Burnishing tools are used to impart a gloss or fine surface finish, often in processes that involve the cold working of metal surfaces. Burnishing tools are also used for the sizing and finishing of surfaces.
Features
A burnishing tool is used as a finishing tool. A burnishing machine is widely used for tool polishing, metal finishing, and ball burnishing. A burnishing tool develops a finished surface on turned or bored metal surfaces by performing a continuous planetary rotation of hardened rolls. The rotation of the rolls increases the yield point of the soft portion of the metal surface at the point of contact. This point of contact results in the deformation of the metal surface to generate a finished metal surface.
Types
There are many different types of burnishing tools. Examples include a roller burnishing tool and external taper shaft burnishing tool.
A roller burnishing tool is a cold burnishing working process tool and is used for generating self-feed and non-feed design surfaces.
An external taper shaft burnishing tool is a burnishing tool used for generating tape shafts in a workpiece.
Other burnishing tools are commonly available.
Specifications
There are several ways in which burnishing tools function. A burnishing tool works on the principle of the planetary system having hardened and tapered rolls, which are placed evenly in a retaining cage of a burnishing machine. A workpiece is tightly held in the burnishing tool and the tool mandrel applies pressure on the hold piece to finish the workpiece surface. Specifications of a burnishing tool vary depending on the type of burnishing tool used for a workpiece. The normal bore diameter of a burnishing tool can range from 5 mm to 26 mm. The adjustment range of burnishing tools depends on bore diameter. The adjustment range for a 5 mm to 7.50 mm bore diameter ranges from -0.1 to +0.20. The shank size for a ball burnishing should be MT-1 and the holder number for a burnishing tool should be H1. Burnishing tools are designed and manufactured to meet most industry specifications.
Applications
Burnishing tools are used in many applications. Examples of burnishing tools usage include:
valve guides
valve rods
motor end covers
fan rotor shafts
In addition, burnishing tools are also used in brake cylinders, hydraulic, pneumatic, and master cylinders. Burnishing tools should adhere to the standard used in the design of COBUM 3050 burnishing machines.
Different Approaches to Finer Roughness Values
Machining a metal surface leaves microscopic peaks and valleys that are called roughness. Surface finishing, whatever the method, is about reducing the roughness average (Ra) value of the surface. Basically, the Ra value is a formula that measures the average distance between the tops and bottoms of these points relative to the mean line, which cuts through them. The finer the surface finish, the shallower the valleys and shorter the peaks.
The first noticeable difference between abrading and burnishing a surface to accomplish this peak and valley reduction is that the former removes metal from the part while the latter does not. Abrasive finishing cuts or tears away the peaks in the surface, thereby bringing the average peak and valley distances closer together. But Cogsdill says that this also leaves sharp projections in the contact plane of the machined surface. Burnishing doesn’t have this problem. At first glance, a burnished part looks as if the metal surface has been smeared smooth. But that would be incorrect. The burnishing tool’s polished and hardened rollers actually perform cold flowing of the surface and subsurface material, which results in a controlled plastic deformation of the part.
Not What It Looks Like
Material deformation comes in two varieties: elastic and plastic. Elastic deformation occurs when stress is applied at a pressure below the material’s yield point; bend a piece of plastic a little, and it bends back. Plastic deformation, on the other hand, is permanent, a result of stress at a pressure above the yield point of a given material. Crushing an aluminum can, for example, is plastic deformation.
To stick with the analogy of a soft cylinder, imagine squeezing the opposite, undimpled sides of a dimpled container to push the dimples out. That’s a bit reminiscent of what happens when roller burnishing a metal part surface.
The diagram in the slideshow above shows a profile of a hypothetical part surface before and after roller burnishing. The cold-flow effect of the rotary tool applying the rollers radially to the surface causes the peaks to flow into the valleys and the valleys to flow upward toward the peaks, creating a plateau profile whose contact plane is much less sharp than it would be after abrasion. This is important, as a common misunderstanding is that roller burnishing simply smears or pushes over the peaks to make the part smoother. Rather, cold flowing actually stimulates the material both at the peaks and a few thousandths of an inch down beneath the surface, flowing them together. Because the finish comes from subsurface effects, a burnished surface is not only smoother but also work-hardened for greater durability. Cogsdill’s Roll-a-Finish burnishing tools can be applied on a variety of ID and OD diameters, flat surfaces, tapers, contours, and fillets for final finishes down to 2 to 4 microinches Ra.
