Weighing all the options is half the battle
September 16, 2010
When shops examine deburring technology, they have myriad options to consider. Carefully weighing the alternatives is paramount.
Fabrication technology has advanced dramatically. Lasers are more powerful than ever; turret punching systems are doing more forming; waterjet machines are commonplace. And all of these blanking systems create burrs or sharp square edges that are potential liabilities. This is where deburring can help.
Before choosing a deburring machine, define the desired edges your parts require. This sounds simple; after all, most prints say that parts must be “burr-free.” But what level of deburring does this spec really require? No industry standard defines what exactly constitutes “burr-free,” and no two people can agree on how good is “good enough.”
Spend too much time deburring, and costs spiral; too little time, and the part may require rework. Today the burden rests with the manufacturer to find the optimal balance.
Those processing sheet metal and steel plate deal with two types of burrs, mechanical and thermal. A mechanical burr is created by shearing, punching, sawing, routing, and drilling. Sharp tools, proper die clearance, as well as precise feed rates reduce the burr substantially, but do not eliminate it.
A thermal burr usually is referred to as slag or dross. This burr occurs during plasma or laser cutting. The proper fuel mixture and cutting speed, replacement of burning tips, and correct optics and gases on lasers can substantially reduce the burr.
Scallops, or nibble marks, are similar to burrs but are on the part’s vertical edges, created during turret punching by the repetitive motion of the parting tool. Traditional deburring covers only the surface work, not those scallops on the vertical edges. Note that no wide-belt deburring machine can remove nibble marks down the entire part edge.
Deburring systems use belts, brushes, or a combination of the two (see Figure 1 and Figure 2). Brushes and belts may be configured for the application. Years ago the single-head abrasive-belt machine was the most common. It removed the vertical burr and created a grain, or brush, finish. But it also left horizontal burrs and sharp edges. In other words, many burrs were knocked down—but not out.
This is why multimedia belt-brush machines have become more popular during the past 20 years. Many today realize the benefit of three or four heads in a machine and want the flexibility that comes from combining abrasive belts and brushes. They effectively remove burrs and produce smooth part surfaces and edge.
The abrasive belt rides around a contact roller, or drum, and an idler roller. The contact roller works with the idler roller to tension and track the abrasive belt. Contact drums, usually made of rubber, can vary widely in size, but are often between 6 and 11 inches in diameter.
The contact drum’s rubber hardness—called the Shore or durometer—also varies, usually from 25 to 85 Shore. The 25-Shore rubber is soft and flexible, which allows it to conform to the shape of the part; the 85-Shore rubber is hard and best-suited for the material removal involved in graining.
Typically, the contact roller is serrated at an angle from 30 to 60 degrees. Thirty degrees is better for light finishing, while 60-degree serration is more aggressive, often used for stock removal or heavy dross removal from plasma cutting.
The contact drum is really the machine’s “cutting tool,” and its speed determines the finish. In years past the abrasive head usually had only one speed, but today variable-speed systems have become commonplace. Most important, those variable speeds allow the system to get the best use out of modern abrasive material. The speed of the conveyor that transports the part through the machine also determines the finish.
Deburring and graining each calls for a different speed and feed. Combine a soft contact roller running the proper abrasives at a slow speed, and a conveyor that slowly feeds the part underneath, and you usually can get an effectively deburred part. This combination produces a 3-D abrasive elasticity that reacts with high abrading pressure on the burrs and edges, yet creates very little pressure on the part’s surface (see Figure 3). This also makes it possible to remove vertical burrs and round off edges when used in combination with a brush. Setting up a machine in the opposite configuration—a hard contact roller that moves the abrasive at high speed, and a fast conveyor—will produce a grain finish on the part surface (see Figure 4).
Should you use a wet or dry deburring machine? It’s an old debate, and in truth, each system has its advantages and disadvantages.
Wet deburring machines flood coolant into the work area. Gravity returns the coolant with dirt and grinding particles to a filter that catches the particles and recycles the coolant to the main tank. Some machines have a filtering system as a stand-alone unit, while others have a filtration unit under the machine to save space. Wet systems eliminate the need for a dust collector, offer long belt life, and minimize hazards when working with different metals.
