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Got burrs?
Making choices for deburring, graining sheet, plate
- By Anthony McCue
- September 11, 2007
- Article
- Finishing
Editor's Note: This article was developed from a seminar on deburring and graining held at AM Machinery Sales Inc., Warminster, Pa., March 20-21, 2007.
So you have the sheet metal burr. Big deal. Run the workpiece through a deburring machine and your problem is solved, right? Or maybe you have a part that needs its surface sanded, or grained. Easy. Run it through a similar machine and that's all it takes, isn't it? In fact, graining usually remove burrs, so you can kill two birds with one stone, right?
Wrong. Wrong. And wrong. Deburring and graining are much more complicated than that. The variations in the deburring process are just as numerous as the factors, or combinations of factors, that cause burrs in the first place.
Burr causes fall into two categories:
Mechanical. These are the result of shearing or perhaps stamping operations in which the die clearances weren't set right. Nibbling is another cause. Of course, in fabricating, there are many other mechanical processes that can cause burrs.
Thermal. These result from processes such as laser or oxyfuel cutting. This type of burr is usually called dross or slag.
Although burrs start from just two root causes, many factors contribute to them—the type and thickness of the metal; the die condition and precise amount of the die misalignment; condition and clearance of the shear blades; laser power and shielding gas selection; oxyfuel pressure, flow, and the nozzle tip condition; and the list goes on. Almost indefinitely.
Similarly, graining is more complex than it seems. The graining process, running a sanding belt over the part's surface, traditionally has been referred to as deburring. The graining process might knock a burr over, but it does not take it off. As a part moves through the machine, the sanding process sharpens the burr. This makes the situation worse rather than better.
Machine Makeup
A basic deburring and graining machine has at least two heads, one with a sanding belt to knock down the vertical burr, and one with a brush to remove the burr and radius the edge. A conveyor moves the workpiece through the machine and pinch rollers hold the part down on the conveyor. Some machines have three heads, and some have four. Your application dictates the configuration and number of heads needed.
Machine Type. Users have a choice between a dry machine and a wet machine.
The basic difference is that wet machines provide a better finish and extend abrasive belt life. However, wet machines typically require more maintenance than dry machines.
Be aware that it's dangerous to use a dry machine for processing stainless steel and aluminum. The dust produced by stainless and aluminum must be kept separate—allowing them to mix can lead to an explosion. Using a wet dust collection system is imperative when mixing metals in a dry machine.
With a wet machine, you don't have this concern. The water prevents an explosion from happening.
You should be careful in selecting a wet machine. Be sure the machine is designed to withstand the harsh environment of wet processing. Some manufacturers simply add a coolant system to a dry machine and call it a wet machine. For durability, it's necessary to keep some components dry, especially the bearings. It's easy to tell if the machine was designed to protect the bearings: If you can see the bearings, they'll get coolant in them.
Another important consideration is flooding the workpiece at its point of contact with the abrasive belt. This is where it does the most good in removing the grit from the abrasive belt, which prevents the belt from packing. If the coolant hits the belt at some other point, it's too late to do much good.
Another chief advantage of the wet deburring and graining process is that it cleans the grit off the parts. With a dry machine, the grit stays with the part and goes to the next process. If the next process is press brake bending, the grit will scratch the tooling's surface.
Drum Characteristics. The sanding belts rotate on drums that have specific characteristics. First, they vary in hardness from 25 Shore (A) to 85 Shore (A). Typically a 25 Shore (A) drum is used for deburring and an 85 Shore (A) drum is used for slag and heavy stock removal. The drums can have a spiral pattern in them, also called a serration. The serration angle varies from 30 degrees to 60 degrees. Thirty degrees is better for finishing ,whereas 60 degrees is more aggressive.
A third consideration is drum diameter. A smaller-diameter drum is more aggressive than a larger one, but a larger one provides a better finish.
Another variable is contact drum speed. Deburring machines typically have a variable-speed drum. A soft contact drum at a slow speed extends abrasive belt life and provides better edge quality. A hard contact drum at a high speed provides a grain finish and is used for surface conditioning.
Finally, some drums are adjustable. Although fixed drums are suitable for graining, adjustable drums provide more versatility. With an adjustable contact drum you can switch from a thin microfinishing belt to a thick Scotch-Brite® belt and avoid the time- consuming process of adjusting the pinch rollers.
Conveyor Systems. Conveyors that carry the workpiece through a machine come in many configurations. A rough-top conveyor belt is standard. Some applications require a special conveyor belt. For example, a smooth-top conveyor is for processing thin material to prevent the parts from being compressed into the belt and causing premature wear. Other examples would be a vacuum table to hold down small parts. This type of system works well if you cover enough surface area for the vacuum to work adequately. Magnetic tables also are used for small parts.
Selecting the Right Machine. Getting the right machine isn't merely a matter of looking at the deburring and graining process. The machine has to fit into the overall fabrication scheme.
