MIMF

Hello, I am having an issue with my Jet 16-32 drum sander. Whatever wood I try to thickness the outer ends always come out thinner than the portion in the middle. Do you know why this happens and how can I avoid it?
Thanks Enrico

How much thinner? Weird, but my first guess would be that the feed table has become slightly concave across its width, so there is less pressure in the center and more on the edges, which get sanded thinner. Put a straightedge across the table near the drum and see if there's a gap toward the center. Or maybe the feed belt is thicker on the edges, more worn in the center.

Has it always been like this, or started sometime in the recent past? I can't imagine it's the drum, unless it was a manufacturing defect. Did you buy it new or used??

Hi Peter, it is about 0,2 mm thinner at both edges. So from your reply I can understand that is is unusual. I thought it could have been a pretty common problem. It’s always been this way. The sander is new. But before this I had another one cheaper that made a decent job, but had exactly the same problem. I thought the reason was because the sander was cheap. I was surprised to see the same happening with the newer machine as well. I have different feed tables for the sides and soundboard but the problem is always there. Even when I sand the fretboard.

Do yo use sandpaper with some kind of backing?

If you commonly run stuff through in the center, the sandpaper will be more worn in that area and won’t cut as aggressively as in the unworn areas. Often, you can feel the difference on the sandpaper with your finger (while the drum is not spinning). Do you have the problem right after you change the paper?

I have a doubt that maybe my question was not very clear. When I say that the wood is thinner at the ends than the center, I do not mean laterally. I mean that at the beginning and at the end of the piece it is thinner than in the middle.

Sorry for misunde4rstanding, but that is even weirder. Is it an even difference in thickness from end to center and back to end, or is it just a few cm on each end? Do you keep it flat on the table when it goes in and comes out with even downward pressure, or does the weight of the unsupported end of it tend to want to make it tip down, which could put more pressure on the opposite end?

Oh! Those ends! Have you heard of “snipe?” Look at this and see if it helps: https://thewoodwhisperer.com/articles/d ... ner-snipe/

Peter, it is thinner for few cm on each end. Maybe 2 or 3 cm. At first I thought it was the feed table because it was quite heavy. Then I just used one of the two sides as a feed table. The problem is present anyway. In your case, are you having constant thickness all over the places?

Bob, I will try the sacrificial pieces hoping that it works. Thanks

Are you taking light cuts?

Enrico Schiaffella wrote: ↑Sun May 17, 2020 5:24 pm In your case, are you having constant thickness all over the places?

Enrico, when I feed the board in, I press down on it near the drum as it goes under it. I also support under the ends of the board as it goes in and out so it doesn't tip down. I haven't noticed any thickness differences, even with stock as thin as 1/16" or less. And I take very light cuts - it takes longer, but does a more even job.

Barry, as far as I know yes I take very light passes. Especially when I get close to my final thickness. The last few passes I do not even change the height of the drum to get it as even as I can.
Peter, ok next time I want to try to exert some light upward pressure both at the beginning and at the end of the pass. Let's see if this helps.
Thanks.

Level the drum to the feed table. Should be parallel to the table. Also take light passes as you near the desired final thickness. Heavy passes can cause the drum to flex up and out of parallel. If the outer end of the drum is wider, as you flip the piece end for end to feed, the inner end of the drum is taking more off that side reulting in thicker in the middle.

The varying thickness as described suggests that something is loose in the frame. Check that all the hardware is tight. Also make sure the frame is not cracked.

You may be able to reduce the snipe by adjusting the pressure rollers. I have mine set as light as I can while still preventing the work from slipping on the belt, and that seems to help some with snipe. I don't think it's possible to eliminate it, though. I just always try to have my rough stock be a bit longer than necessary so I can trim the ends.

To me, this doesn't sound like snipe. Snipe is not gradual. It usually looks like a semicircular divot in the last half inch of the board. It is caused by the board tilting upwards as the end runs under the sanding drum.

The gradual taper Enrico describes is more likely a flexing of the sander's frame.

Yes there is definitely a problem. Yesterday I tried to thickness sand some balsa for my double top. The first and last 2 inches come out thinner by about 0.2 mm. I tried to support the wood but it didn’t make any difference. I will figure out something. Thanks for your help.

I've got an old 16-32 performax. Just took some measurements and the pressure rollers are about 2" before and after the drum. Seems to be a clue about what's going on. While the wood is only in contact with one pressure roller and the sanding drum the .2mm differences occur. Do you have infeed and outfeed tables besides the conveyor belt? Mine are about 3/16" below the plane of the conveyor belt. I also use supporting tables (I call them sleds} and find that their weight can cause them to tip enough on entering or exiting the sander to get what you describe. I support the ends of the sled and push down like Peter described both at the beginning and end of each pass.

Carl, that is interesting. My old 16-32 does not have pressure rollers. I also do not get a thickness variation like that. You might consider ditching the pressure rollers.

I don't need to Barry. I've figured out how to make this old workhorse work fine. I was just musing about what Enrico could think about.
Thanks. I think Andrew suggested adjusting the pressure rollers. Might be part of the puzzle.