Accuracy Versus Precision

“In theory, there is no difference between theory and practice. In practice, there is.”
-Yogi Berra
 
I was going through another back issue of Fine Woodworking today, (FWW #25, November/ December 1980) and came across an article titled “On Precision in Joinery,” by Allan J. Boardman. Boardman didn’t specifically mention it, but there is a difference in some fields between accuracy and precision. It’s possible to be accurate without being precise, and it’s possible to be precise, without being accurate. And I think this is a discussion that is relevant to woodworking, too.

In our context, precision is defined in terms of exactness and fit. When joinery is cut precisely, it fits without strength-sapping gaps. Shoulders close up on mortise and tenon joints. Miters come together all the way around. Dovetails fit perfectly. And the assembly comes together, without needing an I-beam clamp’s tectonic level of force to close everything up. (I love I-beam clamps. But they are a bit excessive.) This doesn’t imply that the entire assembly is made without error. Precise work involves making adjustments that take those errors into account, but still allows everything to come together well. Some dimensions may be slightly off, or the wood may have a slight spring in it. But joints that have been precisely fit will still be solid, and the finished product will be strong.

Accuracy is defined in terms of tolerances and deviations. “Accurate to a tolerance of +/- .00004 smidges,” and so on. It’s not about hitting the mark right-on. It’s about how close to the mark you can reliably get, within a margin of error. And that error is the difference between the two words.

The article explained pretty clearly the same philosophy that we learned at North Bennet: Use machines to do bulk waste removal, and hand tools to precisely fit things together. While the method wasn’t specified as such in the article, It’s what’s currently known as hybrid woodworking: Bulk work with machines, fine work with hand tools.

Hand tool purists may not like the dust and mess and noise of power jointers and planers and table saws. But they still work initially to the tolerances allowed by broad axes, froes, scrub planes, and cross-cut saws, when they’re doing the rough dimensioning.

I think that in a hybrid work environment, it’s fair game to use whatever method you want for bulk waste removal, including combinations of hand and power tool methods. I’ve had gnarly figured planks that I didn’t want to run across a jointer, for fear of massive tearout. So I’ll flatten one face with a jack plane that has a cambered iron, accurate to a margin of error that’s defined by the scoops I took out with the plane. The high points will all be flat enough, and with my roughly jointed face down, it goes through the planer just fine. It’s accurate work, done to coarse tolerances. And the thickness planer doesn’t know or care. Once the other side has been surfaced, the board gets flipped, and my plane strokes will be removed during the next pass.

But however you hog off bulk waste, both camps can agree, I think, that there’s a time to move from accurately used rough work, to doing precise work with the final fitting tools. There’s a convergence in method here, and however you choose to work, the transition between accurate and precise methods is where the magic happens. It’s worth paying attention to, because so much time can be lost.

Precision should take over when the limits of accuracy have been reached... but not before. If you have a fetish for hand planes, and you find yourself taking off an eighth of an inch, 2 thousandths of an inch at a time, you’re going to be at it for a while, and the odds that human error will be introduced will increase. That’s a lot of wasted time, with the potential for wasted time and effort thanks to a part that's ruined after chasing a final dimension around for too long with a hand plane.

In a similar vein, accuracy should be taken to a reasonable degree, but not past that. While high levels of accuracy are very helpful, it’s not always appropriate to try to substitute high levels of accuracy for precision. Margins for error are measured in reference to an ideal, and not necessarily to the conditions present in the project at hand. Highly accurate jigs and fixtures will not necessarily bridge the gap from being accurate tools to becoming precision instruments, because adjusting will still be needed on the back end to compensate for errors in the project.

Some woodworkers will try to reduce the margin for error to zero on their jigs and tools, in ways that aren’t always appropriate, especially when they’re more comfortable with refining their accuracy than they are with being precise. Shooting boards are a prime example. Some people build shooting boards to excessive levels of accuracy, spending hours with dial indicators and feeler gauges. They’ll insist that it’s as accurate as humanly possible, and that no adjustment should be necessary at all. But there’s a discrepancy there between what’s ideally called for, and what’s really needed. To precisely miter two pieces together, especially when you’re working at a large enough scale, it’s very likely that the final angles won’t be exactly 45 degrees. And if all the shooting board will cut is a perfect 45 degree angle, you may need to shim the fence a little bit here and there with a piece of tape, or veneer, to make precise adjustments to the fit of the joint.

