Trick for inspecting plywood

The short version: use a really bright flashlight to shine through the plywood, and you can see inclusions and voids pretty easily, like x-ray vision.

Longer: So, I’ve had a few cuts fail where inconsistency is in baltic birch plywood that I got at the local art store caused the pieces to not be fully cut. Upon inspection, it seems pretty obvious that there was a knot in the central sandwiched layer.

So I took a bright flashlight and looked through my 1/8 inch plywood, and found some surprising things, including what appears to be a void.

Suspect plywood:

Clear section, a nice orange spotlight:

Bright spot, a void:

Suspicious dark area:

Mystery solved, it was the label on the back. Doh!

I figure I’ll use tape to mark problem areas ahead of cuts.

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Very good idea.
One of those things that you probably knew in the back of your mind, but failed to bring to the surface.

Only one question now.
Where did I put that bright flashlight ?

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Hrm… maybe have to make a light table w/a boatload of GU-10 LEDs in a grid.

Or a strip of GU-10 LEDs. They are low temp enough that a laser cut box w/a bit of translucent frosted acrylic on top would work well. May want to put a fan at one end.

<adding to project list and pining, again, for 40 hour days>

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ah this is a great idea!

adds to mental toolbox

thanks!

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BTW if you don’t have a new-ish LED flashlight, they are astonishingly cheap and far brighter than your old mag lite.

Search for “Cree XM-L2 flashlight” or “Cree XP-HI” flashlight. They’re super cheap and cheap, respectively. The XP-HI lights are slightly brighter, but they cost more.

Also, be sure to get one that has a decently large reflector, stay away from ones that look like pure cylinders. The metal bodies are important as heat sinks, the skinnier types don’t dissipate the heat as nicely.

If you want to get really nerdy try to find one that is warm colored, ones that are “daylight” or “5000K” light temperature will be pretty harsh and bluish. Try to find “warm white” or “3000K” or so, it’s a more pleasant flashlight to use.

They are powered by lithium batteries (specifically 18650 size usually)… some have usb charging ports, others require a charger. Caveat Emptor.

Update — adding the details here so it doesn’t get lost in the thread:

So I’ve refined my technique a little here, and have 2 alternate methods.

1: Turns out if you are at home and can make your room nice and dark, a bare light bulb works great.

What I do now is mask the ply, then turn off all the lights except for one table lamp. I hold the sheet up with one hand in front of the light bulb and move it around to identify dark spots, marking them in pencil on the masking material. It’s easier than a flashlight, requires nothing that you probably don’t already have around (a light bulb), and allows for much more thorough and faster inspection (it’s a larger “x-ray” than a typical flashlight).

2: The other is to use the sun with some way to block the light.
Get a box, (or cut one with your nifty laser cutter), cut a small hole in the bottom, then take the whole operation outside in bright sun. Cover the box with your baltic, aim at the sun, and look through the hole on the bottom. Really easy to spot defects this way too, if it is a bit clunky. Sometimes you gotta laser and you just don’t have a dark room, you know?

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That’s a great trick! I don’t use much plywood from local stores for that exact reason…tends to be pretty crappy. (At least this way i can use up what I’ve bought without cussing.)

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I have seen all kinds of stuff used to fill, up to and including Bondo body filler. Never tried to cut that but I can predict the result.

3…2…1… rabbit hole!

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http://www.oveready.com

While the rabbit hole is still open…

There’s something really satisfying about seeing the UPC label chewed up by the cuts.

Thank you for the tip!

Ooo, pretty! But I’m more a $7 light guy.

But in the spirit of the rabbit hole:

https://www.silentthunderordnance.com/

Check out the Lance of Ra. impractical but strangely interesting :slight_smile:

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When the beam throw photo needs a map… you might just be a big flashlight. I like the naming of them out of the mythic supernatural.

I’ve never had an overready, kind of like a Ferrari you just sort of marvel at. Have had and edc’ed two surefires for the last ten+ years though, and despite that company not keeping up with today’s lumen count, I always thought their build quality was nice.

Cool! I wonder if HD would object to someone checking before purchase.

I doubt they would any more than they object to someone (me) going through the pile of 2x4s to try to find 5 that are actually straight and knot free out of the 400 in the pile.

Just don’t make a mess.

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Depends on your definition of astonishingly cheap. :rofl:

When doing my light painting I prefer the $1.99 flashlights at Walmart but I suspect they wouldn’t help much here. :wink: It is getting harder and harder to find flashlights with incandescent bulbs and that makes me sad (again from a photographic perspective).

Then again, my primary go to now is a $500 “flashlight” but that’s a whole different rabbit hole. :smiley:

So I’ve refined my technique a little here, and have 2 alternate methods.

1: Turns out if you are at home and can make your room nice and dark, a bare light bulb works great.

What I do now is mask the ply, then turn off all the lights except for one table lamp. I hold the sheet up with one hand in front of the light bulb and move it around to identify dark spots, marking them in pencil on the masking material. It’s easier than a flashlight, requires nothing that you probably don’t already have around (a light bulb), and allows for much more thorough and faster inspection (it’s a larger “x-ray” than a typical flashlight).

2: The other is to use the sun with some way to block the light.

Get a box, (or cut one with your nifty laser cutter), cut a small hole in the bottom, then take the whole operation outside in bright sun. Cover the box with your baltic, aim at the sun, and look through the hole on the bottom. Really easy to spot defects this way too, if it is a bit clunky. Sometimes you gotta laser and you just don’t have a dark room, you know?

3 Likes