Introduction
You need to cut a keyway inside a bore. Or perhaps a spline with multiple internal grooves. Traditional broaching requires a dedicated machine and custom tools. It is fast for high volume but inflexible and expensive.
Broach milling changes this. It uses a special tool holder on a standard CNC machine to create internal shapes through a series of small, straight cuts. No special broaching machine is needed. You can cut keyways and splines on the same equipment you already own.
This guide covers everything you need to know about broach milling. You will learn how it compares to traditional broaching, how to set it up on mills and lathes, and how to solve common problems like chatter and chip packing.
What Exactly Is Broach Milling?
A Modern Alternative to Traditional Broaching
Traditional broaching pushes or pulls a long tool with many teeth through a part in one pass. The tool gets progressively taller with each tooth. It is fast. But it requires a dedicated broaching machine and custom broach bars that cost thousands of dollars.
Broach milling works differently. A special tool holder turns your CNC mill or lathe into a linear cutting machine. The tool makes a series of straight, shallow cuts. Each pass removes a small amount of material. After multiple passes, the keyway or spline reaches its full depth.
The Key Benefits
| Benefit | What It Means for You |
|---|---|
| Flexibility | One holder machines keyways, DIN splines, ANSI splines, and custom grooves. Change inserts, not machines. |
| Cost Savings | No investment in dedicated broaching equipment. Any shop with a CNC machine can do internal keying. |
| Accuracy | Modern CNC machines provide excellent positioning control. Achieve tight tolerances consistently. |
| Blind Hole Capability | The process can stop at an exact depth. This is impossible with traditional through-broaching. |
How Does CNC Broach Milling Compare to Traditional Broaching?
A Side-by-Side Comparison
| Feature | Traditional Broaching | CNC Broach Milling |
|---|---|---|
| Machine Required | Dedicated broaching machine | Standard CNC mill or live-tool lathe |
| Tooling Cost | High investment for one custom broach | Moderate investment for holder and inserts |
| Setup Time | Long, requires specialized fixtures | Fast, like any standard CNC tool change |
| Flexibility | One broach = one specific feature | One holder uses multiple insert sizes/forms |
| Blind Hole Machining | Not possible | Ideal application |
| Cycle Time (High Volume) | Very fast (seconds) | Slower per part, but eliminates secondary ops |
| Operator Skill | Specialized operator | Standard CNC operator/programmer |
When to Choose Each Method
Traditional broaching still makes sense for extremely high-volume production of a single part with a through-hole. The per-part cycle time is unmatched. But the lack of flexibility and high initial cost make it impractical for most job shops.
CNC broach milling excels for small-to-medium batch sizes, prototyping, and parts with multiple features. A broach milling cycle might take 30–60 seconds per part. But it happens within the primary machining cycle. This "done-in-one" approach eliminates hours of logistical delay, queue time, and the potential for scrap from multiple setups.
How Do You Set Up Broach Milling on a Lathe?
The Power of Live Tooling
Adding broach milling to a CNC lathe with live tooling represents the peak of efficiency. You complete a complex part in a single clamping. This reduces cycle time and improves accuracy by eliminating setup errors.
Step-by-Step Setup Guide
1. Tool Holder Selection
Choose between axial and radial live tool holders:
- Axial holder – Points the tool parallel to the Z-axis. Ideal for cutting keyways inside a bore.
- Radial holder – Points the tool perpendicular to the Z-axis. Used for features on the face of a part.
For most internal keyways and splines, an axial holder is the standard choice. Ensure adequate clearance between the holder, tool, and workpiece near the chuck jaws.
2. Workholding Rigidity
Broach milling creates significant straight cutting forces. Any bending in the workpiece causes dimensional errors, poor surface finish, and chatter.
