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How flat is "flat"?

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billw

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So today I was planing a piece of walnut and fastidiously checking it for flatness after every few passes of the plane. Now to an untrained eye (mine) it looked "alright", and using a square on it showed a slither of sunlight creeping through because of one or two high points on each check along the length. I'd say maybe 0.1 to 0.3mm on average, sometimes creeping up to 0.5mm.

At some point I must have decided that I should try and eliminate these pesky gaps but I soon ran into the endless loop of correcting one gap only to create another, quite often worse, one elsewhere. I finally just figured I'd hit my talent limit - I could have adjusted the plane but it was already taking off such thin shavings that I didn't dare tamper with the setting.

So question for everyone - do you have a point at which you say "yeah, that's flat" and what's the tolerance?
 

Rorschach

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All that bothers me is if something is flat enough for the job at hand. Would I worry about 1mm on a small box? Yes, would I worry about 1mm on a dining table? No.
 

profchris

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I stop when it's flat "enough".

If it needs to look flat, then the test is by eye, naturally.

If it needs to fit against something else, then both are flat enough when they fit well enough. And that second "enough" varies too - joining a guitar top or back, I'm looking for no visible light in the join. Other joints can be closed up to some extent by clamping, so close enough is less close there.

And then there is subsequent movement of the wood ....
 

rafezetter

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So today I was planing a piece of walnut and fastidiously checking it for flatness after every few passes of the plane. Now to an untrained eye (mine) it looked "alright", and using a square on it showed a slither of sunlight creeping through because of one or two high points on each check along the length. I'd say maybe 0.1 to 0.3mm on average, sometimes creeping up to 0.5mm.

At some point I must have decided that I should try and eliminate these pesky gaps but I soon ran into the endless loop of correcting one gap only to create another, quite often worse, one elsewhere. I finally just figured I'd hit my talent limit - I could have adjusted the plane but it was already taking off such thin shavings that I didn't dare tamper with the setting.

So question for everyone - do you have a point at which you say "yeah, that's flat" and what's the tolerance?
Myself and I'm sure many other people new to handwork fell down, nay, RAN down that particular rabbit hole until I finally got it in my head that wood moves even day to day (hour to hour?), let alone season to season. It's a factor we all know yet it took quite a while, probably a year of occasional handplaning, and after buying several forms of graded straightedge, feeler gauges and the like, before it finally sunk in.

I'm not advocating sloppy work, but the 0.1 > 0.3 category of gaps are only worth chasing if the item is something like a drawer front enclosed in another hole with minimal clearance, where the discrepancy will be more obvious; although sometimes not even then if the whole piece is of low value.

Chasing those is all about taking ONE PASS with as little pressure as possible then checking, and a plane set so fine it hardly feels like it's doing anything with the high spots - and having the blade "shaving sharp" - but I'm not going there :)

HTH
 

thetyreman

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it really depends what it is, for dovetails it needs to be very flat where it meets at the joints, which helps the joint be gap free, when you are marking them out and cutting them, for me 0.5mm gap would be unacceptable, also because it moves and changes constantly, that's why I work very fast.
 
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Even if you did get it within 0.1mm of flat, it's going to skew as soon as you start putting clamps on it.

I think square is super important, as it ties everything togeather.
Can't think of a reason where flat is that important, and can using be pulled into place (at least aesthetically) in most cases by clamping.
 

Ttrees

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I know some will puke at this 😛
My bench is my reference and I find its easier than having to muck about with
straightedges or winding sticks, just flip the timber over to look, no tools required other than a good lamp, and basic vernier calipers to keep parallel, (stainless ones without the dial, which have a single locking screw.)

One thin, short shaving can sometimes lift the other side of the timber
a whole lot, like a good few mm off the bench (where jointed previously) and can need only one or two in one spot to bring it back down on the bench.

On the timber I work with, the material differs throughout the length, and "sets" of shavings cannot be relied upon alone, as the plane can follow a twist, which can be elusive to recognise with a shorter narrower straight edge.
I'm sure a machine would nip this in the bud fairly lively.

The way I see it, it would be harder to find with a narrow straight edge.
You can also gloss the high spots (rub the timber for 3 seconds), say on wider stock that you know a wee bump somewhere is causing things to go awry,
Bare aluminium can be used well with a graphite stick to do the same thing..
(you can buy chunky sticks of graphite in art shops and they seem to last for years for this purpose)
I've seen MikeG has a nice lump of u channel for this, that could transfer a good scribblin.

Nothing worse than a gap between a glue joint.
All the best

Tom
 

Blackswanwood

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I used to spend ages chasing every glint of light showing under a straight edge until I was liberated when following one of Richard Maguire’s videos. He checks the flatness by simply sliding the edge of his plane along the face he was working on. It brought home to me how you can end up chasing a spurious level of accuracy that no one notices and then the wood moves anyway.

Woodworking has been more enjoyable and accurate since I learned that lesson.
 

Bm101

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I'm no where near par compared to most of you guys when woodworking of course but I learned a long time ago doing up properties that mostly it's only you that knows. My first flat I spent an insane amount of time on. I had worked on other people's house my whole life and I was going to crack this one. I mitred all the ceramic tiles in the bathroom on a two bob wonky as f**k Tommy Walsh (Garden force aka Alan Titchmarsh ) branded tile wheel. When I look back I think I must have been insane. There was a wonky (Victorian) wall in the front room. The inordinate time I spent getting the coving disguised. Never get that back. Landscaped it with (free) york stone and london stocks. I clocked all the screws on the decking I installed.
I installed a hardwood back door rather than a pine cheapo. By this point I knew we'd be putting it on the market soon mind.
Sold it a year later.
I just made an effort and found some old photos.

Sorry.











Time.
Slips by doesn't it.

What I'm saying is. If you can't tell by eye, then it's flat enough. Just my opinion.
 
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