File an Internal Keyway by Hand

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About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

I needed to replace a motor. The shaft on the old motor was 3/4 inch in diameter. The shaft on the new motor is 7/8 inch in diameter. Expanding the diameter of the shaft hole was not difficult, but I needed to add an internal keyway.* I decided to file a keyway by hand because that fit the tools I have available, and I successfully did it once before on a smaller collar.**

Materials

  • A short piece of electrical metallic tubing (EMT, or thinwall conduit)

Tools

  • Tubing cutter
  • Hacksaw
  • Vise
  • Relatively thin flat file for steel
  • Steel wedge (flat screwdriver)

* As I began enlarging the shaft hole, I believed enough of the old keyway would remain that I could simply deepen it with a file, but enlarging the shaft hole removed the entire keyway. I would have saved a lot of time and effort if only I had made the original keyway 1/8 inch deeper before enlarging the shaft hole.

** An alternate method for cutting an internal keyway is to mount a cutter the size and shape of the desired keyway on a bar and manually crank the tool holder down the lathe bed so the cutter removes no more than 1/1000th inch per cut.

Step 1: Make a File Guide

The internal keyway cut inside the shaft hole must be parallel to the existing keyway on the motor shaft. Anything done by hand with a file quickly wanders off of a desired straight line.

Cut a piece of EMT close to the internal diameter of the shaft hole so it is just 1/16 to 1/8 inch longer than the pulley or collar is wide. A tubing cutter works very well, but a hacksaw will suffice. Cut the tube as squarely as practical.

Lay the EMT piece on a flat surface. Bring a piece of wood up to the EMT and use a marker to make a line along the length of the EMT. Note the orange colored line in the first graphic. Very carefully cut along the line as shown in the second graphic. Use a hacksaw or a Dremel tool with a cutter disc to make a cut that very precisely follows the line you made. Mark the EMT to indicate the very straight cut and distinguish it from a second cut described in the next paragraph.

See the third graphic. Make a second cut to remove a piece from the EMT about a quarter of an inch wide. This second cut may not be as precise as the first cut. That is fine as long as you can always know which is the cut precisely parallel to the side of the EMT.

Step 2: Prepare to File

Fit the EMT to the shaft hole by either compressing or expanding its diameter. (Squeeze it evenly in a vise if too large, or pry the edges of the opening apart a little until the EMT fits the hole somewhat closely.) Insert the EMT in the pulley or collar shaft hole. Rotate the opening you cut until it is where the keyway should be. Normally, there will be a set screw and the keyway will align with it.

Notice the yellow line. Put the assembly in a vise so one of the jaws covers the yellow line. The other jaw will cover the same area at the opposite end of the EMT. The pulley or collar will be able to turn a little on the EMT. Lock it in place by lightly driving the wedge end of a straight screwdriver between the pulley or collar and the vise jaw where the green arrow appears in the graphic.

See the second graphic. The precise cut you made in the EMT is positioned just above the yellow line. Use light downward pressure on the file to keep the cut straight and parallel. Apply most of the pressure during filing against or toward the point of the large green arrow. Loosen the pulley or collar on the EMT by removing the screwdriver wedge, shift the collar or the pulley a bit to make the keyway cut the desired width, and lock in place with the wedge end of the screwdriver. Continue filing.

It can be a slow patient process, but the EMT filing guide allows you to produce a good keyway. The depth of the keyway needs to be even at both ends and through the middle. You may need to do some final adjustments with the file as you attempt to fit the pulley or collar over the key and on the shaft.

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