While bending, cutting, punching, or forming metal tubes, surfaces get some marks, prints, or scratches. These marks are generally impressed by the bending rolls in case of ring-rolling machines, by clamp dies - guides - mandrels or bend-dies in case of rotary-draw benders, or again simply by handling of materials (in manual, or in automatic mode).
In some cases, the defect to be removed is not a mark, but just the so called "orange-peel" at the extrados of the bends, or the porosity of molecular grains in the area of maximum stretching / compression of materials, which occur because and proportionally to the degree of deformation. In some other cases, it's just necessary to remove some very small "craters" emerged on the tube-surface, right before assembly of parts.Sometimes it's just required to eliminate the color-difference of the welded spot, or make even the degree of finish in comparison with the balance of the piece, or again to remove, while polishing the external surface, some little burrs left by a drilled- or punched-hole, or the print left by a metal-chip that was unfortunately clamped-in. All these problems should be considered as solvable by a polishing routine only, without having to go through a more expensive mechanical-, or electrochemical- process.
All operators involved with chrome-plating treatments, or other galvanic coatings, are well aware of the fact that - after these processes - a.m. defects are not hidden, but rather enhanced. In this aspect, one can find a big difference in comparison with different surface treatments process, as epoxy-painting for example, which may easily "forgive some sins", or mask-up some errors, by covering the same under a thick layer of color...
For all above described purposes, some orbital polishing (or grinding) machines for tubes have been properly developed. These machines are capable to work on elbows, simple or multiple bends, in a single plane or in 3-D (also spiralised), with large-, medium-, or small-radii, at the end of the forming routine. Of course, they can also operate on straight blanks. These grinders are called "orbital-type" because the tools - the grinding belts - spin around the tubes, which remain stationary. On these equipment, cross section of materials can be round, oval, or elliptical, and - depending on kind of machine being used - tubes can be bent even more than 140-150°, without interference or collision with the machine frame (exact bending angle depends on tube diameter and on bending radius). Anyhow, machine heads are generally very narrow, with the purpose to allow maximum degree of freedom and different "escape-ways" in case of particularly complex-, or crazy-tube geometries.

Working principle
The orbital grinding operation is performed by two opposite grinding belts, which spin around three pulleys supported by the machine head, while workpiece is being inserted (manually or automatically - but just for straight blanks, in this case) through the same grinding belts. By using some wooden-tips and feeding-plugs is possible to make grinding of parts all through their length, up to the very ends, with no risks. The machine-flywheel keeps on turning, together with the grinding belts, around the tube, so that to dramatically reduce the working times in comparison with any single-brush (or tangential) polishing machine, where the contact between the tool and the workpiece is theoretically just a single point. Advantage in terms of productivity in favour of the orbital grinding machines is huge: payback-time, or return of investment is therefore very short, especially when we compare a given degree of finish, and a given cost of human-labour.
Technical advantages
Orbital-type tube grinding machines don't require any psyco-phisical effort from operators, and not even a specific training, because they are simple to use, maintenance and set-up, safe in operation, and because they perform with results absolutely consistent as soon a certain "procedure" is established. This procedure will include just the use of certain grinding belts (in a given sequence), and will prescribe a previously tested feeding speed / rotational speed for each work-piece.
Again, in order to get a comparison with single-brush polishing machines, when using the orbital grinding machines, the operator doesn't have to provide any particular pressure between the tool and the work-piece, he does not have to spin the tube on its axis getting tired on wrists, and he doesn't even have to support its weight. In fact, thanks to the self-centering action of the machine belts, and to the "V" supports at the entrance and at the exit of the head, parts remain perfectly balanced.
As regards dangers in use of machines, the fact that tube doesn't spin in orbital grinders is already an advantage, with no mention to the risk of abrasion of the operator, which remains an unsolved problem in conventional single-brushing machines, while this risk just doesn't exist in orbital grinders. Here - in fact - there're some safety protection covers and centering (calibrated) bushes that can be left in place without reducing operational access: they don't allow to enter into the grinding belts anything, but the workpiece.

