I used to buy FAS South American mahogany boards 1/4" thick by 6"-12", already wide belt sanded very evenly and precisely on both faces to 150 grit. Now I may have to do the sanding at my location. I need a very flat, precise thickness for musical instruments. Is there a dual drum variable feed speed sander out there for less than $2000 that can do the job? I realize the passes have to be very light - what I've heard and read about burning, rounding at the edges, the various problems with tracking, mounting the sandpaper, getting both drums in synch, and generally getting the whole thing squared up makes me wonder if a drum sander can give me the even, flat finish I need.
A related matter is whether having the lumber company leave 1/64"-1/32" on each face for me to clean up on the drum sander after they resaw and plane S2S will be enough, especially since I can't really control the condition of their planer and their blades, or how they're likely to hog it through the planer. Any suggestions?
From contributor J:
We had a Performax 16". Sander was fine until I realized what I expected of the machine. If you are doing a large volume of sanding, you should look at a wide belt sander. The cost is higher, but the results and time savings far exceed the investment. Try outsourcing as an alternative. The cost of owning/operating a large sander is not cheap.
I use my General sander as a single drum machine, specifically because it's too hard to precisely dial in the second drum, if you're moving it up and down to go from roughing to finish. It's easier to change paper instead. When I got my machine from the dealer, the infeed drum was out by .013" over the width of the drum. I was able to put shims under the upper support brackets to even that out, and now I'm within .001" over 24", repeatedly. I sand everything from narrow strips for edgebanding to 24" wide planks. It would take too long to try to get the second drum within this kind of tolerance, although if you never moved it once it was dialed in, it should stay there.
As far as surfacing goes, taking off 1/64" from each face is easy to do (not all at once), as long as that will clean up the surface left by your supplier's planer. That's something that can only be determined by experience. Mahogany is soft and easy to sand - I do a lot of sanding using only 150 grit, which will surface it down nicely without leaving deeper scratches. If you have to remove significant tearout, 120 grit will go faster, but you're correct in that you have to go slowly (slow feed speed and light passes), use that sandpaper cleaner stick often, and definitely hook the system up to a decent dust collector. You're going to need optimally a 6" duct split into two 4" hoses just above the sander to pull all the dust out.
I have much fewer problems with burned paper now that I've gotten more experience with the machine, but I've learned that once you get a burned stripe, you have to deal with the problem immediately, as it will only get worse if ignored. Sometimes the burned particles can be picked out with an awl or scraper, sometimes it's just better to replace the paper itself. Keeping the paper clean, using a slow feed, and replacing the paper before it's used up seems to be the best approach. There was a review of this machine in American Luthier a couple of years back, in which it was referred to as the hidden gem of drum sanders. The reviewer replaced the conveyer belt with a sandpaper one, to increase accuracy, and he tells exactly how he did it. You might be able to get a reprint of this from the publisher, or a back issue of the magazine.
At American Luthier they are trying to locate the article about converting the conveyor belt to sandpaper. I'm not sure if this would involve eliminating part or all of the existing rubber, or building right on top of it - I can see where keeping the drums parallel to the belt might be tricky. I'd appreciate any further input on the above.
The article in American Luthier is on pg. 36 of issue #70, summer 2002. Its title is "General 15-250MI: The Hidden Gem of Thickness Sanders". The author removed the rubber conveyer belt completely, and replaced it with a sandpaper-type as on the Performax sanders. He gives step-by-step directions, as well as a source for the belt.
I would try the machine with the stock conveyer belt first and see how it works before going to all that trouble. Perhaps using a sanding board under the stock would solve all the problems. He says as much in the article, but also says that he didn't want to continue using one. He had problems with dust pickup, but was splitting one 4" line into two 4" lines above the machine, which won't work. You can't get enough airflow through a 4" duct; like I said in my earlier post, you should go for a dust collector which can drive a 6", which usually means a 2 hp collector or larger. Going with inadequate airflow and blaming the sander is not addressing the real problem. Adjusting the first drum parallel to the bed is very easy. I ran two strips of stock through the machine at opposite sides of the belt at the same time, and used my digital caliper to check them when they came out. The difference in thickness equals the amount out of parallel. The bed assembly is supported above the lower body of the machine by 4 angle brackets, 2 on each side. Take some shim stock equal in thickness to the difference in stock thickness determined above, and insert it under the two brackets under the side of the bed which produced the thicker measurement. Double check your results with two pieces of stock again, and fine tune if necessary. Worked for me.