Grinding Journal - Summer 2007 - (Page 25)

Summer 2007 This helps understand the next crucial point: Many users mistakenly move to a courser grit size when burn is present on a ground component, but that is the opposite of what they should do! Moving to a finer grain can help alleviate a burn. Burns are often caused by rubbing of the grain on the ground surface. By moving to a smaller grit size you create more unit pressure on the grain, therefore allowing for better grain penetration into the material. Grade: Abrasive Grain Grit Size Grade Structure Bond Type Bond Modification 5SG 60 – L 8 V HP Grade is the measurement of the relative hardness of the grinding wheel. Manufacturers create different grades by varying the ratio of bond to abrasive grain in the wheel. Grade is called out by a letter in the 3rd position of the marking system. The letter “A” would be the softest grade and the letter “Z” would be the hardest. By adjusting the grade, you allow the wheel to release grains at an increased or decreased rate. Typical precision grinding operations using vitreous bonded wheels will usually use a grade between D and M. The grade of the wheel affects its material removal rate, as a softer wheel offers the constant presence of new sharp grains and the release of older dull grains. Grade also affects the form holding capability of the wheel. It is difficult to hold a tight part form with a soft wheel grade. The chart below covers these points and more: Wheel Grade Affects on Performance Variables MRR Wheel Life Form Holding Potential for Burn D,E,F,G,H,I,J,K,L,M Higher Lower Worse Lower Lower Higher Better Higher The structure of a grinding wheel is a measurement of its porosity, or the amount of air relative to other components. Structure is represented by a number that generally ranges from 4 to 30. The lower the number, the less porous the wheel. A 4, 5 or 6 structure would be considered a very dense wheel. At the other end of the spectrum, 24 – 30 structure wheels are very porous. Manufacturers typically induce structures over an 8 by putting an additional ingredient into the mix that will burn off during firing and leave small hollows in the wheel. Porosity can be “engineered” into the construction of a Targa wheel, which has an extruded ceramic grain. Natural Porosity Induced Porosity Engineered Porosity Structure: Abrasive Grain Grit Size Grade Structure Bond Type Bond Modification 5SG 60 – L 8 V HP High porosity leads to better coolant delivery into the cut plus better removal of the swarf from the grind zone, so porous wheels are typically used in surface and creepfeed grinding applications. The downside of a porous wheel is shorter wheel life. Grinding Journal 25

Table of Contents Feed for the Digital Edition of Grinding Journal - Summer 2007

Contents My Thoughts Exactly When Lives Are at Stake Walter Consolidating Production The Art of Grinding: Knowing the Source of Grinding Errors and How to Fix Them The How and Why of Conventional Vitrified Grinding Wheel Selection Grinding Glass Flat Problem Solver

Grinding Journal - Summer 2007

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