One Pass Polygons: How Does Rotary Broaching Work?

One-Pass Polygons: With rotary broaching, a polygon is cut in one pass.
Rotary broaching is a fast and accurate method of producing internal and external polygon forms on the end of a workpiece while it is rotating. The entire operation takes seconds and is capable of producing forms to an accuracy of better than 0.0005".

The advantage of performing this operation is that it almost always costs less, because it eliminates a secondary operation on another machine. In some cases, rotary broaching can be performed at the same time as other operations, thereby increasing speed and profitability during production, without sacrificing accuracy.

Unlike conventional broaching, in which a series of polygon forms that increase in size are pushed through a hole until the desired form is achieved, the rotary broach cuts the full form, one corner at a time, in one pass.

Rotary broaching requires two components: a toolholder and a broach. Due to the increased use of titanium and other difficult-to-machine work piece materials for items such as bone screws, the variety of materials used to produce rotary broaches has expanded. Standard broaches typically are made from M-2 HSS, but are also available in PM-4 and cobalt HSS metals like T-15. Broaches are coated with TiN, TiCN and TiAlN to enhance their performance.

Rotary broaching is becoming increasingly popular in the medical, aerospace, automotive and plumbing industries. Due to the large variety of rotary broaches and holders available from stock, the process is practical for small machine shops. By simply switching broaches, they can run a different polygon form with minimal, or no, setup change.

Toolholder Design
The rotary broach holder incorporates an internal live spindle that holds the tool at a 1-degree angle. The spindle spins freely within the holder and is driven by contact with the rotating workpiece. Thus, while rotating, pressure on each corner of the broach is constantly changing. This creates a chisel-type action as the broach is fed through the workpiece to the desired depth. Because speeds are up to 2,000 rpm, most full-form broaching takes approximately 10 to 15 seconds.

The holder is designed for use on any CNC mill, lathe or screw machine and can be operated one of two ways. In a turning or screw machine, the holder is stationary. Its internal live spindle and the broach rotate with, and are driven by, the workpiece rotation. Because the broach is workpiece-driven, rotating synchronizing devices are not needed. Therefore, it is possible to rotary-broach at any desired station inside the machine.

In a milling or drilling machine, the toolholder is mounted in, and rotates with, the machine spindle while its free-moving internal live spindle and the broach remain in contact with the workpiece. This moves the holder around the centerline of the hole at a 1-degree angle, creating the effect that it is "wobbling." Although the holder continues to hold the broach with great precision, rotary broaching is often referred to as "wobble broaching" for this reason.

Finding Form
Form sizes up to 2" can be broached in aluminum, 1.5" in brass and 1" in steel. Rotary broaching is best utilized to create small forms at shallow depths. The practical forming length, or depth, for rotary broaching is up to 1.5 times the smallest diameter of the form to be broached.

Hexagons and squares are the most common configurations. Off-the-shelf tools for these forms are available in most sizes. Broaching a hexagon form into a cap screw is a typical application. Profiles such as serrations, splines, six-lobe shapes and other polygon forms can be easily produced as well.

Toolholding Tips
The cutting edge of the broach must be concentric to the toolholder's internal live spindle and its shank. They can easily be aligned using a V-block and some adhesive tape.

Place a broach in the holder and measure how far the cutting edge extends from it. Then, replace the broach with a setting plug (or drill rod the same diameter as the shank) that extends from the holder the same, or a greater, distance than the broach did. Fasten the holder into a V-block. Tape the spindle to the holder body so it doesn't freely spin and then rotate the entire assembly by hand. Adjust the holder until the setting plug zeros at the same distance from the holder as the cutting edge of the broach. Replace the setting plug with the broach and the holder is now ready to be mounted in the machine.

One nice feature of rotary broaches is that, once set up, they cut equally as well in right-hand or left-hand operations.

Pilot Holes and Chamfers
For internal rotary broaching, a pilot hole with a lead chamfer is required. Start with a clean, true-running pilot hole that's as large as the design allows. The hole diameter should be, at a minimum, equal in size to the distance across the flats or inscribed-circle diameter of the form to be broached. The lead chamfer diameter must be slightly larger than the form being broached.

Except in cases where the spindle can be slowed to a near stop, a minor skid mark is likely to show at the point where the broach makes initial contact with the workpiece. If objectionable, this can be removed by rechamfering the hole. In every instance, a lead chamfer larger than the major diameter of the form going to the minor diameter of the form is required.

Square and hexagon holes sometimes are not required to be full form. In these cases, make the pilot hole pilot as large as possible. The less material to be broached, the less thrust will be needed to broach.

The depth of the pilot hole must exceed the depth to be broached. There must be enough room at the bottom of the hole to allow chips to accumulate without excessive packing. Chips can be removed after broaching with either a boring tool or a flat-bottom drill. A recess at the bottom of the hole allows chips to break cleanly. The recess diameter should be larger than the major diameter of the form being broached.

For external rotary broaching, the turned diameter of the workpiece should be smaller than the major diameter of the form being broached. If full form is required, turn the diameter 0.0008" to 0.0015" larger than the full diameter. For starting the external broach on the workpiece, a lead chamfer is required. A 45-degree chamfer should be smaller than the minor diameter and larger than the major diameter.

Speeds and Fluids
Bearings in the toolholder are designed for normal machine speeds. Therefore, whatever spindle speed other tools are run at should be acceptable for rotary broaching. Normal feed rates should be on the order of 0.004 to 0.008 ipr for brass and other nonferrous materials, and about half that for mild steel. Grades of stainless steel prone to workhardening present special challenges; check with the tool and material suppliers about recommended feeds before assuming broaching is practical. In all materials, the smaller the broach, the lighter the feed should be.

Fluids play a minor role in rotary broaching. Any type of water or oil based fluid is acceptable. The chip is a flowing type and the amount of heat generated is minimal.

When internal rotary broaching in blind-holes, fluid should be applied to the broach tip prior to its contacting the workpiece. Fluid should not be directed into the pilot hole. The fluid may be unable to escape, preventing the broach from reaching full depth. Broaches with vent holes are available to alleviate this condition. For external rotary broaching, fluid may be applied prior to the tool engaging the workpiece or continuously during the operation.