• Abstract and Figures
• Public Full-text

GRINDING OF TOOL STEEL This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our “Material Safety Data Sheets”. Edition 7, Revised 12.2012 The latest revised edition of this brochure is the English version, which is always published on our web site www.uddeholm.com SS-EN ISO 9001 SS-EN ISO 14001 GRINDING OF TOOL STEEL CONTENTS Introduction 4 Grinding wheel design 4 How the grinding wheel works 6 The grinding machine 9 Grinding fluid 9 The tool steel 10 Recommendations for grinding of Uddeholm tool steel 13 Cutting speed and feed 14 Grinding wheel dressing 15 Examples of suitable grinding wheels 15–17 3 GRINDING OF TOOL STEEL tional standard by ISO, indicates the The table below shows how the Introduction composition of grinding wheels. The characteristics of aluminium oxide The high alloy content of tool steel identification consists of numerals abrasive can be varied by alloying it. means that such steel are often more and letters in a particular sequence, difficult to grind than conventional defining the abrasive, grain size, grade ABRASIVE COLOUR PROPERTIES structural steel. and binder. In order to achieve successful Normal Example: corundum Brown, grey results when grinding tool steel, it is Mixed Abrasive necessary to choose the grinding corundum Yellowbrown ougher T wheel with care. In turn, choosing the Grain size Red alumina Red Harder right grinding wheel and grinding data A 46 H V Grade White alumina White requires an understanding of how a Binder grinding wheel works. This brochure provides a quite Unfortunately, the colour of a grind- ABRASIVE detailed description of the make-up ing wheel does not always necessarily of the wheel, of how it works when It is important that the abrasive fulfils indicate the type of abrasive used in grinding and of the parameters that requirements in respect of: it, due to the fact that some grinding determine the final result. It also • hardness wheel manufacturers colour their includes recommendations for • sharpness abra-sives and binders. grinding wheels for use with Udde- • thermal resistance There is also another type of alu- holm tool steel. • chemical stability minium oxide named ceramic or sintered aluminium oxide. This abra- To day, the following four main groups sive has a fine crystalline structure, of abrasives (all synthetic) are used, which means that the grains retain fulfilling the above requirements to Grinding their sharpness better. However, its greater or lesser extents. use requires higher grinding pressure. wheel design 1. Aluminium oxide designation A (SG) A typical application for it is grinding In principle, a grinding wheel consists 2. Silicon carbide designation C tool steel in rigid grinding machines. of the following components: 3. Cubic boron nitride designation B Examples of this type of abrasive are • Abrasive 4. Diamond designation SD SG (Seeded Gel) from Norton and • Binder Cubitron from 3M. • Air pores Abrasives have different application 2. Silicon carbide is an abrasive that is areas, depending on their particular Binder used primarily for grinding cast iron characteristics, as shown partially in and austenitic stainless steel, although the table below. it can also be used for hardened tool Air steel. It occurs in two main variants: pores THERMAL the black silicon carbide and a some- DURABILITY Abrasive HARDNESS IN AIR what harder green variant, which is ABRASIVE KNOOP °C more brittle than the black material. Aluminium 3. Cubic Boron Nitride (CBN) is pro- oxide 2100 2000 Figure 1. The arrangement and proportions Silicon duced in approximately the same way of abrasives grains, air pores and bond carbide 2500 1200 as synthetic diamond, and is an abra- bridges (made up of binder) determine CBN 4700 1400 sive that is used primarily for grinding grinding wheel characteristics. Diamond 7000 650 hardened high-carbide tool steel and high-speed steel. A drawback of CBN Certain special grinding wheels, such is its high price—almost twice that of synthetic diamond. as metallically bonded diamond 1. Aluminium oxide, is the abrasive wheels, contain no air pores. most commonly used for grinding 4. Diamond is seldom used, despite It is the composition and variation steel, and is available in several vari- its high hardness, for grinding tool of the above components that deter- ants. It can be alloyed with other steel as a result of its low thermal mines the characteristic of a grinding oxides, of which the most common is resistance. Diamond is used primarily wheel. An identification system, which titanium oxide. for grinding cemented carbide and has now been ratified as an interna- ceramic materials. 