Sharpening Saws At Home
Material and properties of carbide teeth
In domestic saws, sintered tungsten-cobalt alloys of VK grades are used as material for cutting inserts (VK6, VK15, etc. The figure indicates the percentage of cobalt). BK6 has a hardness of 88.5 HRA, BK15. 86 HRA. Foreign manufacturers use their alloys. VK hard alloys are composed mainly of cobalt cemented tungsten carbide. The characteristics of the alloy depend not only on its chemical composition, but also on the grain size of the carbide phase. The smaller the grain, the higher the hardness and strength of the alloy.
Carbide plates are fastened to the disc by high temperature soldering. As a material for soldering, in the best case, silver solders (PSr-40, PSr-45) are used, in the worst case. copper-zinc solders (L-63, MNMTS-68-4-2).
Carbide tooth geometry
The following types of teeth are distinguished in shape.
Straight tooth. It is usually used in saws for longitudinal fast sawing, in which quality is not of particular importance.
Oblique (beveled) tooth with left and right angle of inclination of the rear plane. Teeth with different angles of rotation alternate with each other, which is why they are called alternately chamfered. This is the most common tooth shape. Depending on the size of the sharpening angles, saws with an alternating cut tooth are used for sawing a wide variety of materials (wood, chipboard, plastics). both in the longitudinal and transverse directions. Saws with a large angle of inclination of the rear plane are used as cutting saws when cutting boards with double-sided lamination. Their use avoids chipping of the coating at the edges of the cut. Increasing the bevel angle reduces the cutting force and reduces the risk of chipping, but at the same time reduces the resistance and strength of the tooth.
The teeth may tilt not only the back, but also the front plane.
Trapezoidal tooth. A feature of these teeth is the relatively slow blunting speed of the cutting edges in comparison with the alternately chamfered teeth. They are usually used in combination with a straight tooth.
Alternating with the latter and slightly rising above it, the trapezoidal tooth performs rough sawing, and the straight line follows it. fine. Saws with alternating straight and trapezoidal teeth are used for cutting plates with double-sided lamination (chipboard, MDF, etc.), as well as for sawing plastics.
Conical tooth. Saws with a conical tooth are auxiliary and are used to cut the bottom layer of the laminate, protecting it from chips during the passage of the main saw.
In the vast majority of cases, the front edge of the teeth is flat, but there are also saws with a concave front edge. They are used for fine cross cutting.
Sharpening angles are determined by the purpose of the saw. those. what cutting material and in which direction it is intended. Saws for longitudinal sawing have a relatively large rake angle (15 °.25 °). For saws transverse sawing angle γ usually ranges from 5-10 °. Universal saws designed for transverse and longitudinal sawing have an average value of the rake angle. usually 15 °.
The sharpening angles are determined not only by the direction of cut, but also by the hardness of the material being cut. The higher the hardness, the smaller the front and rear angles should be (less tooth sharpening).
The rake angle can be not only positive, but also negative. Saws with this angle are used for cutting non-ferrous metals and plastics.
The basic principles of sharpening
When sawing massive workpieces, lateral faces also undergo rapid wear.
Excessive blunting of the saw should not be allowed. The radius of rounding of the cutting edge must not exceed 0.1-0.2 mm. In addition to sharply reduced productivity when working with a very blunt saw, sharpening it takes several times longer than sharpening a saw with normal bluntness. The degree of bluntness can be determined both by the teeth themselves, and by the type of cut that they leave.
The correct sharpening of circular saws is to simultaneously ensure the proper sharpness of the cutting edge, to ensure the maximum number of sharpenings, which in the best case can reach 25-30 times. For this purpose, a carbide tooth is recommended to be sharpened on the front and rear planes. In fact, the teeth can also be sharpened in one frontal plane, but the number of possible sharpenings is almost two times less than when sharpening in two planes. The figure below illustrates why this is happening.
The last pass when sharpening the saw blades is recommended to be done along the back plane of the tooth. The standard amount of metal removal. 0.05-0.15 mm.
Before sharpening, clean the saw from contaminants, such as tar, and check the values of the sharpening angles. On some saws they are written on the disc.
Equipment and materials for sharpening saw blades
When using abrasive wheels (especially diamond), it is desirable to cool them with coolant.
With increasing temperature, the microhardness of abrasive materials decreases. Raising the temperature to 1000 ° C reduces the microhardness by almost 2-2.5 times compared with microhardness at room temperature. An increase in temperature to 1300 ° C causes a decrease in hardness of abrasive materials by almost 4-6 times.
Video: Sharpening Saws At Home
The use of cooling water can lead to rust on parts and components of the machine. To eliminate corrosion, soap and certain electrolytes are added to the water (sodium carbonate, soda ash, trisodium phosphate, sodium nitrite, sodium silicate, etc.), which form protective films. In conventional grinding, soap and soda solutions are most often used, and in fine grinding. low concentrated emulsions.
However, when sharpening saw blades at home with a low intensity of grinding operations, cooling of the wheel is not resorted to so often. not wanting to waste time on it.
To increase sharpening productivity with abrasive wheels and reduce specific wear, it is necessary to choose the highest grit, which ensures the required cleanliness of the surface of the sharpened tooth.
To select the grain size of the abrasive, in accordance with the stage of sharpening, you can use the table in the article about bars for sharpening. For example, in the case of using diamond wheels, circles with a grain size of 160/125 or 125/100 can be used for rough grinding, for finishing. 63/50 or 50/40. Wheels with grain sizes from 40/28 to 14/10 are used to remove nicks.
