Screen not "square". Cannot for the life of me figure why... - AVS Forum
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post #1 of 9 Old 09-03-2013, 05:13 PM - Thread Starter
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Please help...

Let's start with the very basics and see where we end...

Simple 1x4 frame.


Internal vertical ID of frame measures PRECISELY 71 5/8"s. At both the left and right meeting points, the middle, and a spot on both the L and R that are equidistant from the middle.

Horizontally: ID of frame measures PRECISELY 127 1/4 " at both the top and bottom and middle.

What does this tell me? It tells me I have rectangle with opposing sides that are just about dead nuts parallel, therefore, I should yield (4) 90 degree corners when both the mirrored vertical and horizontal identical boards are laid out.

A quick 3/4/5 of the corners yields a dead-nuts 90 degree yield on all 4 corners.

So what's the problem?

I come up with a 147 diagonal and a 145 diagonal on the opposed.

I have been sitting in my garage contemplating this for 45 minutes and I simply cannot come up with an answer to this problem that jives with any geometry/trig I'm aware of.

Equal lengths with parallel sides should yield diagonals that are identical. Simple as that.

After confirming both sides (x2) are identical in length and parallel, I do not know where else to go.

I'd love for someone to point out the err of my way.

Thanks a ton.

James

Actual phone call (see pic to left):

 

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Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

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post #2 of 9 Old 09-03-2013, 05:44 PM - Thread Starter
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Or perhaps more importantly: what does this mean from a projected image standpoint?

I mean if I have one side that measures 71 5/8's and another that measures 71 5/8's and a bottom that measures 127 1/4" and a top that measures 127 1/4" just how "off" can this be?

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

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post #3 of 9 Old 09-03-2013, 08:12 PM
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Measure top left corner point to bottom right corner point. Then measure top right point to bottom left point. If the frame is square, the measurements will be the same. If not, rack the long side toward the short side until they are the same. Even though all your frame members are the same, a 3/4/5 measurement won't account for a frame that is racked out of square. I build frames (cabinets) for a living and this is your best way to check square.
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post #4 of 9 Old 09-04-2013, 04:13 AM - Thread Starter
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I appreciate your assistance, but I have already done just as you have indicated: measure the diagonals. That's where the problem was unearthed.

Since both pairs of sided are IDENTICAL in length AND parallell it should be impossible to end up with anything but a rectangle if the ends are square but the fact of the matter is something went wrong, somewhere...my guess is the assembly slid perhaps a qrtr inch or so during adhesion/construction and that was enough to create the difference over 12+ feet on the diagonals, seeing it will + one direction and - the other. Or my r arm saw is not cutting square.

Not the end of the world, $20 worth of wood. I would just caution others building BIG frames. This would be virtually negligible in a smaller screen but when you start considering a 150" is over TWICE the size of even a 100" you can run into issues quickly.

James

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

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post #5 of 9 Old 09-04-2013, 05:02 AM
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The only way to ensure your frame is square is to measure diagonally before you fasten or gusset the corners. Your chop saw and the lumber are not square or straight enough at the scale youre working at to assume your frame will be square without crosschecking the diagonals.

If you have the space on a concrete floor it would be wise to chalk out your frame before cutting and assembly.
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post #6 of 9 Old 09-04-2013, 05:06 AM
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Are all 4 pieces the exact same width from end to end and perfectly 100% straight with no bow?? How are your corners done, mitered or butt? If butt are both the short pieces on the inside edge of the long pieces or is one of them on the END of the long ends???? 2" sounds like maybe on end of the frame has the sort piece inside the long and the other end of that short piece is on the outside of the other long .. would be out by the width of the board see bad drawing below . If you are just laying out the wood and trying to measure, what is keeping the parts from moving, do you have the anchored with something to hold them tight and keep them still?
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post #7 of 9 Old 09-04-2013, 06:10 AM
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There was a crooked man...who walked a crooked mile.....

Are you applying a Velvet Trim? If so, simply build your Trim so that it matches the projected image dimensions -1/4".

Sometimes Miter Saws are not exactly accurate set a 45 degrees. Or even at 90 degrees. Using a Carpenter's Square of at least 12" on each side is better....18" even more so.

When such a thing happens, it's usally a case where the angle of the Miter Cut is off a wee bit. Use a Triangle Square to check and set the saw to whatever it states is correct.

However, if your butting the ends together, then you should use Metal Corner Braces and use their Straight edges to align the Lumber at each corner. This will eliminate any error due to a Saw's lack of inability to make a "true straight" 90 degree cut.
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post #8 of 9 Old 09-04-2013, 08:21 AM - Thread Starter
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Quote:
Originally Posted by travisbickle View Post

The only way to ensure your frame is square is to measure diagonally before you fasten or gusset the corners. Your chop saw and the lumber are not square or straight enough at the scale youre working at to assume your frame will be square without crosschecking the diagonals.

If you have the space on a concrete floor it would be wise to chalk out your frame before cutting and assembly.


Yep, should have and didn't. Frame is already braced and glued with plywood corners so I can forget about any re-positioning. Overlaying my other 1x4's (or velvet) to ensure a "square" over this "non-square" will just look goofy. I didn't miter, these are simple butt joints. Had I just ensured things before clamping my braces (like I usually do) this would all be a non-issue of course. It's about $15 worth of material so no real big loss. I'll simply cut more lengths and have a do-over.

I'd bet money it's my saw and I didn't take the time to check it...oh well, live and learn.


James

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

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post #9 of 9 Old 09-04-2013, 08:26 AM - Thread Starter
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Quote:
Originally Posted by airscapes View Post

Are all 4 pieces the exact same width from end to end and perfectly 100% straight with no bow?? How are your corners done, mitered or butt? If butt are both the short pieces on the inside edge of the long pieces or is one of them on the END of the long ends???? 2" sounds like maybe on end of the frame has the sort piece inside the long and the other end of that short piece is on the outside of the other long .. would be out by the width of the board see bad drawing below . If you are just laying out the wood and trying to measure, what is keeping the parts from moving, do you have the anchored with something to hold them tight and keep them still?
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Of course both sides are within the interior of the top and bottom...but unless they're perfectly square to them you will (and I DID) have run-out. I had the pieces clamped while measuring, but I never checked the diagonals. The pieces are of uniform width and identical in length...I laid them atop one another. The ends though are almost certainly to be what is off and over twelve feet that could (and does) make the difference.

In a hurry I likely lined an edge up with an out of whack edge which means the ID's would be identical but the diagonal, ummm, no. I suppose this thread was really to vent more than anything else. I knew full well something wasn't perfectly square somewhere, it's just a shame that such a small variance becomes a monster at these dimensions, lmao.

It's alright, I'll take the extra 30 seconds to get it right this time. rolleyes.giftongue.gifwink.gif

James

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

mastermaybe is online now  
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