VERY Basic Question - how to get to 90 degrees

I am very new to woodworking. This question seems so basic, that I am probably missing the obvious.
Here is the question.
How does one create a cut that is 90 degrees from purchased lumber?
For example if I buy a 2'x 4' sheet of wood and there is not a perfect 90 angle on at least one angle, will not all my cuts that are up against my fence on the table saw be off?
I hope I am making sense.
Thanks
Scott
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"Scott Willett" wrote in message

For "practical" purposes the 'factory' edges of sheet goods are considered "square" ... and are generally "square enough" for woodworking purposes.
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As long as you manage to get new sheet, that is. If you've got someone's slightly trimmed bargain leftover you'll still face this problem. One way I dealt with this was to tack an auxilliary straight edge to the sheet so it hangs over the sheet's edge and can slide along the fence. The straigth edge can even be a series of blocks if they line up with one another. You're free to position this edge at any angle you like, of course. Once you've got one straight side, use whatever tools you have (such as a square) to line it up a second time at 90 degrees to the straight side. It's work, but it works.
- Owen -
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"Owen Lawrence" wrote in message

?? ... then obviously you are NOT dealing with a 'factory' edge, are you?
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proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email

Actually the OP was talking lumber. Lots of that out there way off straight or square.
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No, he specifically mentioned sheet good. See Quote below:
For example if I buy a 2'x 4' sheet of wood and there is not a perfect 90 angle on at least one angle, will not all my cuts that are up against my fence on the table saw be off?
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The OP didn't say he was buying it from a factory either--maybe a garage sale. I never bought a sheet 2'x4' myself so I don't know if they come from the factory like that, or if the store cuts 4'x8' sheets. I do know the store (I'm picturing the borg) did a LOUSY job of cutting any wood I've ever seen them cut. But that's really not the point, anyway. Someone just wants to know how to handle a situation with a little more finesse than "buy the solution".
- Owen -
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vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email

and quote below

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vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email
In the end I suppose it's the definition of lumber that matters. I have always thought of lumber as timber, not made sheet goods. But a couple of searches said "timber prepared for the building trade" etc.....

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As a previous poster said, factory edges are considered to be straight and this is generally true for plywood and particle board. For other materials, seasonal movement might cause the edges to warp.. Corners are a different issue. Plywood and particle board corners are generally "straight enough," as has been posted but don't count on the corners of dimensional lumber because of the gain/loss of moisture.
Assuming that your table saw's blade and fence are accurately aligned, it will make cuts that are precisely parallel to the edge that rides against the fence, if the edge is perfectly straight. This is a given and something you can count on.
For square corners (perfect right angles), it is up to you to supply the precision. Don't rely on the miter gauge that came with your saw unless you have verified its alignment. This is something you can do with an accurate framing square. But the miter gauge is only useful for small workpieces (not more than a few inches wide and a couple feet long). For squaring larger workpieces, you will have to construct something called a crosscut sled. You can find details of this device posted here frequently, on many of the woodworking web sites and periodically in the woodworking magazines.
Good luck...work safely.

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Thanks so much for the replies. This helps. My second project is going to be a cross cut sled. I will then use the factory edge perpendicular to the blade. Thanks again fro answering such a basic question.

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You do the basic geometry of 3,4,5 to check for square. Or check with a good 2 foot framing square. If it is not square then square it up. Remember high school math where they said a square triangle has a sides of 3 and 4 and the hypotenuse is 5. A (3) squared plus B (4) squared equals C (5) squared. I square up with a router or circular saw run against a straight edge on that square line I just drew.

