Step by Step Guide to Broach Design for Beginners

Broach Tool Nomencluture

Pull End

The pull end is designed to connect the broach to the broaching machine using the puller head.

Front Pilot Broaching

This aligns the broach precisely in the hole that has to be completed, right before the procedure begins.

Roughing Teeth

These cutting blades are designed to efficiently remove a significant amount of material during the cutting process. Increasing the amount of material removed results in a worse quality surface finish, but it also speeds up the procedure.

Polishing Teeth

These cutting blades are used to remove tiny amounts of material. These tools are utilized for the precise size and final polishing of surfaces.

Rear Pilot and Follower Rest

This is an auxiliary device used in conjunction with a broach to facilitate the completion of the broaching process.

Land

The flank face width of the broach refers to the measurement of its breadth. Typically, it is somewhat inclined to provide a relief angle to the flank face of the broach.

Pitch

Pitch refers to the measurement of the distance between two matching places on consecutive teeth of a broach. Typically, the pitch of the finishing teeth on a broach is less than that of the rough cutting teeth.

Dental height

The height of the roughing and finishing teeth progressively increases from the shank to the finishing teeth. The increment in question is referred to as the cut per tooth, and its value is contingent upon the material being subjected to machining. Typically, the amount of material removed per tooth ranges from 0.01 to 0.2 mm for the finishing teeth, and it can up to 0.2 mm for the cutting teeth.

Broach Terminology

• Front pilot: When using an internal pull broach, both the pull end and the front pilot pass through the beginning hole. The pull end is then secured onto the broaching machine’s pull head. The front pilot ensures that the tool is properly axially aligned with the beginning hole and acts as a check on the starting hole size.

• Length: The amount of stock to be removed determines the length of a broach tool or string of tools, which is limited by the machine stroke.

• Rear pilot: The rear pilot keeps the tool aligned as the final finish teeth pass through the workpiece hole. On round tools, the diameter of the rear pilot is somewhat less than that of the finish teeth.

• Cutting teeth: Broach teeth are often separated into three portions running the length of the tool: roughing teeth, semi-finishing teeth, and finishing teeth. The initial roughing tooth is proportionally the smallest on the instrument. The successive teeth gradually grow in size, culminating with the first completing tooth. The difference in height between each tooth, known as tooth rise, is often larger during the roughing part and smaller along the semi-finishing region. Each finishing tooth is the same size. The face is ground using a hook or face angle dictated on the workpiece material. For example, soft steel workpieces often require larger hook angles, whereas hard or brittle steel components demand lower hook angles.

• Tooth land: The land protects the cutting edge from pressures. A modest clearance or back-off angle is ground into the lands to prevent friction. A back-off angle is used to alleviate the entire area when roughing and semi-finishing teeth. When completing teeth, a portion of the area immediately beyond the cutting edge is frequently left straight so that repeated sharpening (by grinding the face of the tooth) does not change the tooth size.

• Tooth pitch: The spacing between teeth, or pitch, is regulated by the length of the cut and modified by the workpiece material. To handle a higher chip load, roughing teeth may need to have a somewhat big pitch. Semi-finishing teeth may have a narrower pitch to shorten the total length of the broach tool. Pitch is calculated such that two or more teeth can cut at the same time. This keeps the tool from wandering or chattering.

• Tooth gullet: The depth of the tooth gullet is proportional to the tooth rise, pitch, and workpiece material. The tooth root radius is normally built such that chips wrap snugly within themselves, taking up as little space as possible.

• Chip load: As each tooth penetrates the workpiece, it cuts a specific thickness of material. The set chip length and thickness generated by broaching result in a chip load that is determined by the broach tool’s design and feedrate.

• Chipbreakers: Notches, known as chipbreakers, are used on broach tools to reduce chip packing and enable chip removal. The chipbreakers are ground into the broach’s roughing and semi-finishing teeth, which are parallel to the tool axis. Chipbreakers on alternate teeth are staggered, with one set followed by a cutting edge. The finishing teeth conclude the work. Round broaching tools require chipbreakers; without them, the tools would create ring-shaped chips that would wedge into the tooth gullets and finally shatter the tool.

• Shear angle: Broach designers may position broach teeth at a shear angle to increase surface quality and decrease tool chatter. When two neighboring surfaces are cut concurrently, the shear angle plays a crucial role in transporting chips away from the crossing corner and preventing chip crowding at the cutting teeth junction.

• Side relief: When broaching slots, the tool is confined by the slot during the cutting process and must transport the chips created along the whole length of the workpiece. Without clearance, the sides of the broach teeth will scrape against the sides of the slot, causing fast tool wear. Grinding a single relief angle on both sides of each tooth achieves this. As a result, just a small piece of the tooth near the cutting edge, known as the side land, may rub against the slot. The same procedure is used for one-sided corner cuts and spline broaches.

How To Design A Broach?

