleaning, suction and drainage systems.

The width of the abrasive belts used ranges from 2 mm to 3400 mm and belt lengths between 330 mm and 24,000 mm. The frames are often made of cast iron or welded steel , sometimes also made of mineral casting, which enables precise machines due to its good damping properties.

Drives

The drive power is transmitted either by belt or directly from the motor to the drive shaft. Machines that are intended for use with cooling lubricants usually have their motors on the back. The performance depends on the exact processing task and is in the range from 0.15 to 1.0 kW per cm of bandwidth. For particularly demanding tasks, even up to 4 kW per cm of bandwidth.

Deflection and tension pulley

The at least one deflection roller can be shifted in its axis in order to tension the grinding belt and thus simultaneously serves as a tension roller. It can be pressed against the belt either with springs or pneumatic pressure cylinders .

Tension forces

The clamping forces are in the range of 5 to 8 Newtons per millimeter of bandwidth. Occasionally they are a little higher, but then lead to increased stress on the bearing parts and thus to faster wear and tear as well as to stretching of the belts. The productivity is not significantly affected by the belt tension.

Run the ligaments

The transverse forces that occur during processing can pull the sanding belt off the support roller. In order to counteract this, the deflection roller can be made spherical or swiveled out to the side by means of a belt reversal in order to pull the grinding belts back to the center of the contact roller.

Crowned rolls

Due to the crowning of the rolls, the belts are held in the middle of the rolls for belt widths up to 350 mm. The arch height, which indicates the curvature, differs depending on the diameter, but should in any case remain below 0.8 mm. The information in DIN 111, which deals with flat belt pulleys , can be applied accordingly.

Belt reversal

In so-called wide belt sanding machines with belt widths of over 300 mm, the sanding belts are actively controlled. The lateral movement of the belts is detected by sensors such as infrared sensors , pneumatic or mechanical buttons and a signal is sent to a motor which deflects the deflection roller laterally. The support and deflection rollers are then no longer parallel so that the belt runs in the middle of the roller. The control path should be around 5 to 15 mm in order to achieve the most even belt travel possible. The pulley swivels back and forth between 15 and 30 times per minute.

Counter pressure and hold-down rollers

Hold-down rollers are built into the machines in order to prevent the workpieces from being removed from the conveyors when machining flat workpieces during throughfeed grinding . With wide ribbons, high pressure forces are sometimes necessary. These rolls, known as "billy roll", are produced with counter-pressure rolls that press the workpiece against the support roll.

Support elements

Support elements are used to press the sanding belt against the workpiece. They can either be designed as support rollers and disks or as flat elements such as shoes or beams. Both have an influence on the metal removal rate and the achievable roughness.

Support shoes and beams

If the sanding belt is to be pressed onto the workpiece over a large area, flat support elements are required. This is mainly used in wood and lacquer processing to achieve a particularly high surface quality. Support shoes, beams or plates can be used as support elements. Since, in contrast to the support rollers and disks, there is friction between the support element and the grinding belt, they are equipped with friction-reducing surfaces, for example made of graphite.

Support rollers or disks

Cylindrical, rotating support elements are called disks if their diameter is larger than their width (height of the cylinder ) otherwise as a roller. In practice, they are also referred to as contact discs, back-up or contact rollers. They consist of a base body on which a tread is usually applied, which is usually also grooved. Support rollers have a body made of steel or aluminum to ensure adequate stability. The tread is made of plastic , rubber or fabric . When using cooling lubricants, treads made of elastomers are used that are based on polyurethane or nitrile rubber because they are more resistant and thus increase the service life.

hardness

The harder the tread of the support disks (measured in Shore ), the greater the metal removal rate and the greater the roughness. That is why hard support disks with hardnesses of 75 ° to 95 ° ShA are used, especially for rough machining. Bare steel rollers without a tread are sometimes used. Soft discs with 25 to 35 ° Sha are used for fine machining. The panes should always be as hard as possible and only as soft as necessary in order to maintain the required surface quality.

The harder the support disks, the less the disk deforms. The lengths of engagement and contact areas are therefore small and the contact pressures are high. Therefore, with hard discs, the abrasive grains are pressed deeper into the material and can remove larger chips, but also leave deeper grooves that are noticeable as roughness.

The shape of the workpieces to be machined also plays a role in the selection of the roles. Soft rollers adapt better to the contours of the workpiece.

diameter

The larger the diameter of the support disks or rollers, the larger the contact zone between the grinding belt and the workpiece. With a constant contact force, the contact surface increases, whereby the pressures are reduced and the individual grains can no longer penetrate so deeply into the material. As a result, the metal removal rate decreases with increasing wheel diameter, while the surface quality increases.

The shape of the workpieces also influences the choice of pulley diameter. Disk diameters between 300 mm and 450 mm are used for form, profile and cylindrical grinding. For surface grinding, on the other hand, 150 mm to 300 mm are common. For the machining of small components such as turbine blades, diameters of up to 10 mm are used.

Grooving

Support rollers are usually grooved. In the case of smooth, unslotted rollers, cooling lubricant can get between the roller and the sanding belt and trigger the aquaplaning effect there, which leads to the sanding belt slipping. Both the angle of the grooves and the ratio of land width to groove width have an influence on the work process.

Rollers with narrow webs and wide grooves lead to a high contact pressure and thus to a high cutting performance and low surface quality. Ridge-to-groove ratios of 1: 2 are usual for pre-machining, 1: 1 for medium performance and roughness is used as standard and 2: 1 for small performance and good surface quality. Straight grooved rolls lead to the highest metal removal rates and the worst surface qualities. They also lead to air turbulence which leads to a high sound pressure level and is therefore very loud. The grooves are therefore partially filled with soft foams in order to reduce noise emissions. The larger the angle of the grooves in relation to the workpiece, the lower the metal removal rate and the greater the surface quality that can be achieved.

Balancing

Insufficiently balanced support disks lead to vibrations, which can manifest themselves as chatter and deteriorate the quality of the workpiece. After a certain time, the support disks have to be sealed in order to restore even concentricity .

Individual evidence

  1. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke , Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 695.
  2. ↑ Manfred Weck : Machine tools, Volume 1 - Types of machines and application examples , Springer, 5th edition, 1998, p. 277f.
  3. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, pp. 696f.
  4. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 696.
  5. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 696.
  6. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 696.
  7. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 696.
  8. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 696.
  9. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann , Günter Spur : Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 696.
  10. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, pp. 697f.
  11. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 700.
  12. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 697.
  13. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, pp. 697f.
  14. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, p. 698.
  15. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, pp. 698f.
  16. ↑ Wolfgang Heidtmann, Martin Pischel: Belt grinding with abrasives on a backing in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd edition, Hanser, Munich, 2014, pp. 699f.