The accuracy geeks think that this is heresy. They believe that their shooting boards shouldn’t need any adjustment at all. Some of them are actually insulted by the notion. Instead, they will insist that all materials must be planed and dimensioned so perfectly that no error will be introduced into their attempt to cut a geometrically perfect joint with their geometrically perfect tool. If there is a problem, they’ll say that it is clearly with the materials, or the project at hand.

I’ve also talked to finish carpenters here in Boston who are so used to working on old buildings that they leave their highly accurate sliding compound miter saws at home. After over 100 years, none of these buildings are straight, square, or plumb anymore. So a saw that cuts a perfect 90 or 45 degree angle really doesn’t help, because the walls aren’t square to each other, or to the ceiling, and the walls themselves may not be straight. The amount of error presented here is huge, and it may be due to the plaster, the framing, or even the foundations. So instead of bringing in a back-breaking sliding power miter saw, they’ll bring in an old hand-saw miter box. And they’ll make adjustments until the miter in the crown molding comes together precisely, in the context of the room that they’re finishing.

Good woodworking is the combined process of accurately making parts, (however you choose to rough them out) and precisely fitting them together. It’s important to develop your ability to be accurate, to minimize the amount of precision work that you have to do. And it’s important to develop your ability to be precise, because even high levels of accuracy will still encounter errors in the real world.

Second quote from the article:

“Precision can, on the other hand, become an obsession that goes beyond common sense to the point of inconsistency with the nature of the wood itself.”

Wood moves. It’s why we use the joinery that we do. It’s why it’s important to look for clear, straight grain, to let wood acclimate to the shop for a few weeks, and mill to final thickness in stages. Over the years, and from season to season, the wood will continue to expand and contract. The traditional joinery that we all know and love was designed to work with the material, and not against it. It’s not ideal, nor is it perfect. But it works.

And that’s a good thing. As woodworkers, we can’t worry too much about an ideal world. The most we can hope and strive for is to be precise in our work, because the best our material will ever be is accurate... give or take a margin of error.

Quick tip: planing stop for thin stuff

I was trying to find a decent way to hold <1/4" thick material while I planed off saw marks, and this is the solution I came up with.

I sharpened the head of a counter-sunk screw, (meaning I ground the top surface down until it intersected the sides at a sharp angle) and drilled a countersunk hole in the bench top, enough that it will be well below the surface when not in use. The sharpened edge bites in just enough, and holds the other end down well, even if there's the slightest bow in the stock. Thin stuff is notoriously unstable, and the ability to hold the other end down is the solution I've been looking for.

Shop and Project Hierarchy of Needs

One of the things I’ve been confronted with on many levels lately is that there are some things that are really fundamental before higher levels of function can be maintained. The basics need to be attended to before you can start to think about long range goals, that kind of thing. What I’ve found lately is that the needs for shops and projects are inter-related. And the basic needs seem to be solidly rooted in planning things out well.

I’ll give an example. There have been times when a project really grinds to a halt, because I’m doing several things at once, and I’m getting lost. I’ll get two-thirds of the way through working on Part B before I start to think about how it’s going to interact with Part A, and then I realize that I need to stop what I’m doing with Part B until the an unfinished detail on Part A is completed in some way. In general, it’s because I had a pretty good idea of how everything was supposed to work when I got started, but I didn’t get down to the final specifics in the plan or on the drawing, and I left Part A unfinished until I got to that point. The logic at the time is typically something like: "This is tedious, and I really need to get this project moving, so I’ll tweak it when I get to the point when I need to, so I can keep making progress."

