How do I make a square hole in (aluminum) sheet metal? 2nd edition 2019.

By Nick Carter

Why would you want to make a square or rectangular hole in aluminum sheet metal? The short answer is that engineers are perverse, thus they design components with square or rectangular dimensions, which require you to mount them in a square or rectangular hole. You may also want to keep a component that slides in or mounts on a hole from rotating, make a picture frame, or perhaps you just happen to have a round peg that seems lonely. Look around at all the square and rectangular openings on your PC or do a Google search for “square hole”, you’ll understand.

The 2019 answer, for me at least, is analog modular synthesizer DIY.

I've updated this article a bit from the version originally published by Make: Magazine as their website now seems to be slowly disintegrating. 

Here are some methods depending on the tools you have and the size of the square hole and the thickness of the sheet metal. 

The first step for all these methods is to mark where on the sheet you want your square hole, and sometimes the center of the square. Use either a fine tipped sharpie or a metal scriber depending on your tolerances. In the pictures I have covered the sheet metal with layout dye which is thin ink that you can brush or spray on metal to make a scribed line stand out.

1) If the aluminum is thin (pop can thickness) you can just pierce the metal with scissors and cut out the square. For slightly thicker metal you can use sheet metal shears to cut out the inside of the hole.

2) You can shear the hole with a hammer and metal cutting chisel (don't ruin your wood chisels!). Place the work in a vise with the edge to be cut just slightly above the vise jaws and hammer the chisel into the work so the edge of the chisel is parallel to the edge, and held horizontal at about a 45 degree angle out from the sheet. Do each edge and the square should pop out. Deburr the edges with a file, and hammer any distortion flat. Picture #1 shows a chisel cutting one edge of a hole. Notice that you do get a rather large burr on the cut edge with this method.

3) Drill a series of small holes around the periphery of the hole, inside the edges, and use a shear, saw or chisel to remove the waste, and again file the edges. This is a good technique on larger holes. Picture #2 shows a row of holes along the edge of a hole.

4) Drill out the center of the hole using a "Unibit" sheet metal step drill and file the edges square. A power file (air or electric) or a "die filer" could be used to advantage on thicker metal. Picture #3 shows a number of holes drilled with the Unibit chucked up on the drill press, notice the sheet metal has been mounted to a piece of plywood. Never, ever hold sheet metal in your hands for drilling it, if the drill catches on the metal you will end up with a rather scary saw blade whizzing around that will cut your hands to ribbons!

Picture #4 shows a square file being used to remove the rest of the metal from the rectangular opening for a switch. This method is surprisingly fast.

5) Drill a starter hole and use a hand nibbler like the Klein 76011B Nibbler Tool. The Nibbler tool is a hand operated square punch and die that closes on the material when the handle is squeezed. They work well for electronic panels that need small square openings. In use you "nibble" along the edge staying on the inside of the line. If the square hole is relatively large you could also use sheet metal hand shears or an electric or air powered nibbler. Powered sheet metal tools are relatively expensive but very versatile. Most major manufacturers make them. Picture #5 shows the nibbler underneath the metal and Picture #6 shows the cutting head of the nibbler.

Revised for 2019.

6) A jigsaw can be used, drill a hole larger than the saw blade and insert the blade into the opening. Backing the metal edge with some thin plywood will prevent the metal bending from the force of the blade. Picture #7 shows a jigsaw cutting the edge of a hole.

A jeweler’s handsaw will work quite well for thin metal and small holes. Use a jeweler’s bench pin to support the work and finish the hole with files. I had great success with this method as the saw allows great control and a fine cut. Picture #8 shows the jewelers saw cutting out the waste from a hole.

You can do this with a hacksaw if the hole is big enough, holding the work in a vise as with the hammer and chisel method. Most hacksaw frames allow the blade to be set 90 degrees from the frame by turning the blade clamping screws in their square holes. Picture #9 shows the hacksaw blade set at 90 degrees to the frame, cutting the side of a hole.

7) For small holes you can make a square punch from tool steel (available in many common sizes), harden and temper with a propane torch, then hammer the punch into the metal, with a piece of endgrain hardwood on the other side. This does cause some edge distortion. Picture #10 shows a square punch being used to nibble along the edge of a hole. This method requires some upper body strength and a big hammer.

8) Use a Greenlee Knockout die. This is a punch and die set that cut by sandwiching the metal between the punch and die, screwing them together with a bolt that passes through the sheet metal. Knockout dies can be somewhat expensive but they will cut thicker metal than a hand nibbler and yield a perfect hole. Hydraulic power packs are also available that increase the speed and force of the punching operation. Picture #11 shows the parts of a Greenlee die and a hole with straight sides and rounded edges produced with it. Picture #12 shows the punch in use, you simply tighten up the bolt until the hole is cut. The punches are available for most standard electronic component shapes besides square and round holes. They can be very expensive.

10) If you have access to a manual or CNC milling machine the hole can be milled, as shown in pictures #13 and #14 of hole for B type USB female connector, but that will leave radiused corners that will still have to be filed if the part requires it. You can use double sided tape or the clamping method I showed for the drill press to clamp the metal to a sacrificial piece of aluminum, plywood or MDF.

Further possibilities I'm too lazy to demonstrate right now:

10) You can make a router template jig and use a small router bit to mill the hole, again leaving radiused corners that will have to be filed. Cutting wax will prevent the router bit from loading up with aluminum chips.

11) You can send the work out to a machine shop that has EDM (electrical discharge machining), laser or abrasive waterjet capability. Be warned that this can be expensive. CNC punch presses are becoming more common.

11.1) Some older shops may have a set of Watts Brothers square hole drills. These are drills that operate on the principle of the Reuleaux triangle to wobble the bit around the inside of a template that fits over the metal. These also leave radiused corners.

11.2) On thicker metal the machine shop may make the hole using a square broach, which is in essence a square tapered saw blade that is forced through a round hole, shaving the sides progressively larger and squarer. Rotary broaches may also be used to some advantage as well.

12) A plasma cutter can be used if the edge is to be dressed later with a file, or if a rough edge is allowable. An oxy acetylene torch can work on steel  (but not aluminum) and will leave very rough edges.

13) You can use a cutoff disc in a dremel or foredom tool to cut the edges, keep to the insides of the edges and be aware that it is easy to overshoot the corners with this method.

14) If you have access to a blacksmith shop you can "hot punch" a square hole by using a punch made to the same size as your square hole and heating the metal up in the forge. This method only works for steel.

15) And finally, redesign the part to use round holes instead of square ones.

So you can see that there are many methods that will work, chances are you have at least one of the tools that will do the job in hand already.

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Copyright Nicholas Carter 2019.