I’m building a split-top Roubo workbench. The top is hard maple, consisting of strips about 6 ft. long x 2 in. wide x 2.5 in. thick glued up butcher block style to form 2 slabs, each about 12 in. wide. I glued up the top first and then built the base. The top pieces are mortised on the underside to fit onto tenons on the top of each leg, and then will be screwed to the rails with lag screws up through the end rails. But now I find that there is twist in one of the top maple slabs. If I push down on one corner, the diagonal corner on the other end pops up about 1/8 in. What approach would be best to remedy the twist in such a large and heavy piece? Should I should be looking to flatten the underside of the piece so it will sit level? I am also concerned about losing too much thickness in the slab with any fix I undertake; I wouldn’t want to lose more than 1/4 in. at most. The fixes I have read about include ripping it into 2-3 pieces, then squaring up each of them and re-glueing. Or putting the slab on a sled, shimming the uneven corners and sending it through a planer or drum sander. I’m uncertain how I would go about trying to remove the twist with a hand plane. Ideas? Thanks.
Replies
Are the slabs free of any twist on their own -- without sitting on the base? If they are twisted, you'll have to fix that first. And it's the top you'll have to focus on, since that's the part that you'll be working on.
If the top is flat, but the base is making it twist, fix the base. With the base assembled and the top off, you should be able to put straightedges and winding strips across the bases tenon shoulders and see where the problem is. The easiest fix is trimming the high tenon shoulder to bring them in line.
Last, drawbore the top to the leg tenons. You shouldn't be able to press on pooper of the top and have it wiggle on the tenons.
Pooper. Heh. I love JC2's version of spellcheck!
Since no drum sander or planer, I know of, has long enough tables to remove this twist and I'm assuming you don't own a 12" wide or larger jointer along with 2 strong backed assistants you will need to deal with this on the bench.
First it is imperative to know that your reference surface is absolutely flat. When you say you push down a corner and have the opposite corner raise up an ⅛" what is the top sitting on at the time? Floors are notoriously uneven and make lousy reference surfaces and most of us don't have the luxury of having 6' long assembly tables with perfectly flat tops in our shops. I've used frames out of jointed and planed PREMIUM 2x4's, preferably fir, with a couple layers of ¾" MDF sitting on sturdy saw horses and carefully shimmed and checked for flatness in every direction in the past. Keep in mind the 2x4's should be acclimated to your shop for at least a week before milling or you may find yesterday's perfectly flat frame is today's DNA helix.
Once you have established a suitable reference surface, you can shim the high corner to stabilize the top. Now there are 2 ways to remove the twist; handplanes starting with a jack, or build a router sled that let's you level large slabs, you can find plans on FW or the interweb, if you think you may work with slabs in the future this could be a useful addition to your arsenal. Either way I would mark a reference line around the edge showing the amount of material that needs to be removed, barring other appropriate tools take your combination square with the rule edge squarely resting on the reference surface adjust the head until it touches the top at the LOW point then keeping the rule edge on the reference table trace around the perimeter of the top with a sharp pencil underneath the squares head. This line should give you some guidance as you plane away the high points. Keep checking flatness in all directions frequently as you plane.
If you choose to use the router method just set your bit depth to the low corner and start routing until the top is level.
Whichever method you choose the reverse side will need to be worked as well. This can be done by flipping the top over, no shims should be needed if the first side is done right, and repeating, but assuming you have a planer large enough you can simply run it through the planer with the newly flattened side resting on the tables.
Keep in mind the second table top will need to be reduced by a like amount and to prevent more movement it's best to remove a little from both sides rather than taking the entire amount, small as it is, from just one.
The total amount of material you need to remove should be equal to the amount of gap you get when pushing down on the high corner even though you will be taking it from the high corner on both sides it's not additive. If you measure an ⅛" gap then your top should be reduced by roughly an ⅛" when you are done.
The best way to handle this is a router sled.
I would remove the top and start by making the bottom flat.
Mark Spagnolo has a very good video on flattening a bench top.
RobertEJr the best method in my opinion is subjective. Someone who has well tuned and honed planes could easily level an ⅛" discrepancy in a fraction of the time it would take to build and use a router sled and there is something deeply satisfying about watching whisper thin shavings curling up from a good hand plane without screaming machines and respirators and hearing protectors. On the other hand if the OP only has economy or mid-grade planes straight out of the box, unturned he may find the planing process difficult and laborious, or he may see value in having a router sled for future projects.
One of the beautiful things about Woodworking is there are many paths to our destination and we each get to choose the one that's best for us.
MJ,
Heh, heh, heh. "He" is called, "Mr. SpelCzech", as in "He who knows better than we do what we were going to say and how it is spelt".
Thanks to all for the suggestions. John_C2: The base is not the problem; when sitting on a level surface, the twist is apparent. esch5995: I work out of a makerspace woodshop and there are flat surface benches that I have checked for level. There is a router sled available, altho I've never seen anyone use it and I'd have to take a look at it to assess its usefulness for this task. I'm reasonably adept with handplanes so I might best take that route, but the longest plane available is a No.5. Alternatively, the shop has a CNC router machine that is large enough to handle my bench top.