And then I’d get there. And so I’d be forced to leave a mostly-completed Part B on the table saw while I went back to the bench to fix something about Part A, so I could tweak the details that would allow Part B to finally mate with Part A. Sometimes the list of incomplete sections was 3 or 4 things, and I’d be chasing my tail all over the shop, with all of them in play, but none of them in a state that resembled ‘done.’ And then the project starts to develop a blast radius as I look for unoccupied spaces to ‘park’ my half-finished sub-assemblies while I work on others.

Another variation of the need to organize a project shows up as I would be slowly but steadily moving through a process on a part. I’d be sure to get the details figured out before I got to the next piece, but only just before. So as a result, three things were happening. I was moving more slowly, because with half of my brain engaged in the planning process, I wasn’t fully focused on the part I was working on. And in the end, that part wouldn't turn out as well as I'd hoped. And once that was done, I'd move on to the next step, and realize that the plan I'd formulated while working on something else was flawed in some way. I think the phrase I used at the time was parallel processing. It slowed me down, and kept me from doing a good job of either planning or executing my projects.

This has been happening with the book I’ve been working on, too. There were a lot of bits, pieces, and chunks that I knew I wanted to fit in. And the outline had places for all of it. But some of it was left unresolved, and so I would write, and revise, and write, and revise... and got lost. I wasn’t really sure where the thing was going. I had to start all over again, with a clear, refined idea of what I wanted the whole thing to look like, what I really wanted to say, and how I wanted to get there. And I started filling in the details. And all of a sudden, it all started to come together.

What I’ve discovered over time is that paying attention to the details early on pays off in a lot of ways that I could have foreseen, and a lot of other, less obvious ways, too. Once I know what the details are, I can make a plan.

With a finished, detailed plan, I can put together a finished, detailed procedure to make things happen. This is logical, I know.

With a detailed, finished procedure to follow, I can move systematically through the work that I have to do. This is also logical.

What didn’t necessarily follow, for me, is that with that procedural, pre-planned work, I had a clear head. The part of my mind that was normally preoccupied with what came next, was free to look around, and see how my shop was working, or wasn’t.

I could see the trail of tools that were being left behind. You see, when I was in the habit of almost-finishing things, part of that unfinished procedure was putting everything away when I was done. The tools were left with the part that I was working on, since I knew I was going to need them as soon as I went back. I'd gotten into the bad habit of leaving my tools lying where I'd used them.

I could see the mess that I was making, and hadn’t normally been focused on. It wasn’t that I couldn’t see the mess before, it was just that I hadn't finished and gotten to the cleaning up part, yet. With discrete steps, and defined things to do, I had the head space to complete that task, put the tool away, and clean up the mess I’d just made, to get ready for the next step. And if anything in the shop layout wasn't really functioning well, like tool storage, I could make a note of it, and fix it later.

And then it grew. Once I knew where my stopping and starting points were for the day, I had the time to sit down, log everything I’d gotten done, what worked and didn’t, and think about how things were going overall. I could think about a higher level of perspective, other than a cascading pile of “Crap... this isn’t done... because that’s not done, because I haven’t figured that out yet... and I can’t even get to that right now because the first thing is in the way...” My head was clear to get the job done to a higher standard, and with that clear head, I was more aware of what was going on around me in the shop.

I had the head-space to look around and realize that maybe I should implement an end of the day on Friday clean-up. Sweep the entire shop, once a week... what a concept. Find all of the little things I forgot, the circles I didn’t close, and make sure they get closed. I can clean up some of those filthy table saw blades for once. Or put a coat of wax on the cast iron surfaces in the machine room.

Working according to plan has reaped a lot of unexpected benefits, not the least of which is relief that comes from working in a shop that no longer looks like Tornado James has been through it.

I’ve heard people say that most shops have a natural level of ‘patina.’ There’s dust and detritus about, and not everything is really 100%. I used to think that any shop that looked a little too clean was clearly the work of someone who was anal retentive or obsessive compulsive. What I’m starting to understand is that maybe it’s more a question of being organized about the work, not rushing, and not getting stuck in a reactive cycle.

A cleaner shop is a safer shop. Tools are easier to find when they’re put away. Projects are less likely to have dings and dents when they’re not literally in the path of progress. And I have a clear head.

