Precison straight edge- checking for accuracy - Woodworking Talk - Woodworkers Forum
Old 12-06-2019, 09:43 AM Thread Starter
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Precison straight edge- checking for accuracy

I have a precision straight edge on the way from Grizzly. How would you check it for accuracy?

I know how to check a square by drawing a line, flipping the square and then drawing another.

Supposed to be .001 per foot
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Old 12-06-2019, 10:48 AM
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I’d put it against a know flat surface like a jointer table and check for light coming through gaps or to see if it rocks back and forth.
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Old 12-06-2019, 11:31 AM
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For woodworking, sight down a 2x4 to find a straight one, then check straight edge against it. 😀

Wise men speak because they have something to say; fools because they have to say something -Plato

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Old 12-06-2019, 11:33 AM
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Quartz and granite kitchen counters are supposed to be very flat. Set the straightedge on a stone counter and then shine a flashlight behind it to see if there are any gaps between the straightedge and the counter. Try the straightedge in different orientations all over the surface. If there are no gaps, the straightedge is straight.

If there is a gap, see if it stays in the same place on the straightedge as you move it around on the surface. That will give you confidence that the problem is the straightedge and not the surface you are using to test.

If you do not have a "reference flat surface" to test, I suppose you could use "statistics":

Keep testing the straightedge on many different "flat" surfaces. If some surfaces are flat with no gaps between them and the straightedge, then the straightedge is probably straight. It is unlikely that multiple unrelated surfaces will have the exact same matching "curves" as the straightedge. IMPORTANT: If none of the surfaces tests flat, then you do not know anything about the straightedge. It could be straight or not. At that point, you must find a reference flat surface to test.
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Old 12-06-2019, 05:52 PM
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There is no reason that you cannot draw a line. Then flip and see if it still matches the lime.

George
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Old 12-06-2019, 06:00 PM Thread Starter
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Quote:
Originally Posted by GeorgeC View Post
There is no reason that you cannot draw a line. Then flip and see if it still matches the lime.

George
It will be hard to measure the amount of deviation this way. I do have a granite plate about 12" long so I can lay it on that and use feeler gauges. Between that and drawing two lines that should do it.

Thanks
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Old 12-06-2019, 06:20 PM
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I have never had a reason to worry about a straight edge being within .001". Woodworking is not that accurate. Wood itself is not that accurate. I build everything from fine cabinetry to furniture and also a lot of woodturning, and I've found having an accurate square is far more important. In turning, a nice dial caliper works well and it's smallest graduation is 1/64".
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Old 12-06-2019, 06:52 PM
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I expect without a known flat surface 2 feet in length to check it against, you are just shootin' darts. If I was in your shoes and had a 16" combination square, I would pull the rule out and compare it to that. If I didn't have a 16", I would just use the rule from the 12". I might try the edge (not the top) of a cast table saw or mating surface of a cast table saw extension.
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Old 12-06-2019, 08:20 PM
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Depends on what tools you have available. For most shops, the method George proposed is gonna be the best. Either the lines match and youre good, or they dont. Next to nothing in woodworking is actually going to require the advertised tolerances, less deviation than you can see by eye with a fine pencil line will be good enough.

There are better options, but they require more uncommon tools. Personally id set the straight edge up on a surface plate, the edge to be measured resting on a pair of matched blocks, and sweep the edge with a tenths indicator on a surface gauge. Thatd show any deviations the edge has from flat, referenced against the surface plate

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Old 12-06-2019, 10:14 PM
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If I bought a precision straight edge that was suppose to be guaranteed to be accurate within .001 per foot I wouldn’t bother testing it because I probably don’t have anything accurate enough to measure it.

If you can’t trust the manufacturer to produce a guaranteed precision straight edge then I’d look for someone I could trust and buy theirs.
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Old 12-07-2019, 09:27 AM
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Take 3 pieces of cold rolled steel about 1" long and file any burrs off one edge.
Clamp the straight edge to a flat, narrow surface.
Clamp the 3 pieces of steel to the surface with the smooth edges against the straight edge, one at each end and one in the middle.
Unclamp the straight edge and flip it. If it doesn't touch all 3 steel pieces, check gaps with a feeler gauge.
Repeat with the center block at the quarter points.
Clamp the 3 plates at the end, center point, and quarter point between them. Slide the straight edge against the 3 blocks. It should always be in contact with all 3. Then flip it and repeat.
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Old 12-07-2019, 01:32 PM Thread Starter
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Quote:
Originally Posted by firehawkmph View Post
I have never had a reason to worry about a straight edge being within .001". Woodworking is not that accurate. Wood itself is not that accurate. I build everything from fine cabinetry to furniture and also a lot of woodturning, and I've found having an accurate square is far more important. In turning, a nice dial caliper works well and it's smallest graduation is 1/64".
Mike Hawkins
I am not using it for measuring pieces of wood. It's for aligning the tables on my jointer and for setting up other woodworking machinery.

For my woodworking I work to the 1/64
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Old 12-07-2019, 01:52 PM
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Originally Posted by littleboss View Post
It will be hard to measure the amount of deviation this way. I do have a granite plate about 12" long so I can lay it on that and use feeler gauges. Between that and drawing two lines that should do it.

