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Thread: Workbench Structural Anatomy
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21st February 2005, 11:01 AM #1
Workbench Structural Anatomy
During the course of the last year or so, I've read a lot of stuff, and heard a lot of opinions about what does or does not constitued a good workbench construction.
Taking a leaf from "MYTHBUSTERS" there are two myths of construction I'd like to try to dispel, but need the help of someone with a bit of engineering nouse!
MYTH 1: Heavier is better.
Wrong!!
A certain mass is necessary, but after a particular point, heavier serves no purpose, other than to put more load on joints and increase the probablility of racking. (From zero to not much if you get to Myth 2!). Once a structure is imovable it cannot be made more so. It needs to be strong enough to overcome the forces generated by your arms at full tilt, or maybe at worse, a medium hammer blow on the axis of the bench.
Any stronger or heavier, and you've just wasted material.
MYTH 2: A proper bench needs mechanical fasteners so that the joints can be tightened to prevent racking.
Wrong!!
This comes I think, historically when glues were not as reliable as today and mechanical fixings were used, and similarly when the bench needs to be dismantled for removal. When mechanical fastenings are used, the joints are subject to minute movement in the timber, which will allow similarly minute movement in the joints, and hence the need for periodic tightening of the bolt/screw fixing.
Using modern glues, it is very easy to incorporate an adhesive which will be stronger than the timber used, and combined with a reasonable mortice and tenon joint will result in a completely rigid connection, creating a "moment joint"in engineering parlance. Minor movement in the timber will not create joint failure, and the full bracing effect will be maintained.
If one was to create a reliable knock-down bench, it may be better to create the rigid joint, and cut the stretchers in half, connected with bolted plates for their length. (does that make sense?)
I think there is a bit of a parallel here with boat construction, where traditionally bolted and rivetted vessels were heavily built and the structure moves and creaks quite happily but every so often a seam will open and needs attention, while light timber structures and modern construction enable completely rigid, immovable structures.
The above is not a statement of fact (although it could well prove to be but I'd love to have an argument if it's not!!!
Cheers,
P
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21st February 2005, 11:10 AM #2
Myth 1: Perhaps, although if the material is heavier, it is probably denser and therefore potentially stronger. If all other things were equal, it seems a heavier bench should be stronger but I can't prove it. A bench carved from solid stone would be very heavy indeed but probably overkill for most work.
Myth 2: I made a bench for my lathe with the same construction that I will ultimately use for a workbench. It uses no fastners and no glue. It is held together with wedged tenons, wedged dovetail tenons, sliding dovetails and bridle joints. It might possibly need some dowel pins through the bridles but I've found that they are tight enough to not work loose. If any of the wedged joints become loose, they are easily tightened up again with a bit of thumping. Having heavy construction helps here because it gives a bit of dead weight against which to thump."I don't practice what I preach because I'm not the kind of person I'm preaching to."
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21st February 2005, 11:34 AM #3Originally Posted by silentC
Myth 2:It uses no fastners and no glue. It is held together with wedged tenons, wedged dovetail tenons, sliding dovetails and bridle joints. It might possibly need some dowel pins through the bridles but I've found that they are tight enough to not work loose. If any of the wedged joints become loose, they are easily tightened up again with a bit of thumping.
It is the working of the joint that causes the long term rickets, so I am postulating (I must stop eating so much fruit) that dismantling should be done somewhere other than at the right angled joints, contrary to historical logical tradition!
Cheers,
P
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21st February 2005, 11:46 AM #4
I suppose there is a break-even point where extra weight yields no benefit. I think this would depend upon the individual using the bench. Have you seen that bloke who can pull a semitrailer plus prime mover? I bet he would need a VERY heavy bench. If he was a woodworker, which he probably isn't.
Whether extra weight would actually turn out to be detrimental to the strength of the joints is another issue and, like I said, I don't think it would be - but have no way of proving it.
I got the idea for the joinery from Scott Landis's workbench book. I thought it would be fun, yes, but I also believe that a strong joint is a strong joint and doesn't need any glue. If you rely upon the glue alone, then you are stuffed if it fails. I suppose you could make a wedged joint stronger and unlikely to work lose if you glued it. I was really just agreeing that you don't need mechanical fasteners to make a strong bench and adding that you don't need glue for that either.
I'm also too cheap to spend money on bolts and fancy glue. The whole thing is made from a tree, except for the 4 screws which hold the lathe in place."I don't practice what I preach because I'm not the kind of person I'm preaching to."
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21st February 2005, 12:02 PM #5SENIOR MEMBER
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Originally Posted by silentCDan
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21st February 2005, 12:18 PM #6
I don't know what the ideal weight for a workbench is, I suppose you'd have to ask Jenny Craig about that, but I do know that it has to be heavy enough to be "dead". By that I mean when you are pounding something or chiseling, with a suitable block to save the surface of course, you don't get any "boing", all the force is absorbed by the bench. I dont think weight is a factor in the structural integrity of the bench but it is certainly a factor in the boing in the bench.
