Introduction: How to Broach a Keyway
Keys are an important part of mechanisms. They provide a solid and reliable interface between shafts and a wide variety of other objects including pulleys (as in my case), sprockets, levers and gears. There are a variety of different types of keys, but the simplest and most common is the straight key.
For this project, I'll be broaching keyways in some pulleys that I'm in the process of machining (still need to finish up the OD). The material is 6061 aluminum, it's 3/8" thick with a ø3/8" hole and I'll be broaching a 3/32" keyway.
Step 1: Tools and Materials
Broach Kit, including a broach, shims (as needed) and the appropriate bushing.
Arbor or Hydraulic Pres
Cutting Oil, my favorite is Tap Magic
Step 2: Setting Up
This is a small broach in soft material, so I can easily use the 3 ton arbor press. If you're broaching a tougher material and/or a larger broach, you'll need a hydraulic press, as large broaches can require thousands of pound of pressure to cut. Also, larger broaches are much longer and often do not fit in an arbor press.
For this setup, I've selected the ø3/8" bushing, and the 3/32" broach. They should fit together well, sliding easily without any slop. Since the keyway is so small, I will not be needing any shims, which are used with larger broaches.
Insert the bushing into the hole you wish to broach. It should drop in easily. Next, rotate the slot to where you wish to cut the key. Liberally lube up the broach with your favorite cutting oil and and insert the broach into the slot in the bushing. The first quarter or so should slid in easily before the teeth engage with the part.
Step 3: Broaching
Align the part with the gap in the press and ensure the broach is straight up and down. Then apply even pressure, ensuring that the part is not tilting or rocking off the arbor press work surface. In aluminum it requires little effort. Be prepared to catch the broach as it reaches the end of its stroke- it may fall out. If the broach remains stuck in the part, use a small punch or piece of stock to press it all the way out. Do not strike the broach, as you can mushroom the end (sometimes imperceptibly) and cause yourself a big headache. Only press a broach.
Step 4: Resetting & Test Fitting
With the broach removed, use a chip brush to remove the swarf from the broach. It's important to clean these chips before broaching another keyway. Failure to do so can result in the broach jamming in the bore.
Now, we can take a key and test fit the freshly broached pulley with a shaft. It's a perfect fit. Often times I purchased shafts that already have the keyway machined in them. They are available from McMaster Carr.