A feature of sharpening cutters is the relatively large extent and curvature of the cutting edges of their teeth. When sharpening, it is required to ensure the movement of the surface of the circle exactly along the edge. Of particular difficulty in this regard is the sharpening of shaped cutters having a backed angle. To preserve the cutter profile and simplify sharpening, the backed teeth are sharpened only on the front surface (where is the front and where is the back surface, see below). Sharpened teeth having a straight or standardized curved shape are sharpened on the rear surface. Cutting and cutting mills are sharpened on the front and rear surfaces of the teeth. Read about their sharpening in the article Sharpening circular saws.
Sharpening is carried out on specialized and universal machines for sharpening mills, less often manually.
Of tool steels, grades U7A, U8A, U9A, HG, XB5, 9XC, HVG, etc. are used.
High-speed tool steel for milling is divided into steel of normal productivity (P6M5, P9, P12, P18, etc.) and high. The latter category includes steels alloyed with cobalt, vanadium, tungsten and molybdenum (R6M3, R18F2K5, R9F2K10, R9F2K5, etc.).
The hard alloys from which the cutter teeth are made are produced in the form of inserts of standard sizes and shapes, fastened to the mill body with high-temperature soldering (for example, PSr-40 silver solder) or using threaded joints (prefabricated cutters). They consist of tungsten, titanium and tantalum carbides bonded by cobalt. Mills made of tungsten-cobalt alloys (VK2, VK3, VK6, VK6M, VK8, etc.) are used for machining cast iron, non-ferrous metals, non-metallic materials. Titanium-tungsten-cobalt alloys (T5K10, T15K6, T14K8, T30K4, etc.) are less durable than VK alloys, but they have higher wear resistance when machining parts from various types of steel. Three carbide alloys consisting of tungsten, tantalum, titanium and cobalt carbides (TT7K12, etc.) are also mainly used for steel processing.
Cutter tooth geometry
At the backed teeth, which are equipped with shaped cutters, the back surface is made in an Archimedean spiral. Since the machining of the contoured surface is very difficult technologically, the grinding of milling cutters with backed teeth is carried out along the front surface.
Regardless of how many teeth are on the cutter, each of them can be considered as a separate cutter, characterized by standard parameters for each cutter. front (γ) and rear (α) angles, the size of the ground to sharpen (f), the angle of inclination of the teeth (λ).
Pad f represents a part of the posterior surface of the tooth that undergoes grinding when sharpening along the posterior surface. The main wear of the teeth occurs along this surface, its size affects the magnitude of the friction force between the cutter and the workpiece, therefore, it must be maintained in a certain range.
Main rake angle γ. the angle between the tangent to the front surface and the axial plane. It is measured in the plane that passes through this point perpendicular to the main cutting edge.
Main rear angle α. the angle between the tangent to the rear surface at the point of the main cutting edge and the tangent to the circle of rotation of this point. The function of the angle α includes the reduction of friction between the cutter and the workpiece.
Auxiliary trailing angle α1 characterizes the increased clearance between the treated surface and the tooth body. The need for sharpening mills at the auxiliary angle arises with a certain amount of wear on the milling cutter and increasing the area f. Her goal. reduction of friction between the tooth and the processed material. Not all cutters have this angle.
Depending on the shape and direction of the cutting edge, the teeth can be straight or helical. The inclination of the cutter teeth is characterized by angle λ between the unfolded helical edge and the axis of the cutter.
The values of the angles depend on the type of cutter, the grade of alloy or steel from which it is made, and the type of material for which it is intended.
When processing viscous materials, the main rake angle is selected in the range of 10-20 ° or more. For carbide milling steels, it is close to zero or even negative. The rear angle can also vary widely.
Wood end mill sharpening
During sharpening, the whetstone is wetted with clean or soapy water. After sharpening, it is washed and dried.
As the front surface is ground, the edge will sharpen and the cutter diameter will decrease slightly.
If there is a guide bearing on the mill, it must first be removed (if possible) and only then sharpened. An attempt to save a minute will end with a ruined bearing and a damaged mill. Still need to clean the cutter from the remnants of wood resin, using a solvent.
