It’s not uncommon for shop technicians to think that changing out the grinding stone every time they start on a new flywheel is a waste of time. But the reality is that not matching your grinding stone to the material being worked will waste far more time than taking a couple of minutes to switch out the stone.
By choosing the correct flywheel grinding stone for the current application, you will be able to grind the material much faster and get a better finish. By taking the time to change the stone you end up saving yourself time on the length of grind, less wear on the stone because of less need to dress and a better-looking finish to end up with.
What is the Correct Flywheel Grinding Stone?
The most important factor when selecting the correct grinding stone is matching it to the material you are going to grind.
Most American automotive flywheels are cast iron. Most truck, bus and agricultural flywheels are cast steel. And High-Performance along with a few import flywheels are billet steel. To determine the hardness of the workpiece, you can do a “spark test” using the stone currently mounted in the machine.
Performing a Spark Test
Simply briefly touch the stone to the flywheel and look at the kind of sparks you get from it.
- Cast Iron (soft) flywheels make deep orange sparks that travel a short distance before burning up.
- Cast Steel (medium) flywheels make a light orange to golden spark that travels farther before burning up.
- Billet Steel (very hard) flywheels make a bright yellow to white spark that travels even further.
For most cast iron flywheels, it’s recommended to use a “general purpose” grade grinding stone that is made of silicon carbide. These are generally dark gray or black in color. For harder material, a softer stone that is made of aluminum oxide would be appropriate. These are generally white or ruby in color.
The machine operator can affect the way a stone grinds by simply changing the amount of downward pressure applied during the grinding process. If the stone is too soft, reduce the pressure. If the stone is too hard, increase the grinding pressure. However, using this method instead of selecting the correct stone can use up your stones more rapidly. It’s always better to select an appropriate stone for the material being ground.
Dress the Grinding Stone
After selecting and mounting the correct stone for the material being ground, it is important to apply a proper dress to the stone grinding surface. There are 2 types of stone dressers available – the “star” dresser and the “diamond” dresser. Before dressing be sure you have all the proper safety precautions including good eye protection.
The star dresser is used by bringing it in close to the stone, just below the surface. Turn on the machine and slowly make contact by bringing the star dresser up, then traverse the stars under the stone at a medium pace. If you traverse too slowly, you will close the pores of the stone making it load up prematurely. A faster traverse opens the pores and allows the stone to be as free-cutting as possible. This will also reduce the time it takes to complete the job.
The diamond dresser is used in virtually the same way as the star dresser. It can be a little more tricky to get a good dress with a diamond dresser, but when you do, it will a more uniform surface. The diamond dresser needs to traverse slightly faster than you would with a star dresser. The main advantage to the diamond dresser is they tend to last longer and, with practice, will provide an ever so slightly better dress.
Coolant Mix Matters
Grinding coolant typically comes in concentrated form. You mix it with water at the recommended ratio indicated by the manufacturer. Having a proper mix is very important. The coolant not only cools the workpiece and prevents rusting, but it also carries away the stone grit and ground-off material. If your mix is too rich your stone will load up quickly and you will be polishing the workpiece instead of cutting it. If your mix is too lean then your work will get hot and could possibly start a rusting process in the reservoir of the machine.
You can use a refractometer to ensure your mix ration is correct. Water evaporates, so you will have to add more water to your coolant from time to time. But be careful, it’s easy to get lax and end up with a mixture that is too lean if you don’t monitor what you are doing.
Keep your coolant clean! You can use a sludge bag to remove the sludge build-up from the sump every few days or so. Use a deburring stick to remove any rust or burrs from the surface of the flywheel grinding table. Also, clean any remaining debris from the inside cup of the grinding stone after each job.
And lastly, it’s a good idea to periodically change the complete reservoir and start with a fresh batch of coolant mixture. The frequency of your use will determine how often you should do this.
A Profitable Operation
Grinding flywheels is an extremely profitable service for your shop. It doesn’t require a skilled machinist, it’s fast and the machine doesn’t take up much floor space. Flywheel Grinders from Van Norman are affordable because in a very short time period your customers will buy the machine for you through the labor dollars it generates.
Don’t Fear the Grind
Flywheel grinding takes very little time. If you aggressively “attack” the flywheel with steady down-feed, using the correct stone, the typical cast iron flywheel will grind in about 15 minutes. A steady, constant down-feed of about .001 per rotation of the flywheel keeps the stone free-cutting and actually works the stone. Meaning it causes the stone to “breakdown” at the correct amount to efficiently unload the material removed from the flywheel along with the fractured stone grains. If you down-feed at the proper rate with the correct stone for the metal being worked, you won’t have to dress the stone during the grinding process.
Dimensions of Stones
When shopping for flywheel stones you will find different sizes and heights and even thicknesses. A taller stone has more usable abrasive and should last longer but could come with a higher price tag. Ultimately it’s up to you to find the best “bang for your buck” when it comes to buying stones. Just remember, matching the stone to the material being ground, dressing the stone correctly, using a good coolant mixed correctly, and feeding the stone aggressively will give you better results, longer stone life and overall lower cost per flywheel ground.