Why use a rotary broach?
We want hex cut outs or a squared feature on the end of our part. However, perfectly squared features aren’t possible when using a CNC milling machine as the radius of the bit would round the corners. We have to take another approach. EDM is a possibility but that isn’t a simplistic process and requires a dedicated machine, it would also be rather expensive for low-cost parts. A better approach would be to use a broach. Rotary broaches are a economical way of producing square featured cutouts on parts like thru-axles, bolts and cogs.
What is a broach?
A broaching tool is used to make squared or polygonal keyway shapes in machined parts. Common examples would be a spline or keyway. Push/pull broaches use a hydraulic pushing/pulling force combined with shims to apply pressure with incremental teeth that remove material from a workpiece. You can learn about vertical broaches here. While rotary broach tips look just like a hex or Allen key they are actually quite different.
What is a rotary broach?
A rotary broach is a type of broach for producing squared features in work pieces.
Larger at the tip with a taper to the base of the tool.
Mounted in a tool holder with a rotary bearing and an offset of 1°
Incorporate a 1-1/2° clearance angle, to compensate if slightly off-centered.
The broach tip has a small deformation or pressure relief hole or vent to relieve hydraulic pressure build-up.
Made from M2 high-speed steel for softer materials like aluminum and plastics.
Made from T15 with outer coatings offers additional strength and longer tool life for tough materials like stainless steel, Inconel, and titanium.
How does a rotary broach work?
Rotary broaching works differently than a hydraulic broach as it rotates with the workpiece creating a shearing wobbling motion that creates the cut. A rotary broach holder holds the broach at a one-degree angle and internal bearings allow the broach to rotate.
What machines can use a rotary broach?
A rotary broach can be used on a lathe, mills or even a drill press.
We commonly use it in our lathes or Swiss auto lathes. A rotary broach holder can be placed in a lathe machine's turret which makes it convenient for repeating the process during a production run.
In a Swiss machine, rotary broaches are placed into a crossbar, making it possible to broach both ends of a workpiece using the machine's multiple spindles.
The process is different on a mill as the workpiece will not rotate. The machine spindle will rotate both the broach and its holder. Rotary broaches can also be stored in a milling centers tool cage.
The rotary broach process can be performed both internally or externally creating hex, Torx, or other custom shapes.
A video showing rotary broaching on a lathe:
Internal feature broaching on a lathe:
For an internal hex, a pilot hole is pre-drilled to the correct diameter (hex cross-dimension +.10mm or 1%) then a 60-90* chamfer is added.
Aerospace parts require exact concentricity thus the hole must be counter-bored, drilled, and then bored in order to keep the broach concentric.
The broach is then centered and the turret will jog/move along the Z-axis toward the chuck and spindle. The workpiece RPM is set from 500-1000 RPM, consult HSS bit rpm for actual speed and feeds.
As the turret moves along the Z-axis the broach will engage with the workpiece and they will begin to rotate together. When the workpiece and broach rotate together it creates a wobbling, shearing, or chisel cutting force caused by the one-degree offset. The rotary broach makes incremental cuts at each corner per rotation. The max depth of the cut can be up to 2 times the cross dimension.
External feature broaching:
While it's possible to use a rotary broach to produce squared features or polygonal shapes or forms on the outer features. A mill can usually produce these outer shapes unless machining small features with tight tolerances. On these parts, an external rotary broach can be used.
This process requires an external broach, this tool is a negative of the required shape. The process requires the part to be prepared by machining the workpiece slightly smaller than the desired shape. The work piece must also have a machined chamfer.
At high volume, the general practice is to use a press or punch as they are quicker and more economical.
However, using one machine for multiple operations is wise, as staging or setups on multiple machines increase the possible chance of loose tolerances due to variance in machines and environmental conditions.
Going further:
While the information provided is from our team's years of experience working with rotary broaches, it's worth sharing the Polygon Solutions website as they provide in-depth explainer videos and their site is full of detailed information. -I'm also quite fond of their adjustment-free broaching tools.