Remember Jaws? Those rows of jagged teeth in the shark’s mouth kept even the most enthusiastic swimmers out of the ocean for years. Well burrs on sheet metal are like miniature shark’s teeth, ready to slice and dice soft flesh. That’s why people in our shop wear high-quality industrial gloves, and it’s why we take the time to remove sharp edges from your fabricated products.
Practically every metal cutting process leaves a burr. It’s a safety hazard, but it can also affect how your parts go together and perform in service. Some customers put deburring requirements on their drawings, others don’t. We’d like you think about what your parts need, and to help, here’s an essay on the subject.
Burrs And How They Form
A burr isn’t a sharp edge, it’s more like a sharp little tooth sticking up from the cut edge. Burrs are a result of plastic deformation, which is what happens when metal is sheared. Some metals deform more than others, and so form larger burrs. Generally speaking, it’s softer metals like aluminum that create the biggest problems.
Consider a punching operation as an example. The punch comes down and pushes the metal into a hole, the die. When the punch hits the metal the metal shears through around the edge, but before cracking, (which is what happens,) it bends into the hole. As there has to be clearance between punch and die there’s room for a little burr to form.
Any cutting operation that deforms metal leaves a burr. Punching, cutting and slitting are all guilty of it. Processes that don’t involve deformation generally don’t leave burrs. For example, electrochemical machining and to an extent, laser cutting, are largely burr-free. (With the laser it does depend on factors like how the machine is set up and the workpiece thickness.)
Why Burrs Are Bad
There are at least three reasons you don’t want burrs on the edges of your metal parts:
- Safety. Even the smallest burrs can leave deep lacerations on the hands of the unwary. That’s true in the fabrication shop but also wherever the parts or fabrication end up. No one wants people to be injured by burrs.
- Fit. Parts might all be cut to length but if they have burrs on their edges it’s likely they won’t go together properly.
- Function. Perhaps a fabrication did go together properly, but if there are burrs on it, it may not function properly or could fail prematurely. Picture for instance burrs on an edge that fluid flows over. Eventually the burrs will break off and could get drawn into a pump or fan. Alternatively, a hose or cable might rub against burrs on an edge. Any vibration will have those burrs acting like tiny saw teeth, quickly cutting through the sheathing.
Burr Removal Methods
Here we’ll describe three common methods, one that you might find interesting, and a fifth that’s an absolute blast to watch. The three common methods are:
- Hand trimming. This uses specialized deburring tools which are like tiny knives you run along the metal edge.
- Filing, mechanical or rotary. Hand files are those flat blades with rows of tiny teeth. A rotary file is part milling cutter and part flat file and goes in a Dremel or rotary air tool.
- Sanding. This when abrasive paper is rubbed over the edge. You can do it by hand, with a power tool or on a sanding belt, like the Timesaver 1200 which we own in our fabrication shop.
As for the interesting method, well how about chemical deburring? There are two approaches. Either use an acid to erode away the raised burrs or subject your parts to electro-chemical deburring. This is like plating in reverse, with metal being removed from high points.
And the method that’s a blast? Well it’s explosive deburring we have in mind.
Explosive deburring is where you put the parts to be cleaned in a special pressure chamber. The air inside is replaced with an explosive mixture, and then ignited. The resulting heat and pressure vaporize those annoying burrs, leaving the edges clean and smooth.
Unfortunately, we don’t get the opportunity to do explosive or chemical deburring. Those are generally reserved for complex, (and therefore high value,) machined parts like castings and forging.
It is of course possible to be a little over-zealous about deburring. This sometimes happens when we put a trainee on the task. Too much filing or sanding can result in parts that don’t fit together as they should. When that happens, sometimes the only thing to do is throw them in a scrap bin and start again.
Another issue is deformation. Especially with very ductile metals like aluminum, deburring can seem like you’re just pushing the burr from one side of the edge to the other. In such cases it’s easy to either remove too much metal or apply so much force that you deform the part you’re working on.
What this means then is that we need specifications for deburring.
Specs For Burr Removal
So you know you don’t want burrs on your fabricated parts, and you’ve decided to put a note on the drawings to remind us of this. How do you go about specifying burr removal?
While it often seems like every aspect of manufacturing is regulated by codes and standards, burr removal is one area the bureaucrats haven’t got around to yet. Quite simply, here in the US there is nothing formal. That’s in contrast to France and Germany where their DIN 9830 and NF E81-010 standards put limits on acceptable burrs. (Rather cleverly, these are related to both thickness and material UTS.)
However, nature abhors a vacuum, and so we have custom and practice and SME recommendations to fall back on.
Custom and practice has given us drawing notes like, “Break all sharp corners,” and “Deburr all edges,” but those are pretty ambiguous. When does an edge break become a chamfer, for example? You might also hear something about maximum acceptable burr height being 10% of material thickness. (We’d argue that’s usually too big and takes no account of the degrees of plastic deformation experienced by different metals.)
The SME recommendations, as summarized in “Deburring: A standard operation?” (The Fabricator, August 2015,) provide seven levels of deburring. These range from, “not required,” and “Do not deburr,” (levels 1 and 7 respectively,) to “Remove all visible burrs” and, “Remove all burrs visible at __ X magnification.”
Tell Us What You Want
Unlike the special effects in the original Jaws movie, burrs are no laughing matter. Those tiny metal “teeth” can cause serious damage to hands as well as not assembling the way they should. If you always put notes about deburring on your drawings, thank you. And if you don’t perhaps we’ve given you something to think about.