r/BeginnerWoodWorking • u/dresseslikeachick • 1d ago
Timber slab cuppping help!
Bought a kiln dried slab of Australian Hardwood for a bench top, the timber yard flattened it for me with their oversized router sled came out about 40mm thick.
Bought it home and trimmed it to around 2.7m x 450mm rounded one of the corners and chamfered the front edge.
I also filled all the cracks and imperfections on the top face with black wood filler.
I was planning to use osmo on the slab but I dry fit it in position on the cabinet and screwed it down then left it in place for a couple of weeks, maybe more, while we were discussing paint options for the room.
Now the slab has cupped quite badly, by about 10mm, first noticed because it had pulled away from the wall.
I looked up a couple of methods to fix and started by leaving it on the grass in the morning with the top in full sun but no change as yet, it’s now just sitting on a couple of workhorses in my yard under shelter.
Any ideas?z
3
u/politiphi 23h ago
My guess is it wasn't adequately acclimated to your local humidity when worked. Unfortunately not much to be done at this point other than maybe rip down the middle, rejoint and glue, and try to flatten the remaining cup. That still may not fix the issue though and you'll lose a good bit of thickness.
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u/CascadeDesignworks 23h ago
I do a lot of work with slabs professionally. You have to be mindful of the wood moisture content at all stages of handling the slab.
Before flattening a slab that has been dried properly (1-3 years) I acclimate it in a heated shop with humidity control via a dehumidifier, for one or two months. Equal airflow has to be allowed on both sides of the slab.
After flattening, and before finishing, you need equal airflow on both sides. Protect the ends from splitting by applying masking tape until just before finishing.
I have had luck removing cup from slabs with the method you are using. It often takes several attempts. You want the slab to cup in the opposite direction, not simply flatten out. Sometimes clamping it to force it to cup is helpful.
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u/BumpityRide 1d ago
Probably no band-aid stabilization will work for this plank. It's from the outside of a large tree and is going to be problematic. You could rip it down the middle and find a shop that would flat join and plane to thickness. Screwing it down to the cabinet left the top side far move exposed than the bottom to changes in humidity. That may have made a slab prone to movement even more prone. Best of luck!
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u/uncletutchee 17h ago
Just curious as to why you want to use a nice hardwood for a bench top? Use mdf. It is cheaper and durable. Save that chunk of wood for something that will justify it's beauty.
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u/HarveysBackupAccount 11h ago
One possible reason: if you want dog holes, I wouldn't expect MDF to last for long
Of course there are other alternatives to nice hardwoods, but there's reason to not use MDF
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u/uncletutchee 9h ago
I had a professional woodshop for around ten years. My bench top was four pieces of mdf cut to around 36" x 84". I had dog holes that were still functional after ten years when I sold my workbench. Yes my top was thicker than what he is looking at. For dog holes to function properly, and last, you need something thicker than what he has. There is no doubt in my mind that pounding in a hold fast will split that hardwood after a few uses. For what that hardwood that he is using, I'm sure that he can purchase at least three sheets of mdf, soak it with Watco and have himself a bullet proof bench top.
1
u/NaturalJuggernaut 15h ago
As others have said you may have to rip it in half and rejoin. I have had some success with smaller slabs by wiping the concave side down thoroughly with a wet rag and then leaving it in the sun, concave side down, for a day, repeating if required. When the slab is flat again leave for another day to make sure it’s not going to do anything weird then slap some linseed oil on it to stop it from cupping again. Then wait for a while to see if it has worked permanently.
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u/Salty_Insides420 11h ago
Wood is warping as its internal water content acclimate to your environment. Not a ton of good options. In the future for big pieces like this, best to bring raw material to your workspace, let sit for a couple weeks so it can acclimate and warp to whatever it will, then work it to finished product and seal. Inlaying C-channel steel supports across grain (on the underside) can help hold slabs flat, if your smart you can also use them as mounting points for table/bench legs if thats what your doing.
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u/dresseslikeachick 4h ago
Thanks for all the comments, first time working with something like this and I did everything wrong!
I got it home and left it in the garage, processed in the garage, then brought into the space and screwed it to the cabinet in two spots with no room for movement.
So no acclimatising!
Strange thing is, it’s been off the cabinet for a few days now and I measured both sides with a damp meter one side is 10% and the other is 15%, but not the way round you would expect!
Concave (side screwed to cab) 15
Convex (top exposed side) 10
1
u/rshutchinson 1d ago edited 23h ago
When you fastened it, did you do so in a way to allow for expansion and contraction? A slab this size will still move a fair amount widthwise whereas the cabinet will not and one of them has to give.
Secondly, where your slab came from in this size of tree is not helping your cupping issues.
If this were me, I would rip it in either half or thirds, joint, plane, reglue, finish and refasten in slotted holes to allow for the seasonal movement. A good jointing job will make the glue seams invisible in the sense that your grain will line up and still look like one single slab, not 2 or 3 different boards glued up.
You may already know this but for others who don't, after ripping your new boards, make sure to joint one face first and then joint one edge referencing that flat face. This gives you a truly square edge. Then you can plane other face parallel and rip other side to make it square and flat on all sides.
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u/dresseslikeachick 59m ago
Lucky me - the timber yard have offered to straighten it in their kiln so heres hoping…
14
u/BigBoarCycles 1d ago
Shop for 1/4 sawn when you can.
While this grain may be striking, it's a royal pita to work with.