An introduction to the technology

Typical grinding operations include the sharpening of tools and the shaping of hardened and unhardened workpieces. The purpose behind grinding workpieces is to remove uneven spots on surfaces, or to grind round or planar parts to a highly accurate size and surface quality. But what happens during the process of grinding?

Grinding is a chip-forming operation. Generally, a rotating grinding wheel is used as a grinding tool. In some cases the workpiece is brought to a fixed wheel, like a bench grinder. However there also are machines in use where the workpiece is clamped and the spinning grinding wheel, mounted on the tool head, moves linearly back and forth over the workpiece. This setup has the advantage of fewer moving parts and less mass movement, with the result of higher surface accuracy and quality.

The grinding wheel

The grinding wheel consists of hard and sharp-edged grains, referred to as abrasives, which are embedded in a bonding material. The projecting grains cut off chips of the material being ground. When grinding metal, friction between the workpiece and the rapidly rotating grinding wheel causes the chips to become red hot.

There are natural and artificial abrasives. Natural abrasives are natural corundum and emery. Quartz, another natural abrasive, is in sandstone.

Today artificial abrasives are used in most cases for grinding wheels. There are a number of common artificial abrasives. Two of the most common are fused corundum and silicon carbide.

Fused corundum (aluminum oxide) is made of alumina in an electric furnace and available in various grades. Silicon carbide (carborundum) is made from quartz sand and carbon powder. It has a green coloration and contains diamond-like glittering ingredients.

Abrasives are crushed by grinding mills.  For the manufacture of coarse or fine grinding wheels, the abrasive grains are sized by sifting. The granulation is designated with Arabic numerals in accordance with the mesh number of the screen through which an abrasive grain falls.

Coarse granulation offers a significant grinding capacity but results in a rough surface. Fine granulation has a smaller grinding capacity but can be used to produce a smooth surface. FFJ

AN EXAMPLE

Assembly plug gauges are used for true-to-gauge mounting of equal sized holes. The gauge part must be ground to size.

Workpiece: The hardened plug gauge must be fine finished on all sides. Center holes have been drilled and already used in turning. Grinding allowance is 0.3 mm.

Process: Three-stage longitudinal grinding—rough, finish and fine finish.

Equipment: Universal cylindrical grinding machine with 150 mm spindle height.

Preparing the workpiece: The plug gauge dimensions must be extremely accurate and true-to-shape. Regrind drilled centers that are not perfectly round.

Preparation for grinding:

1. Select the grinding wheels for roughing, finishing and fine finishing using a grinding wheel table. Clamp, balance and prepare the grinding wheels for the job.

2. Check the zero setting for the upper grinding table before starting, using a test plug with ground centers. This plug, which is always kept ready for use, is mounted between the centers and ground over. Confirm the machine grinds cylindrically by measuring the diameter at both ends.

3. Set the table feed speed based on the rotational speed of the workpiece and the grinding wheel thickness.

4. Set the table travel length and position. The grinding wheel should travel beyond the end of the workpiece by at least 1⁄3 of the grinding wheel thickness.

Mounting the workpiece:

1. Clamp the driver on the workpiece.

2. Carefully clean and degrease the dead centers and the center holes.

3. Mount the workpiece and gently tighten the feed stock center.

4. Rotate the workpiece by hand and ensure that it is guided without play.

Work sequence:

1. Rough grinding: Grind in three to five passes with an in-feed of about 0.02 mm per pass until a grinding allowance of 0.1 mm remains. Always unclamp the workpiece when changing the grinding wheel.

2. Fine-grind wheel: Grind with an in-feed of 0.01 mm per pass until grinding allowance of 0.02 mm remains.

3. Final finish: Grind the workpiece with an in-feed of 0.005 mm until complete.

Udo O. J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.