Lots of good advice above. I would recommend the hand plane strategy. I would try to take off 1/16" off the two high corners rather than 1/8" off of one. Some have suggested leveling both sides. I think I would flatten the top, then simply shim the bottom and call it a day. Maybe this shortcut creates problems I'm not aware of, tho. Regarding the CNC machine - I have seen these used to flatten slabs. The one I saw had an enormous bed and the router moved over the stationary slab. Not sure what your machine does, but I would be nervous to try it on a bench top where I wanted to preserve all the thickness I could. The longest plane your shop has is a No. 5? Shame on them! They can afford a CNC but can't shell out for a No. 6 or 7? Time to write some letters.
If a router sled is available I would use that right off the bat. Arguments about flattening by hand or machine aside; if the sled and router are available, buy an inexpensive bit and go at it. There will still be plenty of hand planing afterwards to satisfy that need if it exists. ;-)
Martyk
It sounds like you are well equipped to solve your problem and you only need to decide how. The sled or CNC machine are more than capable of doing the job if your prefer the whine of machinery for me the No. 5 Jack Plane, if it's probably setup and sharpened, and the whispery shavings it would create would be my tool of choice. The no 5 is the perfect tool for the job on a 12" wide bench top just slightly skew the plane as you push it forward. I also will repeat since other advice was given you will need to remove ⅛" from the high corner of both sides of the top, but I'm sure you have a planer to make short work of the second side and the second top section.
I would choose the high corner based on grain direction, since a twist board rocks on its diagonal corners, you can choose which corner is high by which corner you choose to push down. So choose to make the high corner the one so that you can push the plane towards the high corner with the optimal grain direction for easier planing and minimal tearout. I'm assuming you aligned your boards with similar grain direct to facilitate planing. Once you've determined this simply start with short strokes close to the high corner and gradually lengthen and widen the area you are planing until you have covered the entire top, using the aforementioned pencil line as a guide and frequently checking with a straightedge, the longer the better in all directions.
Good Luck. Let us know how it turns out.
I'm actually in the design stages of a similar bench using the Benchcraft hardware.
Thanks to all for further replies. esch5995: I'm having a hard time envisioning your description of marking a reference line around the perimeter of the top. I understand the principle, but can't quite picture what you have described. Can you make it clearer? What is meant by the "head" of the combination square? Also, just to clarify, you are saying plane 1/8 in. off BOTH of the diagonal corners on the top side to get it flat, then put it thru the planer with the top side down to flatten the bottom of the slab ?? Thanks.
If you have access to a CNC then to me its a no brainer. Hours of cardio vs. planning your next project while the CNC is running. Plus, I wouldn't flatten a workbench with anything less than a 6.
My POV is having flattened a couple workbench tops, I'm beyond any purist thinking about using handplanes and "whispy shavings" LOL.
I'd rather put my earphones on, peruse a couple Glen Huey books, and glance at the robot doing my work better than I could possibly hope for.
Convinced?
Marty the head of a combination square is the cast portion, usually black, that slides along the rule. There are other tools that could do this task, but a combination square is something every woodworker I know has.
Start by clamping what you determine to be the low corner to a known Flat (flat does not necessarily mean level. Flat means if you put a long straight edge across it in any direction you can't slip a playing card under it.) surface bigger than your benchtop. Remember you want the high corner to have grain that flows from the center of the board to the edge you will be planing in the direction you will be planing. Place a few shims in the gap under the high corner to stabilize this corner. Don't force them in, you don't want to raise the top, just push them in lightly.
Once you have done that, take your combination square and with the end of the rule resting squarely on you reference surface, slide the head down until it contacts the top. Now starting at the low corner and keeping the end of the rule flat on the reference surface and holding a sharp pencil against the face of the head(which is facing downward) scribe a line around the entire bench top until you arrive back at the low corner again. Any material above this line need to be removed with the plane. Just use this line as a reference as you plane starting at the high corner and gradually lengthing and widening the area you are planing, checking frequently with a straightedge in all directions.
Once you have one side flattened simply flip the top over. The side you flattened should be sitting flat on the reference surface with no rocking, while the corner you had designated as the low corner is if you were to measure is now the high corner and should be about an ⅛" thicker than the opposite corner. At this point, as much as I enjoy a good hand planing session, I would run the top through the plane with the newly flattened face resting on the tables.
Now it is important to remember you need to plane both top sections simultaneously, to insure a consistent thickness. I would start with the unaffected top first, because since you will be removing very little material from the newly flattened top you will be inclined to set the planer for a heavier cut which could be detrimental when you put the second section through. I also would take 2 passes (assuming 1/32" cuts) on the unaffected top then flip it over and continue to final thickness on the other side. Do NOT flip over the top section that you just flattened. You could, if you choose, take a final pass on the handplaned side (and the other section) after you are sure the top is flattened, if you aren't sure your hand planing was quite good enough or simply use the hand planed side as the bottom.
In total you should be reducing the top thickness by an ⅛" (your estimate) by removing material from 2 diagonally opposite corners one on each face of the benchtop.
I hope that clears things up for you. If not just ask for more details.
PS. I would make sure the Jack plane is freshly honed before starting since it sounds like a community tool don't assume it's sharp.
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