To grind means to reduce something to small particles by rubbing two objects together. When one pictures grinding, it’s not usually the daintiest image that comes to mind. Grinding tends to be synonymous with crushing, even pulverizing. However, that’s not the case when it comes to grinding machines in the metalworking industry.
Grinding machines don’t demolish and destruct parts, but quite the opposite. They’re a tool for extreme precision, and with many types of grinding out there, it’s possible to get exact results for that perfectly smooth finish your part needs.
The following is a guide to all types of cylindrical grinding—what they are, where they came from, and most importantly, how they can benefit your shop.
Grinding is an abrasive process that makes chips by ripping away bits of material using a grinding wheel that breaks apart as it removes material. Grinding wheels are disc-shaped with a hole in the center for attaching the wheel to a rotating spindle (read more about grinding wheels in the section below). The following are the different types of grinding.
The cylindrical grinder is used to remove material from the outside diameter of an object. It can work on a variety of shapes; however, the object must have a central axis of rotation.
There are several variations of cylindrical grinding:
OUTSIDE DIAMETER (OD) GRINDING
OD grinding occurs on the outside surface (the OD) of a part, while the part’s ends are used to rotate it. The wheel and object move in opposite directions while they work.
INSIDE DIAMETER (ID) GRINDING
ID grinding occurs on the inside surface of a part. In this case, the wheel diameter is smaller than the inside diameter of the part in order to effectively enter the part. Similar to OD grinding, the grinding wheel, and the object rotate in opposite directions to remove material.
Plunge grinding is a type of OD grinding, so it also removes material from the outside diameter of a part. Where it differs is that the grinding wheel comes in on the diameter and grinds the entire width of that diameter. In addition, the width of the grinding wheel is always as wide or wider than the length it needs to grind.
CREEP FEED GRINDING
In creep feed grinding, the full depth of cut is removed in a single pass of the wheel. Successful operation of this technique can reduce manufacturing time by 50%, but often the grinding machine being used must be designed specifically for this purpose. Creep feed grinding is found in both cylindrical and surface grinding.
Centerless grinding occurs when there isn’t workholding or centers to hold the part in place. Instead, there is another wheel opposite the grinding wheel, which keeps the object in place while simultaneously grinding.
Some other types of grinding include:
THE HISTORY OF GRINDERS
The basis for today's modern cylindrical grinder was first built in the 1830s by two men working independently, Jonathan Bridges and James Wheaton. These cylindrical grinders evolved from the first horizontal boring machines and lathes and were further refined in the late 1870s when Brown & Sharpe began manufacturing parts of the sewing machine. They believed the shaft and needle bars should be crafted from hardened tool steel. After much experimentation with the grinding wheel, it led them to build one of the first official cylindrical grinders.
It took another 40 years before further improvements were made to the grinding machine. It evolved with the creation of numerical control (NC) in the 1940s and again in the 1970s and 1980s, with the popularization of computer numerical control (CNC). Grinders' latest change came in the 1990s with personal computers and robotics.
Okuma has been manufacturing and perfecting grinders for over 100 years.
THE GRINDING WHEEL
Grinding wheels are composed of abrasive compounds and are used for various grinding and abrasive machining operations. The abrasive compounds are made from composite material consisting of coarse particle aggregates, which are pressed together using a bonding material. As a wheel’s abrasive compound touches the part, it cuts very small, precise chips, leaving a smooth, incredibly accurate finish.
There are three elements of a grinding wheel:
Abrasive grains - Grinds workpieces as cutting edges
Bond material - Holds abrasive grains
Pores - Lets chips out and cools the wheel down
Which abrasive aggregate you choose will depend on the material being ground and its hardness. The following are the five types of typical abrasive grains:
Aluminum oxide can be fused with other abrasive materials to achieve different degrees of purity to give them certain characteristics for different grinding operations and applications. The majority of pure (95% or above) aluminum oxide wheels are a very popular abrasive. They’re used for grinding steels and other ferrous alloys.
Silicon carbide is harder, more brittle than aluminum oxide wheels, and is typically used for grinding cast irons, non-ferrous metals (copper, brass, aluminum, and magnesium), and non-metallics (ceramics and gemstones).
Ceramic grinds materials similar to aluminum oxide but with the ability to grind at a faster rate and with less frequent dressing of the wheel. Typically, wheels are only 30-50% ceramic abrasive with the remaining being aluminum oxide abrasive.
Natural and manufactured diamonds are used for grinding abrasive. Natural diamonds are expensive, but they are perfect for grinding hard materials, such as cemented carbides, marble, granite, and stone.
Cubic Boron Nitride (CBN)
Cubic boron nitride is produced using a high-temperature, high-pressure process, making it almost as hard as diamond. It’s best used for super-hard, high-speed steels, tool and die steels, hardened cast irons, and stainless steels.
Abrasive grains are held together by a bonding material. Each type of bond has a varying strength, which is used depending on the part and the process.
Strong and rigid, this type of bond retains high strength at high temperatures and isn’t affected by water, oils, or acids. They do have a poor shock resistance.
This type of bond is best used for high speeds, rough grinding, and cut-off operations.
Silicate releases abrasive grains more so than other bonding materials. Its speed is limited.
Produces ultra-smooth finishes and is not typically used in heavy-duty situations.
Rubber bond is extremely tough and strong. It’s widely used in centerless grinding situations, as well as with fine finishes.
Metal is typically used to bind diamonds or when the bond must be electrically conducive.
Like with any machine tool, grinding wheels see wear and tear over time, which can lead to poor surface finishes and quality. Instead of replacing the entire wheel, a process called “dressing” is used to restore the wheel.
Dressing returns the grinding wheel to its original sharpness by removing old grains to reveal the fresh grain underneath.
Dressing removes clogs, dulled abrasive grains, and excess bonding material to help minimize vibration, thereby improving surface finish. Dressing also helps restore the shape of the wheel, which can change the more the wheel is used.
To learn more about dressing, check out this in-depth blog post here.
THE BENEFITS AND PERCEPTIONS OF A GRINDING MACHINE
One of the main benefits of grinding machines is their ability to produce smooth surface finishes. They also allow for a tight tolerance.
Some materials even require grinders—such as ceramics. Ceramic won’t hold up with other types of machine tools. In addition, you can use a lathe for hard turning, but the tooling cost is very expensive. You can reduce the tooling cost by using a grinder.
Grinders are also well-suited for automation because you need less operator intervention. When an insert chips on a lathe, the operator intervenes to replace the insert. However, on a grinding machine, you simply need to dress the wheel, which can be done automatically.
Many times, grinders are overlooked because developing a sound grinding process can be challenging. It takes skilled labor to be able to develop this process. Because skilled labor is hard to find, it can deter many shops from going in the direction of grinding. However, Okuma offers the same OSP control on our grinders as we do on our lathes, which can help lessen that learning curve.
If you already have Okumas in your shop, moving to an Okuma grinder is much easier than you’d expect.
GETTING THE MOST OUT OF YOUR GRINDING MACHINE
As with any machine, control systems and technologies can help run a seamless operation. Grinders have a myriad of technologies that make the process easier. New Okuma grinders have introduced I-GAP+ on all control systems. I-GAP+ is the sheet programming function that can create truing programs, dress programs, and grinding programs by only entering the data on two screens.
Programming is done by entering details of what features are being ground.
Grinding Program Creation Sheet
I-GAP+ adds a level of ease to the grinding machine that wasn’t necessarily there before. It helps machinists that may not have used a grinder before to adjust to the programming and skill that grinders require.