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How to square a round hole with files

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Image of a round hole cut into a piece of steel If you have a round hole cut into a piece of metal that needs to be turned into a square slot, you can do it using files.

Preparing your workpiece

Image of a bottle of layout ink, which you will need to use to be able to mark out an area on a metal surface First of all, you will need to apply a layer of layout ink around the general area where your hole has been cut.
Image of a work piece with a round hole that has been painted with marking ink and has then had straight lines scribed into it with a scribe Using a straight edge and a scribe, you will need to carefully mark out a perfect square. Each edge should touch the circle.
A work piece that has been clamped into a vice and is ready for filing Clamp your workpiece securely in a vice with the marked surface pointing towards you. Don’t forget your filing block!

Reshaping the slot

Image of the three different types of file that are needed to make a square hole out of a round one: three square, square and pillar For this task, you’ll need a three square file, a square file and a pillar file.
Image showing the corner of the round hole that the DIYer will be filing square filrst You’ll be working on one corner at a time.
Image of a DIYer starting the squaring process by cutting a triangular notch with a three square file

Step 1 – Cut notch

The first job is to cut a notch that will give you a platform for rapid material removal. For this you will need a three square file.

Diagram illustrating the acute angle on a three square file The acute angle at the edge of the three square file allows you to cut into the edge of the circle without having to worry about your file cutting into the material anywhere else.
Image to show that a square file is not used to begin with as a three square file is better at gripping the inside of the circular hole This is why a square file is not used for this part of the process, as there would be more corners on your tool that you would have to worry about.
Image of a DIYer using a square file to rapidly remove material from the inside of a circular hole

Step 2 – Remove metal

You can now switch to a square file and file away the metal until it’s close to the lines you have marked out.

Image of a DIYer using a pillar file to work on just one side of the square hole, taking advantage of the pillar file's safe edge

Step 3 – Finish

Using a pillar file, you can now file right down to your lines.

Image showing the location of the safe edge on a hand file The safe edges on the pillar file mean that you can file right up to the corner without worrying about accidentally cutting into a second surface.
Image of a round hole that has had one corner squared off thanks to the use of a variety of different files

Step 4 – Repeat

Once your first corner has been filed square, rotate your workpiece and begin working on the second, then the third and the fourth.

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