In metal fabrication, metal shearing is a primary process that is found in just about every shop. However, in those shops, metal shearing defects are found just as much. Persistent problems like rough for burred edges, deformed edges, twist, bow or camber on parts are all defects that can be corrected with the right machine, the right setup and the right maintenance. In this article describe what these common shearing problems are, what causes them and of course, how to resolve them.
A burred edge is a common defect in the operation of a metal shearing machine. Often this is caused by improperly ‘gapped’ shear blades or blades that are just plain dull. The 'Gap', or the distance between the upper and lower blade, is critical to the successful operation and burr-free results on your shear and need to be set differently for each TYPE and THICKNESS of material you are working with. Too much gap and the edges will be torn instead of cleanly cut or even rolled over if the gap is severely out of specification.Too little gap between the blades and your shear may not make it through the material without stopping due to an overload. Make sure you are consulting your metal shear manufacturers operational manual in order to set the proper clearance for your particular machine, material type and thickness.
Deformed edges when shearing metal can be caused by either heavily incorrect blade shear gap, dulled or damaged blades or also by improper hold-downs or hold-down pressure. To correct deformed or ‘rolled’ edges make sure you have first ensured the blade gap is set correctly for the type and thickness of material you are working with. Next ensure your blades are sufficiently sharp and undamaged. Once you have checked the gap and your blades condition, check your hold-downs ensuring they are securely holding the material in place during the entire shearing cycle and not allowing the material to tip-up or slip. Lastly check your shear's gibs or ram pivot bearings as your machine may have too much play in the ram causing the upper blade to jump back on impact with the material, the only resolution for this situation is machinery repair.
Another common metal shearing defect is material 'Twist’. It is best described as the tendency for the cut-off piece of material to curl into a spiral or corkscrew after being sheared. More prevalent in thin sheared sections, twist is typically caused by excessive rake angle. If your shear has adjustable rake angle, make sure you have reduced it to the minimum for the material type and thickness you are shearing. Although twist can be caused by properties of the material itself, it is much more likely to be caused by excessive rake angle.
Another common shearing issue is 'Bowing'. During the metal shearing process the material can ‘Bow’ downwards. This defect in shearing metal parts is also most prevalent when shearing long thin, narrow strips of material. Typically this is caused by stresses inherent in the material itself and is not necessarily an indicator of a problem with the shearing machine or the operation of it. To ensure you have eliminated any possibilities on the shearing machine setup, reduce the rake angle to the minimum possible. Although unlikely, high rake angles can also lead to ‘Bowing’ during the shearing process.
Cambering is another issue that is created when the material being sheared moves away from the sheet horizontally without twisting. Although this issue is almost entirely related to the stresses in the material itself it is possible to reduce it by increasing the ram speed, changing the grain direction of the material being sheared or by adjusting the rake angle of your metal shearing machine if applicable.
Although we have an extensive article on Shears and Shearing in our helpful info selection, a few factors stand out that should be repeated and considered. The shear load required to properly shear any metals depends on the material type, thickness, blade gap and blade rake angle. However all things being equal it should be noted that when shearing mild steel, for example, that increasing from ¼” thick material to ⅜” thick material increases the load and thus, tonnage by 225% even though the material itself is only 50% thicker. Although increasing the rake angle will drastically reduce the tonnage required, it will also reduce the quality of the sheared material due to the inherent issues with large rake angles.
If you are shopping for a metal cutting shear and want to avoid common shearing problems look for a solid machine with a fixed rake angle of ¼” p/foot or less. These non-adjustable rake angle machines are designed to meet the requirement of quality sheared parts day in and day out whereas their adjustable rake angle counterparts will likely be equivalent to a machine (of that manufacturer’s) with far less capability (look for the Machine Weight, Pump HP and other indicators). Whereas cheaper adjustable rake angle machines will commonly provide poorer sheared edges in the shears maximum rated thicknesses.
Some other common things to avoid to keep your shear in tip-top operating condition are:
At Southern Fabricating Machinery Sales, Inc we are fabrication and shearing experts. We frequently post technical articles just like this and our Hydraulic Shear Machinery Inspection Checklist, below, along with other great resources for machinery. One of these great articles on metal shearing is available in our helpful info section in our Introduction to Shears and Shearing linked HERE. This article goes into greater depth on shears and the shearing process. When you have a shearing need arise, contact us as we have the knowledge and experience to help you take the time to select the right metal cutting shear for your needs and budget. Give us a call today at 813-444-4555 or visit us on the web at www.southernfabsales.com as we look forward to helping you shear the best possible parts.