Video: How to Sharpen a Mill On a Trimmer
As when sharpening any other tool, you need to use bars of different grain sizes, depending on the thickness of the material to be removed and the required surface cleanliness. Before sharpening, you need to make sure that the bar has the correct shape.
Sharpening each cutter, to maintain symmetry, you need to try to make the same number of sharpening movements and with the same pressure.
If the material of the cutter cutters is soft enough, instead of a bar, you can use abrasive paper glued to a flat surface (solid wood rail or a strip of steel).
Wood end mills can also be sharpened on a grinder with a low speed of rotation of the wheel, using the appropriate abrasive wheel.
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.
The use of water for cooling can lead to rust on parts and components of the machine. To eliminate corrosion, soap and certain electrolytes are added to 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.
To increase grinding performance with abrasive wheels and reduce specific wear, it is necessary to choose the highest grit, which provides the required class of surface cleanliness of the tool to be sharpened.
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.
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.
Circle shape for sharpening the rear tooth angle on a cylindrical surface. cup (CC or CC) or dish-shaped (1T, 2T, 3T), rake angle. dish-shaped or flat.
Milling Sharpening Machine
The essence of sharpening the end mill is that when it is longitudinally moved relative to the circle, it simultaneously synchronizes its rotation around its axis. Due to this, the sharpened edge is always in contact with the circle at the same height (the same sharpening angle is ensured). Synchronization of translational and rotational movements is achieved with the help of a copier needle, which abuts against a depression on the front surface of the tooth. Pressing the sharpened tooth to the needle and smoothly shifting the cutter in the axial direction, the operator in one motion makes the tooth sharpened for its entire length.
Side tooth sharpening. In a simplified form, the sharpening of helical teeth looks like this. The cutter is installed in a grip.
The copier needle is installed in the position at which it is in the highest position, and its tip touches the outer edge of the groove of the end mill.
The cutter is installed in the initial (extended) position, in which the needle is located near the shank, resting against the groove of the tooth.
The grinding wheel with the side shift handle moves to a position in which its outer edge coincides with the needle.
The engine turns on, and with the direct feed handle, the circle is slowly brought to the mill until sparking begins. Then, using the feed scale, the thickness of the removed metal is established (usually 25-50 microns).
Sharpening the tooth for its entire length is carried out by retracting the spindle with the cutter until the latter comes off the needle. In this case, you must ensure that the cutter is constantly in contact with the needle. This ensures the rotation of the cutter, necessary so that the sharpened edge is in contact with the circle at the same mutual position.
To ensure clean processing, the cutter pass is repeated one more time without changing the thickness of the removed metal. On this, the processing of one tooth ends, and a similar operation is repeated for all other teeth. To sharpen all the teeth the same, you should not change the thickness of the removed metal, which was first set using the direct feed handle.
By changing the position of the needle so that its tip abuts at different points on the tooth groove (on the edge or middle, for example), you can change the values of the angle α and α1.
Saw sharpening. To sharpen the front teeth, the end mill must be set in a position in which the sharpened tooth would be located strictly horizontally. The E-90 sharpening system is equipped with a graduated ring, which makes it easy and simple to set the end teeth horizontally. If you use a machine for sharpening mills, not equipped with a similar mechanism, you can set the horizontality of the teeth using a square.
Sharpening a tooth set horizontally is done by moving the edge of the grinding wheel along the edge of the tooth. The angle of sharpening is adjusted by shifting the circle vertically or by tilting the spindle with the mill (if possible).
Sharpening quality control
Allowable deviations of the front and rear sharpening angle of all cutters is ± 1 °. Angles can be measured with a special angle meter 2URI or pendulum angle meter.
For standard milling cutters, the radial runout of two adjacent (σcm) and two opposite (σpr) teeth is regulated, as well as the end runout. The permissible values of the radial and face beats of the cutter teeth are shown in the table below (for cutters that do not have end teeth, the allowable runout of the supporting ends is indicated).
Checking the quality of sharpening or fine-tuning is done by external inspection using a magnifier. Cutting edges of cutters should be free of nicks and potholes.
The presence of cracks on the plates of the hard alloy is determined using a magnifying glass, wetting the plate with kerosene. In this case, in the presence of cracks, kerosene acts.