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I've had a Supermax 16-32 for about a year now and learning to set it up correctly has been... difficult. I've finally reached a point where it works pretty well for thicknessing tops and backs/sides but sanding larger stock has been tricky.

I've been attempting to thickness a neck blank (well two) down to 18.5mm (18 is my goal but wanted to use my jointer plane to make sure all surfaces are nice and flat) but after sanding I found the ends to be 18.5 but the center to be 17.5. This seems to be the opposite problem of snipe and I'm curious if anyone knows of any way to remedy this.

BTW, I'm using 120 grit paper and only turning the handle 1/8 turns each pass

Make sure your infeed and outfeed tables arent flexing the wood up in any way.

Also, a similar thread came up awhile back and the results were a warped feed table. So try removing the feeding belt and checking the flatness of the platform underneath.

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-Marcus
http://www.bailieguitars.com
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Sometimes multiple passes on the same setting will help with that. Using a sturdy backing board, which is a pain, can also be useful to reach a critical measurement. I've had a number of sanders with similar issues, but luckily my current one runs pretty close to true. Good luck!-Bob

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“ The meaning of life is to find your gift and the purpose of life is to give it away” Pablo Picasso

If the joint in the belt (if it has one) is thicker than the rest, the blank sits on the joint it will lift it resulting in a thinner area - but 1mm sounds a lot?

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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.

ok Brian
when you turn 1/4 turn how much is it taking off? 120 is too fine. I have used a number of these style machines . You want to dial in about .010 to take off at a time. With 120 that is not very coarse and you will over stress the machine.
I found 60 grit is good on these drum sanders. I would take it to within .005 of what I wanted to allow sanding out bench rash . These machines are not finish sanders .
A good tip is this what is getting warm , Wood ? Paper? or the Dust? You want the heat in the dust . Once you get to hot the belt glazes over and get worse. Learn the feed rate for the grit your using. In this case coarse is better.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I agree with John, 120 grit is too fine for stock removal. I use 60 or 80 grit for thicknessing, and 120 through 180 for cleaning up. A well-tuned machine can leave a very clean and accurate surface if you follow the right progression. My spruce tops come out within .003 across the top and with almost no sanding marks left.

I used to have a lot of trouble of the kind the OP describes when I tried to adjust the accessory in-feed and out-feed tables level with the feed belt. Any irregularity would lift the work and cause uneven sanding thickness. I now use the tables for convenience only and have had no further issues.

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It will probably be alright.

That was me. had the the problem from side to side - the edges were thinner than the middle. Turned out my table was warped down in the middle.

But I agree with the other comments as well. Check your feed tables. Sounds like they may be holding the ends up resulting in a lower middle. This would not really be an issue when doing tops and sides that will flex easily. And I also use 60 to 80 for material removal.

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Tony Thatcher
Bozeman, Montana

The other thing can cause deviations in the thickness is the seam in the feed belt. I try to avoid that if possible.
As Bob mentioned, several passes at the final thickness helps too.

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Jim Watts
http://jameswattsguitars.com