Table flatness

[HopefulSSer]

wouldn't a press straighten it out ?

If you'll note in my original post, I used a press to make it better. But there's quite a lot of springback meaning you have to go well beyond flat in the other direction for it to return to flat(ter). I was hesitant to push it further and risk damaging something so I thought it best to ask the experts.

[JPG]

Depends upon WHAT/WHERE the press is PRESSING. Bending/unbending ALWAYS has spring back.

[HopefulSSer]

Depends upon WHAT/WHERE the press is PRESSING. Bending/unbending ALWAYS has spring back.

Yes, but this was A LOT!

[Hobbyman2]

Depends upon WHAT/WHERE the press is PRESSING. Bending/unbending ALWAYS has spring back.

Yes, but this was A LOT!

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I can not say I have ever seen a piece of metal remain flat once it was bent and straightened , even when heated . molecular realignment can be difficult . a machine shop may be able to bend it back and mill it flat but who is to say it wouldnt bend again and milling may weaken the structural integrity . jmo if it is that bad replace it .

[dusty]

LET IT BE SAID UP FRONT "I am a skeptic about this."

Has it been decided how and when a table will be declared flat?. What are the measurements that represent flat? Then there is the simple question of how flat is flat enough for the sort of work that is to be done. Toys and jigs or "fine furniture". I have made a number of book cases and side tables that I am quite proud of and none of my tables are flat. They all sink in the center to some degree.

Assuming a perfect straight edge - how must it be used to test the table. Corner to corner in all directions. Edge to edge at many locations. Or just examine the table until either satisfied or displeased with the results.

I would start by laying the table upside down on a "known" flat surface and pushing on each corner independently. If the table rocks when pushing on any corner - test is done. Table is not flat and cannot be made to be so (unless you plan is to mill the top surface and I doubt that can be done without compromising the integrity of the table.

[HopefulSSer]

Certainly you'd agree there's a point where a table isn't flat enough, yes? As a thought experiment, if you were to lay a straight edge across the centerline of the table along the axis of the quill, parallel to the way tubes, and there was an 1" drop between the outer edges and the blade slot, surely that's too much, right? Granted that's an impossibly bent table but bear with me. How about 1/2"? Still too much I'm guessing. How about 1/4"? Or 1/8"? Or 0.030"? 0.010"? For me, I think 0.030" is too much. At the end of a 6" square, if you're tipped 0.030" then you're off by almost 3º. Would you be ok with your miter gauge being off square by 3º, or if your rip fence were out that much? I'd be happier closer to dead flat than to the 0.030" or so I started with. What's your limit for acceptable?

And yes to test for completely flat, you'd do just as you say

Corner to corner in all directions. Edge to edge at many locations.

I'd add that that's edge to edge at many locations along both axes. And by "many" I think maybe 5 in each would be sufficient -- edges, centerline, and halfway between. That should be plenty sufficient to quantify any global table distortion. So far I've seen nothing on either table I own that would suggest localized impact damage or for example damage from a clamp or the like.

Milling the table is a non-starter. The miter slots would need to be deepened as well, and the cutout for the blade inserts would also need to be milled, or else the insert(s) milled with the table and you'd sacrifice future interchangeability. I assume there's enough adjustability in the rip fence locking mechanism, but there's at least the possibility of running into problems there as well. And as you point out it could compromise the rigidity of the table (which I guess we could think of as a dynamic out-of-flat condition.)

I get it that you disagree. But to me it seems perfectly reasonable that the better the setup and alignment, the better, more consistent, and easier the results. And if the reference plane you're using for setup isn't reliable, then accurate alignment is going to be more difficult. If that can be remedied, why wouldn't you? If my chisel or plane iron is dull I don't just carry on using it, I sharpen it. I don't see this as really any different -- maintaining tools in optimal working conditions.

[DLB]

In another thread (viewtopic.php?p=761#p761) Nick describes a method (diagonals) and tolerance (0.010" to 0.012") for new tables. That has limited application on used tables, which might be inadvertently damaged in use. The 520 table I mentioned earlier with 0.060"+ low area was measured left to right and front to back. The diagonals would have excluded most of the damaged area, possibly enough to pass that test. If a table is damaged in use it is likely damaged by the force of the quill, so this is where you'd expect to find the worst deviation from flat.

So personally, I apply the 0.010" limit but apply it to any line on the table surface (notionally a plane). If a 510 and on table is not flat, the first thing to do is loosen the trunnion bolts to see if the system is stressing the table out of flat. I like the 0.010" limit, it seems reasonable for woodworking and IMO is a practical number for any attempt at bending the table flat, an inherently imprecise operation.

- David

[dusty]

For the trunnion bolts to stress the table enough to make it "not flat" would require that the trunnions are somehow involved. Are the tops of all four trunnion bolt holes coplanar.

Mount a table and lock it in a position where all four trunnion bolts can be removed. Remove the trunnion bolts and table. Stretch a line diagonally from trunnion to trunnion and check.

[JPG]

Two diagonals do NOT verify a plane.

[edma194]

Flattening a Shopsmith table will take time for the metal to move. You can use a press but you'll have to hold it flat for a long time. I don't know how long, maybe weeks, and you'd probably so as well just weighing down the table on a reinforced flat surface. And there's no guarantee you'll end up with table flat within .0001" or less in all directions. The other approach would be grinding the table flat with a surface grinder if the distortion is not great. But anything you do may end up with a non-flat surface if the supporting structure of the table is under stress because the metal will move again over time to relieve that stress.