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Update 5/25/2013: I went with mike's idea of using corner braces, but with a twist. Instead of 7/8" braces and epoxy, I went with 1" braces and metal framing screws. The results are pretty good (see pic). I initially tried to epoxy several 3/4"braces together as mike suggested, but I think I mixed the compounds together improperly as they would not cure after even 3 hours.

Original Question

I am trying to make a rectangle frame out of this 1" 14-gage steel tubing. I am sorry to spam SE with more of my soundproof door questions, but, if you must know, that's what this is for.

Originally, I was going to route a dado out of some MDF and line the sides, top, and bottom of the inside of my door with these steel tubes. This would keep the door from warping vertically and horizontally, but a user here by the name of mike (I hope you're reading this) informed me that it wouldn't keep the door from twisting.

So, in an effort to create my own elbow joints to connect these tubes, I took a 7/8" oak dowel, cut it into eight 4 1/2" pieces, pounded them into the ends of the tube, and mitered them. I then shot two screws through either side thinking this would really keep it from twisting. Well, unfortunately, the frame still twists, most likely because the screws are bending.

What I need are sturdy, one-piece elbows measuring 7/8"x7x8". I thought of taking two 1/2" sheets of MDF, gluing them together with Liquid Nails, routing 1/8" off one side, cutting out an elbow, and then pounding that into the ends (I can't find any wood that's 7/8" or 1" thick). Failing that, make the elbow joints out of QUIKRETE.

Does anyone have any other good idea about joining these tubes together in a super-sturdy fashion?

Edit: I should probably state that I bought these lengths of tubing precut from discountsteel.com. Unfortunately, I do not have the ability to cut (this includes mitering or kerfing), weld, or bend steel, so I have to do the best with what I've got. I guess it doesn't have to be as sturdy as a rock, but sturdy enough to resist twisting brought on by warping of MDF.

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Latest 1" braces w/ metal framing screws

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Have you considered fiberglass? –  HerrBag May 17 '13 at 2:04
    
@HerrBag: Fiberglass for elbows? Can you explain more? –  oscilatingcretin May 17 '13 at 12:08
    
not for the corners, for those I'd weld on 4 web corners (6 to 8" clipped to a 45. I think a cross rail is needed(mid height, wide enough for a lockset). From there, 1" xps foam, then fiberglass –  HerrBag May 17 '13 at 12:20
    
+1 for the awesome joinery!!! –  Chris Cudmore May 17 '13 at 13:17
    
@ChrisCudmore: It turned out to be not very awesome :( –  oscilatingcretin May 17 '13 at 13:23

3 Answers 3

up vote 2 down vote accepted

EDIT 2013-05-18:

Because it would be super easy, quick, cheap, and sound-deadening, I think the way to go is a cement door skinned in masonite/plywood with styrofoam pillows, built as I sketched out below. On one or both sides, Green Glue a second skin of 1/8" sheeting if desired.

Still, if you are set on the steel tubing and MDF, another way join the four lengths that you already have is to find some corner braces that are about 7/8" wide. They would likely be about 4" long. Use a stack of them at each corner, embedding them in tubing with, say, an epoxy to take up any play.

http://i.imgur.com/LPXkPsT.jpg



EDIT 2013-05-17:

A) For a frame welded out of 1" square steel tubing, the ability to resist torsion boils down to the torsional rigidity of the square tubing over the frame's long dimension, about 6 feet in your case. Suppose the frame is 24" wide. A minor twist of 0.01 inches in a 6' length of the tubing would 'amplify' to 1/4" at the width of the frame.

B) Steel is strong, not particularly rigid. Your project needs rigidity much more than strength. So I suggest making a cement door ...

  • Cast the cement permanently inside a 'pan' made out of a sheet of 1/8" plywood/masonite framed with lengths of two-by glued down around the perimeter, and perhaps a 10" extra piece of two-by for the lockset.

  • When dry, cover the exposed side with a second piece of thin plywood/masonite.

  • Before casting the cement, line the 'pan' with plastic sheeting to keep the wood dry, or apply a generous amount of mold-release spray-on wax.

  • Embed whatever steel you desire into the cement, such as hardware cloth, or rebar.

  • If you calculate the weight ahead of time and consider it to be too heavy, embed squares of 1" styrofoam, or possibly shipping peanuts, though some types of the peanuts dissolve when in contact with water.

C) For the single-length method that I detailed below, the cuts can be made using a hacksaw or using a sawzall with a metal-cutting blade. The bends will not require much strength. As an alternative to kerfing+T-stock splines, you could avoid kerfing by using U-channel that fits snugly over the square tubing.



Make the frame out of a single length of tubing, cutting out four 90 degree wedges at the corners, a half-lap joint to joins the ends, and kerf the inside corners to receive T-stock as splines.

1) Start with 20' length of tubing.

2) Don't dado your door until after you have the frame built and tested.

3) Suppose the outer dimensions of the frame are to be X wide and Y high.

4) Cut the tubing to this length: X + X + Y + Y + 12".

5) Choose a side of the tubing that will be a side of the frame (as opposed to the outside or inside sides).

6) Apply masking tape to this side at four locations: A, B, C and D, where

  • A = 6" + X/2
  • B = A + Y
  • C = B + X
  • D = C + Y

7) With a pen, mark the tape at exactly A, B, C, D.

8) Cut out four 90 degree wedges centered a A, B, C, D from the inside side, leaving intact the wall of the tubing that will be the outside side. The wedges will need to be exactly 90 degrees, but 'fat' such that the cut-outs provide enough gap to bend the tubing to 90 degrees, but not so 'fat' that tubing bends past 90 degrees. Experiment with the scrap remaining from the 20' length.

