Plywood instability


Cygnus A

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Hey guys. I've have a tremendous amount of problems with plywood warping.

Almost all of the jigs I've made have warped to some degree including my cross cut sled. It is pushing me to think of using MDF for my jigs. Has anyone else had this problem? I am using HD/Lowes ply (1/2 and 3/4) and I live in an extremely hot/dry environment (Arizona).

The worst case was my outfeed table. My plan was to screw 2 sheets of ply together to form a 1.5" thick top. While building the base, the ply sheets sat in my shop for maybe 2 weeks. When I tried to build the top, the warpage was enough that I couldn't even force the two sheets to meet flush across their length, even with a significant weight applied. I ended up using 1 sheet of ply and 1 sheet of MDF which worked out OK.

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You need to go to a better lumber yard that carries better quality plywood.

I've had very good experience with Baltic Birch, the real stuff that comes in 5'x5' sheets. More expensive, but so much stronger, flatter & more stable.

Something to watch for with BB though, is that sometimes the sheets aren't exactly square. Like maybe an 1/8" off.

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I agree with Dzaius but find that the the cost of using baltic birch to be more than I am willing to pay unless it is for a purpose that I will get a lot of use out of.  My outfeed table is a double layer of 3/4" MDF screwed together with countersunk screws with supporting structure about 24" apart underneath.  Since it is relatively easy and cheap to replace it someday I don't worry too much about wear and tear.  Most of my jigs, except for TS sleds are MDF.

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BB ply is my goto for jigs that have along life expectancy. Nice and stable while still lightweight.

Before you up the quality of your materials you may want to assess your environment. If your warping is due to moisture the mdf will swell and fail spectacularly.  Even a high quality exterior product can only take so much.

if on the other hand the current ply you are using is just poor I soul try some BB ply and see what improvement you get.  For short term or infrequent use mdf would do fine.

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I've used a decent amount of ply from box stores, neither HD or Lowes though, and haven't had any issues with warping. The store that i go to has multiple different grades on hand and i make sure to buy the highest quality offered. I probably have 4-5 sheets in my basement where my humidity swings from 15% to 60% seasonally and i still haven't had warping. I'd recommend to try a different supplier.

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Let me just get up on my soap box for a moment.

I used to use a lot of MDF for jigs, cabinets, you name it. And I hated every minute of working with the stuff. BB is only about 2x the cost of MDF and for the amount of it that I use for shop stuff that isn't going to break the bank. It's just so much better in almost every way.

Occasionally I'll dig out a scrap of MDF & use it for something & then hate myself after.

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34 minutes ago, drzaius said:

Let me just get up on my soap box for a moment.

I used to use a lot of MDF for jigs, cabinets, you name it. And I hated every minute of working with the stuff. BB is only about 2x the cost of MDF and for the amount of it that I use for shop stuff that isn't going to break the bank. It's just so much better in almost every way.

Occasionally I'll dig out a scrap of MDF & use it for something & then hate myself after.

What he said :)

 

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47 minutes ago, drzaius said:

Let me just get up on my soap box for a moment.

I used to use a lot of MDF for jigs, cabinets, you name it. And I hated every minute of working with the stuff. BB is only about 2x the cost of MDF and for the amount of it that I use for shop stuff that isn't going to break the bank. It's just so much better in almost every way.

Occasionally I'll dig out a scrap of MDF & use it for something & then hate myself after.

Yeah, exactly this. I can't stand working with MDF, and for the number of jigs and other things I usually get from a single sheet of something it's just not worth the cost savings.

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1 minute ago, Mike. said:

I don't make a lot of jigs, but discussions like this make me wonder why people don't use aluminum.    Most of my jigs are single use and I throw them away when done, but If I were building anything for repeated use I would use aluminum or baltic birch.

I have never had good luck with box store plywood.  It is crap.  

 

 

Cheap ply bad for making router patterns because the soft inner plys tend to compress after running the guide bearing over them a few times. And joining 2 pieces of MDF at 90* is just bad day waiting to happen.

Now I've got myself going again...

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I have used big box store ply for most of my shop furniture and jigs.  For the most part it is pretty stable, except for the thin sheets.  The show faces are all thin and there are lots of voids.  It's different all over the country though, you have to learn what to expect from what is available to you.  It seems you have!  Like said above, you need to find a better supplier who has better quality.

If you're like me though, buying sheet goods is a pain in the butt.  I don't have a truck and it is sooo easy to have orange or blue rip their junk down to fit in my SUV, which is the only reason I have used it so often.  I need to figure out another option for future projects.

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I agree with Bleedinblue, not all box store ply is equal. My local stores carry "hardwood" ply in red oak and birch, that is decent for the price ($40-50 per 3/4" 4'x8'sheet in my area).

You ARE using hardwood ply, not construction ply, right?

Also, I never buy MDF from the local home centers, mostly because it always has the crap beat out of the edges. And I hate working MDF. If your dust collection isn't perfect, it makes an incredible mess.

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On 6/16/2016 at 9:49 PM, wtnhighlander said:

You ARE using hardwood ply, not construction ply, right?

Actually, I've found that, within the pantheon of home center junk, the construction grade plywood is actually less bad than what they try to sell as "high quality" plywood.  Not sure why that is.  Maybe my expectations are simply lower so it just doesn't seem as bad.

On 6/16/2016 at 11:59 PM, gee-dub said:

You should check out Pat Warner's site.

Thinking back over the lackluster jigs I've made and then perusing Pat's site made me feel inadequate.

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On ‎6‎/‎16‎/‎2016 at 6:49 PM, wtnhighlander said:

I agree with Bleedinblue, not all box store ply is equal. My local stores carry "hardwood" ply in red oak and birch, that is decent for the price ($40-50 per 3/4" 4'x8'sheet in my area).

You ARE using hardwood ply, not construction ply, right?

Also, I never buy MDF from the local home centers, mostly because it always has the crap beat out of the edges. And I hate working MDF. If your dust collection isn't perfect, it makes an incredible mess.

I am using the nicest HD/Lowes has to offer. I live in the desert so moisture in the shop is not the issue. I will try a couple sheets from the lumber supply, but that is a good hour drive from me so isn't a good option most of the time. I was just curious if others had similar issues.

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My HD started carrying what they call 'cabinet grade ply' a few months ago. It's kind of like BB-lite or maybe on the same level as Aruco. Good deal for $30 for a 4x8 sheet though. It has about 7 plys in a 3/4 sheet and few voids but you still get overlapped plys and a lot of dutchmen and other fills.

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I haven't had any issues with warping of plywood from the big bog store. My issue with their plywood is de-lamination. Even the domestic stuff is low quality and overpriced. 

I have started using baltic birch for everything. My nearest supplier in San Antonio, TX has 3/4" 5'x5' sheets for under $30 a sheet so its very economical as well.

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