Pine plywood - it's cheap, but is it much worse than, say, birch ply?

[Florin Andrei]

I'm making a machine for grinding optical components. The material is 3/4" plywood. There will also be a 1/4 HP motor in it, various moving parts, pulleys, belts, etc. The design is not mine, many such machines have been built and work well. The end result will be a rectangular box just about as big and as heavy as to be comfortably carried by two people (or by one big and strong guy).

I'll use butted joints, with wood screws and epoxy. No fancy techniques, I'm just an amateur.

Now, I'm trying to reduce the cost. I did some (smaller) things before, using baltic birch ply, and it was awesome. Super smooth, solid, kinda heavy but strong and durable. Loved it. But that stuff is expensive.

I went today to a local shop and noticed big sheets of 3/4" pine plywood. It was cheaper than similar-sized other types, and it was a lot of plys too - at least 7-ply, if not 9 (I didn't look too closely). The finish was pretty nice as well. Based on a cursory look, it didn't seem to have any holes or empty spaces inside. It did look a lot like the much more expensive baltic birch ply I used before, except for the price.

My question is, what's the catch? Is it in any way inferior to other kinds of plywood? Is it less strong?

[Mike Cutler]

It all depends on what it's intended use is.

There are about a zillion, it seems, standards and grades of plywood out there. If selected for the proper application all is well, but there is some real garbage out there also. Generally anything sourced from China sold in the big box stores has a notorious reputation for instability. It seems that trend may be reversing as I've seen mostly ply sourced from Canada lately in the big box stores.
All things being equal, a softwood plywood will not be as strong as a hardwood plywood if they are made the same. Again though, how strong does it need to be.
As much as I hate the stuff, it sounds like MDF may be what you are looking for. If the final form isn't subject to stress, MDF being an incredibly stable material may be worth a look.

[Danny Hamsley]

It if is southern yellow pine plywood, it is plenty strong. It is made in the Southern US, not china like a lot of the hardwood plywood. It should do just fine.

[Jeff Duncan]

Agree with Mike, mdf or even PB will save you money, usually less than half the cost of plywood. It will be a bit heavier, and takes a little more thought/planning on how to build with it. But may be a possibility for your design while cutting costs.

good luck,
JeffD

[Mark Blatter]

Not to be difficult, but while I have had regrets using particle board and cheaper ply (due to voids, warping, etc.) I have never had regrets using better products. I love Baltic Birch and believe it is one of the best plys available. I also don't believe it is that expensive. I am paying $27 for a 60" x 60" x 1/2 and am sure other get it for less.

[Duane Meadows]

Not to be difficult, but while I have had regrets using particle board and cheaper ply (due to voids, warping, etc.) I have never had regrets using better products. I love Baltic Birch and believe it is one of the best plys available. I also don't believe it is that expensive. I am paying $27 for a 60" x 60" x 1/2 and am sure other get it for less.

If I could get anywhere near that price, I'd use a lot of it! More like $75 here, and generally special order.

[Mark Blatter]

In 2009 when I was buy 1/2" and 3/4" by the unit I was getting the 3/4" for $29 a sheet and 1/2" for $22. Those days are gone for good.

[Florin Andrei]

All things being equal, a softwood plywood will not be as strong as a hardwood plywood if they are made the same. Again though, how strong does it need to be.
As much as I hate the stuff, it sounds like MDF may be what you are looking for. If the final form isn't subject to stress, MDF being an incredibly stable material may be worth a look.

Actually, there is some amount of torque from the motor to the swing arm. Here's a video of a finished machine in action. You can only see the top side; beneath it there's the box with pulleys, belts, and the 1/4 HP 1725 RPM motor. Everything except the wheels and a few rods is 3/4" plywood.

https://www.youtube.com/watch?v=Vmi3FOs4Zwg

So the strength of the structure is one of the design constraints.

The plans require 3/4" ply, but do not specify the type. There are dozens of machines out there built on these exact plans. I'm sure people use all sorts of different materials. But if there is any concern that the chosen material is not strong enough, I'd rather take more time and do it right.

I don't know much about the various kinds of woods, so that's why I posted the question. Every bit of information helps, thank you.