All from finishing that stupid drawing, and writing up a detailed procedure.

Who knew?

Note to self

I've mentioned before how useful it is to take notes at the end of every shop session. I do it so that I'll be able to more clearly recapture the productive state of mind that I was in, and remember what I was working on when I come in the next day... Or after a few weeks.

It also helps me to remember exactly where to pick up what I'd left off. And, if there's anything that I need, that I don't have on hand, it gives me the chance to make a note to pick up whatever it is, before I come in the next day, so that productive time doesn't come to a screeching halt.

But lately, I've been keeping a notebook on the bench to grab thoughts as they come to me.

'Note to self, next time sand the panel before cutting to size, so you don't dub the edges and end up with a loose fitting panel.' That kind of thing. It helps me to hold on to the stray thoughts that really help me to refine my own process.

Because somehow, whenever I say to myself 'Next time, I really need to remember to...' I almost never listen.

Winding sticks... And straight edges

7 years ago I took a Japanese tool class at the Center for Furniture Craftsmanship, taught by John Reed Fox.

This is a simpler version of a project he recommended, combined with the winding sticks that Chris Schwartz recently made.

The big difference is in the dowels that reference the sticks to each other when they're in the vise, so that you can plane the edge of both at the same time, and know that the sticks will be mirror images of each other when you hold those edges together.

John used dovetail keys when he made his, so that they could slide and lock together. But I've been distracted enough by this project, and chose to use dowels as a shortcut.

When held edge to edge, any discrepancy from dead-on straight will be doubled, and you'll be able to see what adjustments need to be made to compensate. Then you can put the sticks back together, and carefully shave down the high spots until your edges are both perfectly straight.

After that, you'll be able to use these sticks to check for wind, as well as to check for cupping and warping.

In theory, you can make them as long as you want. It'll be more work to get them perfectly straight, and they'll be a huge cost savings over something that's precision machined, but these you can fix yourself if they get damaged. And, you can use them to check things like jointer knife projection, without needing to worry about nicking the knives.

It's an entertaining but small project, and a good exercise in precision planing. And the inlaid bits do seem a lot easier to read than a long, solid stripe.
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More on Wharton Esherick

I'm still digesting this book, but it's a gold mine. This was a nifty tid-bit, though, and I thought it was worth sharing:

"As acceptance and interest in contemporary crafts had grown, in 1956 the American Craftsmen's Council opened the Museum of Contemporary Crafts... In New York. [They] invited seventy-year-old Wharton as the senior advisor, the seer to whom all others could turn to for inspiration and advice.

At a session of woodworkers, one man spoke of the economic need to design for the mass-production market. Sam Maloof, a forty-one year old California furniture maker, responded that he found emotional reward in working with the customer and having a "hands-on" experience. Afterward, Wharton approached him and said, "Young man, I want to speak with you." Expecting to be bawled out for what he'd said, Sam was surprised and delighted when Wharton advised, "Stick to your convictions and don't stray from the way you work and believe; remember what I've told you." And to his dying day, Sam did."

From: Wharton Esherick: The Journey of a Creative Mind by Mansfield Bascom.

John Ruskin on teaching and sharing of knowledge

 The moment we can use our possessions to any good purpose ourselves, the instinct of communicating that use to others rises side by side with our power. If you can read a book rightly, you will want others to hear it; if you can enjoy a picture rightly, you will want others to see it: learn how to manage a horse, a plough, or a ship, and you will desire to make your subordinates good horsemen, ploughmen, or sailors: you will never be able to see the fine instruments you are master of, abused.

From "The Ethics of the Dust"

The market for Esherick

George Nakashima popped up on my radar pretty quickly, as did Sam Maloof. And there's sustained interest in their works, in part, I think, because they had a very recognizable style, and because they were successful enough as woodworkers to have enduring legacies. There may be another reason, too, which I'll get into in a minute. After these two gentlemen, if you spend enough time looking into craft furniture, you'll also run across Wharton Esherick. Esherick made furniture, but he started out as a fine artist... first a painter, and from there he moved on to sculpture, and ultimately to building furniture that has a very sculptural feel.

