August 1, 2011
Tube finishing doesn't have to be a manual operation. Centerless grinding can efficiently finish straight tube, and now planetary grinding machines can aid the finishing of tubes of various shapes,including previously bent workpieces.
The demand for high-quality finishes on tubing has increased over the years, much of it driven by more use of stainless steel in the medical, food, pharmaceutical, chemical processing, and construction industries. Another driving force is the need for painted, powder-coated, and plated tubing. Regardless of the desired result, a properly finished metal tube requires grinding.
A grain finish is achieved using abrasive belts, used individually or in some combination. The goal for many applications is to achieve what is commonly referred to as a #4 finish. Stainless steel tubes almost always call for a grain finish; many aluminum tube applications call for such a finish as well.
A lot of mild or carbon steel tubes receive a painted, powder-coated, or plated finish, so in these applications the coating or plating material determines the final appearance of the workpiece surface. However, a grinding process still must prepare the tube’s surface for coating or plating, mainly to remove imperfections and smooth the tube surface so that scratches and other imperfections are undetectable after the coating or plating is applied.
Finishing tube can be extremely labor-intensive. Grinding with hand tools is difficult enough, and grinding a sensitive, curved, and formed surface makes the job even more difficult. But automated and semiautomated tube finishing alternatives can help speed operations greatly—and often create a cleaner, safer work environment. Centerless grinding can put a grain finish on straight tube, while specialized planetary grinding systems (see Figure 1) can put a grain finish on a variety of irregular tube shapes, including bent tubing.
In centerless grinding, a cylindrical part is supported by a work rest blade and rotated by a regulating, or feed, head. This contrasts with conventional roll grinding in which cylindrical parts are mounted on centers. Modern centerless grinding and finishing machines provide high performance, increased production rates, and improved quality at low consumable costs. Coated abrasives offer safety, fast stock removal rates, high throughput speeds, cool operations, and performance consistency with few rejects.
Three basic elements of any centerless grinder are the grinding (or contact) head, regulating head, and work rest support (see Figure 2). Centerless machines vary in size and configuration, but they all operate on the same basic principles. The workpiece is constrained at all times as it travels through the machine. As the workpiece traverses in the axial direction, it is held either by special stops, adjacent workpieces, or by the friction of the regulating head. This makes the operation suited for both long and short cylindrical pieces.
The grinding head consists of the abrasive belt, the contact wheel for abrasive support, and an idler to hold belt tension. The belt direction is downward, against the workpiece, and tends to push the workpiece away and into the regulating head. The regulating head in turn forces the workpiece back into the abrasive belt. This combination generates high unit pressures, making centerless grinding very efficient.
The regulating head rotates the workpiece at a constant surface speed and controls the through-feed of the workpiece past the abrasive belt. The regulating head can have one of two common configurations. It can consist of a belt (leather, rubber, or abrasive) running against a steel platen; or it can have a rubber or bonded-abrasive contact wheel. With either configuration, the regulating head must rotate upward, opposite the direction of the abrasive belt.
Three parameters—surface speed, helix angle, and horizontal angle—govern the regulating head’s motion, and each can be adjusted to fit the specific operation. The helix angle is the head’s angle in the vertical plane parallel to the workpiece centerline. The helix angle determines how far the workpiece travels in one revolution. Consequently, the helix angle determines how many times a given point of the workpiece comes in contact with the abrasive belt as the part passes through the machine.
The actual through-speed is a function of both the helix angle and the regulating head’s rotational speed. In general, by decreasing the helix angle and increasing the regulating head speed, you can obtain a better surface finish and faster through-feed of parts.
The work rest in a centerless grinder provides firm support and properly positions the workpiece as it traverses between the regulating and grinding (contact) heads. Adjusting the work rest raises or lowers the center of the part in relation to the centerline between the contact and regulating wheel.
The distance between the regulating and grinding heads controls the diameter of the workpiece. Generally, the work rest should be positioned about one-third the distance from the grinding head to the regulating head. When determining the height position of the work rest, note that the workpiece centerline should be no more than 1⁄16 inch above the centerline of the regulating and grinding heads. (Of course, for specific setup parameters, consult the operating manual or contact the machine builder.)
The work rest also plays the largest role in properly aligning the workpiece. In fact, most centerless grinding problems can be traced back to poor alignment of the work rest. If it is not placed exactly where it should be between the regulating and contact wheel, or if it is too high or low, the workpiece will tend to bounce. If the workpiece centerline is at the same height as the centerline of the regulating and grinding heads, the workpiece will be ground to the shape of a curved-sided triangle.vThe workpiece centerline must be absolutely parallel to the contact wheel face. Any out-of-parallel condition will cause the workpiece to traverse at an angle, creating severe pressure on both edges of the abrasive belt. This condition will cause a continuous spiral marking—known as barber poling—around the workpiece.
