For the Woodworker, What Constitutes Adequate Precision Measurement

[Steven Hardy]

I try not to build anything that wont bend to fit.....I figure wood comes from trees...and trees bend???

seriously tho....has anyone ever else ever wished they could buy a metal tape measure
that was flat without any curl and without the loose clip on the end of it???

[Steve H Graham]

anyone who's competent at machining can certainly see .015".

Absolutely. A 0.015" pass with an end mill is fairly big, and in wood, it will make a very obvious slot. I was machining walnut with an end mill the other day, and I found that 0.003" was about as much as I could risk, when sneaking up on a measurement incrementally.

On a metal lathe, an accidental 0.015" cut in an otherwise uniform cylinder would be a glaring error visible from six feet away, and it would justify junking or resurfacing the work. I don't turn wood, but I'm sure an error that big would ruin a pen or bowl. Sanding it out would deform the entire piece.

I was able to see a 0.003" gap when I was joining pieces of walnut, and my eyesight is no longer sharp. A playing card is about 0.004" thick, and if you put one on a flat table and run your finger into the edge, it's extremely obvious.

When I'm checking squareness, a gap of much less than 1/64" will jump out at me and will be big enough to send me back to the saw.

The post-machining movement and dimension changes in wood are frustrating sometimes. It seems like you have to glue it up before it moves. I assume this only matters to people like me, who make silly mistakes that have to be covered with very precisely cut pieces of wood! I doubt a person making a big dining table would care about 0.015", unless it was on a prominent surface.

When I was a physics TA, I had to teach people how to use measuring instruments, and we were told we should always shoot for the nearest half-increment and estimate. I think that would be possible on a 1/64" scale with decent eyesight or old eyes aided by readers.

[Leo Graywacz]

I did a job that I needed to glue up a bunch of little reeds together that were 3/8" thick. I clamped them up, about 70 of them and they were the perfect distance. Then I put glue in them. The thickness of the glue screwed the whole thing up. .002" glue line x 133 pcs equaled about 1/4" oversize.

[Richard Wagner]

You have all been very helpful and have caused me to reassess my approach to this whole thing. I'll double check all of my tools for accuracy first and then make certain that all of my machines are properly tuned. Then I will get a new pair of reading glasses.

Hmmm. Maybe I should get the glasses first.

[Terry Welty]

I struggle with the voices in my head... The Quality Control voice insists on 1/32" and hollers "You'll be sorry" if the other voice, The Half-A_ _ guy wins with his "Aw, it's close enough"... Usually, if they would just compromise and settle within 1/16" we would end up ok with the most of the projects we work on.

Roses are Red,
Violets are Blue.
I'm schizophrenic
and so am I.