Precision Tools in Woodworking

[george wilson]

I hope some of you go up to post #111 and look at some of the sizes the "engineer's chart"says objects are,in relation to the smaller than 40 microns we aren't supposed to be able to see. It is very strange that a spider web is listed as 2 to 3 when we aren't supposed to see smaller than 40 microns. My point is,you cannot just list a number and say we can't see it. The object being viewed may or may not be seen due to the background and/or lighting it is viewed against.

A metal lathe turning full of small scratches from being turned is not going to show a 1/250,000" scratch,but that scratch will show up in a polished,smooth surface. A .002" hair is easily visible under most conditions. So was the 1/2 thou. steel wool strand.

I just hate it when sweeping generalizations are made like that.

[Darius Ferlas]

I agree with all the above, George.
The data I posted comes from various averages arrived it based on some psych studies. Unfortunately I do not know the methodologies used, such as ambient light, contrasts, backgrounds, shape of one papillary lines etc.

[Leigh Betsch]

When I get my plane blades really really sharp, sharp like an electron microscope can't see, my shavings get so thin no one can see them. See attached pic.

[george wilson]

Could you enlarge the picture?

[Jessica Pierce-LaRose]

The emperors new smoother?

[Bruce Haugen]

. A .002" hair is easily visible under most conditions.

Some of mine is much finer than that, George. Heck, some is so fine you can't even see it

[Leigh Betsch]

Last time I try to out smarty aleck you guys.

[Chris Vandiver]

Could you enlarge the picture?

Here it is. I enlarged it for you.

[Chris Griggs]

When I get my plane blades really really sharp, sharp like an electron microscope can't see, my shavings get so thin no one can see them. See attached pic.

HAHAHAHA! You beat me to the joke. I was going to post a picture of my new set of <.004 feeler gauges! I'm always loosing them and keep having to buy new ones

[george wilson]

A micron is about .00004"(4 hundred thousandths),to go with the engineer's chart of sizes. 5 microns is .0005". So,a cobweb at 2 to 3 microns would be about .0001" diameter,give or take. I hate walking into them because I can never see them!!

[John Coloccia]

I think a normal spider web is made up of multiple strands. The single strand is when the spider comes dangling from the ceiling. I'll admit that often times I can't see those until I notice a spider floating next to me and my tuffet.

[george wilson]

you may just not be focused ion the strand,but when it touches you,you will soon see it.

[Wilbur Pan]

Darius: I can see one of my hairs,and they are .002" in diameter. I can feel it,too,if i run my finger over it laying on the table,for example. My cat has finer hair(fur) than that. I can see it too. I can feel a LOT smaller of a ridge on a piece of metal I am turning than .003". .003" is pretty large. I can SEE a sudden difference of .001" on a piece of metal I'm turning.A piece of typing paper is .005". Pretty thick,really,though not a lot thicker than some of those specs.

For visual acuity, it's more accurate to describe it in terms of the arc of the field of vision that can be resolved instead of how thin an object can be detected. You can't really talk about seeing a small object as a measure of visual acuity unless you also know how far away that object is.

It also depends on what the eye is trying to resolve. The limit at which the eye can resolve two closely spaced objects is slightly different than the limit at which the eye can detect a fine dark line against a light background, which is also different than the limit that the eyes can detect differences in depth. To complicate things further, lighting conditions, contrast between the object and the background, and even the shape of the object all influence the measurement of actual visual acuity at any given time.

For the most part, though, 0.5 arc minutes is a pretty good number to work with as far as the limits of human vision go. If you do some basic trigonometry, it turns out that if you have 20/20 vision and lighting conditions are good, you should be able to see a 0.001" line at a distance of 6-7/8".

The eye also can resolve differences smaller than that. The human eye can detect misalignment of line segments that are even smaller: 0.13 arc minutes.

[Chris Vandiver]

For visual acuity, it's more accurate to describe it in terms of the arc of the field of vision that can be resolved instead of how thin an object can be detected. You can't really talk about seeing a small object as a measure of visual acuity unless you also know how far away that object is.

It also depends on what the eye is trying to resolve. The limit at which the eye can resolve two closely spaced objects is slightly different than the limit at which the eye can detect a fine dark line against a light background, which is also different than the limit that the eyes can detect differences in depth. To complicate things further, lighting conditions, contrast between the object and the background, and even the shape of the object all influence the measurement of actual visual acuity at any given time.

For the most part, though, 0.5 arc minutes is a pretty good number to work with as far as the limits of human vision go. If you do some basic trigonometry, it turns out that if you have 20/20 vision and lighting conditions are good, you should be able to see a 0.001" line at a distance of 6-7/8".

The eye also can resolve differences smaller than that. The human eye can detect misalignment of line segments that are even smaller: 0.13 arc minutes.

Thanks for clearing that up.

[Alan Wright]

Stanley,

I really appreciated reading your original post. Not sure why some take such umbrage to some of your points. Seems like you are saying "don't assume your tools are square". If they aren't, I'm just making my goal of cutting and joining truly square pieces of wood more difficult. I'm not sure what I'm gonna do, becuase I don't want to spend the big bucks on the precision tools (i'm a hobbyist), but I am definately listening to you, and agree with your principal points. Gotta think on it some more...

THANKS!

Alan