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On Sun, 2 Jan 2005 13:22:20 -0500, "Scott Willett"

Assuming your edges are straight [and you don't then need an auxilliary piece], then you need only one 90 degree angle. That is because a rectangle is a parallelogram, and having one angle 90 will make the others 90 if the sides are parallel. So, make your first cut using the miter guide [an extension, and perhaps clamping jig would hold it better to the miter.] Mark the cuts and the miter sideas now having a 90 degree angle.
Now cut the other two edges using the fence.
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As a final check, after you cut a panel measure the diagonals. When a panel's sides are parallel (they will be after ripping against the fence) and the panel is square, the diagonals will be equal. I always do this final check.
Frank
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Yes, sort of. The cut will be parallel to the fence and will be no better or no worse than the original corners. Assuming your miter is set up properly, you can then cut the ends and make it 90 degrees.
Also, be sure you square is really square.
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that is exactly the qs that I was getting at. The fence is great but only replicates the edge in relation to the others.
Is this what jointers do?. Do they in essence help you get that first straight edge? Then you go from there.
Thanks
Scott Willett
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Jointers take wood that is warped, twisted, cupped, wavy or otherwise not perfect and make it flat and one edge straight. Other tools take it from there.
Chances are, that piece of plywood is near perfect as most from the factory are pretty good. But what if it isn't? You mentioned you have a 24 x 48 piece. Let's say you want a cut off that is perfectly square and measures 10 x 42.
Step one is to set the fence at a little over at 10 1/4". Turn on the saw and rip the wood 10 1/4" wide. No you have one perfect edge. Set the fence to 10", turn the board around and now cut the other edge parallel to it In most cases you don't have to do this, but there is also a chance the sheet goods had a ding on the side and now it is cut off.
Take the piece and cross cut it, taking off just enough of the end to be sure it is square. Turn the board around, measure and mark the 42" you want and make the cut. assuming your saw is set up properly, the finished piece should be perfect. This is wood though, don't expect the same precision at you get on a $500,000 machine making jet engine parts.
Plywood does not require a jointer and it would not be wise to use one on it. Rough cut lumber is another story and another lesson.
--
Ed
http://pages.cthome.net/edhome /



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Cuts against the fence just create a parallel cut edge to the fence. So, if you are cutting sheet lumber and it's not square to begin with, and you are only using the tablesaw fence, then the cut faces will be likewise out of square.
Most sheet goods are cut pretty close to square, though.
To verify whether it's 90 degrees, there are a couple of ways:
1. You can use a try square that's accurate. To test whether your square is accurate, you can do the double line test -- placing it on a flat edge, draw a line perpindicular to the edge, flipping the square over, and draw a second line in the same spot. A good square ought to produce an overlaid single line.
2. Measure the diagonals on the sheet. They ought to be equal.
3. Measure a 3-4-5 square ... e.g., measure 3 units from a corner on one side, 4 units from the same corner on an adjacent side, and test that the marks are 5 units apart (if you're on a 2'x4' sheet you can divide by two, e.g., 1.5', 2', 2.5')
If it's not square, then you have to square it by any one of several methods:
1. On a tablesaw, using either a miter gauge set to 90 degres or a crosscut sled. 2. By hand, saw along a marked square line using either a handsaw or a bandsaw. 3. With a circular saw, using a straight fence clamped square to an edge.
Probably lots of other ways as well ...
p.s. If you are buying at one of the Borg stores (like Home Depot) then they will cut it the large sheets on their panel saws, which are (usually) darned close to square.
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One additional method for checking your square from Euclid-
Use a compass (this can be a good string tied to a nail and a pencil if you don't have one) to draw a circle on something. Bisect the circle in any direction (just draw a line through the center hole made by your compass) Mark any point on the circle, and draw a line from where the diameter line crosses the circle to that point on each side of the circle. The resulting angle is always 90 degrees (assuming your geometric layout isn't sloppy, that is)
I always like this better than the 3-4-5 method or flipping the square, myself.
Aut inveniam viam aut faciam
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snipped-for-privacy@bellsouth.net says...

Hi Scott
From your post I have to make the assumption you are talking about sheet goods rather then solid stock.
If you do get a sheet that is not square enough for your use and wish to rectify the problem you can attach, double sided tape, hot glue, brads, nails, screws, what ever is appropriate, a straight edge guide to one side. Affix it so that when you run the piece through your table saw with the guide against the fence you get a 90 degree side.
A router with a guide bushing or flush cut bit will also accomplish the task.
--
Mike G.
Heirloom Woods
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