Setting Up and Using a Dual-Drum Sander
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 E:
I have to agree with the previous post. Unless you find a good deal on something used, you're not going to find the quality of machine you're describing in that price range. I bought what I thought was the best for the money, Powermatic's dual drum sander. But it is about $2400 and has two speeds, not variable. Great machine, though. I also started with the Performax 16/32 and would not recommend it for what you are doing. It needs too much fine tuning every time you set up a different grit. Plus it's only a single drum.
Best bet... outsource to a local shop.
From contributor P:
I have the General 24" drum sander, and I'm happy with it. I was using a Performax 24" before that, and it was half the machine for twice the money. The General is still less than $2000.
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.
From the original questioner:
I appreciate your response. I've read mostly favorable reports about the General, and it seems that for the price, it's the best thing out there. The negatives I've read: no separate conveyor belt switch, height of pressure rollers not adjustable, and rubber belt requires a lot of roller pressure, which makes it more difficult to remove cupping, especially in the 1/4" thicknesses I use. Yet overall, the machine seems to outrank others in its price range. A couple of other things come to mind: Is the drum itself soft or hard, and do the edges of material come out rounded or fairly crisp?
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.
From contributor P:
I'll try to answer your questions as best I can. First, there is a separate switch for the conveyer feed, as well as a dial for conveyer speed. The reviewer who said there wasn't was mistaken. Pressure roller height is not adjustable, but roller pressure is. As far as removing cupping is concerned, that's what a jointer is for. You can remove cupping with the sander if you use a sanding board under the stock with shims to build up the space under the cup, which will get one face flat, and then flip the stock, and sand the second side parallel to the first. Actually, at the 1/4" thickness you are using, I doubt if any conveyer system by itself would hold the stock so delicately that it would take the cup out instead of just flattening the stock out by roller pressure. The sanding drums are aluminum with no rubber or Velcro covering. Edges come out crisp.
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.
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