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.Click to go to Dynabrade
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.
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