Designing a broach requires comprehensive consideration of many factors. The following are the general design steps and key points:

1.Determine Processing Requirements

• Clarify the shape, size, accuracy requirements and material properties of the parts to be processed.
• Understand broaching allowances, surface roughness requirements, etc.

2. Select Broach Type

Select the corresponding broach type according to the shape of the part (such as round hole, spline hole, rectangular hole, etc.), such as round hole broach, spline broach, etc.

3. Determine Broach Parameters

• Calculate the broaching allowance and distribute it reasonably to each tooth.
• Determine the amount of tooth lift and select it within an appropriate range based on the factors mentioned earlier.

4. Design Tooth Shape And Size

• Including rake angle, relief angle, blade width, etc.
• Ensure that the cutter teeth have good cutting performance and strength.

5. Determine Chip Flute Shape And Size

• The shape (such as linear, arc, etc.) and size of the chip flute must be able to accommodate chips and ensure smooth discharge of chips.

6. Design The Broach Neck, Transition Taper And Shank

• The neck is used to connect the working part and the shank, and the transition taper facilitates the broach’s entry into the workpiece.
• The handle should match the chuck of the broaching machine.

7. Select Broach Material

• High speed steel, cemented carbide, etc. are usually selected according to the processing materials and cutting conditions.

8. Consider Broach Calibration And Fine Cutting

• Calibration teeth are used to correct dimensional deviations caused by cutting deformation and tool wear.
• Precision cutting of teeth ensures final machining accuracy.

9. Strength Check

• Conduct strength checks on key parts of the broach (such as teeth, neck, etc.) to ensure that they will not break or be excessively deformed during the broaching process.

10. Draw Broach Working Diagram

• Including the overall structure of the broach, dimensions of each part, tolerance marking, technical requirements, etc.

11. Develop Manufacturing Processes

• Determine the manufacturing process route of the broach, including heat treatment, grinding, sharpening and other processes.

In short, broach design is a complex process that requires the comprehensive use of mechanical manufacturing technology, material mechanics, cutting principles and other aspects of knowledge, as well as optimization and improvement based on actual production experience.

Broach Tool Design

For defining the geometry of a broach an internal type is shown below. Note that the geometries of other broaches are similar.

• P = pitch
• RPT = rise per tooth
• n_r = number of roughing teeth
• n_s = number of semi-finishing teeth
• n_f = number of finishing teeth
• t_r = RPT for the roughing teeth
• t_s = RPT for the semi-finishing teeth
• t_f = RPT for the finishing teeth
• L_s = Shank length
• L_RP = Rear pilot length
• D₁ = Diameter of the tooth tip
• D₂ = Diameter of the tooth root
• D = Depth of a tooth (0.4P)
• L = Land (behind the cutting edge) (0.25P)
• R = Radius of the gullet (0.25P)
• α = Hook angle or rake angle
• γ = Back-off angle or clearance angle
• L_w = Length of the workpiece (not shown)

What Is The General Range Of Tooth Lift For Broaches?

Broach tooth lifts typically range from 0.005 mm to 0.03 mm.
However, the specific tooth lift range will be affected by many factors, such as the properties of the material being processed (hardness, toughness, etc.), broaching accuracy requirements, the material and size of the broach, etc.

For materials with lower hardness and higher toughness, the tooth lift may be relatively large, such as 0.02 mm to 0.03 mm. For machining tasks with higher hardness and higher precision requirements, the tooth lift may be 0.005 mm to 0.015 mm.

It should be noted that this is only a rough range. In actual applications, the appropriate tooth lift needs to be determined based on specific processing conditions and requirements.

How To Adjust The Tooth Lift Of The Broach?

You can adjust the tooth lift of the broach through the following methods:

1. Change The Grinding Process Of Broaches

• When sharpening a broach, adjust the tooth lift by controlling the depth and frequency of grinding.
• Use high-precision grinders and suitable grinding wheels to achieve precise grinding control.

2. Redesign And Manufacture Broaches

• If the existing broach tooth lift does not meet the needs, the broach may need to be redesigned and manufactured.
• In the design stage, the appropriate tooth lift is determined according to the processing requirements, and the processing accuracy is strictly controlled during the manufacturing process.

3. Adjust Broaching Process Parameters

• For example, appropriately reducing the broaching speed or reducing the feed amount can reduce the actual tooth lift to a certain extent.
• However, this method may affect production efficiency and needs to be considered comprehensively.

4. Using Adjustable Broach Device

• Some advanced broaching machines are equipped with an adjustable broach clamping device, which can fine-tune the position and angle of the broach within a certain range, thereby indirectly adjusting the tooth lift.

5. Replace Broach Teeth

• For broaches with a modular design, tooth sections with different tooth lifts can be replaced.

When adjusting the broach tooth lift, it is necessary to operate with caution and conduct testing and verification based on actual processing conditions to ensure that the adjusted tooth lift can meet the requirements of processing quality and efficiency.