Table Saw Exension: A Tale of Woe... and now what?

[Erich Weidner]

Well, ever since I saw that FWW article about Phenolic coated Plywood, my existing folding extension table had lost its luster. (Which was made of a sheet of melamine coated MDF) Especially as it serves double duty as my assembly table, cleaning up the stuck on dried glue was taking its toll on its once perfect surface (though it was still very, very flat).

When I recently (last month) sold my old TS and upgraded to a 3HP SawStop Professional, I was decided on building it out of a phenolic plywood top.
The plan was to have a 12" wide table which was bolted to the rear rail of the TS and with two diagonal braces attached to a steel angle which I screwed into the rear of the TS for support further than just at the point where it was bolted to the TS rear fence. (I adapted this from some plans I found online.) There is a second cut of the phenolic ply which is an additional 30" which is attached to the first piece via a piano hinge.
Here are the original plans so you have an idea of what I'm talking about: http://sawsndust.com/p-outfeed.htm

The only source I could find in Austin was Fine Lumber and Plywood (which is fine, this is were I buy my wood from anyway). A full sheet was in the neighborhood of $150. It never occurred to me to check it for being flat. It looked flat. It was $150, it must be flat... (For the curious it is "Finnform", manufactured for the concrete market)

For some reason no matter how much I planed the frame pieces they would wobble end to end when placed against my flat phenolic ply. The best reference I had was the table saw surface itself (which was serving as the work table for the larger of the two pieces of phenolic ply so I couldn't get to it), my Sjoberg workbench and a 2' steel straightedge. (I placed an order for some longer ones from Veritas, but it took several weeks for them to show up, more on this later).

By now I'm sure you figured out what it took me two weekends to get into my head. Indeed the plywood was not flat.
I assumed my 1-1/2" tall x 3/4" poplar frame (attached with pocket hole screws) would resolve this issue and suck everything flat.
After finally getting the 12" piece attached (and it does measure acceptably pretty flat at the edge which is bolted to the big rear "L" steel rail on the back of the TS, I prepared to finish the assembly and get to cutting sheet stock on the TS for the next project...

Fast forward four weeks to last night. With my wife's help I managed to get the "hanging" portion hung and screwed into the piano hinge. Today I went out after work, excited to get started on the pile of baltic birch ply earmarked to become a dedicated SCMS table!

First off, I did a few passes of boards with the saw off to make sure the outfeed table wouldn't pose any safety risks... I had deliberately dropped the first piece 1/16" below the actual TS surface, and aimed to get the 2nd piece about 1/32" below that to ensure no snagging of work being pushed on through the table saw. Well, the cup on this larger section of ply results in the rightmost edge being about 1/16" proud of the section it mates up with.

In the meantime, my Veritas 50" straight edge had arrived.
First I quietly cursed a bit then got back to work...

I've now pulled off the frame from the 48"x30" part, placed it back upside down on the tablesaw and checked it. Neither the frame, nor the ply is flat. Since the poplar is already painted, further planing on it is not an option. I'll consider it expendable and a lesson learned to get a better workbench for jointing... that is another story.

So now what?

Do I get aluminum (or steel) angle bars, or square tube (aluminum or steel) and try to force the plywood flat by screwing this/these in addition to, or instead of a new wooden frame?
Thicker wood? I'd really like to salvage this project if possible.
Photo of things as they stand now...
In the second and third photos I'm holding the straightedge "flat" against the left part of the ply, you can see the gap beneath the straightedge. About 1/8".

223022223023223024

I'm hoping someone has some suggestions.

[Kevin W Johnson]

I'm sure others will chime in with ideas, but my first thought would be to cut relief slots on the underside to allow it to flatten when attched to some steel square tube. It should be easier to find square tube that is straight then it will be for angle iron. You can also use a smaller size square tube than what would be required in angle iron to prevent pulling a curve in the angle iron.


BTW, Welcome to the Creek.

[ian maybury]

That's a bummer Erich, especially as it was so pricey. I guess a bit depends on how the sheet is curved. The phenolic probably stiffens it quite a bit too. It'd could take quite a lot to pull it straight if it's bent in two dimensions. You'd probably have to screw it to a strong grid like frame which isn't the easiest to organise.

If it's just in one dimension, then maybe some good deep section (= stiff) lengths of straight aluminium channel or box would do the job, and could be fitted in around the existing frame?

Lots of trouble, and possibly quite a cost for a work around. Might it be just as straightforward to hand pick some flat sheet and re-skin your existing frame?

ian

PS if it's badly warped (especially if in two directions) is it possible the problem is something to do with only one side being skinned, or some imbalance in whatever is coating both sides>

[Kevin W Johnson]

I'm gonna bet that aluminum isn't going to work on this one unless a lot of relief cuts are made. I'd still go with steel square tube, once built (flat), you should never have to worry about it again, ever.

[Eric DeSilva]

I'm not sure I understand completely. Even 3/4" ply has some serious flex over 30" if it isn't supported. I'm not sure I would assume that ply would lie flat with just a frame around it unless it also had a torsion box-like affair underneath it as well? Or am I too cynical about plywood?

