I have a workbench top under construction, using salvaged red oak cut vertically and glued (typical top). It will be shaker style, with cabinets/drawers underneath. A couple of questions I hope you can help me on:
1) The top is only 1.50″ thick, and I am planning on adding a 3/4″ MDF sheet as an under layer, so the whole thing is only 2.25″ thick. I have cut the cheeks on both ends to accept a breadboard end cap, but am wondering the best way to secure it, given the “thinness” of the top:
– wooden dowels throught the top versus bolts and wooden plugs thru the end caps themselves
2) Do I need to allow for movement, given the top slats are 3/4″ thick (yea, lots of glue to make a 30″ wide top). If so, and knowing I will be adding a front and back apron with either box joint or dove tail joints on the ends, how do I do that?
3) I have seen plans where the top is simply bolted to the cabinet frame, and others where it is configured to allow for movement. Should I be concerned, and if so, what is the best approach
Thanks for the advice!
Replies
I, and a lot of other people, could tell you what you need to know to solve your immediate problem, and it is a problem because a top that wide is going to move a lot, at least an inch I'd guess, between the driest and most humid times of the year.
Instead of a quick answer, I'm going to suggest that if you are going to learn how to build furniture that doesn't self-destruct, you will need to sit down for an evening or two and read about how wood behaves.
The classic book is Bruce Hoadley's "Understanding Wood". You don't have to read the whole book, just the part about wood movement. A short read and also worth having is the Lee Valley wood movement calculator which includes a well written pamphlet on the basics of how wood reacts to moisture changes.
Neither of these books will tell you how to design your benches joinery to deal with the movement but at least you will know how much motion you have to deal with and why. Once you understand how the top behaves it will be a lot easier for you to understand the various joinery options you have for dealing with the movement.
John W.
I have read about movement, and have also been told becuase the slats are thin and the wood is very old and aged, that the movement would only be about 1/16" in width, and almost nothing in length. This was done using calculators.
So, now I need to work with movement anywhere from 1/16" to 1"? Seems like I am back at square one...
Personally I would skip the mdf altogether. a laminated 1 1/2" oak top will be plenty strong. I would attach it to the base with gravity, use a couple of large dowels to keep it aligned on the base where you want it. The breadboard ends won't work if you are planning to put a dovetailed front and back board. most benches that have such a detail have a tool tray and that is where the movement is allowed to takeplace. You should have a look at woodsmith number 50 for a decent bench design as well as Frank Klaus bench for a great design.Tom
Douglasville, GA
Neither the age of the wood, or the thickness of the individual slats, or whether or not it was kiln dried, or the moisture content when the gluing was done will have any effect on the seasonal expansion and contraction of the wood in your bench top. If all of the pieces are quarter sawn that would reduce the movement by a little more than half but it still wouldn't eliminate the expansion and contraction.
A finish will slow down the gain and loss, but over the course of a dry winter and a humid summer the wood will gain or lose just as much moisture as if it had been unfinished.
As for the calculation: A one inch thick piece of red oak will change by .0037 inch for each 1 percent gain or loss of moisture. In much of the U.S., and elsewhere, wood will go through a 10 percent moisture change between winter and summer. A one inch thick piece of wood at a moisture change of 10 percent will move 10 x .0037 = .037. Your bench top will be thirty inches wide so it will move: 30 x .037 = 1.11 inches.
A bench top glued up from a lot of small strips should be perfectly stable without any reinforcement, I would skip the bread board ends because they are only going to create difficulties with no improvement in the top's flatness. By the way, as long as the oak stays reasonably dry you can safely use steel fasteners in it with no corrosion problems.
Hope this helps, John W.
Edited 3/12/2004 2:58 pm ET by JohnW
John -
Check your math. I get 0.111" instead of 1.11" for the expansion of a 30" wide glued-up plank of oak based on your coeffecient of .037"/inch.
...........
From Beautiful Skagit Co. Wa.
Dennis
I get 1.11"
VL
Nope, you, or your calculator, have misplaced the decimal point, it really is 1.11 inches.
10 x .0037 x 30 = 1.11
Misjudging the decimal point location is a common and easy to make mistake and one that I became very careful to watch for after four years at an engineering school. The instructors took real glee in nailing you for putting the decimal point in the wrong place on an otherwise perfect answer.
