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Precision: How close is close enough?

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Forum topic by RogerBean posted 01-31-2011 08:33 PM 1290 views 1 time favorited 18 replies Add to Favorites Watch
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RogerBean

1144 posts in 1607 days


01-31-2011 08:33 PM

Topic tags/keywords: question milling shaping turning joining sanding

When I visited Andrew Crawford in England last year, I thought I was a pretty good box maker. In the course of our discussions, he he suggested that I leave a “little extra”. When I asked how much to leave, he replied: two tenths of a mil. Not being too tuned in to the metric system it blew right by me at the moment, but later I realized: “Crying out loud, that’s .007 inch!” Cows have hair that thick! He, and now me, always has a dial caliper handy.

It was something of a revelation. The visit changed the way I think about “close enough”. And my world of woodworking has changed much for the better. It’s just a new way of thinking about how close is close enough. I have learned, sometimes painfully, that on a box, a few thousands gap can look like a chasm. I also work with metal machinery, but I just didn’t connect up the same level of precision to woodworking.

Later, I began to realize that as a bamboo fly rod maker (purely for fun) the level of precision is even less. (I’ve made a few over the past ten years or so.) What Ron Barch (Editor of the Planing Form, the cane rod maker’s newsletter) calls “The most precision form of woodworking.” The planing form for a fly rod tip is typically set to something like .031 inch at the tip. (i.e. that’s a total of .062 or so at the tip for the finished rod once the six pieces are glued up.) It’s set with a dial indicator before the planing of the cane is started. It’s done with a very sharp block plane. So the tolerance here is, what? ...0001????

Obviously, a Queen Anne secretary does not require quite the same precision, but how close is close enough. How close is good enough for your projects? Is precision a mindset? My own opinion is continually shifting.

Roger

-- "Everybody makes mistakes. A craftsman always fixes them." (Monty Kennedy, "The Checkering and Carving of Gunstocks", 1952)


18 replies so far

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dbray45

2500 posts in 1430 days


#1 posted 01-31-2011 08:38 PM

Well, if you make a dovetail more than about 0.01 too big or you don’t cut the pins or tails straight, you can split the wood—makes a nice wedge—isn’t this fun???

-- David in Damascus, MD

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roninkokomo

14 posts in 1326 days


#2 posted 01-31-2011 08:41 PM

One rule that I have operated by for the last 35 years. “If you have to ask yourself, is this good enough? . It isn’t”
I think more than anything precision is a habit. Having said that we need to remember that the medium actually moves and working in thousandths of an inch in a lot of situations is wasted effort. But the human eye can be trained to detect very small discrepancies that should not be accepted.

I think accurate work is a habit that one learns, but must be governed by a small dose of common sense.

roninkokomo

-- roninkokomo

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dbray45

2500 posts in 1430 days


#3 posted 01-31-2011 08:46 PM

Isn’t that the truth— Ever put something together that barely held and when you get it out of the shop the thing requires a crowbar to get it apart? Remember that set of drawers you did for somebody and a couple of months later you are planing the edges and the next winter the drawers are so loose???

This is what makes this interesting.

-- David in Damascus, MD

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CharlieM1958

15696 posts in 2872 days


#4 posted 01-31-2011 08:48 PM

Certainly food for thought, Roger. I agree that my standard for “close enough” has gotten stricter over time, but I doubt I’ll ever get down to sweating thousandths here or there. I’m not going to say it doesn’t matter…. It’s just that I’ll never have enough practice to get that good.

-- Charlie M. "Woodworking - patience = firewood"

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CiscoKid

317 posts in 1527 days


#5 posted 01-31-2011 09:07 PM

There are always dial calipers, micrometers, and dial indicators handy in my wood shop. A habit brought about by 14 years designing scientific test apparatuses. With that said, it is indeed very frustrating when the piece changes dimensions at the onset of winter.

-- Al, Culpeper VA

View Viktor's profile

Viktor

447 posts in 2072 days


#6 posted 01-31-2011 09:37 PM

0.2 mm is about my standard also. Certainly for dovetails, meters etc. It does not always work out this way on practice though. Yes, wood is unstable, but it won’t change much in the time you are making a joint or a project. For large pieces, such as bed rail I’ll cut at least within 0.5 mm. Nobody will ever notice, but it just feels good.

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BritBoxmaker

4369 posts in 1690 days


#7 posted 01-31-2011 10:21 PM

I usually like to work to 0.1 mm or 4 thou(sandths of an inch). Thing is when you consider how wood moves, expands and contracts am I fooling myself?

-- Martyn -- Boxologist, Pattern Juggler and Candyman of the visually challenging. http://www.theartofboxes.com

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dbray45

2500 posts in 1430 days


#8 posted 01-31-2011 10:25 PM

In my oppinion, what ever works for you – works for you. Its all good! Each board is different and as such, each of us are different in what we see and how we do things. If it works – be safe – and keep doing it.

