Meeting the Demand for High Quality Tube Finishing

On Fabricator Magazine (August 2011), USA, Brett Mandes, writes an article about round tube finishing where quality and flexibility topics are discussed.

“The demand for high quality finishes on tubing has increased over the years.  The increased use of stainless steel in the medical, food, pharmaceutical, chemical processing, and construction industries is a factor in driving this demand.  Another driving force is the need for painted, powder coated, and plated tubing.  Regardless of the desired result, to properly finish a metal tube a grinding process is required.

Different types of tube finishes include a grained finish, painted, powder coated, anodized and plated.  The grain finish is achieved using an abrasive belt, or a sequence of abrasive belts.  In this case the type of belt and the grit used will in large part determine the appearance of the final finish.  A standard in the industry can be commonly referred to as a #4 finish.  A grain finish is almost always used when finishing stainless steel tubes.  A grain finish is also commonly applied to aluminum tubes as well as anodizing.  When manufacturing with mild steel and carbon steel the tube will most often receive a painted, powder coated or plated finish.  When a coating, plated, or anodized finish is used it is the coating or plating material that determines the appearance of the final finish.  However, a grinding process is still required to properly prepare the surface of the tube.  In this case the grinding process is done to remove imperfections and smooth the surface of the tube to a point where the scratches left by the grinding sequence are undetectable after the coating or plating material is applied. […]”

In order to make the necessary tube finishing centerless grinding is a method often chosen.

Centerless Tube Finishing Machines

“The three basic elements of any centerless grinder include the grinding head, regulating head, and the work rest support.  Centerless machines may vary in size and configuration but they all operate on the same basic principles.  The parts are placed in such a way that the work piece is constrained at all times as it travels through the machine.  As the work piece traverses through in the axial direction, it is held either by special stops, adjacent work pieces or by the friction of the regulating head.  This makes the operation compliant to both long bar stock and short cylindrical pieces. […]”

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 presents its own set of challenges.  Many manufacturers choose to finish their tubes after they are bent to avoid damaging the finish during the bending process.  When choosing to finish bent tubes centerless machines are not an option because they require that the tube be not only round but the tube must also be straight.  Traditionally grinding with hand tools was the only available method for finishing bent tubes.  Uneven pressure, difficulty making the finish lines concentric to the bend, operator fatigue, time spent positioning and repositioning a tube, and inherent safety related issues have made this method less and less desirable.  The planetary machine as emerged as a welcome alternative.  A bent tube can be processed without fixturing and without the tube spinning.  There is much less effort required by the operator as the finish is determined by the grit of the abrasive belt and the through put speed as opposed to unit pressure created by an operator.  The result is a much higher degree of uniformity among parts, while creating a uniform concentric finish.  When considering the differences between these two machine designs remember this fundamental difference: with a centerless machine the machine does the work of carrying the part through the machine.  Once the part is in contact with the regulating head and the grinding head the part will begin to spin at a high rate of speed and move forward, for the operator it’s hands off.  With the planetary machine once the tube is in the grinding area the part is being processed as the belts spin around it but it will not move.  It is up to the machine operator to move the part through the grinding area unless the machine is equipped with an automatic feeding device. […]”

“Automation for the planetary design must take into account loading and unloading, but it must also account for driving the part continuously through the grinding area.  This is accomplished in one of two ways.  The simplest way is to fit the machine with what is called an “autofeed” system.  The autofeed system is a set of motors with two sets of rollers above and below the tube at the infeed and outfeed. This requires the operator to load the part into the autofeed and unload the parts after processing but the operator is doing none of the work to feed the part through the machine.  To also accommodate for automatic loading and unloading with the planetary design you can add loading and unloading tables similar to that described for the centerless machines.  To make this work properly the addition of photo sensitive eyes need to be added before and after the grinding area to automatically adjust the variable speed of the abrasive belts to allow the part to enter and exit the machine as described above.”

The discussion on high quality flexibility in tube finishing above has shown two of the available method to polish round tubes: centerless tube finishing machines and planetary (or orbital) tube finishing machines.

Finishing Bent Tubes in MLW100
As shown planetary machines came to solve some of the problems centerless systems: spiral marking, vibration and related safety on the work of long tubes due to its proper rotation, capability of polishing bent and curved tubes and also finishing out of round and elliptical tubes.

Also, planetary machines are able to finish drilled tubes as it is demonstrated on the radiators tube finishing article posted earlier. More, a lower and flexible abrasive belt contact delivers a softer finish quality and lower heat on the tube.

You may get more information about these features of ML Planetary Tube Finishing Machines on the Round Tube Finishing section of this website, visiting www.nsmaquinas.com or emailing us at marketing@nsmaquinas.pt