A misplaced decimal point was an easy mistake to make using a slide rule because slide rules never tell you where the decimal point goes, that part you had to keep track of in your head, it's sort of easy after you do it long enough.
John W.
Edited 3/13/2004 2:00 pm ET by JohnW
Edited 3/13/2004 3:48 pm ET by JohnW
Edited 3/13/2004 3:51 pm ET by JohnW
John -
Ooopppsssss.
Sorry, trying to do things in my head doesn't cut it anymore.
That's absolutely amazing to realize how much the cumulative shrinkage/expansion really is. Do you, or anyone know of a source for coefficient of linear expansion of various species of wood?
...........
From Beautiful Skagit Co. Wa.
Dennis
Try here:
http://www.woodbin.com/ref/wood/shrink_table.htm
http://www.woodbin.com/calcs/shrinkulator.htm
http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=shrinkage
VL
Thanks for the link, Venicia.
I tried the 'shrinkulator' entering 30" as the initial dimension and a change of rel. humidity from 30 to 50% and it returned a dimensional change of 30.16 (radial) and 30.41 (tangential). Thus the radial growth is only 0.16" and tangential growth is only 0.41", according to this source.
The 1+" dimension calculated using the coefficient of shrinkage given previously must be for the change from fiber saturation point to oven dry??
...........
From Beautiful Skagit Co. Wa.
Dennis
Dennis,
I thought that John W would be correct, so I entered a few numbers in the 'shrinkulator'
Here were the results.
We're looking to radial shrinkage, not tangential, due to the way it's cut, I believe, but, then again, I could be wrong.
Shrinkulator Summary Table
Species
Initial MC
Final MC
Initial Dim
Final Dim Radial
Final Dim Tangent.
Change Radial
Change Tangent.
Oak, Southern Red
20.0
6.0
30
29.29
28.25
-0.71
-1.75
Oak, Southern Red
6.0
16.0
30
30.52
31.33
0.52
1.33
Oak, Southern Red
4.0
14.0
30
30.52
31.34
0.52
1.34
Oak, Southern Red
4.0
20.0
30
30.84
32.14
0.84
2.14MC = Moisture Content
Cheers,
eddie
Dennis,
You are getting the lower numbers because you have chosen a relatively narrow range for the relative humidity. In an area like New England, where the winters are cold and dry, and houses are heated for four or five months out of the year, the relative humidity would be around 20% in the winter but will rise to 80% or more in the summer. If you put those numbers in a calculation you'll come out with much larger dimensional changes.
If you live along the coast in the Northwest, you will probably only see a difference of 20 to 30% in the relative humidity over a year and you could design your joinery for less wood movement, but I wouldn't recommend it. You never know where a piece of furniture will end up during its lifetime which can be measured in decades, if not centuries. A lot of antique and modern furniture, that wasn't designed to deal with large moisture swings, cracks badly when it gets moved to places like New England or the Southwestern states. I sleep better at night knowing that the furniture I made will hold up no matter where it ends up a hundred years from now.
John W.
A question.... How are you securing the solid oak top to the MDF?
I planned to use non-ferrous woodscrews so the acid in the wood would not corrode. Just long enough to connect, as the tenon from the legs will reach through the MDF and about 1/4' into the top.
I'd like to follow Howie's question with this comment. You had better put a very good finish, of equal amounts, on both top and bottom sides of your laminated top. It won't matter how you attach the MDF. If you leave the underside unfinished that baby will bow up like a dead fish.
In my humble opionion, you're just about to paint yourself into a corner with this one. I can recommend how to proceed but..... Ken
But what...I understand about sealing both top and bottom equally...I assumed this group's purpose was to assist each other..what I am getting is not assistance..while I have not been woodworking for 20 years, I have built cabinets, tables, jigs, etc.
My question remains true to my first inquiry: What is the best method?
Regarding wood movement, I think movement that differs from person to person by almost 1" reflects that fact that someone, I do not know who, either does not understand my question or is guessing or has not worked with aged red oak, 3/4" thick, glued up for a top..not to be rude, but I need an answer, not pointless inferences and guesses..
