It would be hard to overstate the importance of keys used by various mechanisms. For instance, they provide a reliable interface between shafts and a great many other objects such as pulleys, levers, sprockets, and gears. There are quite a few different types of keys, but the most common single type of key used in most businesses is the straight key.
The process of broaching keyways is therefore also a very common type of procedure which is a necessary part of preparing any keys which you will be using. Below you’ll find a description of a typical broaching procedure that can be used to produce a keyway broach.
When it’s necessary to machine a keyway, a spline, or some other irregular shape into a hole, you’ll need to use a keyway broach, keyway broach shims, bushings, and possibly keyseating broaches to accomplish the task.
Tools and Materials
When preparing to broach a keyway, you’ll need a broach kit that includes the broach you’re going to use, some shims, and whatever type of bushing will be used on the project. You’ll also need a hydraulic press or an arbor press for the project. It will be important to have on hand some cutting oil, for instance, Tap Magic or some other readily available brand of cutting oil that can be used once your project is underway.
Keyway broaches are tools in the shape of a bar that have cutting teeth that are capable of creating precise keyways in the boreholes of washers, gears, pulleys, and many other components mounted on shafts, so they can be locked into the shaft. Each of the many teeth on a keyway broach is larger than the one before it, and that allows the tool to progressively remove more material, as it makes its way into the hole.
Keyway shims help to manage the depth of the keyway when it’s necessary to make several passes with any kind of keyway broach. These shims would be placed between the keyway bushing and the keyway broach, to compensate for any material which gets removed. They also help to ensure that an accurate keyway depth is achieved.
Keyway bushings are used to support the keyway broaches and guide them during the cutting process. These bushings will be placed inside the borehole of any workpiece being used, so that precise keyway cutting can be accomplished. Without the keyway bushings, it would be very difficult to ensure accurate keyway cutting, and the keyway itself would likely be out of alignment.
To be sure that you have a proper fit for all your materials, you should choose shims, bushings, and broaches with the same style designation, and this will usually be indicated by either a letter on its own or a letter and a number.
These tools are excellent for use in short-run production projects, and can also be used for maintenance operations.
Getting Set Up
If you’re using a small broach in soft material, you’ll have no problem using a 3-ton arbor press. However, if you’re broaching a tougher material and a larger broach is required, you may need to go with a hydraulic press, since larger broaches often require thousands of pounds of pressure to make an adequate cut. In most cases, larger approaches are a lot longer anyway, so they won’t usually fit into an arbor press.
Choose a bushing and a broach that fit well together, and so they slide easily with no slop. If your keyway is small enough, you won’t need any shims, as you might when using larger approaches. Your first step should be to insert the bushing into the hole to be broached, and it should drop in fairly easily. Then you can rotate the slot where you plan to cut the key. It will be necessary to lube up the broach using whatever cutting oil you’ve chosen, and then inject the broach into the bushing slot. The first part of it should slide in fairly easily, and then the teeth will actually engage with the part.
Making the Actual Keyway Broach
Now you’ll need to align your part with the press and make sure that the broach itself is completely vertical, i.e. that it is straight up and down. Then you will need to apply uniform pressure to make sure that your part doesn’t tilt or rock off the arbor press working area. Any aluminum object won’t take much effort at all, which means it’s very easy to work with. You should be prepared to retrieve the broach when the end of the stroke is reached because it’s entirely possible that the broach may fall out.
If the broach doesn’t fall out but remains stuck in the park, you can use a small punch or some kind of stock piece to push it completely out of the part. One thing you should not do is to strike the broach because this could flatten out the end of the broach, even if it’s by a tiny amount. This could cause you major headaches later on when you attempt to use your keyway broach. So keep in mind that you should never do anything more than press the broach when you’re trying to separate it from the part.
Resetting and Test Fitting
Now that you’ve removed the broach, you can use some kind of chipping brush to get rid of the swarf around the broach. It’s extremely important to get rid of these chips before you attempt to broach a second or third keyway. If you don’t remove this swarf from around the broach, it could end up jamming the bore and causing you some major problems.
At this point, you’ll be able to take a key and test fit your newly broached pulley with the shaft. Hopefully, you’ll find that it’s a perfect fit and that no rework needs to be done.
Of course, if you purchase shafts that already have keyways machined into them, you can bypass the entire procedure described above. However, this is not always the case, so it’s a good idea to have a working knowledge about how to broach your own keyways.