March 2012 Archives

This is a method of pressing and moving metal brushes onto material surfaces to create directional lines, and is called "Hairline finishing".
Smaller scale brushed finishes maybe done with commercially available metal brushes, but larger scale brushed finishes employ semi-automatic system with wider brushes where the brushes pass over the workpieces or the workpieces pass under the brushes.
Other brush configuration is where the brush wire elements are arranged radially as brush wheels. This type of brush wheels can be mounted on buff lathes in a similar manner as paper buffs, and can create directional lines on surfaces. Table 1 shows the types of wheel brushes and finish types.

[Table 1] Brush types and finishing levels

There are a various diameters of wheel brushes and are used based on application purposes. There are types made of bent metal wire fixed to sheet steel. When used on buff lathes, varying wire lengths result in various finish effects.

In general, longer wire with circumferential velocity will result in dot effects, and lower circumferential velocity will generate scratch effects. With the wheel brushes, sharpness of the tips of the wire is critical. Pumice stones and bricks are occasionally pressed against the wheel brushes to remove accumulated oxides and metal powder.

A notable point on the brush finishing is that the wire fragments and powder of the bushes get ingrained into the surfaces of soft metal such as aluminum. If anodizing or chemical conversion processes are applied on such surfaces, partially blackened defects will result. Pretreatments to remove these ingrained fragments must be applied.

This method is characterized by ease in creating roughed surfaces with direction uniformity. The paper here means sand paper. In the past, the abrasives are bonded on surfaces of paper as the name implies, but cloth is used instead today. In addition, endless paper belts are in frequent use. Various application examples are shown in [Fig.2].

[Fig.1] is an example of a buffing lathe with contact wheels as abrasive surfaces, and is the most fundamental configuration. The figure on the right is a vertically configured simple semi-automatic machine of the same with a feed belt as the abrasive surface.

[Fig.2] shows a various applications of paper belts. The center-less application in the figure can easily automatically grind round objects such as rods and pipes by the adaptation of a simple adjustment wheel. The variable belt application figure shows the ability to grind difficult to access inside surfaces.

The paper belts are purchased, but the paper can be user made. Paper buffs are made by bonding peroration reinforced cloth buff material into certain thickness. This is fixed to a buffing lathe and centered (balanced), and adhesive is applied on the circumference to bond abrasive materials of various grit sizes. Such paper buffs were the mainstay for grinding before the advent of paper belts.
Surface adjustments using the paper belts involve heat generation. The heat generated has negative effects on the surfaces to be processed. to counter this problem, abrasive compounds are used in conjunction. Buff polishing compounds, as discussed later, can be used but oil based polishing compounds are used for friction reduction and cooling effects.

Various surface adjustment methods will be explained hereafter.
First, the blasting methods. Blasting methods employ abrasive mediums sprayed or projected onto surfaces either by Dry method which is in the air or Wet method which uses liquids containing the abrasive materials. Both methods yield non-directional polished surfaces.
The abrasive materials used for the process are sand shots (silica sand, alumina sand, etc.), grit (pulverized steel, etc.), powdered iron (reduced iron powder), cut wire (cut steel wire), and high hardness steel shots.

An example of a dry blasting system is shown in [Fig.1].

An object is placed in the process chamber and the abrasive is sprayed on the surface while keeping the gun tip at proper distance from the object. The spent abrasive material is sent to the cyclone and reusable portion is recycled. The removed powdered material is trapped by the bag filter. As shown, the process chamber is a sealed steel box and the gun is typically fixed stationary. There is a glass window and a pair of rubber gloves on the front side, and the operator holds the work piece with the gloves and apply the process using the foot valve.
There are dry methods where compressed air is utilized to spray the abrasives as well as methods that project the abrasives with blade type devices similar to windmills. This method is used for objects too large to fit inside the process chamber.
The wet methods are typically used for less aggressive removal needs. Surfactants, rust inhibitors, cleaning chemicals, and polishing materials are mixed into water with suspended alumina powder, silica sand and etc. Compressed air is used to spray.

For surface treatments, the same surface treatment will yield varying appearances depending on the preparations used due to the amount, quality, and directions of light reflections. Most metal surfaces contain imperfections such as variations of base metal coarseness, non-uniformity, unevenness, and surface ingrained contaminations. It is not possible to obtain satisfactory decorative finish with such imperfect surfaces. In order to counter such problems, some means of mechanical polishing operations are needed. Such processes are called "Surface Adjustments" of materials.

Various surface conditions are desired based on the products usage purposes. Some surfaces are required to be mirror finished, hairline finished where the lines are formed unidirectional, or satin finish with random light reflections. When the original material surfaces largely differ from what is required for the final surface conditions, several steps of grinding/polishing processes are applied to obtain the desired final conditions.

In order to indicate various surface conditions, Surface Roughness expression is typically used. There are various polishing methods such as: dry blasting, liquid blasting, paper polishing, brushing, steel wool, buff polishing, and milling. The process outcome of these methods differ and they are shown in [Fig.1]. The polishing methods and the surface roughness obtained can be seen in the figure.

(11) Friction coefficient measurement method

Lubricants strongly adhering to metal surfaces lower friction coefficient. Similarly, existence of organic substance, or contaminants also alter the friction coefficient.
This principle can be used to measure the friction coefficients by using a slider on contaminated metal surfaces as shown on [Fig.1] by converting the force acting on the slider into friction coefficient by a strain gauge. The amount of lubricity by the contamination is determined by varying the loads on the slider and by obtaining friction force variations over time.

(12) Contact potential difference test

The bonding strength of organic substances on metal surfaces is related to the organic substance's affinity to the metals. This is a contamination measurement method utilizing this principle by measuring the contact electrical potential differences.
The contact potential electrical difference is the potential difference between clean metal surface and organic substance contaminated metal surface. The differential value would be zero if there is no contamination, and the value would vary depending on the amounts of contamination. The [Table 1] shows potential differences of stearic acid on metal surfaces. Similar single molecular contamination on platinum and nickel oxide would vary in removals, due to the differences in Adsorption power.

[Table 1] Removal of stearic acid single molecular contamination on metal surfaces

(13) Other

There are other methods such as: Weighing method and Ellipsometry method.