Uneven wear across the abrasive belt can cause other problems. Ideally, the entire abrasive belt surface should show even wear. Technicians can read the wear pattern of a used abrasive belt to detect the trueness and condition of the contact and regulating wheels. Improper dressing of the contact or grinding wheels often can cause uneven abrasive wear. Improper adjustment of the regulating head’s horizontal angle also can cause barber poling.
The regulating wheel should be dressed parallel to the centerline of the workpiece only after the proper helix and horizontal angles are set. After the wheel is dressed, these angles should not be changed unless a total changeover in setup is required.
In centerless grinding, the tube must spin. If you attempt to process an irregularly shaped or out-of-round tube, the tube will tend to jump out of the machine, potentially damaging the tube or machine and creating a safety hazard for anyone nearby. Similar problems arise when using the centerless grinder to process very long tubes. While not impossible to process with a centerless grinder, long tubes tend to whip around at the ends if not supported properly.
Finishing bent tubes presents its own set of challenges. Centerless machines can’t finish them because, again, the tube spins rapidly—not very practical for a formed tube. Traditionally, grinding with hand tools has been the only practical option for bent tube. But such a manual process comes with obvious disadvantages: uneven pressure; difficulty making the finish lines concentric to the bend; operator fatigue; time spent positioning and repositioning the tube; and safety hazards, to name a few.
In these cases, planetary grinding systems can be a viable alternative.
Planetary tube grinding machines do not spin the tubes during the process, which creates a safe operator environment and eliminates the possibility of barber poling. Planetary systems also do not require tubes that are perfectly round, and instead can process tubes that are oval or have irregular shapes. Perhaps most significant, they can grind bent tube.
The machines have a main wheel that rotates. In the wheel’s center is a hole for the tube to pass through. Driving the wheel is a V belt connected to a single-spindle motor. Mounted to the back side of the main wheel, on either side of the center opening, are two belts. The belts travel around rollers (three for each belt) to form two facing triangles, with one triangle’s side parallel to the other; this is where the belts contact the workpiece. To grind a tube, the rollers spin the two belts, and the main wheel rotates the entire belt assembly, ensuring that abrasive material contacts all points around the tube circumference (see Figure 3).
Each belt has a tension roller for tensioning the belt; a tuner roller that adjusts the belt for different tube diameters; and a drive roller connected to a dedicated, variable-speed motor that allows these belts to spin independently of the main wheel. The operator adjusts these belts to accommodate specific tube diameters.
Here’s how it works. Picture a planetary grinding machine with the main wheel having a 5-in.-diameter hole in the center, where you insert the tube. When you turn on the machine, you see abrasive belts starting to spin. At first the space between the two belts is too small to allow the tube to enter.
To allow the machine to accept the tube, you depress a foot pedal, and the opening between the belts expands. When the opening is large enough to allow the tube to enter, you feed the tube into the opening and release the pedal, which returns the belts to their original position. They restrict around the tube and create sufficient grinding pressure to remove defects—ultimately creating a grain finish over the tube’s entire circumference. Belts contact the tube in such a way that the tube itself does not spin during the process. This is how it looks in action.
When comparing centerless and planetary grinding systems, remember this fundamental difference: With a centerless system, the machine does the work of carrying the part through. Once the part contacts the grinding and regulating heads, the part begins to spin rapidly and move forward. For the operator, it’s hands-off. On a planetary system, once the tube is in the work area, the belts are grinding away, but the tube does not move. It is up to the machine operator to move the part through the grinding area, unless it is equipped with an automatic feeding device.
Both centerless and planetary systems offer varying degrees of automation. For the centerless machine, feeding is already a hands-off operation, so automation focuses instead on part loading and unloading.
In high-volume environments, a centerless machine can be fitted with infeed and outfeed tables to hold multiple tubes. Infeed tables often have pneumatic tilting devices that change the table angle in relation to the ground; this lets gravity feed tubes to motorized rollers that, in turn, present parts to the machine. Similar setups are designed at the outfeed, where finished tubes are collected and protected.
As with centerless machines, automating planetary machines involves loading and unloading, but it also includes feeding parts continuously through the grinding area. This can occur in two ways. The simplest is to fit the machine with an autofeed system consisting of two sets of motor-driven rollers above and below the tube at the infeed and outfeed (see Figure 4). The operator loads and unloads parts, but does not manually feed tubes through the machine.
Loading and unloading tables are available as well, but these also require sensors (photo eyes) at the infeed and outfeed. The sensors instruct the machine to adjust the belts for tubes to enter and exit the work area.
As always, which technology to choose depends on the application. Centerless grinding excels at efficiently finishing straight tube. It also requires less operator intervention. Once the operator moves the tube into position, the machine takes it from there. Planetary systems do require some feeding force, either from an operator or external feeding mechanism. But unlike centerless grinding, planetary machines can handle bent tubes and other irregularly shaped workpieces.
Regardless, know that when finishing tube, hand tools are not the only option. Today machines can help make what was a difficult job much easier, safer, and efficient.
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