4 GRINDING OF TOOL STEEL ABRASIVE GRAIN SIZE GRINDING WHEEL GRADE Vitrified grinding wheels are those The grain size of the abrasive is an The grade of a grinding wheel refers most commonly used for grinding important factor in selecting the to its hardness, i.e. how securely the tool steel. correct grinding wheel. abrasive grains are held by the binder. Resinoid is used as a binder in Grain sizes are classified in accord- It does not, therefore, depend on the grinding wheels intended for high ance with an international mesh size hardness of the abrasive used in the peripheral speeds, such as certain in mesh/inch, ranging from 8 (coarse) wheel. CBN wheels. to 1200 (superfine). Grain sizes for The grade of a grinding wheel is Rubber-bonded wheels are used for grinding tool steel are generally in determined primarily by the quantity high specific grinding pressures, such the range 24–100 mesh. Coarse grain of binder used in the wheel. A higher as for control wheels in centreless sizes are used for rapid rate of remo- proportion of binder reduces the grinding. val, when grinding large workpieces, amount of air pores and produces a Metallic binders are used for grinding softer materials or when the harder wheel. diamond and certain CBN wheels. contact surface of the grinding wheel The grade of a wheel is indicated Such wheels can withstand very high is large. Fine grain sizes are used to by a letter, indicating the hardness in peripheral speeds. produce high surface finish, when alphabetical order: grinding hard materials or when the E = very soft composition contact surface of the grinding wheel Z = very hard composition. is small. For tool steel, the most commonly The surface smoothness of the encountered compositions are within ground part depends not only on the the hardness range G–K. Indication of grain size of the grinding wheel. The the grade is sometimes followed by a sharpness of the wheel, the bonding numeral, which indicates the spread material used and the hardness of the of the abrasive particles in the wheel. wheel also play a considerable part in determining the surface finish pro- duced. GRINDING WHEEL BINDERS In the case of diamond and CBN The following binders are used to grinding wheels, European grinding bind the grains in a grinding wheel: wheel manufacturers indicate grain • Vitrified designation: V size by the diameter of the abrasive •Resinoid ,, B grains in microns, while American and •Rubber ,, R Japanese manufacturers indicate it in • Metal ,, M mesh size. The photo shows the difference between a CBN wheel and a conventional grinding wheel. As a result of the high price of CBN, wheels made from it consist of a thin layer of abrasive applied to a central hub, usually of aluminium. 5 GRINDING OF TOOL STEEL •A very large number of cutting cease to remove material and gener- How the grinding edges. ate only heat. The grinding wheel is wheel works •Very high cutting speed. The most then said to be burning the material, which can cause cracks in it. Grinding is a cutting process in which common cutting speed for preci- For a grinding wheel to work the cutting edges are formed by the sion grinding, 35 m/s = 2100 m/min., correctly, the stresses in the binder grains of abrasive. The same prin- is far above what is normal for and the strength of the binder must ciples apply for grinding as for other other cutting processes. be so balanced that, as the grains chip-cutting methods, although vari- •Very small chips, i.e. very small become as blunt as can be accepted, ous factors mean that it is necessary cutting depth for each cutting edge. they are pulled out of the binder and to consider the theory of grinding are replaced by new, sharp grains. somewhat differently. GRINDING FORCES The grinding wheel, in other words, Conditions that are special for sharpens itself. Self-sharpening also grinding. The grinding forces that act on each individual grain of abrasive are re- occurs through grain breakage, which • The cutting tool has an irregular ferred to as specific forces. A mean creates new cutting edges. cutting geometry and the abrasive value of the specific forces can be The degree of self-sharpening, i.e. grains are irregularly placed, which obtained by dividing the total force whether the grinding wheel is hard means that cutting, ploughing and by the number of cutting edges, or soft, is affected by the composi- sliding will occur, see figure 2. which depends on the size of the tion of the wheel (its design hard- • The cutting geometry can change. contact area and the number of cut- ness) and by the conditions under The method of working of an abra- ting edges in the grinding path. The which it is working.

Load more

  20

Copy Link