[johnny means]

Erich, your apron wasn't tall enough to hold the ply flat. I would forget about the metal tubing and such. Just use a thicker apron, preferably with some type of plywood. I like 4" for a table support grid. It would be plenty stiff and pull that sheet flat as a sheet of glass.

[Erich Weidner]

That's a bummer Erich, especially as it was so pricey. I guess a bit depends on how the sheet is curved.

With the 30"x48" section laying on top of the tablesaw (blade retracted of course), and tablesaw being dead flat, it is upside down. In this position as I gaze at it it is convex. I am able to press down with both hands on the middle and see it go flat against the tablesaw table, so it looks to my eye that it can be coaxed into place with enough force. I just double-checked, the warping looks pretty consistent. That is, the convex shape seems to be the same.

As I stand behind the saw (as in the photos), the middle of the ply is humped (convex) up. I already have holes from the pocket joints pockmarking this face of the ply so it will need to remain the "bottom" side of the top on whatever the new frame is.

So... a few different opinions on whether to go wood or metal for a frame. I do want to keep weight down as the large panel does need to hang from the piano hinge when it is not in use. I did get a Stainless Steel hinge from Lee-Valley though, so it probably will be able to deal with the extra weight if need be.

[Erich Weidner]

Erich, your apron wasn't tall enough to hold the ply flat. I would forget about the metal tubing and such. Just use a thicker apron, preferably with some type of plywood. I like 4" for a table support grid. It would be plenty stiff and pull that sheet flat as a sheet of glass.

What sort of grid spacing where you thinking? Do you suggest plywood vs metal due to cost, ease of assembly, other? :)
If possible I'd like to add the least amount of weight I need to. It was a pretty heavy top even with just the 1-1/2" poplar frame. Which of course was why I was thinking aluminum channel or angle. I do know I want to only build this one more time. :)

[glenn bradley]

Maybe like so. Strips of ply ripped to dead straight. Counterbore panhead screws from the edge of the grid members so that they extend into the ply top about 5/8".

[Erich Weidner]

Maybe like so. Strips of ply ripped to dead straight. Counterbore panhead screws from the edge of the grid members so that they extend into the ply top about 5/8".

Would counter-boring be better than pocket holes? Or no real effect in strength? (I figure the pocket holes due to their angle might be stronger, with more thread in the ply).

[ian maybury]

Hi Erich. I'm not sure i understand your description of the curve. What i meant was that if the ply has pulled so that the surface curves in every direction like a ball, then there's a likelihood that strips running in one direction like Glenn has drawn might not straighten it. If the on the other hand the curve is in one direction (like the sheet is rolled around a cylinder), then placing stiffeners at right angles to that might help. I missed it in your post, but either way a the guys say an 1 1/2 x 3/4 frame doesn't seem very likely to pull 3/4 in ply straight.

Still think there's an argument for starting again with a flat sheet...

ian

[Erich Weidner]

If the on the other hand the curve is in one direction (like the sheet is rolled around a cylinder), then placing stiffeners at right angles to that might help.



Still think there's an argument for starting again with a flat sheet...
ian
I have a feeling the whole stack is going to end up being like this. But only one way to find out.
Then the next question is what do I do with all this expensive phenolic ply? :)

[glenn bradley]

Would counter-boring be better than pocket holes? Or no real effect in strength? (I figure the pocket holes due to their angle might be stronger, with more thread in the ply).In my experience, pocket holes would be fine. You will maximize the benefit of you can glue the strips on. Is the underside Phenolic as well? If so, epoxy(?). I would do a dry run first to assure you are getting what you are after. If the strips achieve your goal the epoxy will give you superior shear strength and will not creep like PVA can sometimes do. In looking at the pics it looks like you have plenty of room for 2-1/2" to 3" tall strips. you'll have to judge the required strength versus the added weight. The ribs need to be adequate to overpower the curve. Of course if clamped straight and epoxied, you'll be in pretty good shape. I have also done this to overcome weight issues.

[ian maybury]

I wonder if you strip out the current framing would you have enough ply left to skin the bottom too? Then treat it as though you are making a torsion box assembly table roughly along these lines: http://thewoodwhisperer.com/my-new-assembly-table/

Position the bottom skin so that as far as possible its curvature opposes that of the top skin.

Glue it up over a large flat surface like an accurately finished workbench top - with enough weighting down to hold everything flat. Chances are that gluing will hold it 100% in alignment if it's clamped flat while curing.

I'm not sure about gluing to the phenolic, there's a possibility it mightn't work that well - but i guess it wouldn't be too hard to skim the glue lines with a router to expose wood. Epoxy may manage it (but needs checking out), it's pretty compatible with phenolic reins and you could get a slow one giving plenty of time.

Just another possibility..

ian

[Bruce Page]

Erich, I had a similar issue when I made my outfeed table out of melamine covered particle board. My solution was to clamp the table to a flat surface and route slots to just below the melamine coating and then, keeping it clamped, glue & screw some straight stiffening boards to it. It worked like a charm. I used PVA glue but epoxy would be a better choice.