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Best Bet on Creating a *PERFECTLY* Flat Assembly Table

5K views 25 replies 22 participants last post by  bigJohninvegas 
#1 · (Edited by Moderator)
I'm creating what I hope to be my last assembly table and I'm going all out. The last couple I've built have been just a hair off perfect and it drove me nuts.

What's the best way for me to create a perfectly flat 4×8 plane? Right now I'm looking at using 3/4" Baltic Birch vs. what I've used in the past which is 3/4" standard plywood.

I'd even go so far as to use a ridiculously priced sheet of metal if that was my best option but I'm curious what you're using in your shops and what kind of accuracy you expect.
 
#2 ·
i made the two in my shop out of mdf, but they are 3 ft x 8 ft on casters, with shelf under and have laminated a sheet of 3/16 tempered hard board to it, for a sacrifice sheet, slick side up. Has lasted 9 years so far, almost time to replace the tempered board from glue, misc finishes, i used to put a chunk of padding down on it for sensitive pcs, but built the other one with a commercial carpet glued on it for those parts,

used two layer of mdf, 3/4 and the tempered hard board, 4×6 legs, with 2×4 frame, for top sub base. bolted and screwed. you could park a volks wagon on it
and very good casters.
good luck
rj in az
 
#3 ·
I've also made 2, and I used MDF for the skins on each. The first was 4×4, the second roughly 3' by 7'. In both cases the webs were 4" wide. I mostly followed David Marks method os insuring they were flat. That is to first get some boards (I used 2x stock) jointed to be perfectly straight. I then laid them on saw horses using shims to make sure they laid on a flat plane. I started with levels to get close, then used winding sticks to make that perfect. At that point the shims get hot melted in place to insure they don't move. Then I put the first skin on my frame and checked again with winding sticks. Thsi has worked very well for me to get perfectly flat assembly table. The other differences were mostly in the web, I used 1/2" plywood for mine (Marks used MDF) and I assembled it bu crosslapping (or 1/2 lapping the intersections. This made the assembly go a lot faster and using plywood reduces the weight slightly. Heere's pic of my frame and the web assembly.
Wood Wood stain Hardwood Creative arts Plank


Wheel Table Tire Wood Hardwood
 

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#7 ·
Some years ago, I purchased some used lumber core doors to make work/assembly bench tops. They have served me very well and, for my purposes, are "perfectly" flat. But, even if your definition of perfect is more stringent than mine, they are a good starting surface. They are stiff and heavy and, with a hardboard or laminate top surface, very serviceable.
 
#10 ·
Working on a project for one of my sons, making a cabinet for audio equipment; bought a 4×8 sheet of 3/4" Baltic Birch, $186.00 here in my town, at a hardwood speciality store that also caters to professional cabinet makers. Also bought a 5×5 sheet of 1/2", that was $90. I really like BB, but damn.

As for the assembly table, the torsion box idea looks to be your best bet, and going with the MDF as your top surface would probably be the best surface, once sealed. Perhaps a torsion box with a double layer of MDF would give you the least expensive, rock solid option. Good luck.
 
#11 · (Edited by Moderator)
What is your definition of perfectly flat?

A wood top table will never, ever be perfectly flat.
Wood moves with temperature AND humidity changes. Depending on how the table is supported, it can bend/warp as environment changes. So what was flat today, might not be flat tomorrow.

Assembling wood projects does not require a perfectly flat table. A solid core door, or a torsion box, loosely attached to a stand so it can move as needed is best plan. If you want it to stay flat, you have to check and adjust it regularly.

Hence, it's not worth the time and effort to discuss making a perfect flat wood table further.

If you want perfectly flat table, need to use a precision ground; marble table, commercial welding table, or a hunk of aluminum tool plate. These are not cheap, so buckle on your big boy pants before you go shopping.

Best Luck.
 
#12 ·
I think people stress this topic way too much. Most assemblies will come out perfectly flat and square, even on an imperfect bench.

I made a rolling shop cart with a doubled up 1" MDF top. Solid 2" thickness with Formica and hardwood edging. Works great. No wasted space as with torsion box designs.

Good luck with it.
 
#13 · (Edited by Moderator)
If you want a truly flat reference, the gold standard is solid stone.
These are typically the domain of high precision instrumentation and aerospace manufacturing.

