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Hi All! I've been working with my Supermax 16-32 drum sander for several guitars. Overall, I feel it's a great tool, well built, easyt to use, etc. But I am really having a tough time sanding wider pieces (backs and tops) to an even thickness across the piece. The center is always higher - 0.15" in the center vs 0.12" on the edges. Doesn't matter if I'm doing a parlor that I can feed through centered on the drum, or a wider where I have to do passes through the open end.

I take multiple passes in opposite directions before lowering the drum by about 1/8th of a turn. The pieces don't have any cupping. Happens whether I'm sanding the front or back side of the board.

I've triple checked my setup and my drum is as dead level as I can make it. Thicker pieces such as 1/2" ply seem to do fine, so I suspect the top or back is flexing upwards at the edges. I can mitigate it by double side taping the piece to a work board, but this seems to get dangerous with breakage risk pealing it off the tape as the piece gets thinner...and is a pain when doing deflection testing.

So, do any of you run into this? Any tips or tricks to avoid this. I do like to thin my edges a little bit, but I want to be able to control how much I thin them, and do it around the full edge, not just the wings of the lower bout. I've started hand sanding with a wide sanding block once the edges get down to 0.11" to avoid over thinning them.

Thoughts?

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Tony Thatcher
Bozeman, Montana

i would have been expected the opposite, thicker at the flexible edges and thinner in the middle closer to the closed side.

I know some folks use a backing board covered with 100 grit. Run through the machine in a single direction only, cover with sandpaper, and everything placed on it should sand evenly.

However, if you're getting very consistent measurements with the drum dead level, maybe you need to adjust it to be out of whack so that the pieces come out true...

Ed gave you some good tips. Adding a bit to the discussion as I've spent a fair amount of time rebuilding and calibrating equipment over the years.


1) Check flatness of the drum and flatness of your table. You are getting a 0.03" variation and that is a lot. With a drum sander, you should be able to dial variation to under 0.005". Honestly, I would expect to within a couple thousandths or I wouldn't be happy.

2) If the the components are dead flat, then that means you CAN setup the machine to yield proper results. You need to look at the manual and take the time to set up the machine correctly. Google is your friend here - use it to find a thread where someone takes you through the process of setting up the machine.


3) If all else fails, Ed's suggestion is actually a really good one. Using a "sled" can account for any variations. You make the sled by sending it through the sander first and then maintaining that orientation henceforth.

Something to try, take a peice that's less than 16" and put lots of pencil marks on it, run it through 3 or 4
times without changing direction. Check where the marks are being removed most. Flip the piece around
and run 3 or 4 passes the other direction, check pencil marks again. If the pencil lines are being removed
more on the inboard side this will tell you if your drum is wedging open a tad while under load. In that case
you might need to do what Ed suggests.
My dust collector suction tends to lift the conveyoy belt up to contact the drum abrasive when sanding thin stuff
so I always use a carrier board with small strip glued to the rear end as a backstop as a workaround with no problem.
No tape but pressure rollers fore and aft help keep everything held nicely in place.
Ken

Yup. A sled with a sandpaper top, and probably a low wood lip on the trailing edge (just in case the top/back slipped) was my next step. And it's probably the best approach. Though the specs say it can sand down to 1/32" (0.031"), so it would be nice to avoid the sled.

I'm assuming the drum is dead flat, though we all know about assumptions. I'll check it later today. It could be that the paper does not lay completely flat at the ends of the drum where it folds down into the paper holder, though it seems this would be only on the extreme edges, and there would be the same problem with any stock that you run through it, not just thin stock. I'll have to check the feed belt table, too, to see if the belt flexes up at the edges. Though the hold down rollers are plenty strong enough to press it flat.

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Tony Thatcher
Bozeman, Montana

I've been doing just this. My pencil lines are always sanded off on both edges at the same time, that is, no need to turn the board around and run it through in the other direction. Almost like the drum is slightly concave (which I'll check later today). And I haven't thought about the dust extraction lifting the belt. Certainly a sled would address that situation. But as you said, the hold down rollers should do a pretty good job with that.

Really, I think the sled is the natural direction to go. And I shouldn't be so stubborn to go in that direction.

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Tony Thatcher
Bozeman, Montana

Heard the one about the bus stuck under a low bridge?
After hours trying to get it out, a little boy came along and said "let the air out of the tyres"
That helped me with my drum sander.
Look at the bed of the sander, is it sagging a bit, i.e. concave?
Mine was, resulting in thin at the edges, thicker in the middle.
Cured it by making a thin (4/5mm) MDF backer board, running it though and sanding it, so it will take up that shape, and glued 100 grit to it.
maybe?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

It's a simple fix and probably not your problem. But check to make sure your sandpaper is installed correctly. If you don't crease fold the ends where it goes into the slot, they will stick out proud of the rest of the paper. Also, if the paper is loose enough, that may cause your problem, but that is just a guess.

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-Marcus
http://www.bailieguitars.com
Instagram | YouTube

It's not a big, it's a feature!

I purposely have my drum out of square to the table so that plates are thicker in the middle than on the edges. I prefer plates to be profiled that way. I generally use rough 60g paper, so this tool is just a roughing tool. Everything gets cleaned up later using hand planes or scrapers.

We have a winner! The bed is concave by 0.036". Can't believe I didn't check that.

Now, to work with it or have them send me a new bed.


Sent from my SM-G920V using Tapatalk

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Tony Thatcher
Bozeman, Montana

That was enjoyable! And fast! The joys if hand tools.

Thanks for all the ideas!

Sent from my SM-G920V using Tapatalk

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Tony Thatcher
Bozeman, Montana

Hey, graduated tops made easy! Mines the same but not as much. It's not actually a problem for me, though I take more care when thicknessing the sids to do the last oases in the center so it's even.

Also handy for purflings as you can sneak up by a few thou just by running them through a few inches over from last pass...

Crap. now I need to check mine.

_________________
-Marcus
http://www.bailieguitars.com
Instagram | YouTube

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

My sander is adjusted quite well, but still I’ve always used a uni-directional sled — wide for plates, narrow for sides, finger boards, binding. I don’t bother with sandpaper . From time to time, I resurface them with 80G, to make sure they are dead on.


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+1 for sleds. I use the same size variety as Tim but have them surfaced with 100 grit sandpaper. Directional, and I have their outside edge marked with arrows and flip them end to end for an occasional dressing so I don't have to readjust the drum all the time (I've got a real old Performax.)