How straight is straight?

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snipped-for-privacy@milmac.com (Doug Miller) wrote in

Sheesh.
0.0002"=5 microns off straight over a foot 0.002"P microns off straight over a foot
For reference, a human hair is about 100 microns thick. Talk about splitting hairs.
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Nate Perkins notes:

And anyone who REALLY gets their tools set up to .002" is way past where they need to be when working wood.
Results on the best days are going to be within 1/64" or so, with the very, very, very occasional dip to 1/128". Unless you're making a damned small box, moving it from bedroom to kitchen when the dishwasher is in use will make it move nearly that much.
With my eyesight as it is now (fairly good except for excessive floaters), I doubt I could see a difference between 0.0002" and 0.002" on my table saw table even if the straight edge would show it. And if I could, I'd still leave it alone.
Charlie Self "One of the common denominators I have found is that expectations rise above that which is expected." George W. Bush
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(Charlie Self) wrote:

All of which is correct, but misses the point of the thread. The original post asked, in essence, why is the Starrett more expensive, yet less accurate, than the Lee Valley; this was shown to be a misconception. Whether the 2.5x greater accuracy of the Starrett is needed for woodworking is a different discussion.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.
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Yes Doug -- you hit the point of the discussion right on the head. I've noticed that these threads take on a life of their own and usually morph into completely different discussions. In this case, it has, at least, stayed constructive.
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Charlie, You are correct about wood: that 0.002 is way past where you need to be. However, for machining steel parts, 0.0002" is really nice.
Also, seems to me that if your stratig ege reference has an error of up x, then your jointer can be up to square root of (2 x squared), root mean squared (RMS). If you join two pieces that also gets RMS'ed so the error is potentially twice the eror in the original reference straigth edge.
Still not very much, but just for completeness, we ought to make note of this effect -- unless this analysis is wrong -- and it sure could be.....
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It sure is an entertaining analysis. It has all the right components - numbers, multipliers, square roots, abbreviations and a way to put all of those together. Hell, it doesn't have to be accurate, it sure looks impressive as a formula.
--

-Mike-
snipped-for-privacy@alltel.net
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All it needs is an addendum calculating for 220/240/440V, wire gauge, and whether we need three or four leads...
Patriarch, who IS kidding...
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Charlie Self wrote:
Hi Charlie,
Especially when you consider that 200 years ago people were using wooden planes, and big-slab wooden benches - which weren't leveled with a routah and sled - to make furniture that is excellent by any standard.

--
Saville

Replicas of 15th-19th century nautical navigational instruments:
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Why bother with straight? Play with it. Straight lines are boring. Use an angle grinder with a carving blade in it and go for it.

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Charlie Self wrote:

First, a human hair isn't necessarily 100 microns. red heads may be down to 60 microns, lots of black hair is 150 to 200 microns.
Second about 1/128" Hell, breathing on the wood will make it move that much from just absorbing moisture. And working out the math, if you are 1/128" short you are truly a "hair" short or maybe "two hairs" short or more accurately you are one black and curly short. Oh yuck!
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On Thu, 13 Jan 2005 00:09:13 GMT, "George E. Cawthon"
......and in reply I say!:
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What's yuck about black and curly man? My wife has black curly hair on her head. Seems OK to me <G>
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Old Nick wrote:

I thought someone would descend to a lower level. ;-)
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On Fri, 14 Jan 2005 05:47:30 GMT, "George E. Cawthon"

Now, now, trolls' accusations not withstanding, this is a civilized group populated by gentleman (and proper ladies). [besides, that would have been waaaay too easy :-) ]
+--------------------------------------------------------------------------------+ Now we'll just use some glue to hold things in place until the brads dry +--------------------------------------------------------------------------------+
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On Fri, 14 Jan 2005 05:47:30 GMT, "George E. Cawthon"
......and in reply I say!:
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Aaah! That stuff between my toes.......
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On Fri, 14 Jan 2005 05:47:30 GMT, "George E. Cawthon"
......and in reply I say!:
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Damn!
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snipped-for-privacy@aol.comnotforme (Charlie Self) wrote in

Yeah, my point exactly. Most people have no concept how small 0.0002" really is. You can't see 0.0002" with your eye. You can't reliably see it with the best optical microscopes. It's so small that any amount of routine handling will throw it out of tolerance.
So why pay extra for 0.0002" tolerance in Starrett vs 0.002" tolerance from Lee Valley? Beats me.
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True
Yes, you can, easily.

Depends upon your routine.

from
Depends on the intended purpose.
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On Fri, 14 Jan 2005 22:23:42 -0800, CW wrote:

[This keeps coming out sounding argumentative, and it's not meant to be:)]
Can you tell 0.0002 from 0.0003 by eye "easily?" When you say "easily," do you mean bright light shows 'twixt straightedge and work? In astronomy we deal with arcseconds; there are limits to what the eye can see. Older eyes have more trouble. Can old farts here see 0.0002 easily? Is that amount small enough that temperature (coefficient of thermal expansion) matters?
--
"Keep your ass behind you"
vladimir a t mad scientist com
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>> You can't see 0.0002" with your eye.

No, but that was never the issuse.
>When you say "easily," do

References to astronomy don't make you look good. For various reasons, I have learned to dismiss the astronomy types.

I can. YOu would have to live with my eyes for a day to appreciate that.

For a guy in his woodshop, no.
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Really? What types don't you dismiss?

Sure it does. Look up the CTE of a good stainless steel. Assuming a 2ft bar, the linear CTE is about 3x the tolerance for every degree F.
Now try the same with the elastic modulus. :-P
You want to pay extra money for a 5um tolerance, go for it. I'll save my money and buy something useful.
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