How to Select and Use the Best Cutting Wheel for the Job
How to Select and Use the Best Cutting Wheel for the Job
By Rick Hopkins, senior product manager, Weiler Abrasives Group.
Published in the Dec 2016 issue of Welding Journal.
A good weld starts with good preparation, and good weld preparation starts with a clean cut. But cutting is an art that requires both skill and knowledge of how to select and use the best cutting wheel for the job.
The type of cutting wheel to use for a given application depends largely on what material will be cut. However, at the end of the day, there is no substitute for proper technique when it comes to gaining cost savings and productivity.
Welders will always prefer a cut line that is smooth and consistent to make a clean, strong weld joint with as little filler metal as possible. The goal when cutting any material is always to use the fastest and sharpest method to cut smoothly and without contamination or smearing. This is important because a smeared surface can prevent reliable fusion of the joint during welding.
The biggest challenge welding operators face when choosing the right wheel for the cut is whether speed and smoothness are the priority, or whether durability is most important. Lower cost wheels generally have softer bonds — offering a smooth, fast cut. Because the bond is softer, these wheels are typically less durable. On the other hand, harder wheels offer exceptional durability but tend to cut more slowly. Finding the balance between speed, life and cost is often the biggest challenge, and simply selecting the least expensive wheel option can end up costing the user valuable time and money.
Selecting the best cutting wheel profile
Typical cutting wheels range from .030 of an inch up to 1/8th inch in thickness. The two most common cutting wheel profiles are Type 1, which is flat, and Type 27, which has a depressed center, or raised center hub.
Generally speaking, Type 1 wheels offer more versatility, especially when cutting profiles, corners, C channel or anything that requires the operator to cut up and over two different planes of cutting surface. Because the entire cutting surface is flat, users can use more of the wheel for cutting with less possibility of the raised center hub interfering with the workpiece.
Type 27 wheels, on the other hand, are ideal for getting into tight corners or overhangs, or for whenever the operator needs additional offset from the grinder to perform the cut. The depressed center of a Type 27 wheel provides the extra clearance sometimes needed to get the job done.
That said, an operator must be careful not to plunge too deep into the cut, as the raised hub can interfere with the material being cut and ultimately compromise the structural integrity of the wheel. For this reason, it’s most often recommended to use Type 1 cutting wheels unless the application specifically requires the additional offset provided by a Type 27 wheel.
While technique plays a major role in both the speed of the cut and lifespan of the cutting wheel, the main factor behind a wheel’s cutting performance is the cutting grain used.
Selecting the appropriate grain
There are three cutting grains commonly used in bonded abrasive cutting wheels: aluminum oxide, zirconia alumina and ceramic alumina. Aluminum oxide wheels provide an aggressive cut out of the box, but immediately begin dulling from the first cut. Ceramic alumina grains self-sharpen and are the most resistant to heat, providing a consistently high cut rate and longer life. Zirconia alumina grains are harder and sharper than aluminum oxide grains and provide an excellent value. Keep in mind that product cost also typically increases along with durability and cut rate.
Aluminum oxide wheels utilize softer bonds that are less resistant to heat, resulting in a cut that is fast, smooth and easy-to-control. However, they are by far the least durable of the three grains. Aluminum oxide wheels are the most economical and have a high initial cut rate, but the tradeoff is shorter product life. As a result, aluminum oxide wheels are ideal for cutting milder alloys such as carbon steels. White aluminum oxide wheels provide increased durability and cut rate, while maintaining a lower cost of use.
Zirconia alumina is a harder, tougher grain that is more resistant to heat than aluminum oxide. Zirconia alumina grains also maintain sharpness longer than an aluminum oxide grain. As a result, zirconia alumina wheels cut faster throughout the cutting process. Their increased toughness makes them more resistant to heat and allows for harder bonds, resulting in longer life. Zirconia alumina cutting wheels provide an excellent overall value as they do not need to be replaced as often, reducing product changeover while also minimizing costly operator downtime.
