# What flatness tolerance can be achieved over a large surface with hand planes?



## wts (Dec 3, 2017)

I am a little newer to woodworking, and I recently built a 5'x3' MDF torsion box assembly table / workbench. Unfortunately, I found that the top surface is not flat (oops). Using hand planes, I was able to improve the flatness to at worst 0.05" (+/-0.025").

My question to you is: should I try to keep improving upon this flatness, or will I just end up going in circles trying to chase a tighter tolerance? My fear is that by improving the "local" flatness over the length of a 22" jointing plane, for example, I will throw off the flatness with respect to the opposite end of the table.

Also, how flat is typical / necessary for this type of assembly table (and subsequently for furniture that may be built upon it)?

Thanks in advance!


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## Manitario (Jul 4, 2010)

So, your workbench is within ~1/16 of being perfectly flat. I'm not sure how much this will affect what you normally use a bench for; just don't use your bench as your reference for flatness. I have two benches, a hand tool bench which was ~0.005" but I don't think it really made any difference; and a plywood topped bench that I use for glueup and assembly, which I've never checked the flatness.

-panel glue-ups; typically clamping a flat caul at each end and then flattening post glue up not really depending on the bench flatness
-hand planing; typically you have a separate reference to see if the board you're planning is flat
-joinery; most of the joinery is going to be referenced from a square rather than from your bench.


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## rwe2156 (May 7, 2014)

Good enough for ww'ing.

BTW MDF isn't always as flat as its supposed to be. It all depends on how they store it. If its sagging because its not totally supported, you can't get the bow out.

This is why I never buy MDF from HD or Lowes.


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## clin (Sep 3, 2015)

I didn't realize you could hand plane MDF.

I'd concentrate on overall flatness. Then address local flatness (low spots). On option would be to get it overall flat, then throw a layer of 1/2" MDF on top. Or you could even spread something to fill in the the low spots. Though you probably would want to cover that with something.

I could see +/- 0.025" causing problems in some builds. It would just depend on what it it is you're doing. And of course over what length you're seeing this error.

Unfortunately you have discovered that while very rigid for the amount of materials in them, torsion boxes are only as flat as the work surface you build them on, and of course as consistent as you cut the shear webs.


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## Unknowncraftsman (Jun 23, 2013)

That's pretty good for your first one. I think you should use it and see if it's acceptable. Then someday when your feeling frisky use that ones surface to build a new one. That's what I did when I needed a assembly table each one got better.
Good luck


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## wts (Dec 3, 2017)

Thank you all for the responses! It sounds like what I have is sufficient for most woodworking applications, but for some things maybe not. For what it's worth, I am willing to put in the effort to get it right now (if possible) rather than revise it down the road.

To add a little more insight, at the moment the bench seems to be "low" along the outer edges and corners. The interior ~4'x2' surface is actually quite flat (i.e. +/-0.015" fairly local surface roughness). FYI my top layer is 3/4" MDF, which I intend to cover with a thin tempered panel sheet. I like the idea of an extra layer of MDF with gap filling, but I am concerned about weight as the box is already quite heavy. I do need some portability.

Hypothetically, would I be able to bring down the interior area to be planer with the outer edges/corners without throwing off the other side? I am a little worried that I will only make things worse. Is there a good technique? Sorry, I am still a little naive with hand planes.


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## Smitty_Cabinetshop (Mar 26, 2011)

You've got 1/16" variation in your bench top, and I've got (probably) a 1/4"+ variation in mine, front to back, on either end. You're wanting it within +/-0.015" for woodworking, mine is approaching 9 years of use and isn't getting any flatter than what happens once a year (or so) when I go over the surface with a toothed cutter for grippiness.

Does my work suffer? I don't think so.

My .02 is, you're fine and ready to get to work on that new workbench. Others will certainly disagree.


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## mattjrogers (Jan 2, 2013)

Really? You guys think that you need a bench that is not off by more than 1/64" over the entire surface? I acknowledge that you could theoretically get the bench perfectly flat using hand planes and scrapers (think hand-scraped cast iron machine ways), but why bother? Old work benches were made of solid wood and in an unheated workshop. They moved with the seasons and certainly were off by more than a 64th over the course of a year and thousands of wonderfully detailed pieces were produced from 0 AD to 1950 AD when you could conceive of a workbench being in a climate controlled space.

A flat bench is great for doing glueups and checking chair legs and joinery and I do have a torsion box table like this, but do I bother to check its flatness and see if I can shave a few thousands off here and there? No. Not worth the time in my opinion as we are working with wood which moves anyway. If you try to make it perfect one day, it will move some the next day or when you bring it to another building you will find that the floor is not as flat as your perfect assembly table and the chair rocks anyway.

Go have some fun and get covered in sawdust instead!


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## wts (Dec 3, 2017)

Thanks Smitty and Matt! This helps puts the flatness requirement into perspective, and maybe I should just call it good. However, my concern is that I may run into problems clamping work pieces along the outer perimeter of the bench (I cannot reach clamps to the center of the bench of course). Anything wider than 6" that I clamp down along the edges will inherently have a fairly significant amount of mismatch. I think that it may even be closer to 0.10" flatness (+/- 0.05") in a few places relative to the center actually.

Perhaps I am overthinking it?


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## Kirk650 (May 8, 2016)

I have a work bench and an assembly table. Both are pretty flat, with the assembly table being very flat. I used two layers of MDF for the assembly table, with the top being laminated in white. Size is 24×55 inches, and I made sure there was enough edge sticking out for use in clamping. MDF is flat, and no way I could hand plane wood that flat. And when the laminated MDF gets glue or paint on it, I just scrape it off with a paint scraper. Once a year or so i'll Sand it with 220 grit and a ROS to clean the laminate back to clean white.

The work bench is also quite flat, and made like a large cutting board, with 2 1/2 inch wide ash boards face glued. I made it in sections, so that I could run it through the planer, then glued up the sections with clamps and cauls.

A flat work surface is important, and I just did not think I could get there with a hand plane.


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