Flattening wood

[adrian]

I'm flattening a 4' long board (and I have no powered jointer). I removed the rough saw marks and have found that the surface is concave by about 1/8". I'm wondering: what is the fastest, most efficient way to proceed to end up with a flat (or just slightly concave) board?

Does it make sense to work the end of the board on the diagonal with a heavy cut? I imagine that if I do this in the end I'll get two bumps just in from the area that I worked. If I get the ends low enough I could follow with stop shavings along the length. Is there some other better approach?

I thought about taking through shavings to try to make the surface move towards convexity, but I think through shavings cut in the central hollow, at least with the short plane I've been using.

 

[Wiley Horne]

Adrian,

Is the board cupped or bowed? Cupped = concave/convex in the width. Bowed = concave/convex in the length.

Also, is it possible the board has been stored so that one side has taken on more moisture than the other? For example, has the board been stored laying on its side.

Wiley

 

[adrian]

After I took delivery of this wood I stored it in a stack with 1 inch spacers between the boards for about 3 months, so I believe it should be in equilibrium with the environment (my basement, where the shop is). This wood is also quarter sawn, which I think means it should tend to keep its shape.

The defect I'm focusing on right now is bow. There is a 1/8" hollow over the four foot length. I think there is some cupping too, but much less than 1/8". (Board width is about 6".)

From past experience, I know that if I start taking shavings eventually I'll manage, somehow, to arrive at a flatish board. (The first time I tried this I wanted the final thickness to be 1/2" and I started with wood that was 1" thick. And when I finally got a flat surface on the first side, the board was thinner than 1/2" in places. Oops. It took a long time, too.) I've gotten better, but I don't feel like I really have a clear, systematic approach, where I feel like everything I do gets me closer to the goal of a flat board.

 

[Wiley Horne]

Adrian,

This falls into the general category of 4-squaring a board, and I'm sure there are excellent videos to be referenced. And probably Marc has a great photo sequence. However, I'll tell you my version, pointing out where things go wrong in my experience......Caution: opinions follow.....

PRELIMINARIES

1. Twist? Check whether the board has twist, as well as bow. If it does, set it aside and plan on cutting it down into smaller pieces. Spend no further time on it.

2. Thick enough? Likewise, if it is your estimate that the board will ju-u-u-st barely meet your thickness requirement by the time you get it dressed, then set it aside, and figure on cutting it in two for smaller parts. Reason: If you think 1/8" is what is needed to straighten it, it'll turn out to be 5/32" sure as little green apples. Nothing worse than over-thin rails or stiles.

3. Bench edge flat? The edge of your bench (where you'll be doing this flattening) has to be as straight as you want this board to be.

WHICH SIDE TO START ON?

4. I like to start on the convex side, because sometimes removing wood at the 'hinge' will cause the board to straighten a bit on its own. But it's easier to start with the concave side up, because it's easier to determine when you have it flat. So to simplify, let's say you start with the concave side up.

SHIM THE BOARD

5. Catch the board _lightly_ between the dogs, and shim the ends so the board will hold still under planing pressure.

CAMBERED BLADE PLANE

6. When there's 1/8" of wood to remove, doing it 1 thou at a pass will have you thinking 'machine jointer' after about 10 passes with no visible result. Start with a jack or fore plane--I like fore planes a lot--with cambered blade so that you're taking off 10-15 thousandths per pass. Go with the grain, so as not to rip off too much (against the grain with a roughing plane is great if you're really trying to do some demolition, but that's not the case here).

7. Work the two high ends separately, being careful to take the same number of passes across the width. Gradually length the cuts as you remove more wood. If there is cup on this side (usually is some), make a series of passes straight across the width all the way down, then 'smooth' the result with lengthwise passes along either side. Then return to flattening, but take no roughing plane passes in the middle of the board--the low place in the middle of the board is the datum that you're going to take the whole face down to, so you don't want to remove material from the low place, other than what is required to correct cup.

SWITCH TO TRY PLANE

8. When you start getting into range, switch to a longer try plane, and make end to end passes to see how close you are to making an end-to-end shaving. Once you can make an end to end shaving, you're done. If the low place in the middle is still, say, 6" wide, then make another set with the cambered blade plane.