The peripheral speed of the circle when grinding carbide teeth should be about 10-18 m / s. This means that when using a circle with a diameter of 125 mm, the engine speed should be about 1500-2700 rpm. Sharpening of more brittle alloys is performed at a lower speed from this range. When sharpening carbide tools, the use of hard conditions leads to the formation of increased stresses and cracks, and sometimes to chipping of the cutting edges, while the wear of the wheel increases.
When using machines for sharpening circular saws, changing the relative position of the saw and the grinding wheel can be implemented in different ways. moving one saw (the engine with the circle is stationary), simultaneously moving the saw and the engine, moving only the engine with the circle (saw blade is stationary).
A large number of grinding machines of various functionality are produced. The most complex and expensive programmable systems are able to provide a fully automatic sharpening mode, in which all operations are performed without the participation of a worker.
In the simplest and cheapest models, after installing and securing the saw in a position that provides the necessary sharpening angle, all further operations. turning the saw around its axis (turning on the tooth), feeding it to grinding (bringing it into contact with the wheel) and controlling the thickness of the metal removed from the tooth. are made by manual workers. Such simple models are advisable to use at home, when the sharpening of circular saws is episodic in nature.
An example of the simplest machine for sharpening circular saws is the system, a photograph of which is presented in the photo below. It consists of two main nodes. engine with a circle (1) and a support (2) on which the sharpened saw is installed. The rotation mechanism (3) serves to change the angle of inclination of the blade (when sharpening teeth with a beveled front plane). Using the screw (4), the saw is shifted along the axis of the abrasive wheel. This ensures that the required value of the rake angle is set. The screw (5) is used to set the stopper in the desired position, preventing the circle from entering the interdental cavity excessively.
The process of sharpening saw blades
The saw is mounted on a mandrel, clamped with a cone (centering) sleeve and nut, then set in a strictly horizontal position by means of a mechanism (3). This ensures a bevel angle of the front plane (ε1) equal to 0 °. For disc sharpening machines that do not have a built-in angle scale in the tilt mechanism, this is done using a conventional pendulum protractor. In this case, check the horizontal position of the machine.
By rotating the screw (4) of the mechanism that provides horizontal movement of the mandrel with a circle, the necessary rake angle is set. In other words, the saw moves to a position in which the front plane of the tooth fits snugly against the working surface of the circle.
A marker is used to mark the tooth from which sharpening begins.
The engine turns on and the front plane is sharpened. putting the tooth in contact with the circle and several movements of the saw back and forth while pressing the tooth against the circle. The thickness of the removed metal is controlled by the number of grinding movements and the force of pressing the tooth against the abrasive wheel. After sharpening one tooth, the saw is brought out of contact with the wheel, it is turned on one tooth and the sharpening operation is repeated. And so on until the marker mark makes a full circle, indicating that all the teeth are sharpened.
Sharpening the tooth, beveled on the front plane. The difference between sharpening a chamfered tooth and sharpening a straight line is that the saw must be installed not horizontally, but with a slope. with an angle corresponding to the bevel angle of the front plane.
The angle of inclination of the saw is set using the same pendulum protractor. First, a positive angle is set (in this case, 8 °).
After that, sharpening every second tooth.
After sharpening half the teeth, the angle of the saw blade changes from 8 ° to 8 °.
And every second tooth is sharpened again.
Rear Sharpening. To sharpen a tooth on the back plane, it is necessary that the machine for sharpening saw blades allows you to set the saw in such a way that the back plane of the tooth will be in the same plane with the working surface of the abrasive wheel.
If there is no machine for sharpening saw blades
Accurately maintain the necessary sharpening angles while holding the saw in your hands while weighing. the task is impossible even for a person with a unique eye and enviable hardness of hands. The most reasonable in this case. to make the simplest sharpening device, which makes it possible to fix the saw in a certain position in relation to the circle.
The simplest of such devices for sharpening circular saws is a stand, the surface of which is flush with the axis of the grinding wheel. Putting a saw blade on it, you can ensure the perpendicularity of the front and rear planes of the tooth with respect to the saw blade. And if the upper surface of the stand is movable. securing one side pivotally and the other. resting on a pair of bolts that can be screwed in and out. then it can be installed at any angle, getting the opportunity to sharpen an oblique tooth along the front and rear planes.
True, in this case one of the main problems remains unresolved. Exposure of the same front and rear sharpening angles. This problem can be solved by fixing the center of the saw relative to the abrasive wheel in the desired position. One way to implement this. make a groove for the mandrel on which the saw is mounted on the surface of the stand. Moving the mandrel with a circle in the groove, it will be possible to withstand the necessary front angle of sharpening the tooth. But for sharpening circular saws with different diameters or sharpening angles, it should be possible to either move the engine or the stand and the groove with it. Another way to provide the required sharpening angle is simpler, and is to install on the table the stops fixing the disk in the desired position. At the end of the article there is a video demonstrating such a device.
Sharpening quality control
GOST 9769-79 sets certain tolerance values for sharpening parameters. The end runout of the tooth tips should not exceed 0.2 mm (for saws with a diameter of up to 400 mm), radial. 0.15 mm. The runout is checked using a conventional dial gauge.
Deviations of sharpening angles should not exceed:
- for the front corner. ± 1 ° 30 ‘;
- for a back corner and bevel angles of a front and back planes. ± 2 °;