Question about Split Top Nicholson Workbench Build

[Brian Dormer]

I'm ready to pull the trigger on a Nicholson Workbench build (because my first workbench, while it turned out great - I did many things wrong design wise).

I'm MOSTLY following Chis Schwarz design, with a few twists. The big twist is in the top - I'm making mine a split top. 2" gap (because my largest clamps can then be used thru the gap) with a gapstop (wood for gapstop to be determined - but I have some walnut and some cherry that might do nicely.) The wood is Douglas Fir (2x12's and 2x10's culled from the Big Orange Borg) - I've been seasoning it in my shop for about the last 3 months - so it should be close to equilibrium.

The Schwarz version has a solid top made of doubled up 2x12 (so it's 3 inches thick) - I'm going the same route, except for a 2" gap in the dead center. My bench will be 24" wide - so each "slab" is 11" wide. From leg to leg (the legs are per Schwarz design) - is 48".

My concern is that the unsupported center of the bench (both front-to-back center and side-to-side) will have a tendency to sag - or, worse, the bench will be a bouncy when planing stock. I could just build it and retrofit bracing, but like when you are sawing - if you start right, ending right is much easier.

I'm thinking I can get away with one bearer, dead center, so there will be less than 24 inches from leg to bearer on either side. But MAYBE I need 2. I'm noodling in Sketchup and 3 looks like overkill (the bearers end up about 8 7/8 apart).

So - my question is - do the "experts" think I need a bearer across the middle? Or 2 bearers? Or 3?

Weight is not an issue - this isn't meant to be a knockdown bench. I am bolting the legs on (again, following in CS's footsteps) but that's only so I can get the bench out of the shop if/when I move.

TIA -

bd

[Reinis Kanders]

I thought that Nicholson bench by design had multiple cross braces. E.g. http://kapeldesigns.blogspot.com/2014/08/the-nicholson-bench-with-holdfast-vice.html
Main appeal of Nicholson type bench with cross-braces is that there is no need for the double thickness top.

[Brian Dormer]

In a traditional Nicholson - that is correct. However, as I stated, I'm going with Chris Schwarz' design - which calls for laminated 2x stock for the top (works out to 3" thick). Per CS, it makes for a much more robust top, less likely to be bouncy during heavy stock planing. Where I differ from CS design is in the split top - and I'm concerned about sagging because at the split, the top slab are just hanging out in space unless I "reintroduce" one or more cross bearers. My gut (and a Sketchup doodle) say 3 bearers is too many. I'm thinking one or perhaps 2 bearers - but if someone else has "Been There, Done That" - I'd appreciate their input.

[Malcolm Schweizer]

A 24x48" span is not much for a 3" thick top of properly chosen wood. I would think one crossmember would be plenty. The Nicholson he builds in Workbenches is not a laminated top; It is a torsion box 8' long with 16" OC cross members, and the top is one layer of 3 nominal 2x12's ripped and edge glued to 27" width. I'm not familiar with the one you are referring to.

I ordered the plans for the split top Roubo but was torn between it and the Nicholson. I like the split top, but never considered splitting the top on a Nicholson. I chose the Roubo due to the heft. My shop floors are tile (by default; not by choice). I need a heavy bench.

[Brian Dormer]

The Schwarz design I'm referring to is from his recent (last year or so) blog posts at Lost Art Press - NOT from the Workbenches book (my bad, I should have been more specific). Also - if you want to see a split Nicholson, search "Naked Woodworker" and "Nicholson" on the Lost Art Press blog and "Mike Siemsen" (he's the Naked Woodworker) on YouTube. There is an EXCELLENT 30 minute video on YouTube showing how to hold work on a bench with no vises (or is that vices?) and it's a split Nicholson.

For a 24x48 span - I agree. One bearer (if that) would be more than enough - that's what CS builds on the blog with NO bearer. BUT - my concern is that the 24x48 in my case will actually be two 3x11x48 sections with one edge (where the split is) just dangling out in space (the other 3 sides are fully supported. If I was designing a house, I wouldn't think twice about a cantilever that size - but the standard for rigidity on a workbench is MUCH higher - especially where handtool work is concerned.

[Malcolm Schweizer]

Ah I see. I am no engineer, but I think one would suffice. I always say "overbuild and overpower." (Power for boats, cars, etc.). Given that, go with two. They won't really be in the way too much.

[Brian Dormer]

I am an engineer (Computer Software/Hardware - not structural - I could do the math if I knew a bit more about bending moments and modulus of elasticity... but I digress) I think I have found a good answer. On SketchUp warehouse - there is a model of one of the Nicholson Benches from the Anthony Hay Cabinet Shop at Williamsburg, VA (if you are in to woodworking and haven't been, stop what you are doing and go NOW) Their benchtops are 1 3/4 thick and the bearers are appx 22 inches apart. The bench tops are made of hard maple, oak, beech and yellow pine. I've seen those benches up close many times (I live about 3 hours from Williamsburg) and they are nothing if not stout. You can Google pictures of the benches from their blog. I would have no question that I could plane a wide board or chop a mortise without one of those benches getting bouncy on me. I've seen them do it.

Of the species, I'd say the Hard Maple is probably the stiffest, and pine the least stiff - so that's the comparison I want to make.

Googling a bit, I found:

Maple bending strength 15,800 Janka Hardness 1450
DougFir bending strength 12,400 Janka Hardness 660

So just roughly, Doug Fir is about half the hardness, but MOST of the bending strength (which I interpret as proportional to stiffness) of Hard Maple. Since I'm nearly doubling the thickness of the Hay shop benches, my top should be plenty stiff with a single bearer (about a 24 inch span .vs. the Hay shops 22 inch span) - but I'm going to go with 2 bearers (works out to about a 16 inch span) just for overkill. Your estimation is born out by the (admittedly rough) math.