How to test a board's flatness?

[bthere]

As others are pointing out, I think the problem boils down to the use of a 12" straight edge on a 48" long piece of wood. Assuming the bow in the board is even along the lenght, you will not be able to see more than 1/4 of the that bow. If you put two boards with machined faces together and see a gap of about .002", then make the assumption that each board contributes half of that gap, .001". By the time you measure a quarter of that gap with your straight edge, you are down to a mighty hard to detect .00025" gap between the center of your straight edge and the board you are measuring.

[LarryG]

One thing I don't see mentioned anywhere ... just how big IS the gap between the two face-to-face boards? Remember, it's wood; perfection ain't gonna happen. You could mill two boards absolutely perfectly four-square and lay them down and come back an hour later and they'll no longer be perfect.

[Anna]

Okay, I just checked. The gap in the middle is about 1/16", making each board off by about 1/32" in the center. I'll try to run it through the jointer again without pressing on the middle too much. I hope to at least get the face frames done, then I'll start worrying about four-squares and such again later.

I'll also order some new straight edges and levels from Amazon. When all else fails, at least I'll have an excuse to get a new jointer.

Thanks again!

[LCHIEN]

I have a 7' long level from Home Depot (the yellow one that costs more than $60) and I also tested with that. The gap is there, in the center of the board. But I thought my level might not be really level - I've had it for a year and it's been knocked around some. Guess will pick up a new one of those, too.

JR, I think you're right. I've put off finishing this project for two weeks because I've been distracted about getting the faces flat. Might as well get this one over it. Thanks.

I don't think normal "knocking around" will unstraighten a level.
Usually they are made of aluminum or composites in an I-beam shape which gives it strength without a lot of bulk or weight. and they're durable.

We would assume that the two sides of the level are parallel. This is easy to check with calipers... like the $16 harbor freight digital ones.

Once parallel, then its pretty easy to check the level for straightness even with a bowed board. If the board is slightly bowed, place it so the low point in in the middle.

The level has two edges A&B. place A against the board, measure the gap with your feelers at a couple of places along the length.

Now flip your level so B is against the same places that A was. Again, measure the same places with the feeler gages. Subtract the gap

If the gap is the same then the level is flat. If the gap changes in one direction, so that lets say its 5 mils higher in the middle and 2 mils higher between the ends and the middle, then the level has a bow of about 2-1/2 mils over its length (half the difference in gap in the middle).

If everything is the same, then the level is pretty straight.

Here's another way to test:
take a relatively flat piece of plywood at least as long as the level.
Lay the level so its on its side and you are using the bottom as a straightedge, draw a line on the plywood the length of the level.
Now flip the level over so its got the opposite edge against the line just drawn. Align it so the level is at the line at the ends. Draw another line using this edge. If the two lines drawn are conicident the level is straight. If the lines diverge in the middle, then the level are not straight by half the gap between the lines (which you should be able to measure if big enough.) It's very important to keep the pencil near vertical with the actual lead against the edge and maintain this angle the whole length.

I don't think I'd worry about 5 mils or less over 6', probably 10 mils is OK. once you get over 16 mils you're at 1/64th inch and now its a judgement call. Bear in mind that a 0.5mm mechanical pencil lead is .019" (19 mils) thick.

[Jeffrey Schronce]

Is your jointer properly set up? Do you experience this with other boards?
Sounds like the beds are not coplaner.

[Anna]

Loring, thanks for the tip. I'll check out all my levels and straight edges that way.

Jeffrey, yeah, I think there's something wrong with my jointer. I guess I have to try adjusting the infeed table again. I hate this benchtop jointer. I wish I bought the Craftsman pro when I had the chance.

[Jeffrey Schronce]

Loring, thanks for the tip. I'll check out all my levels and straight edges that way.

Jeffrey, yeah, I think there's something wrong with my jointer. I guess I have to try adjusting the infeed table again. I hate this benchtop jointer. I wish I bought the Craftsman pro when I had the chance.

Ya, I would imagine 4' board on benchtop jointer would not be a pleasant experience.

[LCHIEN]

Loring, thanks for the tip. I'll check out all my levels and straight edges that way.

Jeffrey, yeah, I think there's something wrong with my jointer. I guess I have to try adjusting the infeed table again. I hate this benchtop jointer. I wish I bought the Craftsman pro when I had the chance.

You're welcome.

Adjusting the infeed table should only adjust the depth of cut. (assuming they're co-planar)
Adjusting the blade to the outfeed table, that's where the trick pony lies. That affects the bow or curve and snipe.

[Tom Miller]

Anna,

When you have the two boards together, like so (), how much pressure does it take to push the boards together (flat)? If it doesn't take much, then I think you're fighting a losing battle trying to joint out this minimal amount of bow. The reason is because as you push the board over the jointer, you'll invariably be pushing it flat as you go, and the jointer won't "see" the bow you're trying to correct.

Another way of looking at it is that, if there's not much internal force behind this bowing, you'll be able to machine the wood properly, and make your joints correctly, such that the piece will easily straighten out when you assemble and glue/fasten it in place.

