Dan Forman
01-14-2012, 4:54 PM
My new secret weapon: 219790 a Metabo variable speed, 4 1/2" angle grinder.
Occasionally we encounter the solution to a difficult problem through serendipity. This happened to me recently, perusing John Jordan's website looking at hollowing tools, I noticed an article by Bill Neddow posted there, about high speed sanding. Now I have found the sanding of bowls to be a very vexing proposition on two accounts. First, I often discover scratches left from earlier grits, but not until down to the finest grits, then have to go back many steps to try to get them out. Often despite my best efforts, visible scratches remain on the finished piece. Secondly, elm and locust are very common here, and are subject to what Bill calls "starved horse effect" - ripples caused by alternating layers of hard and soft wood. With these woods it's very difficult not to end up with a rippled, if otherwise smooth surface.
Now, the technique and equipment fly in the face of "conventional wisdom" regarding sanding, which is that slower is better. We are talking here about speeds of 6,000 to 8,000 rpms for the sander, and lathe speeds of 250 for large bowls to 800 for smaller bowls, for the grits from 80 to 320, then slower for 400 and 600. Bill has reduce sanding time from 2 hours down to 20 minutes on large pieces. Smaller pieces also are take less time than with conventional methods, but the time differences aren't as dramatic, as the law of diminishing returns comes into play with less surface area. Here is a link to the article, which originally appeared in Woodturning Designs, spring 2010. Published Articles (http://www.billneddow.com/articles2.html) It's the first article "High Speed Sanding", though the second one on sandpaper advances fits in as well.
I decided to give this method a shot, and was very pleased with the results on a 7" black locust bowl. I could only find the firm Roloc sanding discs in 80 and 120 locally, so I used a Skilton pad and combination of Astradot and Vince's blue discs for the rest of the sanding. Consequently, I still got a little of the horse's ribs, but on close examination, there is nary a scratch to be found, inside or out. I found the firm Roloc discs to be unworkable for the inside of the relatively small steep bowl, actually put some nasty scratches in there that had to be removed with the Skilton pad topped with Vince's small radiused intermediate pad. All that extra sanding may have been the cause of the minimal starved horse effect that was present, hard to tell without further trials. I should add that this particular bowl had significant tearout both inside and out, that resisted all of my usually effective tools and techniques.
I was so tickled by the results that I ordered a full set of 2 and 3 inch Powerloc discs from Packard, as nothing in the higher grits is available locally, and I think Roloc must have discontinued them altogether, as I could find none for that system on the web either. I'll leave the rest of the description up to Bill, so read the article and be prepared to approach this with what in Zen is known as "beginners mind", laying aside for a moment all that you have heard in the past that is contrary to the ideas expressed. I'll post a pic of the bowl when I get a chance, maybe late tonight.
Bill and i have been playing phone tag, I hope to get hold of him soon and clear up a few questions, and will post updates to this thread.
Dan
Occasionally we encounter the solution to a difficult problem through serendipity. This happened to me recently, perusing John Jordan's website looking at hollowing tools, I noticed an article by Bill Neddow posted there, about high speed sanding. Now I have found the sanding of bowls to be a very vexing proposition on two accounts. First, I often discover scratches left from earlier grits, but not until down to the finest grits, then have to go back many steps to try to get them out. Often despite my best efforts, visible scratches remain on the finished piece. Secondly, elm and locust are very common here, and are subject to what Bill calls "starved horse effect" - ripples caused by alternating layers of hard and soft wood. With these woods it's very difficult not to end up with a rippled, if otherwise smooth surface.
Now, the technique and equipment fly in the face of "conventional wisdom" regarding sanding, which is that slower is better. We are talking here about speeds of 6,000 to 8,000 rpms for the sander, and lathe speeds of 250 for large bowls to 800 for smaller bowls, for the grits from 80 to 320, then slower for 400 and 600. Bill has reduce sanding time from 2 hours down to 20 minutes on large pieces. Smaller pieces also are take less time than with conventional methods, but the time differences aren't as dramatic, as the law of diminishing returns comes into play with less surface area. Here is a link to the article, which originally appeared in Woodturning Designs, spring 2010. Published Articles (http://www.billneddow.com/articles2.html) It's the first article "High Speed Sanding", though the second one on sandpaper advances fits in as well.
I decided to give this method a shot, and was very pleased with the results on a 7" black locust bowl. I could only find the firm Roloc sanding discs in 80 and 120 locally, so I used a Skilton pad and combination of Astradot and Vince's blue discs for the rest of the sanding. Consequently, I still got a little of the horse's ribs, but on close examination, there is nary a scratch to be found, inside or out. I found the firm Roloc discs to be unworkable for the inside of the relatively small steep bowl, actually put some nasty scratches in there that had to be removed with the Skilton pad topped with Vince's small radiused intermediate pad. All that extra sanding may have been the cause of the minimal starved horse effect that was present, hard to tell without further trials. I should add that this particular bowl had significant tearout both inside and out, that resisted all of my usually effective tools and techniques.
I was so tickled by the results that I ordered a full set of 2 and 3 inch Powerloc discs from Packard, as nothing in the higher grits is available locally, and I think Roloc must have discontinued them altogether, as I could find none for that system on the web either. I'll leave the rest of the description up to Bill, so read the article and be prepared to approach this with what in Zen is known as "beginners mind", laying aside for a moment all that you have heard in the past that is contrary to the ideas expressed. I'll post a pic of the bowl when I get a chance, maybe late tonight.
Bill and i have been playing phone tag, I hope to get hold of him soon and clear up a few questions, and will post updates to this thread.
Dan