Wood breathes, and expands and contracts with changes in moisture. Precision depends upon the amount of compensation needed to adjust to these changes, whcih depends on the size and length of each piece of wood involved. And how well the joint will hold, and the quality of construction.
Doll house miniatures versus timber framing has completely different requirements for precision/accuracy.
If you start out with multiple boards of the same moisture content, then they're all pretty close to the same thickness the next morning, regardless of the actual dimensions. That's usually all that matters.
An accurate digital readout on the planer is really nice if you need to go back and make a new board to replace one that you've wrecked. Measure the other boards, find another board with the same moisture content, and plane the new one to the thickness of the others.
Chris
In general, woodworking isn't dependent upon absolute measurements so much as relative measurements. When jointing boards, 1/32 would be unacceptable since one wants the joints "light-tight". On the other hand, 1/32 is way ridiculous in overall project dimension tolerance. For joints, the absolute dimensions don't make near as much difference as the dimension of the joint components (mortise to tenon,etc.) relative to one another.
The nature of the material seems to drive this -- wood moves and changes with humidity. It's better to work to relative fits than absolute dimensions.
I prefer to use a hand held, digital caliper.
Measure the board you ant to duplicate, then run new board thru planer in 1/16 increments, measuring as you go.
Final passes are 1/64.
You end up dead on for at least an hour
Lew
I certainly don't agree with that. There's been many projects I've built where 1/16" is simply too great a length to properly assemble my project. When necessary, (not always such as my recent picnic table), I've calibrated and cut my parts on the tablesaw to 1/64" of an inch.
My ex left her 'just for women' toolkit. When neighbors ask to borrow tools, I just hand them the nicely organized pastel case full of poorly made tools. They seldom ask twice.
Tolerances are most important in the area of joints. I think as long as you hammer the nails to below the wood surface, that's fine. Paint can fill small hammer dings, bondo fills the bigger ones, but they won't hide protruding nail heads very well. Silicone caulk smooths out the joints where the board ends don't match up well or aren't square.
Damn... why didn't I think of that! My wife has one of those that someone gave her, and since she uses real tools, she's never opened the pretty pink case..
mac
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Wow.. I admire your precision... This also reminds me why I'm a turner...
Ok, that one's round, what's the next project? lol
mac
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Leaving the "my tolerances are smaller than yours" brigade to carry on measuring each other! in practice, the tolerances are about the width of a pencil line. You can sharpen (or not) your point if you really need better than 1/2 mm or so. Most people just tighten the clamps a bit more if the tolerances don't work in their favour.
Just my 2p worth.
Metal and wood are very different. Wood changes shape so tolerances don't make sense. Understanding wood movement is crucial when making larger projects if you want good results.
One of the beauties of specializing in Shaker style pieces is that being off a little can be chalked up to aging and all those boogers in the finish lend character to the distressed look.
I like it when a new piece looks 60 years old.
People always say I must be rally skilled to get that look. They don't know it is due to a limited skillset.
Tolerances usually are included when drawings are submitted in an industrial environment. You won't see tolerances in any hobbyist magazines or books. The topic is "beyond their scope". But, if a cabinet shop gets a set of drawings from an architect, there is generally a definition of tolerances. Sometimes it is implied and sometimes it is explicit. In general, tolerances are as follows unless otherwise specified:
- Dimensions in fractions: +/- 1/64"
- Dimensions with one decimal place (X.X) +/- 0.10"
- Dimensions with two decimal places (X.XX) +/- 0.01"
- Dimensions with three decimal places (X.XXX) +/- 0.005"
- Dimensions with four decimal places (X.XXXX) +/- 0.003"
- Angles +/-0.5 degrees
You'll find drawings with tolerances in furniture factories (even for the cheapie knock-down furniture). They will even have inspection stations with SPC data collection and control charts. Just about any environment where interchangeable parts are being made you'll see drawings with tolerances. Ask Henry Ford why it doesn't pay to run a factory full of "artisans" who insist on custom fitting everything.
If you can't read prints and build something to spec (with tolerances like these), then you're probably better off working with designers and consumers. Drawings from designers rarely include dimensions, let alone tolerances. It's usually up to the craftsman to engineer the solution from a sketch. And consumers want you to come up with the drawings based on an idea in their head or pictures from magazines, etc. In these cases you're on your own to figure out how much accuracy is needed to produce the desired results.
The hobbyist making something from plans out of a magazine or book usually gets dimensions but is pretty much on their own for everything else. You learn by experience what needs to be accurate and what doesn't.
Unique skills are required for success with all of these disciplines. But, I wouldn't want to confuse "trial and error" with any of these skills. It's better classified as a coping mechanism used by those who need to develop some skills. It usually comes from having no idea what accuracy is needed or how to produce it. You just try something and see if it works. If it doesn't, you try again. You keep trying until you get it right or you get fed up with getting it wrong so often.
Ed Bennett
Interesting.
Just for comparison, the standard (implied) tolerances on engineering drawings at the aerospace firms I was associated with throughout my career was X.XX +/- .03, and X.XXX +/- .010. Any tolerances other than those, looser or tighter, had to be explicitly specified in the dimension callout. Often those non-standard tolerances, and virtually all 4 place dimensions, had to be justified to the group leader or project engineer and, unless the need was obvious, would frequently be questioned by the manufacturing engineers or planners.
Tom Veatch Wichita, KS USA
Hmmm... sounds like my marketing line when I tell them that each turning is unique.. lol
mac
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Hi Tom,
Different industries, different customers, different vendors, different tolerances. In some areas a particular company dominates the market and their standards tend to prevail. For example, here in Boise it's Micron. Just about every machine shop in town does something for Micron. I'm sure in Seattle it's Boeing. I'd bet that Detroit is dominated by automotive needs. And, some companies have a formal review process (sounds like your experience) while others let engineers run open loop. It's always uncomfortable questioning tolerances from one of these open loop guys. I've met a few who have never built anything in their entire life.
Ed Bennett snipped-for-privacy@tablesawalignment.com
Thanks to all who replied. I get the picture. I reverse engineer (metal)components for a no longer-produced all terrain vehicle for my son. I have to create drawings for myself when I make the parts and for others when he farms things out (higher volume than I want to deal with). Particularly for the outside stuff, I am the only guy alive (probably) who knows what fits with what, so I have to figure and apply tolerances as needed. The game I play with myself is to make the drawings good enough that the vendors are happy, my son's costs are minimized and the parts NEVER fail to fit and function.
Pete Stanaitis
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