ROTARY BROACHING BASICS
This guide provides some basic rules and tips for successfully producing forms using the rotary broaching process.
Broaching can be performed on a manual or CNC lathe, mill or other turning center. The only difference is that in a mill, the rotary broach tool holder is rotated in the machine spindle and the part is stationary whereas in a lathe the tool holder is stationary and the part is turning.
Tool Holder Set-up:
The Polygon Solutions tool holders have completely sealed bearings. Therefore, there is no need for constant greasing.
Polygon Solutions’ tool holders are completely adjustment-free and require minimal set-up. Centering the rotary broach as close as possible to the center of the workpiece is extremely important. Oversized forms or uneven form configuration can result from improperly centered broaching. As long as the toolholder block on your turret, or machine spindle on a mill, is centered with your workpiece, simply insert the Polygon Solutions tool holder and clamp it down. IMPORTANT: DO NOT attempt to locate center off the rotary broach or any part of the tool holder.
Coolant: Rotary Broaching is generally a very low heat operation. However, it is recommended that coolant or cutting oil be used. When broaching tough materials like stainless steel, titanium or Inconel, Polygon Solutions recommends the use of our specially formulated cutting fluid. This cutting fluid is applied by adding a few drops to the tip of the broach prior to it engaging the workpiece. The Polygon Solutions cutting fluid greatly reduces thrust requirements and tool wear. Order Part #: CF-004.
Part Preparation Guide
Pre-Drill Hole Diameter:
Internal rotary broaching requires a pre-drill hole. It is strongly recommended that the hole diameter is larger than the minor diameter of the form being broached. Below are formulas for recommended pre-drill hole diameters for hex and square forms. When broaching forms such as serration, spline or hexalobular (Torx), it is recommended to pre-drill a hole 2-3% larger than the minor diameter of the form.
*These percentages may be reduced for free cutting material and should be increased in materials with tougher machinability.
Pre-Drill Hole Depth:
If not broaching a through hole, the pre-drill hole must be as deep as possible. The pre-drill hole depth must be greater than the broaching depth to allow for chip accumulation. The minimum depth Polygon Solutions recommends to avoid excessive chip packing is 1.3 to 1.75 times the depth of the form being broached. If possible, an undercut just beyond the depth of the broached form will allow the chips to break away cleanly. Alternatively, chip removal can be achieved using a drill or boring tool.
A 45° lead-in chamfer, slightly larger than the major diameter of the broach, is essential for successfully rotary broaching an internal form. In tougher material a 30° chamfer may work better.
Speeds and Feeds:
Many factors affect speeds and feeds including material being broached, major and minor diameters and the form being broached. Polygon Solutions recommends starting with a minimum feed rate of .001-.003” IPR at 700-800 RPM.
Broaching Tougher Materials:
When broaching material such as stainless steel or titanium, a slower RPM and Feed Rate is recommended. Slow the speed down to 100 RPM until the part is engaged to a depth of .010” or until the part makes full contact with the broach and then increase the RPMs. This will prevent the broach from skipping around on the chamfer area when engaging the workpiece and reduce the risk of chipping the broach. PM M-4 broach material is also recommended when broaching these tougher materials.