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Old 09-21-2016, 02:01 PM   #15
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Originally Posted by gomotomoto View Post
What's the best way to cut a 3" hole in the airstream? I'm installing a heater vent pipe, and I've nave cut the skin before.
There are 2 tools that can be used, A 3" circular saw tool chucked in your drill motor. With this tool you drill a 1/4" hole first. Then the arbor of the tool fits through the hole and the hole is used to center the 3" circular tool as you cut it with the drill motor. Yes cut slowly and cut that 1/4 inch guide hole with a Unibit to get an nice round 1/4" pilot hole. The 3" tool and unibit are available at Lowes or Home Depot.
Another tool is a Flycutter. You will also need to cut a 1/4" gyide hole. This tool is designed to work in your drill also, but you can cut the hole by simply turning the tool by hand. It takes some patience but it works. Since your not using the drill motor it's a very safe but slow method. I use a Harbor Freight tool it can be adjusted to the exact diameter you need. Use one hand to hold the Flycutter in the 1/4" hole and turn the cutting in with your other hand as you press the cutter into the aluminum. Make several rotations and you will see the metal peel away as it cuts into the aluminum.I've built two aluminum airplanes and used both methods.
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Old 09-21-2016, 02:08 PM   #16
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Cutting holes in sheet metal

To get a perfectly round, burr-free hole, use a Greenlee punch. They are available at electrical supply houses and I think at auto parts outlets as well. They are a little pricey, so if you have an electrician friend, perhaps you could borrow one (or get him to help you with it.)
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Old 09-21-2016, 02:11 PM   #17
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cut out on AS Skin

Use a 1/4 drill to locate hole on each side of wall, drill all the way through. Then on the inside use your jug saw with fine teeth to open up the cut that you need. For the outside hole, use a Electrical knockout punch of the the size that you need, the outside hole will be clean with out any narrely edges. The knockout punch is a two part tool that comes in sizes from 1/2'' EMT to 6" EMT, it is used in the electrical trade to knockout holes in meter boxes, ect......
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Old 09-21-2016, 03:07 PM   #18
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Originally Posted by us3rnam3 View Post
There are 2 tools that can be used, A 3" circular saw tool chucked in your drill motor. With this tool you drill a 1/4" hole first. Then the arbor of the tool fits through the hole and the hole is used to center the 3" circular tool as you cut it with the drill motor. Yes cut slowly and cut that 1/4 inch guide hole with a Unibit to get an nice round 1/4" pilot hole. The 3" tool and unibit are available at Lowes or Home Depot.
Another tool is a Flycutter. You will also need to cut a 1/4" gyide hole. This tool is designed to work in your drill also, but you can cut the hole by simply turning the tool by hand. It takes some patience but it works. Since your not using the drill motor it's a very safe but slow method. I use a Harbor Freight tool it can be adjusted to the exact diameter you need. Use one hand to hold the Flycutter in the 1/4" hole and turn the cutting in with your other hand as you press the cutter into the aluminum. Make several rotations and you will see the metal peel away as it cuts into the aluminum.I've built two aluminum airplanes and used both methods.
I have used a flycutter (adjustable beam hole saw) with good effect on thicker aluminum. You can also use the fly cutter to scribe a perfect circle then cut it out with aviation tin snips.
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Old 09-21-2016, 03:14 PM   #19
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I would use a hole saw bit of the size you want the hole and turn it with a 1/2 inch variable speed drill motor. A 3" cutter should be turned about maybe 200 RPM or so not 1500 RPM and this would be punishing your 3/8 drill unless it's the kind with a low/high speed gearbox. Feed lightly through the first skin and remove the first plug then push the insulation aside and check for wires or pipes. The pilot bit should have made a hole through the second skin but if not hopefully the cutter body will be tall enough to center the bit in the second skin. If you have a choice buy a long pilot bit or a deep cutter body, most home centers and large hardware stores sell these hole saws but they are not cheap. Expect to pay $50 to $100 for one if you can't borrow one from a tradesman friend. Be careful not to cut into a rib or anything else!
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Old 09-21-2016, 03:43 PM   #20
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Consider trying a small Diameter Punch versus Hole Test

