Metal ductwork question

[RobHannon]

I think I know the answer to this but maybe someone has a brilliant idea I have not found. Is it possible to un-crimp a metal fitting? I got some 6" wye fittings that have the crimped end on the wrong end for smooth airflow. I could adapt it but then I will have the crimps going against the airflow collecting dust and creating more turbulence.

[retired2]

It's not possible to do it in a way that looks like it had never been crimped.  However, I've had a little success using a flat nosed pliers.  I'm not sure what it is called or where I got it, but the face of the jaws are about 3/8" wide with a throat depth of about 1" or 1-1/2".  It takes patience, but I just go around the pipe pinching each crimp.  That won't get it out totally because the crimps have a memory.  The other thing I do is hammer it on my vise.  The top of the jaws of my vice have a large convex radius.  I slip the pipe over the closed jaws of the vice and just keep tapping the crimps with a hammer.

Neither of these methods are fast, and neither work perfectly.  In fact, you probably need to use both methods to achieve the results you are looking for. 

There may be commercial machines out there that would do a really nice job, but that is not a solution for a home shop.  You might call a sheet metal fabricator and see if they can do the job for you at a reasonable cost.

[RobHannon]

I care more about how smooth the fittings are on the inside than how they look outside. Mastic and tape can handle the outside. Hammer and something to act as a anvil horn is a good idea and 6" is big enough diameter to be able to get some tools in. I will try that.

Fortunately I don't have a lot of fittings to do this to.

[retired2]

Here is a photo looking into the side outlet of one of my wye's.  My wye's were crimped in the right direction, but the lateral branch was assembled with contour cuts and a series of metal tabs to lock the branch in place.  The tabs were approximately 3/8' square and there were a lot of them around the circumference of the joint. And naturally the tabs faced in the direction opposing air flow.  If I did nothing they would have caused a plug in no time.

So, I carefully tapped down all the tabs as flush as I could get them.  After that I wiped the joint with a high tech body filler.  Then with some good old fashioned  elbow grease and sandpaper I smoothed the joint so there would be nothing for stringy waste to snag onto.  The tabs are all covered by the body filler.

I would suggest you give some thought to this idea as a way of smoothing the inside of your connection.  That could get a little tricky if you are attaching a 6 fot length of pipe.  What you might want to consider is adding a piece of straight pipe, maybe 4"-6" long to your crimped end.  Then you could easily reach in  to add body filler and smooth it with sand paper.  Then you have a straight un-crimped end to mate up to your pipe that is crimped in the correct direction.

Hope this idea helps. 

p.s. I cleaned all the galvanized pipe with something, not sure if it was vinegar or acetone, before applying the body filler.  I don't know if that was necessary of not.   

[RobHannon]

Not a bad idea. I normally clean duct with an acetone rag before using mastic anyway. I will try that with the first set of fittings going into the tophat and see how much time it adds.

[retired2]

Another thought.  If your crimped section is on the wrong end, doesn't that mean it is on the longest leg?  If so, why not just hacksaw the crimped section off?  Doesn't that leave enough straight pipe to engage the end of the next section of pipe?