Offset Rotating Broach Information

Since, as the name implies, broaching can be performed on a part that is rotating, turned parts requiring a broached hole can very often be completed in the original setup on the screw machine or CNC turning machine. This eliminates the need for secondary operations and improves part uniformity.

Typical Somma broach tools and holders.

The rotary broaching principle can also be applied to broaching on a CNC machining center or vertical mill. The only difference being that the broach holder is rotated in the machine spindle and the part is stationary instead of the reverse condition on a turning machine.

As manufacturers of rotary broaching tools, we soon realized that in order to produce parts successfully there are a few basic rules that must be followed. These rules are based on understanding how and why rotary broaching works.

1. Cutting Principle

The basic principle that makes this tool work is the same for all tools regardless of the manufacturer: The centerline of the cutting tool is offset at 1 degree from the centerline of the work piece. This causes the broach to shear its way into the part with a scalloping effect as it is advanced into the work piece. We have found that the easiest way to visualize this cutting action is to picture it as if the work piece were stationary and the broach holder were rotating, instead of having the work piece rotating and the holder stationary as is normally the case with a screw machine.


As can be seen in Figure 1, the broaching cutter spindle is driven into the part at a 2 degree included angle of its cone of rotation. This causes the broach to cut only on its leading edge, not its full end surface as it would if it did not have the 1 degree offset. This eases the load of the cut and creates a shearing, rotational cutting action so that cutting tool is actually spiraling its way into the part.

2. Construction

Since all holders operate on the same principle, the only real difference between broaching tools is holder and cutting tool life.

The prime cause of holder breakdown is bearing failure. Following experimentation with many types of bearing, our current bearing types, construction, and quality have resulted in longer bearing life than for any other tool available. Because of superior bearing construction, our tool is more rigid, contributing to longer cutting tool life and more accurate and uniform parts.

3. Feed Rate

The Somma broach tool is offset 1 degree, and it is essential that the helix angle of the feed rate not exceed one degree. If it does, the broach will bottom out, crowding the metal instead of cutting, and causing tool wear or breakage. See Figure 2. Therefore, the best feed rate should be the equivalent of 1 degree helical maximum. To maintain this 1 degree helical feed rate, the feed rate per revolution can be calculated by multiplying the diameter that is to be broached by .016. (EX: .250" part diameter x .016 = .004" IPR)  The feed rate, however, must never be less than .004 IPR or else a counterboring effect will occur.

Rotational speed (RPM) has very little effect on cutter life, since the cutter rotates with the work piece. However, life of the spindle bearings is affected by the speed of rotation, therefore, we do not recommend speeds over 1200 RPM. Cutting tool material and use of coolants is not critical for this same reason.

4. Blank Preparation

For internal broaching, the hole should be drilled 0.005"-0.015" larger than smallest diameter of the broach, and countersink it at 90 degrees to slightly larger than largest dimension of broach. Drill the hole as deep as possible to leave room for chip accumulation.


For external broaching, turn the O.D. to be broached 0.005"-0.015" smaller than the largest dimension of the broach and form a 45 degree chamfer on the front end down to at least the smallest dimension of the broach. This 45 degree angle is essential for easy starting of the broach; if the part design does not permit it, it can be removed after broaching.

5. Broach Geometry

Internal broaches must be ground with a 1-1/4 degree maximum back clearance on all surfaces. The face should be dished at a 7-10 degree clearance. This can be done with a carbide centering-type drill. The larger end of the broach should be made to the high side of the part tolerance since the broach gets smaller as it is sharpened.

External broach dies must be made with maximum 1-1/4 degree back taper (draft) on all surfaces. The front end of the opening in the die should be made to the low side of the part tolerance since the hole gets larger as the broach is resharpened.

The face of the die should be sharpened with a 10-15 degree face angle. When broaching squares or hexes, this clearance angle should be surface ground in facets to coincide with the points of the form, rather than ground cylindrically. See Figure 3.


6. Part Configuration

Hex and Square shapes can usually be broached without much difficulty. Problems may be encountered when broaching other shapes, such as splines or keyways, especially when the depth of the cut is greater than half its width. If the chip being cut is too thick and does not curl away from the cutting edge of the broach, the broach can jam up.   Please note: On Internal Broaches, countersink the hole at 45° to a diameter larger than the cross corners dimension of the broach.  

On external broaches, chamfer the bar end with a 45° angle smaller than either the across flats dimension or the I.D. points of the broach.

7. Spiralling

Spiralling of the broached form is caused by the back taper on the broach. Since the broach is driven by the leading edge of the hole (ID) against the nearest surface of the broach (BB), the space between the broach and the hole caused by the back clearance allows the broach to rotate slightly and cut a spiral as shown in Figure 4.

The greater the back clearance on the broach, the greater the spiralling action. For this reason, a maximum 1-1/4 degree back clearance is recommended instead of the 1-1/2 degree more commonly used. Ideally a 1 degree back clearance would eliminate all spiralling, but, due to machine or holder inaccuracies, dragging action might cause other problems.

Spiralling will also occur if the broach is not properly centered. The more the form is shifted off to one side, the greater will be the amount of spiralling.

If the job layout permits it, the spiralling can sometimes be reduced by reversing the spindle rotation half way into the part. This causes the broach to be driven by the opposite wall of the hole thereby reversing the direction of the spiralling and making the hole straighter.


8. Setting the Holder for Center

Nothing is more important than having the cutter centered as close as possible to the center of the work piece. Improper center setting will cause uneven hole configurations, oversize holes, spiralling, excessive cutter wear and excessive holder wear.

Therefore, it is essential that you follow the setup instructions that come with your broach holder.

Offset Rotary Broaching Setup Instructions

If you have a specific broaching problem you need help on, please do not hesitate to contact us.