The pneumatic Random Orbit (ROS), a.k.a. Dual Action (DA) sander has been around for a long time, and is the predecessor to the electric version. These are less common amongst hobbyist woodworkers for a fairly good reason: You must have a good compressor to even operate one. This is a far different animal from what you find in any of the electric sanders.
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The DA sander which I strongly prefer is the Dynabrade 5" Model#57015, shown without optional vacuum attachment. You will find these sanders in more production wood shops than (possibly) all other types combined. I suppose that is probably overstated, but to date, I have not been in a production shop which did not use these sanders--ya I know, but I don't get out much.
I have found this sander to provide virtually swirl-free sanding at even fairly coarse grits (120 grit), and completely swirl-free sanding at no higher than 180 grit. The sander has a much longer life-span than the electric types.
I never find the need to go coarser than 120 grit, nor finer than 180 grit. I have one sander always loaded at 120 and the other at 180. That's why I own two sanders (It is a production shop). The 120 grit sander gets much more use, so periodically, I will swap them. I don't know if these are available in a hook and loop pad design, but personally, I think H&L contributes to swirls and non-level sanding, plus has a shorter pad-lifespan. Why? Because it is soft and thick (relatively speaking), and does not provide as flat of a surface as a standard PSA pad.
So what are the negatives? Why doesn't every hobbyist own one? Because you need a 5 hp compressor or better to operate one. Even though the manufacturer says the sander requires 16 CFM, I find that to be overrated. I have operated a unit on a portable compressor which only delivers 9 CFM @90 PSI. Granted, with a compressor this small, the duty cycle was 100%.
One of the biggest benefits of this sander is that the speed is infinitely variable. It has a built-in air-valve to limit air volume internally, or I can do it externally with the compressor's regulator. I can operate the sander from a couple of orbits per second to 10,000 orbits per second.
The bottom line is, if you have a 5 hp compressor, you can't buy a better sander. If you don't have the compressor, it won't do you a bit of good.
What makes the Dynabrade so good (further details)
I really don't know the specifications on the various electric sanders, so some of this may or may not apply. And frankly, I don't have all the answers about the Dynabrade either. I just see the final results, not the reasons.
- Less swirl-marks.
When compared to other pneumatics and also with the various electric sanders, I have found the Dynabrade has a noticeable reduction in swirl marks. At 120 grit sanding, swirl marks are not noticeable unless the stain color is dark. At 180 grit (the highest grit I bother using) there are no noticeable swirl marks on any species with any stain color. On a recent project using birch, I sanded a column's cap-plate at 180 grit. The final unfinished surface was almost glazed and rather shinny considering there was no finish applied yet. (The surface had a sheen--light cast--almost comparable to a lacquer finish. I didn't expect this myself, and was impressed with the results.)
I think one of the biggest reasons for near swirl-free sanding is the movement of the eccentric cam and pad system. I have noticed that as soon as I turn on the power to an electric sander, it is spinning in a "grinder-mode" or full free-wheel. This is why some of the units have clutch-disks. On the Dynabrade, the pad is in "full-orbit-mode" for the first 1-2 seconds, and takes 3-4 seconds to go full-free-wheel due to momentum.
If your sander pad is not visibly spinning (free-wheeling) while you are sanding, then you are not getting the full dual-action from the sander!! However, it should be the orbital motion of the eccentric which causes this spinning, not a sticky bearing. If your bearing is sticky (poor), then the sander will free-wheel as soon as you turn on the power.
While it may sound as though I just contradicted myself, having the sander free-wheel at startup is a sign that it will not free-wheel "naturally" during sanding. Furthermore, these clutch-disks themselves, may be contributing to swirl marks if they limit the natural free-wheel motion of the sander.
Another reason for the reduction in swirl is the small (3/32") orbit path. The orbit path is how far side-to-side the eccentric cam displaces the pad. The smaller the path, the smaller the swirls. The Porter-Cable also has a small orbit path, but otherwise, most sander orbits are much larger (3/16" is common).
The Dynabrade is small. Really small. At only 3 5/8 inches tall, it is almost half the size of most electrics, yet has more horsepower. At 1/4 hp, only a couple of the larger, heavier sanders can compare. The reason why it is small with a lot of power, is because of the pneumatic motor versus the heavy electric. Your 300 pound compressor sitting in the corner is providing the power "remotely". Being a true palm-grip sander, a large hand (like a basketball player--not me) can completely conceal the entire sander in the palm.
- Low Maintenance:
With an electric sander, the longer it runs, the more heat gets built up. Heat is bad. With the pneumatic, the more it runs, the colder it gets. The air keeps the whole unit cool.
With a cooler operating temperature, the bearings and motor will last much longer. My two sanders have thousands of hours on them with no maintenance until last week.
I noticed one of my sanders was hesitating when started, so I stripped both sanders down for cleaning. After years of service, grime and pitch had built up in the wing-vein motor. It took 45 minutes per sander to strip them completely down, clean the veins and ports, replace the lower, main bearings (off-the-shelf bearings) and reassemble them. Had I replaced all wear-and-tear parts (normally I would have, but this wasn't pre-planned) my total cost would have been about $10-$20 each sander to buy the manufacturer's wing-veins, and aftermarket bearings. After cleaning each sander, they are performing in "as-new" condition.
What are the drawbacks?
Even though I can operate one of these sanders on a craftsman portable compressor (5 hp rated, 2.3 hp actual), the compressor will operate at 100% duty cycle. On a stationary 5 hp compressor, it will operate at 75% duty cycle. An industrial 5 hp compressor will operate 2 sanders at a 50-75% duty cycle.
Trying to use these sanders on a small compressor will give you moisture problems. I bought the portable compressor because it was large enough to operate the sanders at a construction site, but I wouldn't use it for daily work. The minimum compressor size should be a retail-sized 5 hp or better. If you don't already own the compressor, then buying one of these sanders would be an expensive proposition. But if you already have a 5hp compressor, it's a great investment in tools that I Guarantee you won't regret.
Even for work outside the shop, for me, buying a $300 portable compressor for site-work was preferred over buying a $150 electric ROS.
Rick Christopherson - 3/20/98