Wet systems do, however, require a strict maintenance regimen. Certain coolant mixtures work best with certain materials, and excessive corrosion occurs if proper water and lubricant mixtures are not maintained. A number of factors affect the water chemistry in a machine. For example, regular tap water has sodium, calcium, magnesium, and sulfur dissolved in it, and as water evaporates from the tank it leaves these harmful minerals behind.
Daily cleaning and monitoring the chemical makeup of the coolant are vital for optimal machine performance. Some wet machines have stainless steel in critical areas to prevent corrosion. A few machines are made entirely of stainless steel to suit even the most rigorous applications.
Bearing life is the most troublesome problem with wet machines. On some newer systems, pressurized bearings prevent moisture from entering, but this is a relatively new, somewhat untested technology. As always, proper maintenance is the most effective solution.
Dry machines can require less maintenance, provide a longer machine life, and can be less expensive to purchase and operate. However, it is very important to be aware of potential problems. Aluminum dust can accumulate in ducting, and all it takes is one spark to start a fire. Again, maintenance is critical, and cleaning the machine after changing from one material to another is essential.
Dry machines also can leave dust particles and grit on the part surface. If those flat parts are not cleaned, they can prematurely wear press brake tooling, which can be expensive, especially if you’re using hardened, precision-ground punches and dies.
Dry systems also require either dry or wet dust extractors. Dry extractors are adequate for collecting dust from various processes. They must be changed regularly and disposed of properly. However, they do pose some serious hazards if they aren’t maintained properly. Dry filters consist of cloth, paper, synthetic, and other materials. Various metals, abrasive grits, brush fibers, and oil residue can get into the filters, and a hot spark could cause a fire. Check with your fire department for local laws on dust collection.
This is why wet dust extractors have become so popular. They eliminate the cost of regular filter changes and disposal. But more important, the filter medium—usually just tap water—saturates the particles and quenches any hot sparks. The dust is pulled into a water wash or scrubbing system, and particles settle to the bottom where they can be removed easily. Most wet filters are stainless steel, so rusting isn’t a problem.
Most heavy-duty grinding applications are done dry. When running both aluminum and steel in the same machine, a wet dust collector must be used to prevent fires caused by ignition of the dust. Regardless of what collector you use, though, it is imperative that you check and clean ductwork daily to eliminate any fire hazard.
Steel wire, stainless steel wire, Scotch-Brite™, nylon fiber, flap wheel—the list goes on when it comes to available brush media. And variations of each of these brushes abound. There are 40 types of Scotch-Brite brand brushes alone.
Steel wire brushes, an economical choice, work well if your applications involve only cold-rolled steel. Avoid using such brushes on wet deburring machines or if you also run stainless steel. Otherwise, cold-rolled particles will impregnate the stainless. Stainless steel brushes, the most versatile and longest-lasting available, come in various wire diameters and densities.
Scotch-Brite brand brushes do an excellent job of deburring all materials. They come in many different types, from those that provide a satin finish to a grain, or No. 4, finish. Finally, nylon fiber and flap wheel brushes work well for extrusions and up-forms, as well as hard-to-reach places that require the brush to be flexible.
Top, or cup-style, brushes as well as barrel brushes are the most common. Top brushes (see Figure 1) deburr a range of ferrous and nonferrous materials, galvanized, coated, and up-formed parts. Top brushes with stainless steel wire also remove oxidation from laser-cut edges. Barrel brushes work well for parts that also require a straight-line grain finish (see Figure 5). With a four-head machine set up with a sequence of two belts and two barrel brushes, a stainless steel piece with a 2b mill finish can be deburred and a No. 4 finish applied in a single pass.
Of course, a brush alone will not deburr your parts. The proper choice of contact drum, abrasive belt, abrasive speed, conveyor speed, and brush selection all work together to produce the proper finish and edge quality.
When choosing a deburring system, be sure to evaluate a few basic criteria. First, consider the complexity of the design; high complexity often can cost more to operate and maintain. How difficult is it to replace the brushes when the brush media wears out? The easier brushes are to change out, the less your downtime.
Some machines allow for the brush style to be changed in each individual head, which can benefit operations that demand versatility. For example, a job shop processing laser-cut parts with stainless steel wire top brushes with a “deburr only” specification can complete the job, then quickly switch to a barrel brush and process a completely different order, like one involving aluminum coming off the punch press to be deburred and grained in one pass.