CONTROL ACROSS THE BOARD

9. On the first side you do, you can control how much material you take across the width, simply by equalizing your passes across the width. Once you're finished with the first side, you will scribe the thickness around all the board edges, and plane to the scribe line, and that is your true control over thickness in the width.

TURN THE BOARD OVER

10. Having finished one side, and scribing the thickness line around the board, put the convex side up. Take the jack and/or fore plane, and work from the inside in. The board is higher in the middle. Watch your progress against the scribe line around the edge, and lengthen your passes in order to equalize your progress toward the scribe line. When you start getting nervous about the cambered blade removing too much material, switch to the try plane.

The width....This side will probably have 'reverse cup'--it'll be high in the middle across the width. As you begin to get in range on the lengthwise flattening, make a set of passes directly across the width from end to end, removing the reverse cup. Then switch back to lengthwise flattening, to remove the widthwise planing marks and complete the flattening.

As you approach the scribe mark, you decide whether you're going to wait another day, to see if the board wants to move more, or whether you're going to proceed to joinery that day. If it's a rail or stile, I'd wait another day, and leave a bit of the scribe there as a witness. But if its' going into a glued-up panel, I'd plane to the scribe line and proceed right away with the glue-up, and not give the board a second chance to move. Let the glue hold it straight.

Wiley

 

[adrian]

I decided early on that it was easier to shim a the timber with the convex side down than it was to shim it with the concave side down. So I've been in the habit of starting with the concave side up. Now the method Charlesworth presents with stop shavings would seem to be better suited to starting with a convex side. One could just take stop shavings until the plane doesn't cut. I suppose there'd be the question of when to switch to a finer depth of cut.

In spite of what people say, I've managed to flatten boards to within .004" (the tolerance I have generally aimed for) even though my bench is much less flat than that. Particularly if the bottom side of the timber isn't flat anyway, it's not clear to me how flatness of the bench can make much difference. (My bench is a built in plywood structure. Maybe someday I'll rip it out and replace it with a proper bench, but not now.)

So it sounds like your recommendation is to take heavy cuts in the direction of the grain at the ends only, gradually lengthening the cuts. I'm suspect that if I do that I'll end up with bumps at the ends, and maybe a risk that the ends go lower than the middle. In the past I've found it very easy to take too much off the ends.

 

[tnimble]

I tend to find starting with the convex side up easuer and more effective. Shimming is rather easy, first shim at the higher gap, then repeat for the lower points. Then determine the location(s) of the higest spot(s) (this includes cheking for wind and include this as a high spot) and plane them down. When having that under control, take the longest plane you've got and start making stop shavings. When the plane stops or you have a consistent shaving along the entire board length and have a slight hollow take 2 couple of through shavings.

Then flip the board over, there should be only shimming needed of about a sheet of paper in the middle. First attack the ends and any very high spots, again take the longest plane and make the stop shavings and finish with a couple of through shavings. You should set for a finer shaving as soon as you've got a consistant shaving and are near desied board thickness.

 

[adrian]

It seems like shimming with the convex side up should be easier, but I vaguely recall that the shims would rapidly work loose whereas it seemed much more solid and stable if I shimmed the concave side up. I'll have to revisit the shimming.

 

[Chris Knight]

In spite of what people say, I've managed to flatten boards to within .004" (the tolerance I have generally aimed for) even though my bench is much less flat than that. Particularly if the bottom side of the timber isn't flat anyway, it's not clear to me how flatness of the bench can make much difference.

If you are prepared to shim everything all of the time, I guess you could manage with a very uneven bench, however, life is too short for that. Besides, I use my bench for assembly a lot and in that mode shimming is a total pain - much better to have flat bench, although I confess mine leaves much to be desired.

Also your tolerance of 4 thou might be fine for what you want - it certainly is for a lot of mine. However, there are also plenty of times when I'm aiming for smaller - say one to two thou. I find that even on a board as short as 18 inches that if the bench has a hollow in it bigger than my desired tolerance, a long plane won't cut the centre section of the board with a fine set blade because the board bends into the hollow. I keep some paper and a few thin cards handy to shim boards for this reason.

 

[adrian]

But shimming is unavoidable regardless of your bench when neither side of the wood is flat. As it happens, I have a thickness planer that I use for the second side so I have not yet been in the situation of trying to work a face whose other side was flat. (Well, ok, I did a 6"x6" piece and a set of 6"x2"---figured wood that the planer couldn't handle-- but the short length made bending less of a problem. I was able to get 1.5 thou shavings, but no smaller.)

