A common question among plastic injection mold makers is: “What is the best grinding wheel for tool steel?” Obviously, there are many variables that have a direct impact on the answer.
Here are some user-friendly, experience-generated guidelines that will at least point the surface grinding toolmaker in the right direction.
This article covers the type of grinding commonly found in mold making and tool-and-die shops, as well as many general precision machine shops. More specialized grinding, such as the jig-grinder, Blanchard grinder, and tool-and-cutter grinder are commonly used, but such specialized methods are not the focus here.
The typical tool shop has manual surface grinders, automatic wet surface grinders, cylindrical grinders and internal grinders. These all share similar metal cutting mechanics, wheel composition, coolants and general good practice techniques.
Grinding Wheel Specifications
Understanding the basics of the numbers and letters on the grinding wheel blotter paper will help the toolmaker intelligently decide which wheel to use for each job. True, you can “get by” with just a few general purpose wheels, but, to be effective you need to work smart and use the best wheel for the job.
Here are some general rules for understanding the markings on the wheels.
RA46-G800-VOS — Radiac Abrasives Ruby Red surface grinding wheel
- R stands for Ruby, which is a very hard grade of aluminum oxide, designed for tough tool steels.
- A represents Aluminum Oxide, which is by far the most common type of abrasive in typical grinding wheels.
- 46 stands for the grit size. Grits generally are between 32 and 220 grit. This number is the result of an international mesh size. Think of a screens with varying numbers of openings per square inch: the more openings, the greater the number, the smaller the grit.
- G represents the grinding wheel grade. The grade refers to it’s hardness, or how hard the abrasives are held by the binder. Most wheels fall between G-K, with lower letters in the alphabet indicating softer grades.
- 800 means that it has induced porosity. This particular wheel is quite porous, and is known by Radiac as Por-Os-Way.
- VOS indicates the method of binding the grains together: vitrified. The V is for vitrified, and the OS stands for Open Structure.
32A46-H8VG — Norton general purpose tool room grinding wheel
- 32A is Norton’s proprietary type of aluminum oxide, known as Alundum
- 46 specifies the grit size. The smaller the number, the coarser the grit.
- H is the grade, or hardness. The lower the letter alphabetically, the softer the grade.
- 8 indicates the structure, or how close the grains are together. The more tightly compacted the grains are, the lower the number. 8 is a rather open structure and has been shown to be the best structure for this type of Norton wheel.
- VG is vitrified in the Norton type.
As is evident, Radiac and Norton have similar, but slightly different markings on their grinding wheels.
Mold making or tool-and-die shops generally limit the number of suppliers to only a few, which makes the process of choosing the right wheel for the job simpler.
Other grinding wheel specifications
There are 4 main abrasive groups that concern toolmakers:
- Aluminum oxide (designated as A or SG)
- Silicon carbide (designated as C)
- Cubic boron nitride (designated as B)
- Diamond (designated as SD)
Reputable grinding wheel companies include: Norton, Radiac, Camel, and Winterthur.