Ceramic alumina grains are designed to self-sharpen and are also more durable, making them the most expensive choice. Ceramic grains are manufactured with thousands of fracture points designed to fracture and chip during use so they self-sharpen — rather than dull — throughout the cutting process. As a result, ceramic alumina wheels maintain their sharpness and cut rate for roughly 75 percent of their lifespan before an operator feels a decrease in cutting performance.
Calculating upfront costs vs. downtime and turnover
When selecting the best cutting wheel for the job, it’s important to consider not only the up-front wheel cost, but also the costs associated with downtime and changeover. In many applications, selecting a zirconia alumina or ceramic alumina cutting wheel provides a lower overall cost of use because operators can perform significantly more work with the same wheel and spend much less time changing wheels to finish a job.
When given the opportunity to do a time study, it is often a big surprise to the line leader or foreman to see how much time their operators spend changing wheels and walking to the tool crib for replacements. One such study found that a single cutting wheel changeover took more than 30 minutes, given that each operator had to walk to the tool crib, sign out another wheel, wait for the wheel, return to their cell and then change the wheel before continuing the job. This loss of productivity can be staggering to any business. In cases like this, there is a significant benefit to selecting a higher quality, more durable product. The individual wheel cost more than pays for itself through minimized downtime and reduced overall consumable use.
What wheel thickness to use and when
Generally speaking, thicker wheels last longer, but that increased durability comes at a price. Thicker wheels cut slower and generate more friction and heat through the cut — often discoloring the workpiece and requiring additional steps to finish.
Always consider the material when selecting the best wheel for the job. On harder steels and alloys — such as armored steels, titanium, stainless, high nickel alloys and cast iron, which generate significant heat throughout the cutting process — it’s cost effective to upgrade to a cutting wheel that is more resistant to heat. Zirconia alumina and ceramic alumina are good choices in these applications.
Using proper technique
Heat and pressure are the enemy of any abrasive product. The smoothest, fastest, cleanest cuts are performed by minimizing surface contact and heat. Minimizing heat and friction also helps maximize wheel life.
Never bump, drop or use impact to enter the cut. Always enter the material from a corner or section with less surface area, and use even pressure and consistent motion through the cut. Let the wheel do the work — pushing too hard increases cut speed but also generates heat and friction, which ultimately reduces wheel life.
Angle of approach matters. Round stock is considered by many the easiest to cut because the surface contact is the same no matter where the operator enters the material. However, when cutting angle iron, I-beams or square stock, start the cut on a corner where the material is the thinnest to reduce friction and produce a clean, straight cut.
Understand the thickness of the material being cut and take care to plunge the wheel only as deep as necessary for the thickness of that material. This also will reduce heat and friction to extend product life and minimize binding.
Cutting wheels should never be used without a guard that is properly rated for the size of wheel and mounted correctly per manufacturer instructions. Type 1 cutting wheels require the use of a two-sided guard that offers 180 degrees of protection.
Before mounting any cutting wheel, check that the maximum safe rpm rating on the wheel is higher than the maximum rpm rating of the tool being used. Using a wheel with an rpm rating lower than the tool, improper mounting or tool misuse compromises operator safety, as well as the safety of coworkers, and can result in serious injury. It’s recommended to let the wheel free-spin for 60 seconds prior to beginning a cut. This allows the operator to detect any balance or structural issues with the wheel prior to use.
Finally, operators should ensure the cut line is as close to the clamping point as possible while still allowing enough room for the guard, the tool and their hands. The further an operator moves from the fixed point or clamp, the more vibration will be transferred through the workpiece, which reduces wheel life and increases the risk of wheel failure. This can also make the wheel more difficult to control.
The proper angle for holding the grinder depends on several variables, including workpiece size and orientation, tool size and user comfort. Before cutting, bring the wheel to the workpiece with the grinder stopped and look for potential interference. When the operator has determined the most comfortable angle of approach, adjust the guard so it is positioned between the body and the workpiece. Giving some forethought to the overall size and thickness of the workpiece before cutting can help ensure the guard won’t interfere with the cut halfway through.
As with any skill, practice makes perfect. It’s important to consult with a trusted abrasives manufacturer if there are questions about safety considerations, selection or technique.