In order to make the necessary tube finishing centerless grinding is a method often chosen.
Centerless Tube Finishing Machines
However, the author enhances some disadvantages of this method.
“If the rest, although parallel, is not properly spaced between the regulating and contact wheels, or is too high or too low, the work piece will have a tendency to bounce. If the centerline of the work piece is at the same height as the centerline of the regulating/grinding heads, the work piece will be ground to the shape of a curved sided triangle. The centerline of the work piece must be absolutely parallel to the face of the contact wheel. Any out of parallel condition will cause the work piece to traverse through at an angle creating severe pressure on either edge of the abrasive belt. This condition will cause a continuous spiral marking around the work piece known as “barber poling”.
“Another potential problem is uneven wear across the abrasive belt. The entire abrasive belt surface should show even wear. The wear pattern of a used abrasive belt can be used as a tool to detect the “trueness” and condition of the contact and regulating wheels. A used abrasive belt is essentially an imprint of the condition of the contact wheel and regulating wheel. The most common cause of uneven abrasive wear is the improper dressing of the contact and/or regulating wheel. Barber poling can also be caused from an improperly adjusted horizontal angle on the regulating head. The regulating wheel should be dressed parallel to the centerline of the work piece only after the proper helix and horizontal angles are set. […]”
According to the author “centerless grinding and finishing works well for straight tubes but has two distinct disadvantages: (1) the tube spins at a high rate of speed as it is being processed and (2) you cannot process bent tubes. “
“Sometimes centerless grinding is not possible. If your tube has to be finished after bending or assembly centerless grinding will be impossible. Similarly, if your tube is too far out of round or has an irregular shape you will not be able to achieve optimum results with a centerless machine. As an alternative you can consider the use of a planetary machine.“
Planetary Tube Finishing Machines
“One of the challenges tube fabricators say they experience is not being able to automatically finish tubular components after they've been bent or end-formed. The tube's surface can get scratched or gouged as it is clamped and handled during bending. With a planetary machine the grinding and finishing process can be applied after the tube has been bent. The other two distinct advantages gained by the use of a planetary machine are: (1) the tubes do not spin which creates a safer environment of the operator and eliminates the possibility of barber poling and (2) the tube does not have to be perfectly round. With a planetary machine you can process tubes that are oval or have an irregular-shape. The machine is designed around an aluminum main wheel that carries six tensioning rollers and two abrasive belts. […]
|Planetary Wheel of ML100|
As the belts slow the opening between the belts expands. When the opening is large enough to allow the tube to enter the operator feeds the tube into the opening and releases the pedal. When the belts return to full speed they restrict around the tube and create sufficient grinding pressure to remove defects and create an even 360 degree finish. With this planetary design the belts spin around the tube so the tube itself does not spin as it is being processed.”
Bent tubes and Out-of-Round Tubes Finishing
“As mentioned above one of the advantages of the planetary design is the ability to process tubes that are bent, out-of-round or with an irregular-shape. By the very nature of the centerless machine design the tube must spin as it is being processed. If you attempt to process a tube that is not round the tube will have a tendency to want to jump out of the machine causing potential damage to the machine, the tube or the operator. You can encounter a similar problem when processing very long tubes in a centerless machine. Although not impossible to do with a centerless machine, it can be very difficult to get a consistent finish on very long tubes because they have a tendency to whip around at the ends while they are spinning if they are not supported properly. This problem is eliminated with the planetary design. Since the tube does not spin, for long tubes all that is needed is support to hold the weight of the tube at each end. This is often accomplished with a very low cost low tech solution.
|Finishing Bent Tubes in MLW100|