9) Make a lap joint for the ends of the tubing by cutting off a 12" length of the inside half of the tubing on one end, and a 12" length of the outside half of the tubing on the other end.

10) Rip a 12" length of hardwood to square, such that it fits snugly inside the tubing (for safety, work with a longer piece, then cut 12" off).

11) Cut eight ~4" long kerfs on the inside of the tubing, a pair at each of the future corners.

12) Cut four 8" lengths of T-stock, say 1/2" x 1/2" x 1/16". The width of the eight kerfs needs to closely match the gauge of the T-stock.

13) At their mid points, cut the 'upright' all the way to the top. This will allow the lengths to be bent to 90 degrees with the uprights on the outside.

14) Bend the tubing into a frame shape.

15) Bolt the 12" lap joint together, inserting the 12" length of hardwood to to keep the joint aligned.

16) Slip the bent T-stock splines into the kerfs at the 4 corners.

17) Test the frame for its torsion. If satisfied, place the assembly on the MDF door blank to layout where the dados need to be routed.

My friend and I are wondering what nefarious acts you have planned for such a secure and sound proof room.

But I'm not convinced of the design - even if fully welded, the 1" steel tubing frame will still twist 1/4"+ out of plane with a small amount of force. So my answer is that the frame can't be made sturdy enough to counteract the forces the door will encounter due to the MDF warping due to your moisture differential.

I'd go with a sand floor covered by a free floating double layer of 1/2 plywood, and walls and ceiling lined with acoustic foam. The walls and door could be pocketed to hold sand inside of them as well. Sand is cheap, enviro friendly afaik, and dampens like nothing else.

But you seem set on MDF and metal tubing, so there must be good reasons. So I tried to answer your question as best I could within the limit of your previous statement of not wanting to weld ...

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I really like the idea, but I don't really have the ability to bend or kerf steel (I updated my question). I was hoping that some kind of elbow joint would offer the sturdiness that I need, but it appears that this is just impossible. Also, I am going with MDF so that I can use Green Glue between the layers. Sand may also settle over time, leaving an air-filled gap to transmit sound. Plus, the added weight of sand would probably collapse my room! –  oscilatingcretin May 17 '13 at 12:28
    
What if I lined up my tubing in a rectangular shape, secured an L-shaped mold at the corners using spare wood, and poured QUIKRETE into it so that it would flow into the tubing and also fill the mold? Do you think a concrete elbow would eventualy break or crumble? I can drill into steel, so what about mending strips fixed diagonally? –  oscilatingcretin May 17 '13 at 12:31
    
edited my post, adding more ideas –  mike May 18 '13 at 20:33
    
mike, you're a true thinker. I actually love your recent idea of stacking corner braces with epoxy. I will probably have to go with 5/8" because the inside of the tubing is probably 1/16" shy of 7/8", hence why I had to whittle away the end of the 7/8" dowels and then pound them in with a hammer (how I am going to get them out at this point is another story if i decide to get them out). Yet another reason to come to stack exchange before you jump the gun =\. Anyway, I might take a stab at drilling them out with a flat-bottom bit because they're serving no purpose right now. –  oscilatingcretin May 19 '13 at 1:44
    
1) cut the dowels flush, then pound em in deeper or 2) cut the dowels flush, predrill their centers for a 3-1/2" or longer sheet rock screw or lag bolt, drive in the screw leaving the head protrude, clamp the tubing to work bench with the tubing flush at one end ,then use a claw hammer against the workbench edge to pull the screw+dowel out, blocking under the hammer with one-bys or two-bys as you go such that the pull remains co-axial, or reclamp as you go such that the end of the dowel remains at the edge of the bench. –  mike May 19 '13 at 4:02

Miter the ends of the tubing itself and then weld the corners.

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Even when welded, the 1" steel tubing frame will still twist 1/4"+ out of plane with a small amount of force. –  mike May 17 '13 at 8:00
1  
@mike - I totally agree with you. A 1" square tubing frame the size of a door will still twist some. Welding is however the only really practical way to get the frame as sturdy as it could be. To get a frame that would be more suited meeting the needs of oscilatingcretin it is going to require something more like 1.5" or even 2" square tubing that was welded up. Welding is really the only way to transfer the full lever force of one axis into a torsion force in the adjacent frame edge. Any other type of corner is going to introduce some flexure at the corners - the shown dowel pegs for example. –  Michael Karas May 17 '13 at 10:45
    
I ordered these precut from DiscountSteel.com. Unfortunately, I don't have the ability to miter steel, let alone cut it. However, thanks for the advice and I am sure it's the best route. –  oscilatingcretin May 17 '13 at 12:06
    
Also, I am testing the twisting my laying the frame down flat as you see in the pic and lifting up one corner. This entire frame weighs probably 20 lbs, so I can see how this twisting is occuring. In application, though, the door will be upright. Are you both saying that, no matter what I do, the potential warp strength of the door will be stronger than any joint method I use that doesn't involve welding, bending, or kerfing the steel? –  oscilatingcretin May 17 '13 at 12:17
    
@mike: I am impressed with your calculations. I used c-clamps to hold all the joints tightly together so there was as little bending as possible. Indeed the corner raised up just over 1/4" before its parallel corner lifted along with it. –  oscilatingcretin May 17 '13 at 16:24

Have a look at these:

Modular Connector 3 Way Corner 25mm (3 WAY) Modular connector 3 way corner25mm

http://www.ruralfencingsupplies.com.au/index.php?main_page=index&cPath=395

I don't know if you can get these in States but we have them in Australia and you can make anything you want to with 1" square tubing. I bought a ton of them to make a cabin.

Best of luck!

S.

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