Different adjustments
The orbital grinding machine represented in picture 1 is capable to operate on tubes from OD 10 mm. (and less, using some special devices), up to OD 76 mm. It can also operate oval-, elliptical-, tapered-shapes, etc., provided that they can be inscribed in a max. circle of 76 mm. of diameter. If diameter variation from one piece to the other is just of a few millimeters, no adjustments are required, because drive pulleys are elastically mounted and they allow to compensate small differences.
In case very different cross-sections have to be operated, shifting the drive pulleys on the machine flywheel is necessary. This operation is made in 5 minutes, through a single central screw, in such a way to get an optimal positioning / contouring of grinding belts on the work-piece. When having some big differences in tube-sizes, it's also necessary to replace the guiding bushes at the entrance and at the exit of the machine-head, and also to adjust the height of the "V" supports, to suit the new dimension of tubes to be polished.
In order to get different degrees of finish, some different grains of belts shall be used.
Therefore - no matter if it takes of grinding, satining, buffing, fine finishing, etc. - always the same machine can be used, that is therefore to be considered very flexible in use, and suitable from small production runs, up to continuous large batches. Replacement of grinding belts is made - after having switched-off the motor - simply by opening the protection shells of the flywheel, without any spanner or key (in 30 seconds). Generally, conventional grinding belts from grain-size 60 up to grain-size 600 can be used (brilliant finish), but also Scotch Brite ® belts and special hi-efficiency belts can be used efficiently. Materials that can be operated are solid bars and tubes in standard- or carbon-steel, stainless steel, aluminum, brass, copper, and even wood - where operation is made in "dry" conditions, of course.
Wet grinding
Here a very important issue: tubes are generally operated wet, with the purpose to get best polishing degrees (brilliant finish), to extend lifetime of grinding belts (that are constantly washed by the water-emulsion, and therefore may keep their abrasion properties for longer), keep down the temperature of parts, and - above all - to reduce and collect even the thinnest dust and small particles generated by the process, while keeping a clean- and healthy-working environment.
Everybody's not strictly involved with polishing operations, will find almost unbelievable the difference in terms of finish between a "dry" and a "wet" grinding job, especially if we fix some other parameters, such as the tube type, rotation- and feeding-speeds, kind of grinding belts, etc. With the purpose of working wet, orbital grinding machines are generally built with special-electric motors, -switches and -wiring resistant to water.
In order to make wet-grinding, these machines are supplied with a mechanical water-pump included, a professional decantering-unit with a paper-filter, and a re-circulation of liquids, which consist in some emulsions of water and suitable addictives (having properties of lubrication,refrigeration and rust-prevention). The whole system is very similar to the ones installed on precision-grinding machines.
Some models are equipped with an electronic inverter to adjust the grinding speed.These versions are most suitable for Scotch Brite ® belts, large dimension round tubes, or elliptical and oval tubes as well, and for very tight elbows. In all a.m. cases, a slower grinding speed is generally preferable, in order to reduce harmful vibrations on the work-pieces, while allowing for a better finish in a lower time.
On-site tube polishing
What about polishing of a tube already on-site, which needs just a last "touch-of-finish" ?
Since just a few years, a new series of handy-electrical tools are available. These have compact dimensions, yet allowing an easy access even to most difficult corners of tubes.
Thanks to some special pulleys which contain the grinding belts, these portable polishers are suitable to operate even on tight bends, and close to hooks, supports, flanges or beads connected to the main tubes, without loosing contact with the belts.
Through these machines is also possible to cover a wide range of materials and diameters, let's say from 10 mm. up to 100 mm. and over, yet with a polishing angle of 180° and over, in order to improve the process efficiency, thus reducing the cycle times (see picture 2). These grab-on tube finishers are considered a useful complement to the orbital-tube polishing machines, and - generally talking - they are delivered in a unique package of supply to get a certain degree of finish on bent tubes.
Application fields
Staircase handrails and tubular furniture
Boilers and dairy industries
Pharmaceutical and petrochemical plants
Water-taps and door-handles
Motorbike-handles and -exhausts pipes
Balustrades for off-shore boats, etc.