What's new
What's new

Question On Tapping 1/8" Aluminum

JIMMIEM

Plastic
Joined
Jun 3, 2018
I have a ceiling mounted central a/c Aluminum cold air return that I would like to cover during the winter months. The outside dimensions are 26" X 38". The outer frame is 1 1/4" wide and 1/8" thick. There are some predrilled holes in the frame which have a diameter between 11/64" and 3/16". The cover that I have made has a wood frame with plexiglass in the center. My plan is to drill holes through the wood frame that align with the holes in aluminum frame. I did a test with a #10 self tapping sheet metal screw. The problem was that the screw would bite and hold but if I removed and then reinserted the screw it would not hold. I assume that if I tap it then I can use a machine screw which can be installed and removed every year.
Cut tap or Forming tap? What size tap? What size screw.
Note: the aluminum frame is tight to the finished sheet rock ceiling.
Thank You.
 
Rivnuts are a good choice. 1/4-20 should be plenty.

If you can access the backside at all, a riveted or glued or taped in place steel nut plate works great as well.
 
Rivnuts are a good choice. 1/4-20 should be plenty.

If you can access the backside at all, a riveted or glued or taped in place steel nut plate works great as well.
Backside access would be in the attic. I pulled up some of the attic insulation and found wires and pipes around the cold air return shaft. I was hesitant to drill up through the ceiling sheetrock fo fear of damaging a wire or pipe. I did a quick search for rivnuts and I saw some with lengths that will allow installation without breaking all the way through the sheetrock.
Thank You for your response.
 
Rivnuts require a setting tool so one option might be to use the steel expansion anchors meant for plaster and sheetrock. They have barbs meant to keep them from spinning while tightening but you could either cut them off or even bend them back flush with the flange. These anchors have slits in the sides so they will expand out against the back of the sheetrock as the screw is tightened. Once the anchor has been tightened once you can remove and replace the screw repeatedly. Just use a bit of oil or grease if you plan to reuse them every year so a bit of rust won't lock them in place.

I don't know if they still sell them but you used to be able to buy aluminum rivnuts with a disposable mandrel that you could use with an ordinary Pop Rivet tool. The mandrel is good for maybe 5 rivnuts and they are packaged that way.
 
IF you are using a cheap rivnut tool you'll want aluminum. A fully pressed aluminum rivnut is much less frustrating than a stainless or steel rivnut that spins because it wasn't fully seated.
 
1/8" thick aluminum will provide sufficient thread depth for use with a 10-32 machine screw. Use enough screws for the weight of the cover and if the existing holes are too big, make new ones to achieve 75% thread. Don't over-tighten, lubricate the threads lightly and the threads will last as long as you need them to.
 
I have a manual pop riveter that I've used for blind pop rivets. I saw a video where the person had an accessory that he used to insert a rivnut using the pop rivet gun. The draw bar which was like a blind pop rivet had a threaded section on the end which treaded into the the rivnut. The draw bar fit in the gun, just like a blind rivet, and he installed the rivnut. I saw another video where the 'guts (spring, etc) of the pop riveter were removed and a threaded bolt was inserted through the top and out the nose. Rivnut screwed onto the nose. Are these things for real or should I just buy a threaded insert rivet tool?
 
My experience with RivNuts was all back in 1976, but I remember that they take a hefty pull to set properly. I'd not consider using a standard rivet puller unless it is a really good one designed for larger rivets.
 
I'm not sure how many holes you have to work with, but if there are enough I think that sheet metal screws would work. I see two problems with the self-tapping one you tried. First, a #10 was probably too small. And second, a self-tapping screw can cut a new thread on it's second use. That leaves very little metal between the original thread and the second one.

A sheet metal screw would be the choice. It will form the thread instead of cutting it. So it should follow the original thread on the second and subsequent uses. Electronic enclosures have used sheet metal screws for a long time and, with due care, they can be removed and reused many times. I would think with the diameter holes you have a 1/4" sheet metal screw could be used.