Here is just a suggestion:
Try getting a Greenlee hole punch in a much smaller diameter and a hole saw of the same diameter and try both on a scrap piece of aluminum.
Consider picking up a small diameter punch at a local Lowes / Home Depot such as something around the 1 inch diameter. Big enough to start to get into the wander/tilt/bite/grab/bend problems of hole saws. 3 inches is a lot of torque when the hole saw wants to bite through the far side of the hole.
Experiment with an affordable punch.
Try both.
See how much cleaner and perfect the cut is with the punch.
Then consider how much you have invested in the new heater project to see if the purchase of roughly a $250 punch would then be worth it compared to the total price of your project.
It is amazing how clean a hole the the punches produce.

"Do you Fell Lucky"

Please, let us know how it turns out and which method you chose.

If nothing else, we all like reading about what others have done to their Precious!
Maybe it makes all of us feel better.
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Old 09-21-2016, 04:40 PM   #21
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When I had to trim my water heater hole a bit taller, I grabbed my aviation tin snips after marking the line. Used the left and then the right hand ones to trim away the skin in the way, filed off the sharp edges, and drove on. No hesitation once I triple checked measurements. It's an AS, not solid gold....


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Old 09-21-2016, 05:06 PM   #22
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Originally Posted by gomotomoto View Post
What's the best way to cut a 3" hole
Yes, one of the most important tools in my toolbox, is a hole saw kit with different mandrels . The best to cut clean holes.
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Old 09-21-2016, 06:05 PM   #23
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Originally Posted by BambiTex View Post
I have cut four round holes in my vintage AS skin (shore power, water fill, and two for the furnace). I used hole saws for all four. Once the agonizing measurement process was complete, cutting the holes was relatively easy.
If using a hole saw select a metal cut saw and drill slowly counter clockwise to prevent the the teeth from digging in the aluminum.
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Old 09-21-2016, 06:24 PM   #24
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The basic RotoZip tool has a circular guide. Set the tool at a 1.5 inch radius, drill the pilot hole on center, then set the tool to cut the hole. Metal cutting bits are available.
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Old 09-21-2016, 09:12 PM   #25
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Originally Posted by GeeSag View Post
Here is just a suggestion:
Try getting a Greenlee hole punch in a much smaller diameter and a hole saw of the same diameter and try both on a scrap piece of aluminum.
One of the best hole saws I've ever bought was this Greenlee carbide cutter. I've used it on aluminum up to 5/16" thick, usually in a milling machine but also freehand, and it does an awesome job. The carbide teeth make for a very smooth cut and a bit of tilt once the cut is started lets you steer clear of the ripping that can occur when you use a dull cutter, or too much speed.

Budget permitting (the 1 1/8" cutter was around $100 if I recall correctly) I would get this 3" cutter and the matching quick-change arbor.
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Old 09-25-2016, 05:22 AM   #26
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I definitely over-thought it. Skin is thicker than I expected... Made it easier to deal with.

I simply used high speed, and some "feel" ...making sure not to dig down too hard and poking the teeth through systematically. Ended up with two perfect little circle shaped tin can lids!
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Old 09-25-2016, 12:10 PM   #27
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I have possibly observed a tendency of some folks to over think things on this forum....


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Old 09-25-2016, 03:40 PM   #28
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Quote:
Originally Posted by gomotomoto View Post
I definitely over-thought it. Skin is thicker than I expected... Made it easier to deal with.

I simply used high speed, and some "feel" ...making sure not to dig down too hard and poking the teeth through systematically. Ended up with two perfect little circle shaped tin can lids!
Hole saw?

I think the hardest part is making sure you have the right spot and the thought that you're putting a big hole in your trailer.
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