For the Woodworker, What Constitutes Adequate Precision Measurement

[Tom Rick]

Interesting observation you have made Richard. In 38 years I have never seen a professional or serious amateur that was obsessed with accuracy. The use of story sticks and fitting joints to fit will almost insure accurancy to 1/32" which is enough in most cases. And... if the professional or serious amateur makes a mistake that can be seen with the eye.. they still don't get there shorts in a wad as they know how to fix it. That is my observations and own theory with accuracy.

Have a good father's day...

Sarge..

Exactly right- I believe most experienced workers don't give it too much thought.
If the work calls for no visible gap- just cut the piece to fit and carry on.

The obsessive part I believe is the domain of the novice.
The experienced man has enough technique to bring projects together without struggling over every cut.

I frame to a contact fit- loose framing leads to lifted tape and popped screws as the mess settles. I have swung a circular saw around enough that it is no issue to drop the saw to a board and split the line.
A standard call to a good cut man is to "leave" or "take" the line. Things are framed up tight but no one is giving it much thought.
A furniture builder working by himself does clean work as a matter of practice. Why would he place the cut anywhere other than where it needs to be?

An answer to your question might include some reference to the degree of precision tooling required by the wood worker.
I have always felt that the craftsman's greatest tools are his heart and mind.
Primitive tools can yield superlative results in the hands of the master:

Stradivari_Marking.jpg

Stradivari's tools

[Chris Fournier]

This has actually been a useful thread about this topic - which is very rare.

I have found on these woodworking forums that there is a certain personality who will claim anything to establish their mastery of the craft. If the plans call out for "+/- 0.10" they'll tell you that they hit .0005" all the time, with a hand plane no less.

You typically don't get this kind of nonsense on the machinist's forums which is ironic because they often do work to these infintesimal tolerances. The truth is that machinists have been taught how to measure and they know how expensive it is to just buy equipment that can reliably read to "tenths". They'll also admit very quickly that owning fine measuring instruments doesn't necessarily mean that they are able to use them properly and repeatedly to the resolution that they're capable of.

I think that precision is something that a consciencous craftsman will continue to creep up on during the length of his/her career. Early on the payback is huge, later on you may literally be splitting hairs.

Precision is costly on a sliding scale: low = cheap, high = expensive. I think that we all intuitively apply this correlation when we are working on any given project.

A good craftsman can split the line with a circular saw while building a home, slot a guitar fretboard that produces a well intoned instrument or make a cane fly rod that follows a taper formula measured in .001" resolution. The same craftsman can perform all three of these tasks but it is unlikely that he would be foolish enough to apply his standard of precision from any one of the projects to another.

[John Coloccia]

A good craftsman can split the line with a circular saw while building a home, slot a guitar fretboard that produces a well intoned instrument or make a cane fly rod that follows a taper formula measured in .001" resolution. The same craftsman can perform all three of these tasks but it is unlikely that he would be foolish enough to apply his standard of precision from any one of the projects to another.

+1

I think the debate comes in mostly with precision squares and dial indicators when setting up machines. I strive to set all my machines to +/- .001". I know it won't stay like that for long, and it's also kind of silly when the fence on my table saw isn't even that good, but I do the best I can. That doesn't mean I fit all my woodworking to that. All it means is that as I move from machine to machine, I'm not introducing more errors as I go along. For example, if my jointer fence is out, and my table saw blade is tilted slightly, and then I bring the work over to my router table and it's out a little bit, by the time I'm done, a couple of "within woodworking standards" errors can creep in and I end up with a finished product that is noticeably cockeyed. Think about all the trouble people have just making a simple 45 degree miter for a frame or moulding. If the tool is setup once to be dead nuts square and the miter bar is 45 degrees, you end up with a tight joint. That's rarely the case and people are frustrated to no end making a seemingly simple cut. Maybe .010" is acceptable for the final product, but if you're out that much already at the tool, getting there in the final product is an exercise in frustration.