For woodworking, you can get really good with a torsion box, but any stable, relatively flat surface free from twist or sway back would be good.
My current table of the last ten years, or so, is a 3' X 4' sheet of 3/4" thermofused melamine shimmed on top of an overbuilt 2×4 Douglas Fir base.
I used my 4' level to level the table but also as a straight edge reference to shim the top on the base.

The table has only moved twice in the last ten years and each time is was recalibrated flat and level.
It is showing it's age and I probably need to rebuild it, or at least replace the top, but it still works great for my purposes.
 
#16 · (Edited by Moderator)
I'd suggest an aluminum torsion box built with 1/16" skins and a honeycomb core. That's what we use in the shop. It's not perfectly flat (estimated .01" of sag in the center of the 5'x8' table) but it's closer to flat than any other large table I've seen. It used to be some sort of vacuum table fixture but ended up as an assembly table in another woodshop before being sold to my business.

Check vacuumtables.com to see similar tables.

Don't use baltic birch. It moves a lot. Appleply is higher quality and warps less.
 
#19 ·
I think people stress this topic way too much. Most assemblies will come out perfectly flat and square, even on an imperfect bench.

- pintodeluxe
It really helps me to have a nice flat top for cabinetmaking.

That said, yes, flat is t doesn't mean a workbench shouldn't be as flat as possible.
 
#20 ·
Perfection can be pursued but not attained (here anyway) so we have tolerances. As previously mentioned, stone is the best we've got so far, the precision you desire is only limited by your budget and the load bearing capability of your shop floor.
 
#21 ·
The point is that if you can detect that is not flat or notice that the un-flatness is affecting the results, then it is not flat enough. If you cannot detect it, then it is flat enough.

For assembly tables, it is helpful to have a surface where glue drips can easily be popped off; otherwise, the drips themselves will make it un-flat. A sheet of plastic laminate of some kind makes for a surface that almost all types of glues used in woodworking can be easily scraped off. Another option is to put a sheet of Masonite on the surface and apply something like polyurethane on the surface and keep it well waxed. When the Masonite gets too beat up, replace it.

I modeled the base of my relatively small assembly table after the 4×8' one that Norm Abrams built in this NYWS episode ; however, I just used a thick laminated particle board desktop that I salvaged. It is simple, sturdy and the aprons provide plenty of support to prevent sagging. Norm shows how he made a large flat torsion box in the episode. He even makes it mobile with a simple jack system.
 
#23 · (Edited by Moderator)
I don t think it is necessary for woodworking, but…

find an old (slate) pool table and remove the side bumpers and baize.
It weights something between 650 and 2700 pound.

or a granite top for kitchen island.

- Sylvain
Pool table slate is not that accurate, ground to only .010 inch flatness. I'm working on a Brunswick Gold Crown IV as I type this with 1 inch thick slate, a commercial model table that's built like a tank. My Oneway Multi Guage with a precision ground 4.5×1.125 base was wobbling in some areas of the slate.

Also slate is surprisingly flexible. One of my 3 piece slate has a .005 inch bow. I can press it to flat with one hand.
 
#25 ·
For assembly tables, it is helpful to have a surface where glue drips can easily be popped off; otherwise, the drips themselves will make it un-flat.
My glue up table…5/8 inch thick steel flat to .002 inch across it's length 90% of the table is dead flat at .000 inch. Precision CNC drilled with about 1,000 clamping accessories. Note the Nitride coating, glue, epoxy, whatever pops right off with a putty knife. Plus she's north of 300lbs.

Table Outdoor table Rectangle Wood Outdoor furniture
 

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#26 · (Edited by Moderator)
I used two sheets of 3/4" baltic birch ply for my bench top. and 1" baltic birch ply, and a little poplar I had around for the base. It is rock solid, and flat.
I do know a few Professional woodworkers that keep a sheet of 3/4" MDF on top of there bench. Both very flat, and protective. When it does get screwed up, flip it or replace it.

This was when it was brand new. No dog holes yet, not so clean now, and maybe soon it will be time to put a new top sheet on it.
Work bench, assembly table and at the time outfeed table. New shop layout, it no longer serves as an outfeed table.
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