The first piece of his that really caught my eye was a 3 step, spiral library ladder, that I found in a book from the 1960's. (As it turns out, it's a pretty well known piece, of which he produced several. The one pictured above was sold at auction last year for just shy of $120,000.) Some of Esherick's furniture is very clean and approachable, and some of it is very... experimental. But his experiments clearly drove him in a very good direction. And in the end, that's inspiring. Not every experiment will come out as planned... that's why they're experiments. And if you're paying attention, you'll learn from them, as Esherick clearly did. I'm still reading about him, so I'm not going to say much more about him for now. But I wanted to put his name out there, because I don't see it enough.

While Esherick's work is clearly well known among those with a taste for fine craft furniture, I don't read much about him in mainstream woodworking literature. I suspect that the first reason is because his work is sculptural, and it doesn't lend itself very directly to hobby woodworkers, because it doesn't lend itself well to marketable machine techniques: the bread and butter of the advertising industry. There are DVDs and source material out there on how to build Sam Maloof's chairs, and you can even buy the special tapered router bits he used to join the back legs. This is all for sale. Looking at Nakashima's work, his taste in wood, and his ability to use it to best effect, it's clear that he was a true artist. But it's not hard for a hobbyist with basic machinery to reverse engineer and make an inferior version that still looks reasonably attractive. Much like the rash of Roubo benches I wrote about, I think these are potential examples of more consumer hobby woodworking, because the directions are out there. The industry promotes specific projects and styles that lend themselves to a technique or tool that's for sale, so that the readers at home can "Build Your Own! In a Weekend!" I'm not knocking on anyone who wants to build their own furniture. But I do feel the need to draw a line between following directions, and original work.

But the other issue I see at play here is that there isn't really a good market for trial and error, and experimentation.  Most hobby woodworkers I've talked to bought their jigs and tools with projects in mind. They weren't really interested in the jigs, so much as they were interested in the chair that the jig would help them build. The process is just an obstacle to be overcome with an off the shelf tool, and not necessarily worthy of admiration or study in its own right. Consumer woodworking in this regard is a sort of middle man for many people who really just want better furniture. So, the magazines and catalogs will provide them with projects (products) that are already designed, drafted, and come with stock lists all made up, so there's not much thinking or creativity required. The creative part has all been done, and so most of the 'bugs' (and, by association, much of the risk of error) have been worked out. It becomes an exercise in kit making and assembly. These folks aren't really engaged in figuring out how to make the thing they're making: They're making the parts they were told to make, and assembling them in the way they were told, and hoping that nothing goes too horribly awry.

Esherick's work doesn't really lend itself to that kind of mentality.

Design Element: Make sure they get the joke.

I’ve been looking around lately at various pieces and styles of furniture, and there seems to be a correlation between how much people like the piece, and how much they understand what they’re seeing. A machine made table is pretty easy to use, but it’s not really inspiring, and there’s a lot of information about how it was made that isn’t really obvious. By comparison, a slab table evokes the shape of the tree, and there’s a very sharp understanding on the part of the observer, that they’re looking at part of a tree. And connections that are made in the mind that enable a more intimate understanding of the object that’s being viewed. It’s a slice of a tree. And the general idea of the process that led from tree to slab is fairly clear.

It’s like a joke: You hear the lead in, you hear the connecting material. But a really funny joke leaves the audience to figure out the leap from the build-up to the punchline. Oh, the guy was dumb, or the girl was easy, or it’s funny becuase we’re lampooning a group or person that we know well enough to get the joke. Or we have our attention called to something that doesn’t quite make sense, but that we do anyway. Our eyes are metaphorically opened, and we get that flash of understanding, that helps us to understand something a little more deeply, or see it more clearly.

I can see that the table was a slice of a tree. It’s not quite the same thing as walking under an actual tree, but the mental image of ‘tree’ is evoked in a pretty clear way. I ‘get’ how the final result came about, more or less.