For example, you might find that a two-head machine will handle your deburring or graining work, but some parts might require two passes. Depending on the part volume, a four-head machine might be a better choice so you can run all your parts in one pass. It's a matter of evaluating all of your processes, and costs, to find the best machine for your particular applications.
The Snag Test
Curious to know just how smooth a finish you can expect from a deburring machine?
One clothes dryer manufacturer uses a deburring machine on the part that becomes the machine's interior drum. The company verifies the part's smoothness by running a nylon stocking over the part. It's a simple go/no-go test: If the stocking snags on the part, it's not smooth enough.
Why Not Use Manual Deburring?
While manual deburring has been around for decades, and the people who are good at it are regarded highly for their manual dexterity and patience, this simply isn't a glamorous job. It creates a lot of metal dust, it's repetitious, it's boring, and therefore it is a good candidate for automation.
ICON Health & Fitness came to this conclusion recently.
"With a deburring machine, we could get rid of five deburring positions," said Cody Kelley, a plant manager for the company's club division. "Then we could assign those people to other jobs," he added.
Kohl Burmester, a manufacturing engineer for the same company, explained that many of the components that go into the company's products are cut with a laser or a plasma cutting machine. The oxidation has to be removed before powder coating, and this is where the deburring and graining steps come in. Many of the fitness machines ICON manufactures for the health club industry get grained, textured, and sent out for chrome plating.
The company has a lot riding on the appearance of its products. You might not be familiar with the company name, but you probably recognize a few of the company's brands, which include Nordic Track®, Pro-Form®, Weider®, Healthrider®, and Freemotion Fitness®.
"All of it has to be precision work," Burmester said. A small scratch can have a big impact. "A scratch, even a small one, means that we have to remove the coatings and rework the part."
The company also provides quite a bit of equipment to professional sports teams. You wouldn't think a linebacker or a goalie would care how their fitness machines look, but it is a high-profile market and the demands are strict.
"They also want really nice-looking equipment with no flaws," Kelley added.
Deburring and Graining Tips
Operators have control over five factors: conveyor feed rate, abrasive type, abrasive speed, brush composition, and brush speed. A slow feed rate is good for deburring because it gives the abrasive more time to work over the edges. A high feed rate is for graining, or finishing the surface. Slowing the process increases the life of the belts and brushes, but at the risk of sacrificing edge quality. It's all about balance.
To break an edge, slow the machine and use the abrasive belt only. For even belt wear, run parts across the entire width of belt, not just down the center.
To be sure all edges get a thorough deburring, send parts through at angles. Aligning a square or rectangular part with the feed direction means that two of the edges are perpendicular to the direction of brush rotation and get a good deburring. The other two edges don't get much, if any, deburring action at all. To effectively grain and deburr small parts, be aware of the machine's limitations.
- For graining, the smallest part dimension is the same as the distance between the machine's pinch rollers. This goes for all machines of this type.
- The smallest part size for deburring depends on the machine. For instance, some wet machines transfer the parts from the conveyor to a set of squeeze rollers to squeeze the moisture off the part. The gap between the conveyor and the squeeze rollers determines the smallest part size. Parts that are too small fall into the coolant tank.
- If the parts are too small, make a tray to hold them. Take a rectangular or square piece of sheet metal and fold the edges over 180 degrees to make a lip that runs around the tray's perimeter. Then load it up with parts and run them through. Another strategy that works in some instances is to leave the parts in the skeleton and run it through the machine, then remove the parts from the skeleton.
Galvanized parts are not a problem. With a proper setup, a deburring machine can remove the burrs from a galvanized part without removing the galvanized layer (the zinc that provides the corrosion protection).
Keep an eye on the big picture and think about increasing your shop's productivity. If you're running parts through a laser, you could set up a deburring machine for a one-time-through process that removes the oxidation, deburrs the part, and prepares it for painting. Relying on the deburring and graining machine to do a little more of the cleanup might allow you to run the laser a little faster, thereby increasing your output.
Choosing abrasives
Choosing abrasives is like choosing a machine—you have quite a few choices.
The first choice is whether to use a belt or a brush. Belts generally are used for graining, whereas brushes generally are used for deburring. However, these are just guidelines. In some cases, these roles are reversed. If using a brush, your next choice is its composition: Brushes are available in steel, stainless steel, aluminum, and Scotch-Brite®.
Determining the correct amount of pressure probably is more objective than many would expect. It's a matter of measuring motor load when the machine is running parts. This is done by measuring the amperage draw of each motor or determining horsepower per inch of the workpiece.
The abrasive speed and conveyor speed are two other factors that determine the quality of the edge or finish. Their combined speeds make up the abrasive dwell time, or the amount of time the abrasive has to do its job.
Information provided by 3M
Anthony McCue is president of AM Machinery Sales Inc.
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The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.
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