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Shop Talk

December 2018 Issue

Plane and Simple

Making interior cuts in sheet metal.

Oscillating multi-tool with cutting blade.

Creating a rectangular access opening in a sheet metal panel begins with selecting the proper tool to make the cuts. If you are new to cutting metal skins in an interior area, your first inclination might be to grab your familiar metal shears. Hopefully, you won’t ruin your project using these as you will find that cutting a metal skin in an “enclosed” area doesn’t yield the same results compared to cutting at the skin’s edge and ending at the opposite side.

We are all familiar with the “curling” metal strip that gets created on one edge of a metal sheet when we cut using hand shears. In fact, this is why there’s a choice of “green” and “red” metal shears (also known as left- and right-handed) as this selection determines which side of the cut curls away from the other side. When starting a cut in the interior portion of a sheet, as is required when creating an inspection panel, there is little “give” for one side of the cut to curl away. The result, if we continue with a cut like this, is a highly distorted skin as the metal has no place to go.

Holes at all four corners are needed to relieve stress.

Since this discussion is attempting to convince you that the common metal hand shear is not the right tool for interior cuts, then what tools can be used? There are several possibilities, and we will discuss one of them now: the oscillating multi-tool.

This power tool has become very popular at home improvement stores as a means to perform detail work with all kinds of materials: sanding, cutting wood and drywall, and grout removal. To this list, we can add making relatively clean cuts in aluminum sheet metal.

These tools can be purchased from Harbor Freight or Amazon for as little as $40. They typically come with an assortment of cutting attachments, each suited for working with a different material. We will be using the metal cutting blade that contains tiny small teeth. My favorite aspect of the tool is its inherent safe design; incidental contact with your finger by the blade while running will not inflict injury! The blade oscillates with a tiny motion at high speed, so without pressure there is no cutting. I was told these tools were used by doctors for removing casts on broken bones long before they appeared to the general public.

For creating a rectangular opening, holes need to be drilled in each corner of the desired area. These are important as they provide needed stress riser relief that occurs in sharp corners to prevent future cracks in the metal. We now have four straight cuts to make that will connect these holes, as well as a place to insert the blade to begin each cut.

Practicing on scrap material builds skill quickly.

Applying firm pressure and following marked lines, the tool will make a steady, easily controlled cut from one hole to the next. I find that about 1 inch of cutting progress can be made in about 5 to 10 seconds on .025-inch aluminum sheet. By practicing your cuts on scrap material, you can quickly learn how to use the tool for best results. If you are reasonably careful and take your time, very little if any sanding of the final edge is needed. A wide variety of interchangeable blades are readily available, and it is worth experimenting to find the ones that cut best for your project.

There are other tools that also do a good job for these interior cuts, but this one is a winner when it comes to safety, speed, and ease of use. Plus, this is the kind of tool that will not be limited to just the realm of aircraft building. It is called multi-tool for a good reason!

As the founder of, Jon Croke has produced instructional videos for Experimental aircraft builders for over 10 years. He has built (and helped others build) over a dozen kit aircraft of all makes and models. Jon is a private pilot and currently owns and flies a Zenith Cruzer.

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