End Mill Step Over Calculator | Scallop and Pass Guide

Step over is the distance that a CNC cutter will travel lateral between passes along the same path. As the cutter pass over the workpiece, the rounded edge of the cutter will leave scallop behind. If the step over distance are too large, the scallops will be highly and noticeable on the workpiece.

If the step over distance is too small, there will be too much passes to complete the machining operation in a timely manner. The height of the scallops is based off the diameter of the cutter and the distance between the step. If the operator reduces the step over distance, the height of the scallops will also be reduced, leading to a smooth surface on the workpiece.

How Step Over Affects Surface Finish

The type of material being machine will affect the step over distance that is selected. Aluminum materials will clear chip easy, allowing for a wider step over distance. Stainless steels will work harden during the cutting process, requiring a smaller step over to prevent the cutter from break.

Wood materials has a grain direction, which will impact the surface finish of the cut piece. For example, a step over of 14 percent might be used when machining wood to achieve the desired finish. Additionally, different operations will require different step over distances.

For example, contour finishing will require a low step over distance to ensure the smoothness of the walls of the machined piece, while adaptive clearing will use a larger step over of 20 to 35 percent to remove material quick from the workpiece. Feed rate and chip load is also related to step over and the desired surface finish. Chip load is the amount of material that each cutting tooth will remove from the workpiece during one rotation of the cutter.

High radial engagement between cutter and workpiece will create high level of heat during the cutting process. High levels of heat will cause the cutter to deflect from it’s cutting path, potentially leaving marks on the workpiece. The step over distance and chip load must be balanced to prevent these type of marks on the workpiece.

Many people attempt to improve the surface finish of a workpiece by increasing the spindle speed. However, the step over distance is a more effective parameter to control. The number of passes that will be required to remove the material from the workpiece can be calculated by dividing the total width of the workpiece by the step over distance.

For instance, using a 1/4-inch endmill with a 12 percent step over on a 3-inch wide face of aluminum will require approximately twelve passes to complete the task. A smaller step over will require more passes, and a larger step over will require less passes. The diameter of the endmill that will be used in the machining operation will also factor into the step over distance that is program into the CNC control system.

For instance, if using a 1/4-inch endmill, the step over distance will be smaller than for a 3/4-inch endmill of the same cutter type; the larger the endmill, the less noticeable the scallops will be on the workpiece. The number of flutes that are present on the cutter will also impact the settings that is used during the CNC process. Two flute cutters are best use on softer materials that will clear chips easy.

Four flute cutters are best used on stainless steel; four flute cutters allow for an increase in feed rate. When testing a new cutting process, it is important to begin with conservative settings. Test pieces of scrap material can be used to test different step over percentage.

For example, an 8 percent step over can be tested against a 25 percent step over. The finish of the cut scrap materials can be inspected under a light to determine whether or not the scallop height are acceptable for the requirements of the job. In general, there is a balance that must be struck between the desired smoothness of the workpiece and the time required to complete the task.

For very smooth workpieces, the step over distance will need to be small. For roughing operations on a workpiece, however, a larger step over will allow for more rapid removal of the material. The step over distance, the tool diameter, the material to be machined, and the chip load will all need to be adjusted to meet the requirement of the specific project.

You should of consider the furnitures size when planning the layout. It is alot of work to get the perfect finish.