[Tai Fu]

If I could even get a 5' x 5' sheet of 3/4 birch ply for 75 dollars I would totally jump on it. Price for me is more like 120 dollars for 2 half sheets (a 5' x 5' sheet cut in half to make transportation realistic). They are incredibly heavy and stable though. I can't even source it from a big box store, or any conceivable stores I can find in Taiwan... I have to special order it from a guy who builds amp cabinets, and he only has it in 3/4" sheets.

Thing is, 3 x 6 crap plywood is only half the price so for me it's not that much cheaper for a much worse product (the crap plywood is lauan and it would often warp badly, and the thing is light and splintery to boot, it also does not hold a screw at all... even MDF could hold a screw better than that) but it is the ONLY plywood available through retail channels.

[glenn bradley]

There are dozens of machines out there built on these exact plans. I'm sure people use all sorts of different materials. But if there is any concern that the chosen material is not strong enough, I'd rather take more time and do it right. I don't know much about the various kinds of woods, so that's why I posted the question. Every bit of information helps, thank you.

After watching the video it appears to be a pretty low force situation. If the ply you are looking at is reasonably flat, low in voids and stable I would go ahead if the savings were significant. I have kind of a 60% rule. If I have to pay 60% of what a "good one" would cost, I buy the "good one".

[Jeff Duncan]

I have to agree, that seems to be a pretty low force system comparatively speaking. Compared to some of the cabinets that get used and abused on a daily basis that is! ....that looks like something that will be handled with a great amount of care.

So my first thought when you say pine ply is stuff made for sheathing and similar applications which is usually rife with voids and fairly rough....even the better sanded stuff! If that's what your talking about then it explains why it's cheaper and also why it's not something I use very often outside of it's normal applications;) You can also tell b/c the plies will be softwoods as well. The quality of the particular sheet you get is likely going to be hit-or-miss based on who made it. Oh and the stuff can start moving on you as you cut it! Good quality hardwood veneered ply usually has a more consistent core and is usually composed of hardwoods like poplar....usually! There's still a good possibility you'll end up with voids....it's just the nature of modern ply. Baltic birch is more refined and generally pretty void free. A 5x5 in 1/2" runs about $40 in this area.

If I was making it and it had to be pretty accurate particle board would be my first choice, mdf my second, and plywood my 3rd. If I had to go with ply I think I'd buy whatever was easiest and not worry about the voids so much, you can fill them as needed. Keeping things flat with ply will require more of your focus than the voids in my opinion.

good luck,
JeffD

[Aleks Hunter]

+12 on the USA or euro made plywood. Examine the edges for voids, a sign of cheapness. IF you want to stiffen it up a it get some fiberglass cloth and epoxy resin. Boat building supply houses are good sources.

[Curt Harms]

I don't know why it seems hard to find but there is a U.S. made ply called Appleply. It's supposed to be equivalent to Baltic Birch. I don't see any prices. Here are places in CA. that are supposed to stock it:

La Mirada Forest Plywood (714)-523-1721

Northridge Far West Plywood (818)-885-1511


Van Nuys Valencia Lumber & Panel (818)-786-5800


Rancho Cucamonga Answer Wood Products (909)-532-8000


Tustin Weber Plywood (714)-259-1100


Anaheim Reel Lumber (714)-991-7121


San Leandro Moore-Newton (510)-317-5157


Berkeley Macbeath Hardwoods (510)-843.9378


San Jose Macbeath Hardwoods (408)-526-9663 95131


Windsor Mount Storm Forest Products (410)-820-8773


Sacramento Capitol Plywood (916)-922-8861


Sacramento National Wood Products Inc. (916)-381-7311

http://appleply.com/pdf/AppleplyStockingDistributors.pdf

[Rich Engelhardt]

My question is, what's the catch? Is it in any way inferior to other kinds of plywood? Is it less strong?After seeing the video of what you're doing, I have a couple of questions.

How much time, effort and money go into the actual cost of grinding a mirror?

Is it something that, if the case moves around a tiny bit, is going to result in a piece of scrap glass?
If that's the case, then how much are you out if that happens?
Is it only a couple of bucks or are we talking a serious investment in money and time?

[Jacob Reverb]

I'd use the cheaper plywood and instead of epoxy, I would use screws and glue blocks on the inside corners fastened with a brad gun and Titebond II. If your joints are tight, it should be plenty strong.

[Florin Andrei]

I don't know why it seems hard to find but there is a U.S. made ply called Appleply. It's supposed to be equivalent to Baltic Birch.

