Engrave Back or Front

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ALMARQ
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Engrave Back or Front

Post by ALMARQ » Sat Mar 19, 2016 1:43 am

Hello to the Community,

Need some advice before engraving my next project - I glued up a solid birch panel - approx. dim. 29" x 29".

In the attached images - please take note of the cupping of the panel.

My question - should I engrave on the side that is opposite of the cupping direction - perhaps it may relieve some of the stress on the panel??
I am thinking if I engrave the Concave side - the cupping will increase - if I engrave the Convex side, it is possible it may reduce the cupping.

Depth of engraving approx. 0.180"

Anyone - please share your thoughts on this subject. Thanks.
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Cupped-Panel-2.jpg
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Cupped-Panel-1.jpg
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Atonwa
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Re: Engrave Back or Front

Post by Atonwa » Sat Mar 19, 2016 2:37 am

Well here's how I would tackle it, maybe right or wrong as others can chime in.

From knowing that you cut glyphs (I'm guessing that's what your planning here) and your depth of .18 deep then you can probably get away with leaving your bow as it will never show a difference in your pattern with your depth of cut. It just not may lay flat on the wall.

Three Options:

1. Secure in place leaving the bow up as shown in pics. Zero your Z on the flat portion and maybe lower your depth pass slightly to compensate for the bow when the bit gets to the high part.

2. Secure the board down down hard enough to get rid of the cup with the bow up. Machine as usual and and when it releases the bow will spring back but the depth of cut will be uniform.

3. Secure without removing the bow and face off the board with the bow ends up. To get the board perfectly flat you could then flip and do the same thing to the other side. My guess is it may still bow again although probably not as much. If this is wood from a big box store than it's moisture level may be high or it may be clamping too hard during glue up.

Here is a good thread on the Vectric forum with furniture makers discussing how to do glue ups and get flat panels. It may be easier to go back to birch furniture grade plywood and deal with the cut.

http://forum.vectric.com/viewtopic.php?f=27&t=15995
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ALMARQ
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Location: Gilroy, CA

Re: Engrave Back or Front

Post by ALMARQ » Sat Mar 19, 2016 6:18 pm

Thanks for the link Atonwa. I will look into the use of the furniture grade plywood and make some test pieces.

After reading some woodworking books on my bookshelf - (something I should have done first) - The reason for the cupping, aka. warping, is that Plain-sawn boards will have a tendency to cup in the direction opposite their rings as they dry out.

On this panel, 4 pieces of wood were glued up - one half, the rings are in opposite directions and have the least amount cupping.

The other half of the panel - the growth rings are both in the same direction and have the maximum amount of cupping.

Therefore, for future glue ups of panels - I will need to pay attention to the direction of the growth rings and adjust accordingly or look into purchasing quarter sawn wood.

I will prepare the opposite side of this panel and engrave - I am hoping the engraving will relieve some of the cupping. Will post my results.

Thanks.

woodworkerbob
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Re: Engrave Back or Front

Post by woodworkerbob » Sat Mar 19, 2016 9:25 pm

ALMARQ wrote:Hello to the Community,

Need some advice before engraving my next project - I glued up a solid birch panel - approx. dim. 29" x 29".

In the attached images - please take note of the cupping of the panel.

My question - should I engrave on the side that is opposite of the cupping direction - perhaps it may relieve some of the stress on the panel??
I am thinking if I engrave the Concave side - the cupping will increase - if I engrave the Convex side, it is possible it may reduce the cupping.

Depth of engraving approx. 0.180"

Anyone - please share your thoughts on this subject. Thanks.
Isn't this project perfect for auto leveling? Auto Level lets the "machine" note how the board is warped, then the G-code, the Z axis values, is modified. The theory is that the warp is compensated for won't effect the engraving.

At least that's why I'm trying to get Auto Level to work. See: http://www.shapeoko.com/forum/viewtopic.php?f=6&t=7849

Bob
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atrueresistance
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Re: Engrave Back or Front

Post by atrueresistance » Tue Mar 22, 2016 4:29 pm

Agree with Bob, autolevel would work perfectly. Chilipeppr and bCNC both have that functionality.
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AnonymousPerson
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Re: Engrave Back or Front

Post by AnonymousPerson » Tue Mar 22, 2016 5:11 pm

As a possibly "out there" thought... would it be practical to attempt straightening that board, similar to how people bend wood by heating/steaming it first then applying pressure?
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woodworkerbob
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Re: Engrave Back or Front

Post by woodworkerbob » Tue Mar 22, 2016 11:51 pm

AnonymousPerson wrote:As a possibly "out there" thought... would it be practical to attempt straightening that board, similar to how people bend wood by heating/steaming it first then applying pressure?
Simpler to just use a big sander. The one in our wood shop is 22 inches wide, with an open end so it's possible to sand up to 44 inches. It's perfect for this task, but you could make the board much (perhaps unacceptably) thinner.

Bob
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Brian Stone
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Re: Engrave Back or Front

Post by Brian Stone » Wed Mar 23, 2016 2:54 am

AnonymousPerson wrote:As a possibly "out there" thought... would it be practical to attempt straightening that board, similar to how people bend wood by heating/steaming it first then applying pressure?
I tried that and posted the results of the experiment, but it has caveats. The reply from krtwood in that thread is dead on. The wood is probably bent because one side is more swelled than the other side. If you remove that moisture from one side, then the cells shrink and the wood is going to contract on that side. It will work, but only temporarily. If you let the board sit for several days, then those cells are going to draw in moisture and you'll have a bent board once again. So if you're going to use this method, then you will very likely need to mill the board relatively quickly after flattening it. A better solution is to plane the board flat.
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LTEPM
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Re: Engrave Back or Front

Post by LTEPM » Wed Mar 23, 2016 3:12 am

Looking at the photos, I would suspect the jointed edges glued up are not 90 degrees which is causing the majority of the problem. I don't see significant cupping in the boards. Too much clamping force or uneven clamping can also produce similar results.

Sounds like you have an excuse to purchase a table saw, 12" jointer, thickness planer, and drum sander to correct the issue. :D

WillAdams
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Re: Engrave Back or Front

Post by WillAdams » Wed Mar 23, 2016 10:39 am

Or, a workbench, some clamps, a try square, a jack and a jointer plane and some winding sticks.

http://www.leevalley.com/us/wood/page.a ... 1182,52515
http://www.leevalley.com/us/wood/page.a ... 1182,52515

Remember, precision has to start somewhere.

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