Thanks

What do you gain by measuring the amount of deviation?

For aligning your jointer blades you only need something around 6" long. I use one of my try squares for this function.

George
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Old 12-08-2019, 02:25 AM
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Don't throw rocks. . . . .

Lay the straight edge on a flat surface and slide another straight edge against it. If there any gaps, you have a problem.

As has been said said, you don't need 0.001 inch accuracy in woodworking. The only place in woodworking where it is nice for that kind of accuracy is setting the table saw fence. I like to set the fence about 0.002 to 0.003 inches wide. This gives less likelihood of pinching the cut.

Rich
In furniture 1/32" is a Grand Canyon
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Old 12-08-2019, 08:56 AM
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Originally Posted by GeorgeC View Post
There is no reason that you cannot draw a line. Then flip and see if it still matches the lime.
George
This is the only suggestion in this thread that does not make or require the assumption that you have a perfectly flat surface to work off of. And, probably have no way of testing it.
Sometimes the easiest way is the best way.

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Old 12-08-2019, 12:22 PM
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What is 0.001 thousands of an inch?

To visualize it look at the edge of a sheet of letterhead paper and try to image something 1/4 that thick and then ponder if it will really have an effect on your life.

As for setting your table saw fence what happens to your precision when the fence is moved to another position on that chunk of stock metal tubing commonly used for a rail?

Wise men speak because they have something to say; fools because they have to say something -Plato

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Old 12-08-2019, 01:22 PM Thread Starter
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I guess none of have bought a new tool and wanted to see if the cast iron tables are flat? Or how do you set up the infeed and outfeed tables on your jointer if you don't have a good straight edge?
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Old 12-08-2019, 01:38 PM
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Originally Posted by littleboss View Post
I guess none of have bought a new tool and wanted to see if the cast iron tables are flat? Or how do you set up the infeed and outfeed tables on your jointer if you don't have a good straight edge?

Yes, and it is very easy to do with the standard tools in my shop that I have bought from Sears, Home Depot, Lowes, etc. No big deal to determine if they are adequate to the job requirements.

George
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Old 12-08-2019, 04:22 PM
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Originally Posted by littleboss View Post
I guess none of have bought a new tool and wanted to see if the cast iron tables are flat? Or how do you set up the infeed and outfeed tables on your jointer if you don't have a good straight edge?
Yes, and I created my own thread about it, searching for a good straightedge to do the job. See:
https://www.woodworkingtalk.com/f12/...36-inch-212929

I was very careful to flatten and measure the flatness of my SawStop table saw. I freely admit that I invested more time and effort than was necessary, but I treated it as a once-only task. It was personal quest that was important to me. I did not expect others to agree, approve, or care.

I bought an Empire model "4004 M" ruler, which was also labeled "Straightedge". It did not cost much. There was no easy way to test it in the store. I tested it for flatness on a flat surface and it was perfectly straight on both sides, at least to whatever precision the flashlight test will yield. I used it to optimize the flatness of the table saw.

It was a valuable learning experience. I learned how much deviation there is in my table saw, and I learned how much 1/1000 and 10/1000 inch really are in terms of visualizing the gaps. Sure, I can look at and touch feeler gauges, but it is not the same as seeing the light shining through the gap (and measuring it) to truly understand what those measurements mean.

It gives me pleasure to use my well-flattened table saw with its nearly invisible seams in the cast iron, but that's me. Many people here would say that I wasted my time, but I gained experience and knowledge that I can use elsewhere.
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Old 12-08-2019, 04:56 PM
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You flattened the top of your Saw Stop?

Quote:
Originally Posted by Tool Agnostic View Post
Yes, and I created my own thread about it, searching for a good straightedge to do the job. See:
https://www.woodworkingtalk.com/f12/...36-inch-212929

I was very careful to flatten and measure the flatness of my SawStop table saw.
I freely admit that I invested more time and effort than was necessary, but I treated it as a once-only task. It was personal quest that was important to me. I did not expect others to agree, approve, or care.

I bought an Empire model "4004 M" ruler, which was also labeled "Straightedge". It did not cost much. There was no easy way to test it in the store. I tested it for flatness on a flat surface and it was perfectly straight on both sides, at least to whatever precision the flashlight test will yield. I used it to optimize the flatness of the table saw.

It was a valuable learning experience. I learned how much deviation there is in my table saw, and I learned how much 1/1000 and 10/1000 inch really are in terms of visualizing the gaps. Sure, I can look at and touch feeler gauges, but it is not the same as seeing the light shining through the gap (and measuring it) to truly understand what those measurements mean.

It gives me pleasure to use my well-flattened table saw with its nearly invisible seams in the cast iron, but that's me. Many people here would say that I wasted my time, but I gained experience and knowledge that I can use elsewhere.

What was your process and how much was it off?

When I assembled/combined my 3 Craftsman table saws, I wanted the joints to be flush for certain, and I leveled out the top by shimming the legs as best I could, but I never removed any material from the top surface or "flattened" it.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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