How do we carry out grinding for the crankshaft?

An engine is knocking and losing power. A worn crankshaft could be the problem, a high-stakes repair where a single mistake can ruin a valuable engine block.

Grinding a crankshaft is a precise machining process. You use a specialized crankshaft grinding machine and a vitrified grinding wheel to remove a small amount of material from the journals. This restores them to a perfectly round, smooth, and undersized dimension, ready for new bearings.

The crankshaft is the heart of an internal combustion engine. Its job is incredibly demanding. Getting the grinding process right is not just a job; it is a critical engineering task that demands precision down to the micron. In our business, we supply abrasives for all kinds of applications, from rough cutting to the finest finishing. We have seen firsthand how critical the right abrasive tool is for high-stakes components like crankshafts. The process is much more than just simple grinding; it is about restoring the core an engine relies on for its power and longevity.

Why do you grind a crankshaft?

Your engine is running poorly with low oil pressure. The mechanic says the crankshaft needs grinding. Why is this necessary instead of just replacing the bearings?

You grind a crankshaft to correct wear and damage. Over time, the smooth bearing journals become worn, oval-shaped, or scored. Grinding restores their perfect roundness and smooth surface finish, allowing new, oversized bearings to re-establish proper oil clearance and engine performance.

Think of the crankshaft and bearings as a system. They ride on a micro-thin film of oil¹. As an engine racks up miles, this system wears down. The constant pressure and rotation slowly wear metal off the crankshaft’s main and connecting rod journals. The journals are no longer perfectly round. This increases the clearance between the crank and the bearings. The oil film can no longer be maintained, leading to a drop in oil pressure and that dreaded "engine knock²." Simply installing new standard bearings will not fix the problem because the crank itself is damaged. Grinding is the solution. It machines the journals down to the next uniform "undersize." This creates a fresh, perfectly round surface. You then fit corresponding "oversize" bearings to match this new dimension, restoring the precise clearance required for the oil film. Grinding saves a valuable crankshaft from the scrap heap and is the foundation of a proper engine rebuild³.

What is a crankshaft grinding machine?

You need to grind a crankshaft, but you know your bench grinder won’t work. What makes a crankshaft grinding machine so special and different from other grinders?

A crankshaft grinding machine is a large, highly specialized industrial lathe. It has two headstocks and precise chucks to hold the long, awkward crankshaft. Crucially, it allows for the precise offsetting of the machine’s axis to grind the off-center connecting rod journals.

A crankshaft is not a simple cylinder. It has its main journals, which run along the center axis, and its rod journals, which are offset from the center. A standard lathe or cylindrical grinder can only work on the center axis. The crankshaft grinder is engineered specifically for this challenge. It is a massive, rigid machine to prevent any vibration that would ruin the finish. It holds the crankshaft at both ends. The most important feature is its adjustable chucks. These allow the operator to physically shift the crankshaft’s center of rotation. To grind the main journals, the crank is spun on its natural centerline. To grind a rod journal, the operator adjusts the chucks to make that specific journal the new center of rotation. This precise C-axis adjustment is what defines the machine. It is a piece of precision equipment that requires a skilled operator to set up and run correctly, as each journal must be ground to an exact diameter and location relative to the others.

What is the grinding process of the crankshaft?

Taking a worn crankshaft and restoring it to perfect condition seems like a complex task. What are the exact steps a machinist follows to ensure a flawless result?

The process involves cleaning and inspecting the crank, then mounting it in the grinder. The operator first grinds the main journals to a uniform undersize. They then re-adjust the machine’s offset chucks to grind each connecting rod journal to the same precise dimension.

The crankshaft grinding process is methodical and requires constant measurement and verification. It is a true craft that combines machine operation with precision metrology⁴. As a manufacturer, we supply the wheels, but we know the machinist’s skill is what guarantees success. Here are the typical steps involved.

How does the grinding process work?

You know grinding uses an abrasive wheel, but how does that action produce the mirror-smooth finish needed for an engine? What is happening at the microscopic level?

The grinding process works by using a spinning wheel made of thousands of tiny, hard abrasive grains. These grains act like microscopic cutting tools, shearing away tiny chips of metal from the crankshaft’s surface to achieve a precise dimension and a very fine finish.

Grinding is fundamentally a material removal process. The vitrified bond grinding wheel is a complex tool. Each abrasive grain on its surface acts like a blade. As the wheel spins at high speed and presses against the workpiece, these grains cut away metal. The quality of the finish depends on the grain size, the wheel’s hardness, and the operator’s control. However, for critical applications like engine components, the process does not stop there. We call this final step honing or superfinishing⁵. After the crankshaft is ground to the correct size, it needs an even better surface finish. A ground surface, when viewed under a microscope, has tiny peaks and valleys. Honing uses a much finer abrasive, often in the form of a honing stone or abrasive tape, to smooth out these peaks. Our factory produces high-quality diamond honing tools specifically for this purpose. These diamond tools provide the ultimate level of precision, creating the perfect "plateau" finish on the journal. This specific surface texture is designed to retain oil, ensuring the bearings are properly lubricated from the very first engine start.

Conclusion

Crankshaft grinding is a precise machining process that restores worn journals. It uses specialized machines and fine abrasive wheels, and is often followed by honing for a perfect finish.