Reader writes about boring and keying a tapered bushing

The following letter to the editor is from Kevin Quitberg.

Remember that before there was high-tech there was low-tech. And there still is low-tech! For example, boring and cutting a keyway in a tapered bushing is one of the most often performed tasks a maintenance machinist will perform.

Tapered bushings come in several different configurations, but they all perform the same function: the transmission of torque from a driver to a driven component. They will come with shallow keyways or standard depth keyways, with or without a flange and some small ones will come with no split on the axis.

Typically, a larger bushing comes in a class 30 gray iron. Some of the smaller bushings are sintered. I will almost always run a machine slower when drilling and boring the sintered material as it seems to be quite a bit harder and less friendly to machining than a simple gray iron casting. The exception to this is in an emergency situation, and the drill bit is considered a throwaway item, time being of much greater value.

When setting up a bushing for machining, you will notice that the flange and the taper on the bushing is rarely concentric. It is tempting to turn your chuck jaws around and chuck the flange, then drill and bore your bushing. Doing so will assure you that the sheave or sprocket the bushing is to fit will not run true. Resist doing it this (the easy) way. Use a 4-jaw chuck to clamp the bushing’s flange and indicate the face and OD of the taper. The bushing’s flange has no bearing on whether the bushing will be true in the component it is to fit. That is totally dependent on the tapered portion of the bushing.

It is also tempting to use a 3-jaw chuck and grip the bushing near the flange on the taper. Sometimes there is a small straight area near the flange that is not tapered and it is tempting to grip the bushing there with a 3-jaw chuck. You may get away with that, and you may not.

Before chucking the bushing, you will need to insert some shims in the split line of the bushing. You want enough shims here to keep the bushing from distorting under chucking pressure. So, the shims will be tight when inserting them into the split, but not so tight the bushing bore is actually deformed slightly. Don’t use a screwdriver or wedge to open the bushing to accommodate more shim pieces than necessary. I use the thin ½”-wide blades from a set of feeler gages. You can cut them to a convenient length, and it doesn’t hurt if they extend into the bore of the bushing as they will be machined away. Insert the shim pieces perpendicular to the bore of the bushing, preferably in the flange you are chucking.

I keep the shim pieces in my toolbox after finishing the bushing as they can usually be used over again. When inserting a thin blade, say 0.002” thick, sandwich it between a couple thicker blades. If you do not shim the split line and end up compressing the bushing during chucking you will not have a round bore after machining. This is important!

The drilling and machining itself is pretty straightforward. I rough-out cast iron with about anything handy, but I like to finish with a cut of approximately 0.005”/side with a C-20 carbide with a 1/32” nose radius and a sharp honed edge with approximately a 0.005” feed/revolution. I like that finish in gray iron.

A bushing without a flange is a little different, as there isn’t a safe and convenient way to hold the piece during machining. The split line will need to be shimmed during machining as well, just like the flanged bushing.

In this instance, you will use the sprocket or sheave as the "mandrel" to hold the bushing. After shimming the split, insert the bushing into the sheave and tighten it as you would during installation. In this case, you would indicate the face and OD of the sheave or sprocket rather than try to indicate the bushing. In a worst-case scenario, you could make a dummy piece to pull the bushing into. If your lathe is not big enough to hold the sheave or sprocket you can do the job in a vertical mill with some gage blocks and toe clamps, indicating the OD of the sheave to find center. Then drill and bore from that point. I like to keep worn out sheaves that will hold various size bushings on a shelf so I don’t have to try to hold a big piece in an awkward manner.

Some of the smaller bushings come with no axial split line. You will not need to shim them, but you absolutely do need to remember to cut the split after boring and keying the bushing. I like a vertical bandsaw for this. Wait until after you cut the keyway before doing this operation.

Now for cutting the keyway. In a maintenance shop the keyway will either be done with a broach or a keyseater. For me, the keyseater was almost always quicker on larger bushings. Many bushings will have "standard shallow" keyways. They will be 1/16” or 1/8” deep. The widths on standard shallow keyways can be up to 1¼” wide. Bushings are effective for transmitting torque. In a nonreciprocating or shock-loaded application, you often don’t even need a keyway. This is why a shallow keyway is all that is usually needed.

When using the keyseater (or a broach), position the split at 180° to the keyway. Clamp the bushing as you would any other item being keyed.

When broaching a bushing with the pre-existing split, you will need to put a hose clamp around the flange after inserting the broach carrier. Occasionally, a bushing will break at the split if the wall is relatively thin. It is more prevalent if the broach is a little bit dull and exerts a lot of pressure on the bushing sidewall as it travels through while cutting the keyway. You don’t want to have invested the time and effort thus far only to have the bushing break. So, make a mental note: remember the clamp.

The bushing you bored while contained in the sheave or sprocket should be broached in the same manner. You can take it out of the sheave to cut the keyway if using a keyseater.

I don’t know if your readers have any experience with machining cast iron, but I really like kerosene for a cutting fluid on gray iron. I use it when tapping and cutting keyways, rarely ever when just turning. The graphite in gray iron really makes a mess. Other than the mess, cast iron is a wonderful material in the applications it is designed for.