A tip on reusing a sheet metal screw: Since the thread is already formed, you should insert the screw with moderate force only until it contacts the metal surface. Then, with that moderate force, turn it backwards until it "clicks". Stop immediately and turn it forward to tighten it. This ensures that it will find the original thread and not create a new one. Of course, do not over-tighten it.

And yes, whatever you choose, do use stainless.



I have a ceiling mounted central a/c Aluminum cold air return that I would like to cover during the winter months. The outside dimensions are 26" X 38". The outer frame is 1 1/4" wide and 1/8" thick. There are some predrilled holes in the frame which have a diameter between 11/64" and 3/16". The cover that I have made has a wood frame with plexiglass in the center. My plan is to drill holes through the wood frame that align with the holes in aluminum frame. I did a test with a #10 self tapping sheet metal screw. The problem was that the screw would bite and hold but if I removed and then reinserted the screw it would not hold. I assume that if I tap it then I can use a machine screw which can be installed and removed every year.
Cut tap or Forming tap? What size tap? What size screw.
Note: the aluminum frame is tight to the finished sheet rock ceiling.
Thank You.
 
I have a manual pop riveter that I've used for blind pop rivets. I saw a video where the person had an accessory that he used to insert a rivnut using the pop rivet gun. The draw bar which was like a blind pop rivet had a threaded section on the end which treaded into the the rivnut. The draw bar fit in the gun, just like a blind rivet, and he installed the rivnut. I saw another video where the 'guts (spring, etc) of the pop riveter were removed and a threaded bolt was inserted through the top and out the nose. Rivnut screwed onto the nose. Are these things for real or should I just buy a threaded insert rivet tool?
I used to buy packages of 8-32 Pop Rivet brand aluminum rivnuts at the hardware store that were something like a half dozen rivnuts and one disposable mandrel. Because the mandrel gets chewed up by the serrations on the tool jaws they only last for a few uses.

A quick web search seems to show that they no longer offer these very economical kits. If you have a blind rivet tool and a lathe you could turn down screws or all-thread to make a few mandrels similar to what they used to offer.
 
Another option which I have used in the past is to chuck a larger hex head screw with the head facing out and tap it for the same threads as the inserts. Use a deep socket with greased washers and a heavy nut to pull the improvised mandrel as you turn it. This method requires a steel plate with a clearance hole between the rivnut and improvised tool and a means such as vice grips to keep the socket from turning.
 
Good application for tech screw. Available in fine or coarse thread. Use battery powered impact driver at the right setting so you don't strip them. Run the next larger size screw in the same striped out hole. Use screw with rubber washer and stop when washer is just compressed a little.
 
I'm not sure how many holes you have to work with, but if there are enough I think that sheet metal screws would work. I see two problems with the self-tapping one you tried. First, a #10 was probably too small. And second, a self-tapping screw can cut a new thread on it's second use. That leaves very little metal between the original thread and the second one.

A sheet metal screw would be the choice. It will form the thread instead of cutting it. So it should follow the original thread on the second and subsequent uses. Electronic enclosures have used sheet metal screws for a long time and, with due care, they can be removed and reused many times. I would think with the diameter holes you have a 1/4" sheet metal screw could be used.

A tip on reusing a sheet metal screw: Since the thread is already formed, you should insert the screw with moderate force only until it contacts the metal surface. Then, with that moderate force, turn it backwards until it "clicks". Stop immediately and turn it forward to tighten it. This ensures that it will find the original thread and not create a new one. Of course, do not over-tighten it.

And yes, whatever you choose, do use stainless.
The frame came with 10 holes already drilled...although I could always drill more. From what I have read there are Cut taps and Forming taps. I just used regular sheet metal screws that do not have the cutting type point. Was this the wrong type? I can get some of the others that have the cutting point. If I use these initially what do I use going forward? The frame I will be attaching is 1 1/4" thick. Do these type of screws come in long lengths in oreder to match the thread pattern that has been cut?
 








 
Back
Top