Tom is right. Skit the MDF and the ends. I would use some 3/4" plywood, and depending on the leg dimension, cut it to around 6" x 28". Bevel the ends. I would attach these plywood pieces to the legs securely using large screws. I would then attach the top to the plywood with screws via slotted holes so the top could "float". The top must be finished equally, top and bottom.
You didn't say anything about the base. Be sure the legs/frame are assembled with plenty of weight and rigidity. Good Luck, Ken
Ok, now that makes sense. Only thing is, I really want the breadboard ends for the look, so how about wood dowels vertically through the breadboard caps and throught the cheeks, with sloted holes in the cheeks to allow for movement but still give me the look I want?
Breadboard ends will be fine for this bench as long as you don't dovetail or boxjoint boards to the front and back. Glue the breadboard at the center an allow the top to move as it should. Tom
Douglasville, GA
Would it work to joint the front corners (dovetail/box), use dowels as previously stated, and not joint the back. Perhaps do a tool tray that lets the breadboard and top slide past the back tray wall?
I would do a dovetailed joint on the front, use a spline to keep the cap aligned with the top (no glue), a bolt through the center of the cap with a slotted hole on the top (see the woodsmith design) and put a tool tray on the back and dovetail the back of the tool tray to the end cap. Actually that is almost exactly the way my bench is built. It just sits on top of the base.Tom
Douglasville, GA
cc
I have th flu (I'm whupped) and won't add to what Tom has already told you on that end. I will just add the fact that the last work-bench before the current was recovered red-oak flooring. I have used it quite a bit as it's easily accessible in the Atlanta area.
If it's 1 1/2" thick I have never seen it move as much as an inch. I won't say it couldn't (anythings possible given the right circumstances I suppose) but I would have my doubts with cured red oak. We do have a bit of humidity here in Atlanta in the summer, so I would not be overly concerned with that much movement.
What I would do is pay atttention to what Tom said about the end caps. If you get more concerned with styling over basic engineering, you got a potential for failure. I also use bullet headed dowels in the top of my base which correspond to holes cut in the bottom of the top to correspond. It ain't going anywhere with that method if the base is right.
Good luck...
sarge..jt Proud member of the : "I Rocked With ToolDoc Club" .... :>)
Combine what you are saying about the slotted dowel holes with what Tom said about glueing only in the middle, say around 4"-6".
If you really want to do like the old world guys offset the holes in the cheeks about 1/8" and use dowels that have a slight taper. You'll have to do that by hand. This method will pull the ends on tight and hold them there without glue.
See if you can find an old dowery chest with breadboard ends. Many of them were assembled this way, still very intact. Ken
Edited 3/11/2004 8:15 am ET by Ken's Shop
If you dovetail or finger joint the end of the breadboard (really an endcap) then you don't want to glue the middle. Use a bolt with a slotted hole. I beleive the woodsmith plan and Frank Klaus's bench use the same type of end cap arrangement.Tom
Douglasville, GA
I have a comercially made bench that has breadboard ends. The left edge of the front vice is even with the edge of the breadboard end. Wood movement in the top is enough so that some times of the year the left side of the vice couldn't close completely (when the top shrank), it was so annoying that I finally trimmed the breadboard when it protruded the most. Now the vice is effectively shorter. No matter what you do, if the breadboard is wider than the bench top part of the year the effectiveness of the front vice will be comprimised. I would put the glue not in the middle as has been suggested by some but much closer to the front edge so that the wood movement will be toward the back of the bench where it wont effect the front vice.
Rod
Thanks for the suggestion...I think I have a good idea of some options now...currently working on re-acquanting myself with a homemade mortise machine I built last year so I can start on the end caps. In my design, the edge of the vise face is about 1/4' shy of the edge of the end cap, so movement should not be an issue for the vise...
Again, thanks for the info!
If you can possibly obtain a copy of the FWW 2003 Tools & Shops issue, there is a great workbench article that shows the best way to attach end caps to a glued up workbench. They are glued at the front only and the quite considerable movement is permitted at the rear. The ends are dovetailed into the front edging and there is no rear edging, thus allowing free movement of the slab. The slab is secured to the end support trestles only in the center, screwed from below.
