Strange Drum Sander Result

[Bruce Thompson]

Jet 1020 drum sander, 6 months old; claims it can handle stock as short as 2 3/4"
180 grit abrasive. .003 to .004" stock removal per pass. (1/16 rotation of drum height wheel where one rotation = 1/16")
Boxes 9 1/2" L x 4 3/4" W x 1 1/8" D

I'm making a travel chess set that is two small boxes to be hinged such that when opened, the chess pieces inside are removed and the two halves of the hinged box open up to a playing surface.

After gluing up the box-jointed sides, I put them through the drum sander to bring the proud edges of the sides down flush to the playing surface. Ran the boxes through, short side leading, about 10º off perpendicular to the drum, so 80º or 70º. Jet manual says not to run perpendicular; 60º angle is best. They looked good, so I thought I would turn them over and sand the open side edges smoother with a few light passes.

Afterwards i discovered that all of the corners are lower than the rest, by about .012" tapering to nothing about 3/4" in from the corners.

No one at Woodcraft, Rockler or Austin Hardwoods, nor the local Jet warranty repair guy, can tell me why that happened or how it is even possible.

IMG_4014.jpg IMG_4013.jpg

Note the gaps between the saw table and corners at the bottom and between the straight edge and corners above. I can slide more than 4 sheets of note paper under the corner, and that mikes out to .0012+. When the hinged box halves are closed, the gap is double that, so about 3/64"

I can laboriously hand sand them to get them true, but that doesn't help me to understand the problem and to avoid it recurring.

(Tried to edit the title, but can't. Obviously I am using a sander, not a dander.)

[Leigh Betsch]

I wonder if the box was tipping when either entering or exiting.

[Zachary Hoyt]

I find that for flattening a coarser grit works better. I keep 80 grit in my drum sander almost all the time. The surface is not finish ready, though.

[Dan Hahr]

Unfortunately, you've discovered the limitations of a drum sander. I have a Supermax 19-38 and I've had similar results. It is basically snipe. I think it is related to two things. One, when you first feed in the box, the drum is spinning at full speed and first hits a corner which progressively becomes wider as it goes through the sander. The first and last corner to go through gets a few more revolutions of sanding and get thinner. Two, when the first corner goes in, the hold down bar/roller presses it down but not as far down as both rollers which together push twice as hard. I've found that I can push work pieces down even more with moderate finger pressure as they enter the sander and make this a little easier.

I'd stick down a sheet of 120 or 150 and have those edges in the same plane in just a few minutes. I save the drum sander for those pieces that won't have parallel edges nearby to judge them by.

Dan

[Dick Strauss]

Is your sandpaper loose? I think this can be a problem with leading edges until the paper is forced to occupy the space between the drum and wood. In other words, the sandpaper flapping ever so slightly away from the drum as it spins make a larger effective diameter temporarily until the paper is forced between the drum and wood.

If the taper stops at a distance into the piece of the drum radius, this may be your issue.

[Maurice Mcmurry]

Leigh Betsch makes a good point. An open box is not going to respond to the pressure of the feed rollers and drum the same way a solid board will. We had velcro on the drum of the one that I have the most experience with. It tended to take a few extra thousands off of the edges.

[Bradley Gray]

I have flattened similar things by using the conveyor belt. Turn on feed motor only, hold the open side against the conveyor.

[Sam Force]

I would butt up a leading scrap piece of wood in front of the piece and another scrap piece to the back. When sanding multiple pieces I always try to have a piece under the drum at all times