When it comes to buffing and polishing, compound application is often overlooked as a way to improve your processing...
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The common theme one often hears these days in U.S. manufacturing tends to involve so-called “lean manufacturing.” In today’s economic environment it is imperative that we—as manufacturers—re-examine all aspects of our manufacturing practices in order to maintain their competitive advantage.
When it comes to buffing and polishing, the area most often looked at for improvement usually involves buffing media, meaning buffing wheels and buffing compounds. After all, if you can find an everlasting buff and a compound that allows you to polish your parts in 50% of your current cycle time, you’ve cut your costs tremendously. But we all know it’s not that easy.
One area that is often overlooked is the application of compounds to buffing wheels. While it may seem like a small piece of your buffing operation, it is perhaps one of the most important step. Buffing compounds can be broken up into two types: solid bar compound and liquid buffing compound.
Typically, solid bar compound is used in manual or semi-automatic buffing operations. Usually, a simple motor-driven buffing jack is used. However, this method is sometimes used on buffing wheels as large as one-meter wide, usually in the cutlery industry. The operator presses a bar of compound onto the rotating buffing wheel for a short interval. Once the buff is loaded with compound, the operator then buffs the workpiece. Often, the operator must load the buff with compound many times during the operation, depending on the size of the piece or the finish requirements.
While this method may at first seem inexpensive—since no application system is needed—it actually has many disadvantages.
First, it is unsafe. While it is difficult to put a dollar amount on the cost of an injury, it more than outweighs the relatively small investment of an automatic bar application system. Second, it can lead to inconsistent finishing results due to its subjective nature of application. Third, the time spent by the operator loading the buffing wheel with compound is essentially machine idle time.
In comes the automatic bar feeder. These devices come in various configurations. They can range from simple cylindrical tubes with a pneumatically activated bladder that clamps the bar and uses pressure from above or gravity to feed the bar compound into the buff, to more complex systems such as the device shown in the box on the right.
This system uses pneumatic clamps to hold the bar in place while in contact with the buffing wheel. It also uses a pneumatic cylinder to index the bar into position in increments of 0.004-0.008 inch.
Regardless of the type of bar feeder one chooses to use, the advantages are tremendous. It goes without saying that this is obviously safer than a manual application. The operator will apply a consistent amount of compound with each application, leading to more consistent finishing results. The biggest payoff however will be productivity, since the operator can now use all of his time at the buffing station for buffing and not applying compound. Depending on cycle times and amount of compound applied, productivity could easily be enhanced by 50% and up. That is real savings!
Liquid buffing compound is most often applied using a spraying system that uses either low-pressure atomizing spray guns or, more recently, high-pressure airless spray guns.
Low-pressure systems are perhaps the simplest and least expensive systems to operate. They consist of a compound source that is pumped using a drum-style pump, diaphragm pump or, in some cases, a pressure pot. The buffing compound is pumped to the spray gun at a typical pressure of 80-400 psi, depending on the type of delivery system used. The spray gun is activated via a solenoid valve that is triggered either using a footswitch or a timer. Once the spray gun is activated, air pressure is introduced to the spray gun and is used to atomize the buffing compound as it exits the gun. This forms a “mist” that is sprayed onto the buffing wheel or directly to the actual part being buffed.
Low-pressure spray guns operate relatively trouble free due to the relatively small number of parts contained inside the gun, as well as the low pressure at which compound travels through the gun. Consequently, maintenance costs and downtime of these guns are relatively low.
While a low-pressure system is sometimes the preferred method, such as on extrusion and sheet buffing, there are some disadvantages of these systems.
Compound use is typically quite high with a low-pressure system because of the air turbulence created by the rotating buffing wheel. Compound exiting the gun at a relatively low pressure simply cannot penetrate the wind barrier surrounding the buffing wheel, leading to a large amount of compound throw-off. In most cases, a majority of the buffing compound ends up on the machinery or shop walls and not on the buffing wheel where it belongs. Challenges also arise when trying to cover wide buffs. Even when using large-angle spray nozzles, the compound simply can’t penetrate the air curtain surrounding the buffing wheel. This calls for either a gun-mover (usually a pneumatically operated device which traverses the gun across the full width of the buffing wheel) or more spray guns.
High Pressure Systems are used more in recent years due to their many advantages. There are two types of common high-pressure spray gun systems.
The first type applies liquid compound through an airless spray gun via a complete high-pressure system. This consists of a high-pressure spray gun, a high-pressure pumping system and high-pressure manifolds, hoses and fittings.
The advantage of this system is that the high pressure by which the compound leaves the spray gun allows the compound to penetrate the air turbulence around the buffing wheel. These types of guns can generate pressures up to 4,000 psi. Another advantage of these guns is the ability to control the spray time using a timer or other device. These guns tend to be compact, making them easy to mount in confined areas, especially on complex automated systems. In some cases they are easily changed over due to the mounting mechanism that uses a simple manifold mounting plate and only a few screws.
While the compound savings can be substantial, it is important to recognize that these systems do have their drawbacks. Due to the high pressures inside the gun, significant wear and tear is inevitable on the spray gun. In addition, the high-pressure pumps that feed the compound to the gun must periodically be re-built due to the pressurized pumping of the abrasive buffing compound. It also needs to be noted that all hoses, fittings, and manifolds must be able to withstand these extremely high pressures that are present throughout the system.
The second type of high-pressure spray gun is the “high/low” spray gun. These systems use normal shop compressed air (70-100 psi). Through a specially designed chamber, the spray gun internally multiplies the spray pressure from ratios of 14:1 to 20:1. This means that incoming air pressure of 100 psi would result in spraying pressures up to 2,000 psi, which is more than enough to penetrate the wind barrier of the rotating buffing wheel.
The major difference of this type of spray gun when compared to either a low-pressure gun or a true high-pressure gun is the amount of compound dispensed during each cycle. The compound cavity inside the gun determines the volume of compound dispensed with each cycle. This can be an advantage, as it will dispense a precisely measured amount of compound with each shot, allowing for some control over your process. However this can also be a disadvantage, especially in applications that require large amounts of buffing compound, as the gun body will not have time to refill in between spraying cycles.
Other advantages to this type of spray gun include significant compound savings, low compressed air use, and more consistent finishing results. As with any system using abrasive compounds at high-pressures, wear and tear on the spray gun is evident and replacement parts and maintenance will contribute to the total cost of ownership. Studies have shown, however, that this expense is far outweighed by compound savings.
Which Method To Choose? Unfortunately, there is no clear-cut answer.
If an operation is currently buffing and polishing in a manually oriented environment, it certainly should look at automatic solid bar compound feeders, or perhaps consider evaluating a liquid compound system as an alternate. Manual application of bar compounds is simply unsafe and unproductive.
Those who are currently using a liquid compound system might want to re-evaluate their current system and perhaps consider alternatives. For instance, it is quite simple to connect a low-pressure spray gun system to one compound source and connect a high-pressure spraying system to another compound source. Analyze factors such as: compound use, replacement parts costs, downtime, and finish quality over a period of three to six months. Compound savings may outweigh the cost of replacement parts and maintenance. It’s also possible that the volume of production or application simply can’t justify a particular system.
Often I ask customers why they are using their current method of compound application. Many times the response is “because this is the way we have always done it.” While there is no “perfect system” in the world of buffing compound application, there are alternatives. Take a look—you might be surprised what you’ve been missing out on.
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