I have a small tool chest that I made in 2004, that used maple for the drawer fronts, that was tapped for maple syrup. The build up: what are the holes? The punch-line, that’s where the physical tree was tapped for sap, to make maple syrup. The bridge to the punchline: this was a maple tree. It’s not lumber, it’s not just a factory product, it was part of a tree. There’s a direct connection to something real and comprehensible.

Simple understanding doesn’t always come easily. Sometimes, when we’re learning something new, there’s a leap of faith that’s involved. The payoff comes after all the effort, when things start to make sense. It’s the point of math homework: After a while, you understand what’s going on, things click, and you’re ready to learn something new. And there are similar parallels with woodworking. After a while, certain operations make sense.

I bought a Leigh D4 dovetail jig pretty early on. I figured it would come in handy, and it has. But there’s one major selling point that some of my customers were uncomfortable with: It has an enormous user’s manual. Learning new things requires a certain amount of effort, and a certain amount of blind faith that the effort will be rewarded. The manual gave some of them the mistaken impression that the tool was more complicated than it is. In fact, the book is that big because it breaks things down to a very basic level. So I like that the jig includes a book that will take you by the hand and walk you through the process. The book establishes a path to understanding. After a little practice, you understand what’s going on.

You can’t keep re-telling the joke about the chicken crossing the road. Even wise old Zen masters can’t get away with stroking their beard, striking a pose, and telling that joke more than once. People learn, and evolve, and some things simply become old hat. At the same time, a contemporary child of 5 is not going to understand ‘the hippy-dippy weather-man.’ And in general, if you have to have the joke explained to you, it’s not going to be funny. So there is clearly an evolution that takes place. The breakthroughs come when we understand a subject enough to make a joke about it that’s funny.

And that’s the point of this entry. When you’re designing something, whether it’s a tool, or a piece of furniture, or software, or whatever else, make sure that the end user is going to be able to see what’s going on, and be able, with some effort, to figure it out. Make sure it’s familiar enough to them that they can bridge those little gaps. Even if what you’re doing is something that you think is groundbreaking, and all brand new, like Rietveld's chair, make sure they’re going to get the joke.

Two simple sleds

These are sleds I made to cut parts for simple mitered boxes. In use, they're very simple, but I thought the geometry behind using each one was interesting enough to write something about it. Both are used with the blade tilted at a 45 degree angle.

The first one is a simple crosscut sled. The 45 degree detail actually makes the process simpler. On a 90 degree crosscut, you have to make the first cut over-size, to allow the other end to be trimmed back, too. In this case, the first cut cuts the board to length at 45 degrees, and the second cut is made without any change in settings, but makes an undercut. So, the board remains the same length, but the second cut miters the underside of the board.

The second sled is a little more involved. This one is used for cutting sockets for the splines that will hold the box together. The socket has to be 90 degrees to the miter, or 45 degrees, the other way. But placement of the cut is in reference to the corner of the miter, which is hanging off of the sled. I considered having a hook that hung out over the end of the sled, but I wanted to keep things simple. So, I use a block that's clamped to the fence of the table saw to set the board position before making the cut.

The idea is that this block sets the specific overhang off of the end of the sled. Then the sled moves forward from there, and the cut gets made.

This is a solution I've used on other jigs before, where the critical distance is short, and it makes more sense to hang the work out into the air like this. It's also useful when I cut the actual splines. The splines need to be cut to exact length, and it's a cross-cut, because the grain will need to run parallel to that of the sides, to maintain glue integrity over time. So in essence I'm cutting 1/4" long splines off of the end of a wide board that's already been milled down to 1/8" thick. Because the board is so thin, and because it's all too possible that the part will split and jam up between the blade and the fence, this is  not something that I feel safe doing against the fence of the table saw. So having a stop block like this makes the operation a lot safer, and very repeatable: I use a regular cross-cut fence, set the end of hte board against the stop block, and cut one spline off at a time.

Etymology

It's funny how the English words 'Experiment,' 'Experience,' and 'Expert' both have a common root in the Latin, Expertus, which means 'to Try.'
I found this last night after I wrote about old magazines and message board learning, and I thought it made for an interesting post-script.