-- David in Damascus, MD

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Pop

419 posts in 2600 days


#9 posted 01-31-2011 10:28 PM

Folks, wood moves. It moves a lot. A human hair is 3 thousands in diameter. I don’t think thousands is a word for wood. That’s a metal word. I remember back in drafting class ” + – .0005” that’s a very highly finished hunk of metal. Wood is more in the range of 100s. 0.0156 inch or 0.397 MM is 1/64 of an inch. If you use that measurement as a clearance for a drawer is ‘ante gona open very well.

Pop

-- One who works with his hands is a laborer, his hands & head A craftsman, his hands, head & heart a artist

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PurpLev

8476 posts in 2302 days


#10 posted 01-31-2011 10:45 PM

I think for woodworking 1/64 or 0.016” is as close as you want to get without having to worry about parts getting wedged within.

-- ㊍ When in doubt - There is no doubt - Go the safer route.

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Seeharlez

83 posts in 1646 days


#11 posted 01-31-2011 10:49 PM

It’s all relative! At my job I design parts that are steel fabrications, machined parts, welded assemblies and so forth. There can’t be one standard tolerance for everything. Within a single joint you may need very tight tolernaces in the range of thousandths of an inch, but that’s why you usually need to trim to fit. Where as you wouldn’t expect the overall dimension of a cabinet or table to be that tight. In that case you may be dealing with 32nds or even 16ths or 8ths.

What’s really important is where you dimension from. Dimensions should be relative to the surfaces that matter to that particular feature. In engineering this is part of what is called geometric dimensioning and tolerancing (a.k.a. GD&T). it can be quite complicated but the basic principals are very relavent to woodworking, be it for the professional or for the amateur hobbyist. The myth is that GD&T makes things more dificult to make but the reality is the it often reveals that some dimensions can be toleranced much looser and it hightlights the dimensions that really do need to be made precisely. Anyways, thers is lots of info available online on the subject. Wkipedia for example.

-- Greg - Vancouver, BC

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canadianchips

1831 posts in 1650 days


#12 posted 01-31-2011 11:39 PM

Finally all above comments .
Striving for perfection is a great goal. Accepting a tolerance level when the job is finished is reality.
I have been laughing and grinning for the past couple of moinths at people “Setting their fences”, “Using micrometers” calibrating special CNC tools, “thinking they are getting the high end jobs because their reveals are within 1/1000”, if you can look at my house and 1 of the 30 cabinets doors reveal is 1/1000 of an inch smaller than the rest———you should be doing brain surgery with out the magnified monacle, or building swiss watches. (I sleep well knowing it is that crooked) l
Seriously though, when we built 100 ft x 500 ft barns and the diagonal was outta square by an inch oor two, THAT WAS accepted, when we did finish work and the cabinets were outta square by 1/16 of an inch, THAT was accepted, I guess if I am building intricate small boxes or marquery work an even smaller tolerance would be accepted.
It really boils down to what we are comfortable with at the end of the day. I know Iam not going to look at another persons work and say to them, YIKES it’s out by a hair !

-- "My mission in life - make everyone smile !"

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itsmic

1419 posts in 1772 days


#13 posted 02-01-2011 01:03 AM

Very interesting subject, all have good points, and each project presents it’s own specification criteria. I work out in the cold, my supplies of wood stay outside also, under cover of the barn, but susceptible to the ambient humidity and temps. It’s a real hassle when I bring the stuff in for finishing and it starts to change. The up side to this, is I account for movement in the construction, making things extra tight, joint wise, and leaving extra space for things to shrink and expand, floor wise, they usually shrink when I bring them into warmer, dryer conditions. I always use a caliber, just to see how close I am getting to exact, it really helps to consider all the aspects discussed above, there is no one rule to follow, as stated. Hey, if any of my boxes ever end up in a storage area for a few winters, they will survive, they have already stood the test of extreme humidity and temperature changes, making something that will last a long time means it has to withstand some abuse. ie. there’s a lot of antiques out there with some sort of damage. Of course some of the beautiful and priceless things I have seen here on lj’s will probably never have to be tested in those ways, thankfully. thanks for bring it up

-- It's Mic Keep working and sharing

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shipwright

4966 posts in 1451 days


#14 posted 02-01-2011 03:50 AM

I guess it comes from all the fitting to strange angles and curves in boats but I never use measuring instruments other than a steel tape no matter what tolerance I’m working to in terms of fit. I fit the pieces until they’re right. To me it’s not often important, within 1/16” anyway, how big something is as long as all the parts fit each other well enough. By well enough I mean that to the human eye it looks appropriate. I applaud those of you who venture into the thousandths of an inch, I guess I just don’t understand why.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fiberglass trees. http://prmdesigns.com/

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bigike

4031 posts in 1942 days


#15 posted 02-01-2011 04:20 AM

wow, The only time i think about precision like that is when I’m setting up my saw as far as my work goes I’m still in the 1/16-1/32 range. I wouldn’t mind doing more polish work with a caliper gauge and or mm’s just to learn it. My math is garbage though so I don’t think I’ll ever leave the 16th-32nd range I’m trapped. I actually like this topic though.

-- Ike, Big Daddies Woodshop, http://www.icombadaniels@yahoo.com

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