My wood calculator program shows that, for a 30" wide slab of red oak, a swing from 5% to 95% Relative Humidity will result in approx 1" of movement radially or a bit over twice that tangentially. It matters not the thickness of the individual strips, only the total width of the slab.
If your 3/4x1-1/2 strips were initially quartersawn, the glued slab will be effectively flatsawn and exhibit tangential movement. If initially flatsawn, the resultant slab will be effectively quartersawn and exhibit radial movement. Either way, the movement is substantial.
I also would not try to mix a glued up slab and MDF in the same top. The differences in the response of each to humidity changes are just too different. Also the MDF would block the moisture transfer through the bottom surface of the slab, while the top transfers moisture freely, even with a finish. That would just be begging for a cupped slab.
Have fun building your new bench and post some pictures of your progress.
--
Lee in Cave Junction, Oregon;
Gateway to the Oregon Caves
If you do a google search, you can find some wood movement calculators. You have to provide some information regarding how the wood is sawn (i.e. flatsawn, quartersawn) and information regarding beginning and ending moisture content (this may be where a book like Understanding Wood by Hoadley would help). But, just playing with the numbers, assuming the wood glued up is oriented like flatsawn lumber from front to back on the bench, you'll see that you get quite a bit of movement.
Personally, I've never made too much effort to figure out exactly how much wood was going to move in a project, I've always just assuming that the "length" of wood will change insignificantly (where the long dimension would be oriented vertically in a tree) and movement along the "width" should not be restrained.
This general rule becomes more complicated when you're not simply working with wide, flatsawn panels, because there a really 3 axis of potential wood movement (up and down the length of the tree, a flatsawn movement and a quartersawn type movement). Quartersawn lumber moves less across its width than flatsawn. If your bench would be glued up so that the top would present a quartersawn surface, you can take that into account in the wood movement calculator.
I'm not too sure this explanation will make sense or even be completely accurate (that's why professionals write books on the subject, to stop yahoos like me from perpetuating bad information), but I think you can count on enough movement across the top from front to back (unless the front and back are end-grain surfaces) that you're not going to want to restrain movement in that direction. This means no bread board ends unless they allow for movement and no large sheet of mdf or other stable material screwed to the top unless it allows for movement the wood top. Breadboard ends and an mdf top can still be used you just need to make sure that they consistently let the top move and nothing else in the construction of the bench restrains the required movement.
One final thought, no matter how dry your wood, it will still absorb moisture over time in an evironment with more moisture almost regardless of the type of finish applied.
Hope this is somewhat helpful. Sorry for rambling. If you want to see some great benches and ideas in detail, take a look at Workbenches by Scott Landis (I think that's right).
Matt
The MDF will, in the long run, add very little to the stiffness of the top. The MDF is, compared to the oak, a very stable material. The oak is going to cylcle through its annual shrink/swell events and that will loosen the screws and in a few years they'll loose their abilty to transfer the shear forces between the oak and the MDF. Unfortunately, I don't think a glue bond in this case will solve the problems over the long run either. If you glue the oak to the MDF, either the top will warp excessively, or the glue bond will eventually fail or either the oak or MDF will fracture. MDF really is fairly weak stuff and wont add much in the way of strength no matter how you use it.
But ...
IF the MDF is firmly attached to the frame and the oak top is fixed to the MDF along the front edge only. Then I would expect that the MDF would lock the legs in space and thus aid the rigidity and stiffness of the whole bench, support the front edge of the bench (the area where traditional benches are thicker to allow for chiseling and pounding).
the oak top would then float on top with expansion only occuring along the rear edge and the bread board ends (if only pinned at the front) would add the design touch the original poster is looking for without comromising the longevity of the oak glue up.
Ian
If the MDF was attached on edge and we assume a perfect transfer of shear between the MDF and the OAK, it would add some stiffness to the system, but not much. I don't know what the modulous of elasticity is for MDF, but it can't be more than a small fraction oak's. It's like bolting some wood onto a steel beam to make it stronger. Yea, the composite might be stronger, but not much. An earlier post offered the opinion that the 1.5 oak top would be plenty strong and I agree with that. I don't know what purpose the MDF would serve in this case.
This forum post is now archived. Commenting has been disabled