Doup28704?s Top Hat Build

[Dougp28704]

03-24-2012: Making the transition intake. The first half of the diagonal bends went really well. The second half is giving me trouble. As Ron said, the curve ends up hitting the bottom of the bench.

After a short break, I went back to it. Finished off the diagonal bends. It looks pretty good.

[Dougp28704]

03-25-2012: Today I made an assembly jig for the round to rectangular intake. It is two .75 thick 5.06 diameter pieces of mdf screwed and doweled together. Two .75 thick pieces of 3.58 x 5.95 pieces of mdf screwed and doweled together. The round and rectangular parts are screwed and doweled to an 8 inch long 2x4. The rectangular end of the sheet metal fits the form fine. But I can?t get the round side to wrap all the way around. I quadruple checked my ?round to rectangular transition drawing?. Ron advised to try not to have your diagonal bends go into the round section. Well, I couldn?t help it, the creases did. I figured I could flatten them out once I got it on the jig. I just can?t get the round part to be round yet. I think a sheet metal shop has a device that has two rollers that clamp the sheet and you push it back and forth to take out the crease. Obviously I don?t have that. Trying to figure out a way. I gave up for tonight and decided to post my status.

Its hard to believe that the creases would take up so much space on the circumferance. But like I said, I am sure my drawing and sheet metal cuts are correct. Any ideas out there?

[phil (admin)]

Maybe a large enough hose clamp that will pull the thing closed around the form?

[RonS]

Doug, To help straighten the small bends just place the round end  on a flat surface (edge of bench) with the form installed and tap the creases with a hammer. 
How much of a gap do you have on the round end?
From your drawing I see you were trying to fit the uncrimped end of 5" pipe inside of the round end. If you are off to far you could go with the crimped end which is about 4.97.
If you have a 4" to 5" or 5" to 6" round transition try and fit the 5" end over the round end of your transition and see if that closes the gap. If it does you could just leave it there.

[Dougp28704]

Good idea Phil. I put hose clamps on and it clamped round to the form.

Ron, I still have about .5 inch gap. Tapping the remaining creases out with hammer may close the gap a little more, but not enough to fasten. I cant figure out what went wrong. My plan was for spiral pipe to fit inside the inlet. It has a 5.03 O.D. I think I will be able to forget the spiral pipe and use the crimped end of regular duct.

[Dougp28704]

I have been thinking about this. Over thinking probably. My drawing was for the inside dimensions. While folding, I needed to see the lines. So, the inside became the outside. Does it make sense that the material thickness offset multiplied by all the bends caused the gap?

Anyways.... I plan to cut a sheet metal strip, from the scrap left over, and rivet it in place to close the gap. And Tape the seams with foil tape.

[RonS]

Doug, Good idea using a SM strip to cover the gap. I think that will be ok.

If you put the out side dimension's ( your ID 5.06 + 2* your SM thickness) into the spread sheet you should be able to see what the difference in the short/long tic marks would be.   

I looked at my transition and all the bend markings are on the inside.

[Dougp28704]

Yeah, down the road I may remake my trasition. This thread should help me remember my mistakes so I dont repeat them. And hopefully help others that plan to make a similar Top Hat.

[Dougp28704]

I riveted it together tonight. Thanks Ron. It is very satisfying to make a custom round to rectangular transition from a $10 sheet metal piece from Lowes. And I still have a good size piece left over from sheet to make something else.