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.
Which Parts Work for Roller Burnishing?
In the slideshow above, there is a photo of a sample part, with the unfinished surface on the left and the finished surface on the right. There are a couple things to note here. First, roller burnishing is fast and repeatable; the finish seen above was accomplished in less than 2 seconds. Cogsdill counts the fact that its tools can size, finish and work-harden parts in a single seconds-long operation as a principal benefit over grinding, honing or lapping.
Something else to note is part preparation. Not every part is suited to finishing by burnishing. Cogsdill outlines some requirements for parts that are ideal for optimum roller burnishing results:
Material: Though almost any metal can be successfully roller burnished, ductile or malleable metals are best (for example, steel, stainless steel, steel alloy, cast iron, aluminum, copper, brass and bronze).
Hardness: Hardness should be less than 40 HRC, ideally. (Some materials as hard as 45 HRC can be effectively burnished as well.)
Uniformity: As evident in the sample part above, the finish depends on a uniform and tear-free surface in order for the peaks and valleys to cold flow correctly.
Roughness: An initial 80- to 120-microinch (2- to 3-micron) surface is ideal for roller burnishing. The rougher starting finish allows for a more dramatic change in the final finish because the rollers can apply greater pressure to more of the surface.
Cogsdill’s tools are versatile and can be used on lathes, drill presses, machining centers and rotating spindles. The standard tool models are designed for right-hand rotation, with either tool or part rotating. An adjustment collar on Cogsdill’s roller burnishing tools enables the operator to set the rollers to the required diameter. The operator unlocks the collar from the interlocking bearing collar and rotates it, altering the position of the tapered mandrel relative to the tapered rolls, thereby changing the effective tool diameter. The tool can be adjusted in 0.0001-inch increments. The company’s focus lately has been on its smaller-size tools, whose compact design and rear-located adjustment mechanism enable use on multi-spindle automatic lathes and Swiss-type machines.
How to Burnish Metal
Burnishing is the process of polishing metal to give it a smooth shiny finish. It is often used on soft metals such as brass or aluminum and is an alternative to diamond dragging.
Trophy brass is usually lacquered with a thin layer of paint or plastic that must be abraded to expose the underlying metal without actually engraving it. The following steps will show how to burnish metal.
Select a burnisher. This is a special engraving tool with a blunt end instead of the usual sharpened tip, illustrating its purpose of polishing metal without cutting into it. The spindle motor causes the burnisher to rotate rapidly and will provide a smoother appearance than diamond dragging.
Set up the engraver for burnishing. Install the light touch burnishing adapter and insert the burnisher into a solid collet or top load spindle just like any other cutter.
Remove the nose cone and micrometer. They are not needed for burnishing and removing them will provide a better view while burnishing.
Loosen the set screw and position the burnisher so that it extends at least 1/4 inch past the spindle and tighten the set screw.
Push the burnisher with your finger to see if it hangs up. If it does, loosen the set screw slightly and rotate the burnisher 30 degrees. Repeat this until the burnisher moves back and forth. The idea is to have the set screw as tight as possible and still allow the burnisher to move freely.
Different Deep Hole Drilling Tools and Technologies
Gundrilling is a deep hole drilling process that uses a long, thin cutting tool to produce holes at high depth-to-diameter ratios. Gundrilling is typically effective in diameters from 1 – 50 mm [~0.04 – 2.00 in]. A gundrill differs from a conventional twist drill by its unique head geometry; a standard gundrill has a single effective cutting edge that removes chips as it advances into the workpiece.
The gundrilling process can drill deep holes beyond what is possible with conventional machinery and tooling, such as twist drills, by using high pressure coolant for clean chip exhaust, even at extreme depths.
There are different types of gundrills, including the following:
Single Flute Solid Carbide (g. botek type 110): have one effective flute, which creates unbalanced cutting forces that push the drill away from the cutting action. To overcome this unbalance, gundrills are equipped with built-in guide pads that help create an accurate hole and achieve the desired surface finish. The cutting action pushes the tool and its guide pads against the opposite side of the hole. Various contours and nose geometries are available depending on the application.
Two Flute (e.g. botek type 120): features two flutes placed 180 degrees apart. This enables the cutting forces to be balanced, keeping the drill tracking straight and helping increase the feed rate.
Indexable (e.g. botek type 01): these gundrills can be single fluted, featuring one screw-down insert with one or more cutting edges and brazed-in or replaceable guide pads; or, they can be two fluted, with multiple insert When used on traditional CNC equipment or on dedicated gundrill machines, indexable gundrills can offer a big advantage in decreased cycle time due to increased cutting data.
BTA stands for Boring and Trepanning Association and is also sometimes referred to as STS (single tube system) drilling. BTA drilling is a deep hole drilling process that uses a specialized drilling tool on a long drill tube to produce deep holes with a typical diameter of 20 mm [~0.80 in] and larger, and depth-to-diameter ratios of up to 400:1.
BTA drilling heads are threaded and are mounted onto long drill tubes. BTA drilling heads have multiple cutting surfaces to remove chips efficiently, exhausting them using high-pressure metal-working fluid through holes in the tool head, then out the drill tube and through the machining spindle. BTA tooling is available in brazed or inserted carbide configurations.
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