Help me visualize where to remove material to flatten?

[Probablygrigsby]

I have cut a rectangular steel tube as close to having true corners as I can manage with the tools I have. Thanks to people who provided help on an earlier thread about this!

What I have is probably good enough for my application, but I’m enjoying learning, so I’d like to further refine a bit more.

When I place the cut tube on a glass sheet, it rocks diagonally from one set of corners and not the other. Here is a picture to illustrate:



It rocks back and forth with the O points alternately touching. The X points don’t rock.

I’m having a tough time visualizing where I’d need to remove material to make it not rock.

To make it easier to answer, here is a diagram. I could remove material at the points, or on parts of a line. So you could say: file at point X or remove material on the line XY closer to Y, for example.


Just to say it, I know that flattening it might leave it not square/could have a tilt.

 

[Probablygrigsby]

BTW, my guess would be B and D, since I think they create the fulcrum, but I’m not sure how to localize it more than that.

 

[DavidR8]

If B and D are the X's then they need to be relieved for A and C to touch the surface. And they need to be relieved such that they end up on the same plane as A and C.
And yes, even if they touch there's no guarantee that the end is square.
Honestly, the easiest way to do this is with a good square and a sharp scriber to mark the end square to the side.

 

[RJSakowski]

I am assuming that the end is square enough for your purposes. Place a feeler gauge under A or C. The would be the amount needed to be removed from either B or D or remove half that amount from both B and D. Check your progress to make sure you don't overshoot.

 

[C-Bag]

Honestly, the easiest way to do this is with a good square and a sharp scriber to mark the end square to the side.

My first step is to mark them like the drawing so you don’t lose reference. Then like David said a known good square(preferably a machinist square) as reference. I would use said square set to the sides with the blade across to examine B and D to see them with light behind the blade. Then I’d have a better idea where I need to file. I’d also not trust a glass sheet as it can flex. A small granite shop grade plate is pretty cheap and doesn’t really care if your table is warped. I think you’ll find when in search of precision it can get very involved or you just keep chasing the weakest link throwing you off.

 

[francist]

This is where a surface gauge really shines. You can shim the piece to get it nice and plumb and with even contact so it’s not rocking then run the surface gauge around all sides to get your line. Surface gauge also doesn’t care if the corners have radii or not. In a pinch or if you find yourself in a place with no surface gauge, a little block of wood and a scriber does the same thing.

 

[Jake M]

These guys are right. Eyeballing stuff on a flat but flexible plate of glass is not a good way to get high precision. On the other hand.... When you're using something that's WAY flatter than what you need, to make a result that's a little less flat than that, but is still flatter than you need it to be, and flatter than you can see by eyeballing it- Flat enough to be overkill flat for the application... Well, a good granite surface plate with lab grade credentials is going to come with a mountain of documentation to tell you exactly how crooked and exactly how flexible that plate is too. Every time you think you have something that's "dead nutz", you're wrong. You just don't have good enough measuring tools. You need flat enough to work seamlessly and transparently. As long as your makeshift reference is better than you need the final product to be.... I say It's a valid reference.

I'm gathering that you (probably) don't have a machinist's square. You're gonna be lost without that. But just like the plate, so long as it's more accurate than the project, then square is square, right? If you've got a framing square, woodworking combination square, or even one of those sliding angle bevel marking gauges with the awkward wing nut on it... We can get you square... The framing square and the combination square (probably) won't be square, but that's fixable. An adjustable angle gauge be made square with next to nothing as well. Do you know how to prove those as square?

When I place the cut tube on a glass sheet, it rocks diagonally from one set of corners and not the other.

Well, that tells you something. You are on the right track (somewhere...) where you said that B and D are high, or low I guess, if it's upside down. Protruding more and creating a fulcrum. Which one is high? Unless it's visually obvious, you have no idea, beyond that one of the two is high. Further, One of those two points must be higher than the other three. It might be that B and D are both high. It might mean that only B or only D is high. That's where a square reference comes in.

Just to say it, I know that flattening it might leave it not square/could have a tilt.

If you just wanted to flatten it, but could compromsie on square... You could just stick some sandpaper on that glass plate.....

You really need a square reference to find square. You could use that square to scribe a reference on the side of the tube to file/sand/grind or otherwise remove material to the line. Or you could use your square reference, on your flat reference (the glass plate), and play with some reasonable deductions about which way the part leans. It'll lean away from the highest spots.

You must have "something", "somewhere"... What have you got laying around that is dead square?

 

[Probablygrigsby]

Thanks, everyone. Lots of good ideas here. I do have a machinists square, but after using it to scribe and cut, my brain checked out and I didn’t consider it at this step. Duh! I also appreciate the technique ideas e.g. removing half difference of the feeler gauge measurement from each side. Once again, I am grateful for the help. Love this forum.

 

[dbb-the-bruce]

Saw a comment dissing a sheet of glass - yeah not that ideal but if it's thick enough it's way better than a bench! Seriously, a quality thick piece of glass is not a bad choice depending on what else is available. A granite counter top will also do. Visit a countertop shop and you can pick up sink cut-outs on the cheap side.

Neither option is high precision but will work well enough over small distances / areas

(and forgot to add - buy an assortment of plastic shim stock. It's a little pricey but I'm still working my way through a $50-ish purchase that's 10 years old. If I ever buy another pack I'll have to decide who to leave it to in my will. It is immensely useful for making comparative measurements. I keep a multi-day pill container with little snips marked by thickness.)

 

[twraska]

Depends if you’re just wanting it to sit flat or if the end needs to be squared with the sides. Remember, a angle cut could sit flat. If it needs to be square, a square is gonna be required.