Machines with variable-speed brush heads can accommodate the huge variety of abrasives now available. A top-style wire brush running at optimal efficiency will have a speed specification that differs from many types of barrel brushes. Oscillation can also be important when using both top- and barrel-style brushes. Many deburring applications require brushes to oscillate at the proper frequency.
With any kind of machining or grinding application, through-feed speeds and cutting tool speeds work together to produce the required finish. This is also true for wide-belt deburring machines. A variable-speed conveyor belt can help you achieve the proper part finish in one pass. A slow conveyor belt is used to remove burrs and can be adjusted for the severity of the burr. A fast conveyor belt is used for graining, as it takes the S surface grain pattern imparted by the abrasive-belt tracking and stretches it out. This and other machine features make that S pattern difficult if not impossible to notice.
Many machines offer a single, high-speed motor; others come with a variable-speed motor. The high speed is necessary in a graining application as it produces a shorter scratch and a more pleasing appearance to the part. The high-speed abrasive belt also can remove the hardened dross that accumulates on laser-cut stainless steel parts.
A slower abrasive-belt speed gives greater control over stock removal and allows for the abrasive belt to flex around the edges of the part. Having control over speed can help greatly when deburring galvanized or prefinished metals that have protective coatings.
Lasers and high-definition plasma machines have brought about new deburring machine configurations. Laser dross on stainless steel is tempered and sometimes rewelded back onto the part surface.
Removing this dross requires high grinding pressure at the point of contact. This can be achieved only by using a harder contact drum, between 55 and 85 Shore. A drum this hard will produce a secondary, horizontal burr. So using a 35-Shore drum as a second head and then a brush, or two counter-rotating brushes, may produce the best results.
Volume can, of course, influence machine configuration. The decision to include a hard drum on the first head should be based on the percentage of parts that have a sawtooth edge or rewelded material on the part surface.
Slag is the molten metal left to harden on the bottom edge of a part that has been cut by a plasma machine. Slag grinders in years past were much more popular than today because of the technology improvements in the cutting process. Still, to this day some use a hand-held chisel or grinder to remove the slag, which of course is very labor-intensive and inefficient.
Slag-grinding machines can remove great amounts of slag from large and heavy workpieces, often weighing more than 1,000 lbs. with thicknesses from 0.5 to 6 inches. For these applications, most use a hard, solid rubber contact drum with a coarse abrasive belt and high-horsepower motors. Most applications require multiple abrasive belts. Slag-grinding machines usually remove just the slag and not the mill scale on the material surface.
Slag grinding is one of the most extreme applications for a wide-belt grinder. It tests the limits of the machine and the abrasives. Various machines remove slag in different ways, but all systems require high rigidity and power to perform the task.
You would not use a straight-blade screwdriver to drive in a Phillips screw. Maybe you could get it to work, but it sure would be much easier using the proper tool. Think of abrasives the same way.
Abrasives are like shoes: One size does not fit all. And buying on price can be a mistake, because using the proper abrasive for your application can save you money in the long run. Most machine builders can help with this process, but seeking help from an abrasives manufacturer is always advisable.
Ultimately, the type of machine and options you choose will depend on the type of burr removal and finish you require. The decision between a wet or dry machine, and the options needed, should be made after samples have been run by the machine builder. A spreadsheet including all the features, specifications, and costs is the smartest way to compare different systems.
But there’s a larger question: Why deburr in the first place? The answer seems obvious. After all, the process can produce high-quality finishes; remove sharp, dangerous burrs; and produce a smooth edge. No burrs or sharp edges on parts also leads to safer material handling and increased personal safety. Paint adheres better. And, of course, products just look better.
But there’s another benefit: reduced liability. Unlike the single-belt machines used decades ago, modern deburring machines smooth both the surface and edges. Historically, many shops used a single-belt machine to scratch the surface, remove the vertical burr, and create a grain or brush finish. This, however, created a horizontal burr or sharp, 90-degree edge. This is what most shops called “deburring” 30 years ago. Today it’s a potential liability.
Modern systems reduce this potential liability considerably by completely removing burrs from part surfaces as well as interior and exterior edges. In essence, they help produce parts that are safe, aesthetically pleasing, and ready for smooth processing downstream.