While form turning tools cannot replace drills due to the larger engagement area involved in drilling, which exceeds the cutting depth capabilities of such tools, the capability of form milling cutters to directly cut into the workpiece solves a troublesome issue in machining: the need to pre-drill a starting hole before rough machining.
Because conventional turning tools cannot directly cut into the material along the Z-axis, a starting hole needs to be pre-drilled. Another approach is using an inclined entry, often requiring the application of CAM software. However, with the use of form turning tools, this step can be eliminated.
Characteristics of Form Milling Cutters
Form milling involves the use of cutting tools equipped with indexable inserts that have circular cutting edges. These milling cutters come with either full circular cutting edges (used for circular face milling cutters or ball nose milling cutters) or partial circular cutting edges. Form milling cutters are categorized into modular milling cutters, milling cutters with helical shanks, and modular (helical) milling cutters. Form milling cutters utilize form turning tools, which enable them to have several advantages, including small back engagement, high feed rates, serving as a complement to the trend of high-speed machining. Form milling cutters have the following characteristics:
Strong Feed Capability
Some form milling cutters can directly feed into the workpiece, similar to a drill, to cut into it.
By combining form milling cutters with helical interpolation, it becomes easy and fast to machine large diameter holes.
Higher Cutting Edge Strength
Due to the absence of sharp corners, form turning tools can tolerate larger tool offsets and vibrations. This allows for increased spindle speeds and feed rates during machining while reducing the risk of tool breakage.
Number of Cutting Edges
Form turning tools have more available cutting edges. Depending on the size of the insert and the back engagement, form turning tools can have 4-8 effective indexable positions, resulting in at least twice the material removal rate compared to conventional diamond and square inserts. This advantage reduces the need for tool changes, leading to higher efficiency and cost-effectiveness.
Using form turning tools allows for high technological cutting rates without requiring extremely high machine power. Because of their strength, form turning tools can accommodate larger feed rates for machining compared to square-end milling tools, and even handle heavy roughing on lightweight machines.
Higher Surface Finish After Roughing
Surfaces milled with form turning tools exhibit fewer noticeable irregularities compared to surfaces roughed with square-end tools. The residual height of the surface irregularities is lower. Workpieces that undergo roughing with form turning tools have a higher surface finish, allowing them to proceed directly to semi-finish machining.
Applications of Form Milling Cutters
Choice of Machining Stages: Roughing is typically carried out using form milling cutters (shown in the upper image), while finishing involves the use of solid carbide ball nose end mills (shown in the lower image).
Using form milling cutters for roughing provides a better “preparation” for semi-finishing or finishing operations. When roughing is performed with square-end milling tools, there is a step left when cutting downwards. The larger the cutting depth per pass, the more pronounced this step effect becomes. Such uneven surfaces on the workpiece can result in uneven tool forces during semi-finishing, causing tool impact and deformation. This makes a direct transition from roughing to finishing impractical. Not only is semi-finishing necessary, but multiple finishing passes are also required.
The use of form milling cutters greatly reduces the occurrence of the aforementioned issues. Instead of leaving a step like square-end tools, there are only small “wrinkles” with very low height that can be easily machined away. Form milling cutters are an optimal choice, especially for small cutting depths, where the height of these “wrinkles” is minimized. The surface of the workpiece after roughing is relatively smooth, allowing for easy semi-finishing. In some cases, it might even be possible to proceed directly to finishing.
Selection of Form Milling Cutters
1Choosing the Insert Geometry
When machining a complex part, the fundamental requirement is that the cutting edge can access the regions corresponding to the part’s contours. This necessitates selecting the appropriate insert shape, main rake angle, secondary rake angle, front angle, and back angle. When choosing the insert shape, it’s essential to consider the insert’s strength. Circular inserts generally have the highest strength. For non-circular inserts, a larger nose angle increases their strength. Due to clearance angle considerations, form milling typically employs 35° or 55° diamond inserts. The choice of insert holder depends on the required cutting path. For intricate form milling, a J-type holder with diamond inserts can be chosen to achieve a larger back angle.
2Insert Back Angle
The main and secondary rake angles of the insert determine the back angle between the insert’s back surface and the workpiece. Different materials require different back angles. For instance, when machining tough materials, especially nickel-based alloys, there is significant springback. These alloys tend to deform ahead of the cutting edge and spring back after cutting. This springback causes the workpiece to scrape against the insert’s back surface, generating substantial cutting heat. Additionally, the work hardening of nickel-based materials produces cutting heat, leading to tool thermal failure. The failure mode might be tool chipping, but thermal expansion of the cutting edge results in tool fracture.
Titanium materials can spring back by 0.05mm to 0.08mm, which necessitates a back angle of 14° or 15° between the insert’s back surface and the workpiece to prevent thermal failure when machining such materials. However, titanium and plastics have similar springback characteristics. Using an insufficient back angle when machining titanium can lead to tool thermal failure. Such a tool, when used for plastics, would generate cutting forces and heat due to springback, melting the plastic workpiece. The insert’s back angle shouldn’t be too large, as excessive back angle reduces insert strength. Inserts without a back angle have sufficient strength but must be mounted on a holder with a negative rake angle to create an adequate back angle. Using an insert with a positive rake angle and no back angle groove ensures the required insert strength while maintaining a positive rake angle cutting.
3Cutting Force and Chip Control
Changes in the relationship between the workpiece, tool, and other factors within the machining system will affect effective chip control. For instance, in form milling, as the insert moves outward from the workpiece center, chip thickness decreases, cutting depth increases, and chip control worsens. One solution is to split a single pass into two, changing the outward feed to an inward feed to achieve the final contour.
Thin-walled and elongated parts are difficult to clamp, and cutting forces can cause workpiece deformation, poor surface finish, or even part scrapping. A specialized insert designed to control chips can minimize such deformation. If the machinability of the workpiece material complicates turning operations, parts made of two different materials can double this complexity. Therefore, when machining parts composed of multiple materials, one approach is to select insert grades capable of machining different materials. For example, when machining a part with an inner 4340 steel and an outer nickel-based alloy, the programmer must insert a pause to change the insert. To address this, using two different insert grades is recommended. When tool life remains low for both insert grades, Sumitomo Electric Industries’ AC2000 CVD-coated inserts from Japan can be used. By adjusting the feed rate and cutting speed, both materials can be machined without changing inserts, significantly increasing tool life.