Great suggestion, thank you, I added Appleply to my list.


How much time, effort and money go into the actual cost of grinding a mirror?
Is it something that, if the case moves around a tiny bit, is going to result in a piece of scrap glass?
If that's the case, then how much are you out if that happens?
Is it only a couple of bucks or are we talking a serious investment in money and time?

A finished telescope mirror that passes all quality tests and is more than mediocre is probably the most precisely manufactured object that you own. The reflective side must follow a paraboloid shape with a precision better than 0.1 microns (1 micron = 0.001 milimeters) over the entire surface.

However, this precision is not obtained by applying extremely precise strokes with grinding / polishing tools. If that were the case, it would be impossible to make a mirror by hand - in reality, mirrors made by hand can be very high quality indeed. Instead, the very precise shape is obtained as an average of a very large number of strokes, each stroke performed a bit "loosely", but all strokes following a general shape, more or less. One problem with machines is that they are too precise, so you have to design it in such a way as to avoid printing a pattern into the mirror due to the motion being too regular.

Initial grinding is not very precise, you just need to remove lots of material quickly. But in the final polishing and figuring stages, very sensitive tests are required to show you the shape and tell you how to modify the polishing strokes so that the surface is gently nudged towards perfection. It's very high-volume, painstaking work. Polish, test, adjust stroke, polish again, test again, and so on.

The grinding / polishing machine slowly shifting shape in time (e.g. from plywood absorbing some humidity) is probably not a problem. This is akin to working by hand and not sitting always in the exact same position. That's okay.

What's NOT okay is having any amount of play in the hinges that keep the swingarm in place. 1 mm of play when the swingarm turns around would "print" a microscopic pattern into the mirror that would be quite unusable. It's very important that the motion is smooth, even though it's not necessarily precise. The whole assembly must be pretty rigid, so as to avoid fits and starts in the final stages of polishing due to machine parts flapping around on loose hinges or bearings. Again, there are many machines made of 3/4" plywood that work great, so this is obviously doable.

Time investment: an absolute beginner working alone, by hand, not full time, should expect to spend about 6 months making a small mirror (6" ... 8"), or longer for a bigger mirror. With experience, time is reduced. A machine also helps reduce time a lot. In any case, a large mirror is a significant time investment no matter what.

Money: A low-expansion glass blank for a 12.5" mirror, plus all the grinding / polishing supplies, is about $400 ... 500. Most mirrors made on a machine are even bigger, so therefore more expensive. It's not a trivial investment.

Most surface errors can be corrected if you own a machine. It's pretty hard to ruin a mirror, except if you drop it and it breaks. Other than that, a loose machine printing a pattern into the glass - just fix the bearings, tidy up the joints, and start over. But you need to own and spend large amounts of a currency called time.


I'd use the cheaper plywood and instead of epoxy, I would use screws and glue blocks on the inside corners fastened with a brad gun and Titebond II. If your joints are tight, it should be plenty strong.

Thanks for the idea, I'm taking notes.

[George Gyulatyan]

I don't know why it seems hard to find but there is a U.S. made ply called Appleply. It's supposed to be equivalent to Baltic Birch. I don't see any prices. Here are places in CA. that are supposed to stock it:

La Mirada Forest Plywood (714)-523-1721

Northridge Far West Plywood (818)-885-1511
Unfortunately Appleply as well as Europly (mfg. by Columbia Forest Products) are more expensive than Baltic Birch. I got shop-grade 4 x 8 sheet of appleply from Far West Plywood and it was around $120. Europly from Philips Plywood and Anderson Plywood is anywhere from $115 to over $150 depending on grade, thickness and unfinished vs finished.

Having said that, they typically have rotary cut maple veneer which is much smoother than BB faces and anything above shop grade has no knots or filled voids on the face veneers. Shop Grade appleply can be pretty ugly though.

Florin, if the pine plywood looks like a multiply similar to Baltic Birch you're probably talking about Araucoply: http://araucoply.com/index.asp

I've seen them at one of the home centers recently and it looked pretty good. Woodsmith magazine had an article on it in their December 2012 issue (#204) and rated it rather well.

[Rich Engelhardt]

But you need to own and spend large amounts of a currency called time.That would pretty much answer things for me.
I'd gladly pay a premium price for Baltic Birch knowing it was the best material and the price wasn't too far out of line.