4 Factors Influencing Cut Edge Quality (and Why it Matters Anyway)
When cutting metal there’s usually a lot of thought given to speed and accuracy. Those are important but we’d argue edge quality is another factor to consider. It’s also one that doesn’t get the attention it deserves, except here at Wiley.
We think about edge quality a lot, and just so you don’t think us odd we’re going to explain why. We’ll start by addressing what we mean by edge quality and why it matters. Only once we’ve driven that point home will we share the four main factors that influence cut edge quality.
Where is The Edge?
This might be a question Bono asks a lot, (yes, that’s a U2 reference,) but here we’re interested in fabrication and not music.
Whenever a piece of sheet metal, plate, tube or beam is cut to size, shape or length, that cutting leaves an edge. On sheet and plate it’s the thickness of the material. On a tube or beam it’s the thickness of the wall or web.
Defining Edge Quality
With sheet metal edges all we need consider are straightness (or conformance to the desired curvature,) and burrs. With plate, bar and tube the edge has some surface area so, in addition to the two characteristics above, we can measure roughness and perpendicularity.
Edge surface finish can be measured the same way as any other surface – by stylus if you have the equipment, or with the aid of a surface finish comparator.
When measuring roughness with a stylus, the usual surface parameters, such as Ra and Rz, apply. These are defined in ISO 4287. In addition, ISO 9013 defines edge quality for thermal processes like laser and plasma cutting.
Burrs are hard to quantify, other than under a microscope, but if you run a fingertip along an edge with burrs you’ll certainly know they’re there! Part prints often call for all burrs to be removed.
Perpendicularity or squareness is easily checked by putting a set square against the flank and the cut edge and looking for light coming through the gap. Laser cutting often produces a noticeable taper or curvature, which is due to how the beam is focused. Punching thick sheet or plate can also leave a taper, which is a function of how the metal shears when struck by the tool.
Plasma cutting is especially interesting, or confounding, depending on your point of view. With plasma the arc spins clockwise. This leaves a better edge finish on the right side of the direction of travel than the left, so it matters in which direction you cut your plate.
Why Edge Quality Matters
Having explained what is meant by “The Edge,” here’s a list of reasons for caring about its quality:
Let’s take a dive into each of these.
When metal is cut quickly the edges often have a ragged appearance. This could lead people to conclude the rest of the fabrication is as rough as those edges. In fabrication design it’s common to hide these edges with folds, bends or covers, but this isn’t always possible. In weldments for example, lap and edge joints leave the exposed edges showing. The cleaner they look the higher the perceived quality of your product.
Edge Quality and Fit
In tee and corner joints the cut edge of one piece of metal butts up against the surface of another. If the edge is uneven or not square, that’s going to mean more fit up for the welder. Plus, gaps will need extra filler metal. Addressing these slows down the welding process.
The issue here is burrs. These can seriously lacerate the unwary hand, and no one wants an accident. Sturdy gloves help protect against burrs but it’s always best to minimize their creation and remove what are thrown up.
Edge Quality and Cost
Poor quality edges need finishing to get them smooth and square. That’s another step in the fabrication process. But don’t do it and welding becomes more awkward and appearance suffers.
Another issue is with yield. When cutting sheet or plate, parts are nested in a way that maximizes material utilization. Ideally this is done with a “common edge,” meaning one cut forms the edges of adjacent parts. However, common edge cutting requires both very precise and consistent kerf width, (the width of material cut or melted away,) and good edge quality. Plasma cutting is especially challenged in this regard. So, to avoid making scrap parts it’s often necessary to leave material between each piece being cut.
The 4 Factors Influencing Edge Quality
Okay, here we go with the list.
- Cutting process.
- Shearing and punching bend metal down before it shears and often leave a burr.
- Sawing and grinding remove chips and often throw up burrs where the blade or wheel exits the part.
- Laser and plasma cutting leave “striations,” (somewhat vertical lines in the edge that create roughness and often angle back away from the direction of cut in an effect called “lag.”)
- Laser can also leave a “heat affected zone” (HAZ) where material properties are altered.
- Laser and plasma may cut curved edges depending on beam or arc quality.
- Abrasive waterjet tends to cut very cleanly but can leave striations.
- Material properties
Some materials are more ductile than others, and so more prone to forming burrs. Those with greater thermal conductivity take more energy to cut with heating processes and edge quality can suffer. When laser cutting steel silicon has a strong influence on edge quality, facilitating a smoother surface but increasing out-of-squareness.
- Cutting speed
In the cutting and melting processes reducing the feed rate usually reduces burrs and the severity and lag of any striations. However, in punching and shearing going faster tends to reduce the size of burrs.
- Process parameters
The cutting tool – saw blade, laser or any of the other options – must be optimized for material properties and thickness. Too many teeth in the cut and they’ll load up but engage too few and cutting forces will go up. In punching and shearing use the correct clearances to allow for controlled deformation. Laser and plasma cutting have multiple parameters, all of which must be adjusted to ensure good edge quality.
Get a Good Edge
Edge quality is an important factor in fabrication. If neglected it will affect quality, appearance and cost. There’s even a safety aspect to consider.
Here at Wiley we give a lot of thought to the edge. Not U2’s lead guitarist The Edge. Nothing rough or out-square about him, (just in case he’s reading this.) The Edge is great, but if you want quality edges on the metal you’re cutting, think about cutting process, material properties, cutting speed and process parameters.