I've often said that I would gladly pay for a service to come in and setup all of my machines once a year so I didn't have to bother.

[Neil Brooks]

I tend to think .... if the outcome was good (you produced a fine looking piece, with tight joinery), then ... whatever you did, worked -- no matter how whacked your machinery might be

But I DO I think it's a useful conversation -- particularly when it comes to the Diminishing Returns of that last (whatever it is) thousandth of precision.

I think it's useful to understand how the Lowest Common Denominator (or weak link in the chain) influences the level of precision that's ever worth seeking -- in our case, movement of the wood, itself.

And I think it's useful to draw distinctions between "precision," "accuracy," and "consistency," in these discussions.

I've often said that I would gladly pay for a service to come in and setup all of my machines once a year so I didn't have to bother.

It's actually a rather zen thing, to me -- one that I rather enjoy. It's also kind of an annual ritual, but .... with (+/-) weekly checkups and tweaks !

[David Giles]

If I can't feel it and I can't see it, it's fine. Since I usually mess up during the first 30 minutes of a project, everything else is adjusted to fit what has already been cut.

[John Coloccia]

If I can't feel it and I can't see it, it's fine. Since I usually mess up during the first 30 minutes of a project, everything else is adjusted to fit what has already been cut.

ROFL. David for the win.

[Rod Sheridan]

When setting up machinery, I like it to be accurate to a few thousandths of an inch.

For example my digital height gauge on my planer is accurate to 0.02mm which is great.

When planing pieces I can set it at 20mm for example, and receive the same thickness I had on the original pieces, makes repeats easy.

For length cutting I have the crosscut fence length calibrated to 0.1mm which is just fine for repeat measurements.

When I design furniture I might want 2 table aprons 1,400mm long, which is what I set my crosscut stop to.

If they're 1,400.02mm or 1,399.98mm who cares, at least they're both the same.

That's the key, all the parts that have to be the same size, are the same size.

If my cabinet is 0.02mm too wide or too narrow it's no sweat, unless it's a built in.

I machine most parts without reference to the other parts, based upon drawings I've made, then dry assemble the cabinets, there may be something like a drawer bottom, that I'll check the dry drawer for size before cutting, as it's a cut to fit condition.

It may not be the right way to work, however it's how I was taught.

Regards, Rod.

[Matt Winterowd]

Richard, I know that you expressed amazement on a recent thread about precision squares, and I want to make an observation about requiring accuracy from your tools vs. requiring accuracy to some arbitrary measurement in a project. If I'm building a cabinet, I don't really care if a side is 18" or 18 1/8". I just care that the two opposing sides are equal, and that the joinery is tight. However, I do want my reference tools to be accurate, so that if I do need to try and achieve squareness, for example, I know that I can rely on the tool giving me an accurate reading. Just like people strive for tight tolerances setting up their machines, I am willing to pay a bit extra to guarantee that my tools are telling me the truth, and maybe even more importantly, that they are all in agreement with each other.

[Greg Portland]

As long as it looks good, I'm okay with it. Many times, I don't measure at all, or use something like a story stick.

Mike

+1. I know I'm in trouble if I have to start pulling out measuring devices. Make your cuts to existing pieces and use a story stick to setup your project dimensions (esp. helpful for built-ins).

[Steve H Graham]

The folks on this forum recently let me know that for some jobs, no amount of perceptible error is acceptable. I was trying to glue six pieces of walnut together to make a solid guitar body, and they told me a 0.002" gap was no good. I've also learned that snipe which is too small to show up on a dial caliper will light up beautifully when you take sandpaper to it.

The more woodworking I do, the more I appreciate my milling machine. I'm trying to put some inlays (structural, not thin decorative stuff) into the front of my guitar body, and I've found that the mill gives faultless measurements, straight edges, and perfect roundovers with very little effort.

