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Got burrs?

Posted by: Mu Ju 2019-03-22 Comments Off on Got burrs?

Making choices for deburring, graining sheet, plate

By Anthony McCue and Brett Mandes

Using a machine to remove burrs from and put a surface finish on sheet metal and plate, also known as deburring and graining, is more complicated than it seems. The factors that go into selecting and setting up a deburring and graining machine are almost as numerous as the factors that cause burrs in the first place. Some of the important factors you have to consider are machine type (wet or dry), setup (the arrangement of sanding belts and deburring brushes), sanding drum hardness and diameter, brush type, and feed rate.

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 (see Figure 1). 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.

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