Regards,
Tom

[eddy merckx]

Hey Anna

How about making some extension wings for that jointer. I did that to my benchtop jointer last year. It makes all the difference. There is a thorough article on how to do it in Shopnotes 48. Total cost is around $30 and maybe 2 hours shop time.

Eddy

[Anna]

Eddy, I already added the extension to the infeed table using the same Shopnotes plan. I thought that will solve the problem I have, but it didn't. I've also done the adjustments to get the tables to be coplanar by adjusting the screws under the bed. Guess that didn't work either.

Tom, I just checked and it doesn't take much to push the boards together. They are kinda springy. I run the boards through the jointer by pushing on it on the outfeed side, instead of where the blades are. I really wanted a nice flat surface. I took woodworking classes at Woodcraft and I didn't have any problems with the jointer at all, so I don't think it's my technique. But then, I've been wrong before.

[eddy merckx]

I think you deserve a new jointer!

[jspelbring]

Jointer

Anna,

I feel your pain. I've been locked in a battle-to-the-death with my POS HF jointer for the past month or so. So far, I've replaced the blades, and set/tuned it several times. At the moment, I'm thinking seriously about getting this one from Grizzly.

[SARGE..g-47]

Good morning Anna...

I've been watching this thread with a bit of interest to see when someone nails it shut. And I believe someone did! You have been given some excellent advice.. using levels (yours is probably fine as stated by Loring).. checking from all angles as opposed to one point across.. etc., etc.

Go back and read Tom MIller's post and you have your answer IMO. I have a 75" jointer bed and I can assure you it is flat. I can run 8' stock through it and it will be trimmed flat as it goes across the flat jointer bed. But.. sometimes it would not appear to be flat if you face two idenically jointed pieces together that went across the same jointer. .... ???? ....

Wood can contain compression. Some species more than others and that can vary from one piece of stock to another in the same species depending on how the tree grew. Sun.. wind.. slope.. crowding.. etc.

No matter how flat you keep the stock on your jointer, if it has compression it will move as soon as comes off the table. Flat sawn will be more likely to bow than qaurter-sawn and how many of us can get our hands on that much quarter-sawn these days as the sawyer is going to get more board feet from flat-sawn and money motivates the market.

So... we as Wood-workers are not dealing with metal that can be machined to .001 tolerances as that might really matter depending on the application. We are dealing with wood and wood "is" going to move. Do you suppose our fore-fathers that built some excellent period pieces that appear in museums held up a project two weeks with worry over concerns that the wood was not perfectly flat after hand-planing? They did not have jointers or highly sophisticated measuring devices and they still got the job done remarkably. They just had a pair of winding sticks and that proved to be good enough in end results.

So... my suggestion would be to heed the facts passed down for 5000 years from craftsman to craftsman and face the reality that wood is not a perfect species to work with. The solution is simple... Your faces are going to flatten when you join them properly to the carcass as you stated yourself, "they are springy".

So... don't overly depend on the high tech and advanced tools we have today in an attempt to get high tech, advanced results. Turn yourself into a wood mechanic that is aware of what he or she is working with and knows how to fix the problems that you face in reality when working with wood.

That is the difference between a "craftsman" and one that would like to be as I see it. Many may not agree as we are all entitled to our opionions. You just got mine... by all means form your own based on what you experience!

Regards...

[Anna]

Do you suppose our fore-fathers that built some excellent period pieces that appear in museums held up a project two weeks with worry over concerns that the wood was not perfectly flat after hand-planing? They did not have jointers or highly sophisticated measuring devices and they still got the job done remarkably. They just had a pair of winding sticks and that proved to be good enough in end results.

Sarge, that gave me a LOL moment and put things in perspective at the same time, thank you.

I am very spoiled by my other stuff, and I also have a bad tendency to get distracted easily by little things when caught in the right mood. I get this itch, sometimes, to do something about it, and can't/won't stop until it's done. I've caught myself a couple times and actually had to tell myself to shift gears. A recent example was a part of the front lawn that was really bothering me, so I got out the weedeater, and before I knew it, I've done all of the edging for the front lawn, the backyard, the side yard... until I ran out of the little plastic thread things the fourth time, and I can't find any to replace it anymore. Same thing happened when I was powerwashing the driveway. I ended up powerwashing the sidewalks, the fence, the deck, anything else that wasn't moving in the backyard. I even offered to powerwash my friend's deck! My husband claims I'm compulsive.

As for the coplanar problem, I was thinking about it last night. If the tables are not coplanar, and I'm pushing on the outfeed side, I think I should be seeing a snipe on the front end more than a dip in the middle. So maybe the tables themselves are not the problem. I've also extended the tables so that I now have a total of 48 inches, which is about as long as the tables for the full-sized jointers.

I'll go ahead and mill the rest of the maple stock I have and finish my project. The jointer has been a pain in the derriere the last few months I've been using it. And it is embarrassing putting off finishing the cabinets for two weeks because of it. I have the carcases all ready to go and they just have to be installed now. I thought I should work on the face frame before dragging everything inside the house, and that's where I got stuck (figuratively). I'll get back onto it right away.

Thanks again. I really feel much better now, too.