I'm amazed at how precise my table saw can be. If I weren't so lazy, I'm sure I could get precision of a thousandth or less by ginning up an adjustment rig with a dial indicator on it. And it edge-joints perfectly, with a simple jig someone here suggested.

Mark Duginske has a funny part in his woodworking machines video, in which he ridicules people who blow wads of cash on "precision" doodads that get you within 1/32". He holds up a bunch of Post-its (his shims) and notes that each one is about 1/8 of 1/32" thick. Personally, I like doodads, because they are useful to people like me who have no skills, but I think he demonstrates more wisdom than I do.

Machinists use rolling papers to do even better. If you put one between a cutter and a surface and bring the cutter just close enough to make the paper tear when you pull on it, you're within a thousandth.

[Keith Outten]

Any measurements made to a tolerance of less than 1/64th inch need to be made in a temperature controlled environment.
Your measuring instrument will not provide the same results from an inspection you make at 65 degrees when your shop warms up to 75 degrees later in the day. If you intend to work to very close tolerances you need to keep your workshop at a constant temperature, otherwise your wasting your time. I have CNC machined inlays at eleven pm that fit perfectly that did not fit the following morning when my shop temperature had dropped by ten degrees.

The American Society of Non Destructive Testing specifies that the unaided human eye cannot discern a scale measurement of less than 1/64th inch which is 0.015" (fifteen thousandth of an inch).

Note that 0.005" is the diameter of an average human hair.

Calibration labs generally keep a constant 68 degrees F for the purpose of calibrating precision instruments for good reason.

When you start working to very close tolerances you must then consider the thermal coefficient of expansion for the material you are using. Wood will expand and contract, however you cannot pin down a close tolerance engineering specification for thermal growth on a material that is not man made.

Lastly, adjusting some machine surfaces to a tolerance of 0.001" is mute when you consider that the bearings in the cutter heads or blades may very well be manufactured to a run-out tolerance of +- 0.008" which is very common.
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[Van Huskey]

Any measurements made to a tolerance of less than 1/64th inch need to be made in a temperature controlled environment.
Your measuring instrument will not provide the same results from an inspection you make at 65 degrees when your shop warms up to 75 degrees later in the day. If you intend to work to very close tolerances you need to keep your workshop at a constant temperature, otherwise your wasting your time. I have CNC machined inlays at eleven pm that fit perfectly that did not fit the following morning when my shop temperature had dropped by ten degrees.

The American Society of Non Destructive Testing specifies that the unaided human eye cannot discern a scale measurement of less than 1/64th inch which is 0.015" (fifteen thousandth of an inch).

Note that 0.005" is the diameter of an average human hair.

Calibration labs generally keep a constant 68 degrees F for the purpose of calibrating precision instruments for good reason.

When you start working to very close tolerances you must then consider the thermal coefficient of expansion for the material you are using. Wood will expand and contract, however you cannot pin down a close tolerance engineering specification for thermal growth on a material that is not man made.

Lastly, adjusting some machine surfaces to a tolerance of 0.001" is mute when you consider that the bearings in the cutter heads or blades may very well be manufactured to a run-out tolerance of +- 0.008" which is very common.
.

What Keith said is accurate (to within .0000001) and wise. However, I often find myself setting machines and making measurements to the best of my tools and my ability. In the end I tend to only sweat machine setup but it just makes sense to me even with the other tolerances of the machine, in some directions the lack of perfection can just stack on top of each other.

[Leo Graywacz]

1/64" is closer to .017" and I routinely do measurements by eye around .007".

Wood movement is more about humidity than temperature.

[John Coloccia]

I can certainly see better than 1/64". 1/64" is the difference between slight gap and no gap. That's not to say I always work to those tolerances, which would be silly, but to say I can't tell the difference is silly as well. 1/64" is 15 thou, and anyone who's competent at machining can certainly see .015".

[paul cottingham]

For me, if it's close enough it's close enough.

Seriously.

+1 dude.
Seriously.