The acronym “SFM” means Surface Feet per Minute and it relates to how far and fast a tool travels across a working surface to accomplish its assigned task. Not a hard concept to grasp, to be sure, but a concept that can become a little confusing when you’re talking about multiple machining processes occurring rapidly to the same workpiece. CNC machines calculate SFM as part of the protocols when operators are inputting tool travel data on control terminals.
Time Travel
The calculations required to determine tool travel are measured in both distance and time. The time required for a tool to complete its work properly is affected by the time the operator programs the tool’s work protocols. Too little time to complete the tool’s work and there will be skipping, chaffing, and other indications that tool movement is too fast to complete the assigned task. Conversely, deformation of workpieces may indicate allowing too much time for a tool to travel across a work surface causing heat to build up and deform the material. Testing new tool protocols on prototype workpieces can ensure proper tool performance when it comes time for going into full production.
SFM vs RPM
While SFM is one means of measuring tool productivity, RPM is another factor to consider. RPM (aka Revolutions Per Minute) determines tool turning speeds. Turning speed and travel speed work hand-in-hand to produce a proper output. Understanding the number of turns and the time that is required to complete those turns to produce a proper cut is important. Again, the issue is proper tool performance, and failing to achieve the right amount of turns will produce subpar workpieces.
How to calculate SFM in machining
Surface Feet per Minute can be calculated using the following equation: Stock Diameter (in) x π x 1/12 x RPM ≈ Stock Diameter (in) x 0.2618 x RPM
Trying to figure out all the revolutions, travel, and ultimate results when multiple tools are being used on the same workpiece can make your brain explode. Thank goodness the majority of CNC machine control programs come from experienced programmers who understand the conditions the machine will function under. CNC machine manufacturers provide “canned” programs that eliminate the need for calculation, however, it’s still a valuable skill that should be developed by every CNC machinist. The ability to fine-tune applications to meet production requirements will be greatly accelerated if the talent to make the changes is in-house. Production stops due to CNC machining adjustments cost time and money, so having the capability of making changes quickly is extremely valuable.
Exceptional Results Requires Exceptional Expectations
The problem with CNC machining is that its only limitation is the imagination of the operator. Once upon a time, machinists thought the leap from three-axis production to four-axis capability was phenomenal. Then came five-axis CNC machines and everyone’s jaws dropped to the floor. We are now into the next iteration in what has become an ever-expanding world of production capabilities. CNC machining creates amazing workpieces with such precision, speed, and complexity that it boggles the mind. Rather than being boggled, smart machinists are figuring out new ways to use CNC technology, and understanding basics like Surface Feet per Minute is an important tool in every machinist’s toolbox.
