How straight is straight enough???

[beagles]

Another post asks about jigs to cut straight edges on long boards.

That got me thinking because I have been jointing boards to make a dresser top and the sides of drawers. You take a board off the jointer and place its edge on the TS surface and sometimes there is a big gap or it rocks. Sometimes there is practically no gap or it is dead flat (at least in reference to the flatness of the TS top).

As I'm doing this, I wonder "How flat is flat enough???" Those who tell you to make a sprung joint say leave a 64th (16 mils).

A related question: When people tell you to measure the diagonals of a piece to determine squareness, they always says "equal" rather than practically or essentially equal.

How equal is equal enough???

In the diagonal measurement, I'm happy with being within a 32nd.

Your thoughts?

[Beamer]

Good questions, Jim.

Ultimately, it comes down to your goals and what you feel is an acceptable tolerance.

A buddy of mine always says "if you shoot for perfection, you'll get a lot closer than if you aim lower" so that's something that goes through my head every time I consider whether something is flat or equal or precise enough.

I don't have super high precision standards, but I do shoot for them. I set out trying to get that perfect joint. If I don't hit that mark, I determine how 'off' it is and then decide if that's worth the effort it'd take to redo it.

For example, I have a teeny gap where an apron meets a leg on my coffee table. When I found it, it was a bit over 1/64. It was in a place that would bother me enough so I went after it with a chisel. Now it's not perfect, but it's much better.

Same table, I was trimming an arch to a pattern with my router and the router tipped. Dug a little divot there. Curses! Well you can't tell from the face side - so I decided as long as nobody goes feeling the arch, they'd never notice. I decided the work involved in getting a good grain match was more important to a nearly invisible defect. So it stays....

My standards are flexible - i aim for high precision, but i'm not bent out of shape if I don't hit it. Doesn't stop me from shooting for it, though. I wanna get as close as i can!

[mduren]

Second Beamers comments

[Stretch]

It all depends on scale of a project. What may be an insignificant gap on a large cabinet will be a huge gap on a jewelry box. It also depends on what part of the piece you're talking about. When edge jointing 8 foot long boards, you might be able to get away with as much as a 1/16" gap in the middle. If you can close that gap with clamp pressure, you'll most likely be okay. Now if you line up four boards for a table top and there are gaps at each joint, you'll have more problems because you won't be able to close those gaps with clamp pressure. For shorter edge joints, your tolerances get tighter. There is no reason for there to be any gap on an 18" edge joint. The same goes on diagonals. On a big cabinet and 1/8" difference on the diagonal measurement is no big thing especially if you don't have a clamp long enough to pull it square at glueup. On a drawer or small box, I want the measurements to be as accurate as I can read on a normal tape measure which is about 1/32".

[Leo Voisine]

Jim,

Lets put it this way - you have a better chance at catching a moonbeam and putting it in a jar, than you have at making anything straight, square, flat, round or parallel, out any material on earth. It simply cannot be done.

I work in a company that makes gages that measure all of those characteristics - to within millionths in an inch. We can't do it either.

All things have a tolerance. Cut a piece of wood 1" long please. Yeah Sure -- It just cannot happen. It's impossible. So please cut me a piece of wood between 31/32 and 1-1/32 long. OK, yeah I can do that!!

Make a board flat, straight --- NO WAY, I cannot do that with the most sophisticated machine in the world. So - make it flat within 1/32 so I can glue two boards together - Yeah I can do that.

Tolerance - to everything there is a tolerance. Even the standards - that are kept in environmentally controlled rooms are made to a tolerance.

Tolerance -- it what you can tolerate. REALLY - what can you live with? What can you tolerate?

to glue up two boards - a little space will be EASILY clamped together. THAT, can be tolerated. To force those boards with clamps should not be tolerated.

What is the number? Nobody knows that - except the designer. When you make something - you - are the designer. Sooo - what can you tolerate? THAT - is what your tolerance will be.

Generally - 1/32 is a good tolerance to shoot for.

However, sometimes "matching" cuts is a more exact way to do it. Cut the length of both boards in ONE cut - the two boards will be as exact as you can get - tolerance is not an issue in that.

[Joe Lyddon]

I think it's more important to get pieces all uniform in thickness for a given project...

ie: All face frame parts must be of the same thickness; all planed at the same setting, etc.

Whatever That thickness is, it's not so critical... to be exactly any given thickness in not so critical.

Uniformity, where it counts, is the critical (nice) thing.

Yes?? ;D

[TDHofstetter]

To quote a certain ol' hippie buffoon of the last century...

"All things are relative".



Get it as perfect as you reasonably can, within the limits of your equipment, and within the quota of attention you are prepared to expend for it. Within an inch is usually perfectly acceptable to the common man when you're dealing with timberframes, as long as the joinery works. Within ten thousandths (one hundredth) is just about makin' it when you're talking about machine setup for woodworking.

It boils down to willingness. How far are you willing to go to perfect what you're doing? Your work's acceptability in your OWN mind is all-important. Other folks will like anything from hatchet-carved to fine-tuned, for different reasons. Witness: Both grades of work (and anything between) SELL TO PEOPLE.

[Gecko]

Doing boxes, I shoot for perfection...and miss every time. Please never check to see if one of my boxes is square. It won't be. It will be close but not there. The thickness of a piece of wood for one is aimed for but most time is somewhere close. It will change do to how much sanding is needed at the final stage as well. Lengths start out within a 32nd but sometimes issues cause them t be shorter. Heights vary from the plan due to how much sanding is needed to flatten the bottom so they do not rock. As long as pieces that go together are the same height and opposite sides are as close to the same length as I can get, all is good. I aim for perfection but am happy if the joints are tight on the outside and the end product looks good to the eye and feels good in the hand.

[elizabeth]

Dido. How flat and square depends on the project. Slapping a workbench together with scraps only needs mostly flat, mostly square, and mostly sized wood, and plenty of screws. The tolerances for a puzzle box, where every piece moves just so, are much greater.

I'm with Joe. I think one of the most important parts in milling is to have boards that are to be jointed to be equally thick. This came home to roost when making a laminated blank to use on a project and is equally important with dovetails.

[JBark]

Well, I agree with some of what has been said here and disagree with some...as well as how it was said. Beagle, one thing I would address if I were you is why you are getting inconsistant jointing of edges. If the jointer is not set up correctly fix that. If it is your technique work on that. Grab a board and bandsaw the edge so you know it is "rough" and joint it straight. Do it over and over again until you always get it straight. By the way, don't assume that TS table is flat...it might be off.

I've always felt that straight and square were goals that had flexibility depending on where they are being used. Example; if you are edge jointing two boards to be glued together you need not set your fence at 90 degrees you only need to mark your boards in such a way as complimentary angles meet. In other words joint one face to 91 degrees and one to 89 degrees. If you mark the faces appropriately you will mate these correctly by flipping one board. Along the same lines if you are joint the front edge of a drawer blade does it need to be absolutely 90 degrees? If you are jointing a table top how unflat can be observed?

I try for getting edge glued boards as straight as possible, mainly because I don't know how much gap is acceptible. I also try for very, very close when measuring diagonals but there are many cases where overkill is not necessary. Sides not square in a chest of drawers amounts to a little bit of tweaking of drawers...fitting, as they say. I'd say that 1/16" off in a chest of drawers is a lot easier to tweak than 1/16" in a small box, and less noticeable to the eye.

John