I agree, a flat bench would be better. I could rip out my existing bench and construct something better. But at least at the moment I want to get on with the project at hand. Because it's built in the existing bench is absolutely rock solid. I could see building a new bench easily taking several months time for me. There's always that question about whether to try to proceed with the tools at hand or whether to acquire new tools for a given task. And while it's temping to constantly get new tools at a certain point I think I have to just make do with what I've got and try to make something that will have a life outside the shop.

Unless you're going to glue the surfaces together I can't think of a reason why 0.004" wouldn't be adequate. When would you need a finer tolerance than that?

 

[Chris Knight]

Adrian,

You are quite right, without a plane surface, shimming will usually be needed even on a dead flat bench, although if you put concave side down, you can often get way without it. However, I guess I was thinking of what comes next.

I am guilty of it - but talking of tolerances without saying what's being done or how the tolerances are measured probably only confuses. What I am getting at is that as soon as more than one piece is involved, we need to think of a tolerance stack-up rather than individual tolerances and in a built-up structure for example, a 4 thou error on individual parts can quickly become a very much larger error that might well be objectionable or even catastrophic.

Imagine for example a leg of a simple table, 1 inch square in cross section where a 4 thou error is present in the length of one side of the square (section). A mortice cut square to that face will tilt the rail with its tenon in the mortice, away from its position for mating with the opposite leg such that if the rail is 12 inches long, the error at the far end will be 48 thou - relatively huge in my book.

I am sure this is not what you had in mind when speaking of a 4 thou tolerance in flatness but it is the sort of thing I had in mind when speaking of needing better accuracy. It applies even when jointing up a few boards to make a table top. If the errors turn out to be all additive you can end up with a badly cupped top even if individual board edges were cut square to the faces that were 4 thou out.

 

[adrian]

I was flattening faces of wood which I eventually glued up into a panel. I considered them flat enough when my straight edge would not admit the 4 thou feeler anywhere. I cut dovetails at the ends. The boards were 2.5-3 feet long and 6-8" wide, so a 4 thou error in that length isn't going to change the angle much. A 4 thou error in one inch is much bigger.

Outside of joinery I don't think a 4 thou error should be a problem. I was unable to use hand planes to prepare the edges to glue the board up because I couldn't achieve a straight enough edge. I ended up doing that with a powered router and a plywood template. I also joined the board using cauls to hold the panel flat.

 

[tnimble]

It seems like shimming with the convex side up should be easier, but I vaguely recall that the shims would rapidly work loose whereas it seemed much more solid and stable if I shimmed the concave side up. I'll have to revisit the shimming.

As shims I use some ordinary writing paper, thick heavy paper, coure sandpaper, some pieces of laminate ad hardboard. Never had any of those slip or loosen. I find it particular easy and fast to shim with.

 

[adrian]

I cut a pile of wedges to use for shims. It seems like having constant thickness shims would make it difficult to get it just right. Do you put in some hardboard and then several sheets of paper, say.

 

[MarcW]

Adrian,

This falls into the general category of 4-squaring a board, and I'm sure there are excellent videos to be referenced. And probably Marc has a great photo sequence...
Wiley

Thank you Wiley for remembering

In the past I fought the idea, what David would say on this one as I maybe distorted some aspects of his technique...

 

[tnimble]

I cut a pile of wedges to use for shims. It seems like having constant thickness shims would make it difficult to get it just right. Do you put in some hardboard and then several sheets of paper, say.

Depending on the gap. Could be one piece of hardboard plus 3 pieces of heavy paper like business card. Or just place a stack a around 10 to 20 along side and push it under, the pieces that are too thick will just remain along side.

 

[David C]

Marc's photo sequence seems VG.

As Wiley says, when machining boards, removing the bump from the convex side will often cause some relaxation and straightening, this has saved crucial thickness many times for me.

When working by hand, I like to work locally on the gross errors, on both sides alternately, constantly flipping the board, as it is not possible to get a flat surface untill the opposite side is firmly supported on the bench.

Use of shims or wedges, helps a lot, in the early stages, and in later stages strips of card or even paper.

David C