As for how the bench is held together, thats up to the individual. If rubber bands work for you then go for it.
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21st February 2005, 12:23 PM #7
Interesting Thread Midge.
I think work benches need to be immovable and stiff.
Imovable so when you are chiseling horizontally or handsawing you don't have to chase the bench around your shed. My bench is not bolted to the floor but has sufficient mass so it's intertia resists any horizontal forces.
The stiffness is so the bench doesn't flex even if it stays in the one place on the floor.
It turns out that the materials we use add mass to achieve the above two requirements. So a heavy bench is not necessarily a good bench unless it is immovable and stiff.
If a new super light material was discovered that achieved my two requiements, was soft enough not to damage sharp edges on tools or timber then I would probably consider using it for a workbench.
BTW, consideration should also be given to where the mass is located so the workbench is stable even when overhanging large heavy projects are attached to the bench. I suggest a fair bit of the weight being down low.- Wood Borer
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21st February 2005, 12:39 PM #8I dont think weight is a factor in the structural integrity of the bench but it is certainly a factor in the boing in the bench."I don't practice what I preach because I'm not the kind of person I'm preaching to."
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21st February 2005, 12:48 PM #9Originally Posted by silentC
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21st February 2005, 12:55 PM #10Originally Posted by silentCIf I do not clearly express what I mean, it is either for the reason that having no conversational powers, I cannot express what I mean, or that having no meaning, I do not mean what I fail to express. Which, to the best of my belief, is not the case.
Mr. Grewgious, The Mystery of Edwin Drood - Charles Dickens
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21st February 2005, 01:25 PM #11
The ends of the mortice are cut on an angle, then one side of the tenon is cut on the same angle with the other side straight. The wedge locks the tenon in place. See attached pic. It's good for stretchers.
"I don't practice what I preach because I'm not the kind of person I'm preaching to."
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21st February 2005, 01:33 PM #12
sitting on the fence :
the bench should suit its desired design purpose.
it should go "boing" if required, it should not move unless you want it too, if you move house a bit it should knowck down unless you are superman, another outside underpant wearing superhero or his homosexual sidekick.
with my work bench I wanted a nice long bench that was massy enough not to move but wasnt so hard to move if I decided to move it. therefore the drawers are removable as is the bench top. thus in a pinch I will whip out the drawers, unbolt the top from the stretchers and move the whole lot myself in 6 phases (4 x drawer, top then carcase). altogether I reckon the sucker weighs in at close to 200kg. What I like about the carcase is that the hardwood construction means its strong enough for me to lift it in the middle and as long as its balanced 9which it is) I can move it without anything breaking - albeit slowy - it is very heavy after all....
I havnt whacked it with anything yet so cant comment on the "boing-ability".
another thing since its so heavy I can use it as a stable table for my machinary (such as the 12' disc sander) and grinder.
I will finish it one day and post the last entry on the relevant thread - I promise!Zed
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21st February 2005, 02:35 PM #13
Sounds like a good idea Zed - the removable mass.
I once cleaned out the scraps from the shelf under my bench, I put them back immediately after the bench realised it could escape. I realised just how important the mass was even though I hadn't intentionally placed it there in the first place.
As far as bonking on your workbench, I wonder how many people would own up to such a seemingly interesting activity.- Wood Borer
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21st February 2005, 02:50 PM #14Originally Posted by Wood Borer
Immovable is as easy to achieve as a couple of screws in the wall or floor. (Holes aren't that hard to repair when you move house). I have used the screw into the wall trick to stabliize the fumping bench, which is very stiff, but proportionally able to be toppled by me bending a bit of pipe in the vice.
Failing convincing anyone about the floor screws, fill the base with sandbags to your own specifications.
A plywood box frame with modern adhesives and minimal structure would give a very stiff structure. (Refer the tea-chest, which has semi flexible joints yet still remained in shape despite copping a pounding in transit).
The top would only need to be a torsion structure with maybe a 12mm upper skin, if stringers were also of 12mm and say 300 mm max in each direction, apart from the sound I doubt whether you'd feel the difference.
I have a feeling that the next round of responses is going to lead us back in a circle!
P
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21st February 2005, 03:07 PM #15
Yes, it sounds like a light-weight bench for light-weights. Probably the type of person who would drink light beer and caffeine-free coffee. Nah, I'll take my thumping great solid b@stard of a bench any day
What is it with arty architect types that they want to make things that are stronger than they look? They seem to have this obsession with making people feel uneasy
Yours in solidarity, brothers."I don't practice what I preach because I'm not the kind of person I'm preaching to."