- Grip the part with maximum surface contact in the chuck jaws
- Support long or thin-walled parts with a tailstock or sub-spindle
- Consider supporting from both ends for the highest rigidity
3. Spindle Orientation and Locking
The main spindle must be stationary and rigidly locked during the cut. The sequence is:
- Command the C-axis to orient the part to the correct angle
- Activate the C-axis lock or spindle brake using a machine-specific M-code
- Perform the cut
Failure to lock the spindle leads to part movement and catastrophic tool failure.
4. Programming the Cut
For an axial holder, the Z-axis performs the straight cut. The basic G-code sequence involves feeding the Z-axis along the keyway length at a programmed feed rate and depth of cut.
For splines, repeat the process:
- After each pass, retract the tool
- Unlock the C-axis and rotate to the next spline position
- Re-lock and make the next cut
A Pro Tip for Better Surface Finish
Program a small relief move before retracting the tool fully. For example, after completing the Z-axis cut, move the X-axis by 0.1 mm before retracting in Z. This prevents the insert from dragging on the finished surface during rapid retract, which creates drag marks.
How Do You Machine Blind Keyways?
The Challenge of Blind Holes
A blind keyway does not exit the other side of the part. This creates two main challenges:
- Chip removal – Chips have nowhere to go
- Tool clearance – The tool cannot pass through the bottom
Broach milling excels here because traditional broaching cannot handle blind holes at all.
Step 1: Machine a Relief Groove
Before any broaching begins, machine a relief feature at the blind end. This is typically a groove or small cross-hole created with a standard endmill or drill.
The relief groove must be wider and deeper than the keyway itself. It serves two purposes:
- Gives the broaching tool a place to "cut air" at the end of its stroke
- Creates a channel for chips and coolant to escape
Without a relief groove, chips pack into the corner. This leads to tool breakage and damaged parts.
Step 2: Use a Pecking Toolpath
A straight, single-pass cut is not viable for deep or blind keyways. The solution is a pecking toolpath that breaks the operation into manageable cuts with intermediate retractions.
The sequence:
- Rapid the tool to the start position just outside the hole
- Feed in along the Z-axis for a short distance (the peck)
- Retract the tool slightly within the keyway (e.g., 1 mm back in Z) to break the chip
- Rapid the tool completely out of the hole to clear chips
- Rapid back into the hole to the last peck depth plus a small overlap
- Feed in for the next peck
- Repeat until final depth is reached
Step 3: Set the Right Depth of Cut
Depth of cut (DOC) per pass depends on material, tool rigidity, and machine capability.
| Material | Recommended DOC per Pass |
|---|---|
| 316 Stainless Steel, Inconel | 0.05–0.08 mm |
| 6061 Aluminum, Free-Machining Brass | 0.15–0.20 mm |
An overly aggressive DOC leads to tool pressure, chatter, and failure. Too small a DOC causes rubbing and premature wear. Listen to the machine. An audible change in cutting sound often indicates parameters need adjustment.
Step 4: Manage Chips Actively
Even with a pecking toolpath, active chip management is crucial:
- Through-tool coolant – Ideal solution. Directs a powerful jet at the cutting edge to remove chips.
- External coolant – Position nozzles to aim directly into the bore for flushing action.
- Air blast – Programmed air bursts can clear chips between passes in difficult cases.
How Do You Choose the Right Inserts?
Matching Inserts to Material
The insert is where machining happens. The right combination of carbide substrate and coating separates a smooth job from broken tools and scrapped parts.
TiAlN Coating
| Attribute | Details |
|---|---|
| Best for | Carbon steels, alloy steels, stainless steels, Inconel, titanium |
| Why it works | Excellent thermal stability. Forms an aluminum oxide layer at high temperatures that protects the carbide substrate. |
| Use case | General-purpose job shops with varying materials |
DLC (Diamond-Like Carbon) Coating
| Attribute | Details |
|---|---|
| Best for | Aluminum, brass, copper, magnesium, soft non-ferrous materials |
| Why it works | Extremely low coefficient of friction prevents built-up edge (BUE) where material welds to the cutting edge. |
| Important | Do not use on ferrous materials. Carbon in the coating reacts with iron at high temperatures, causing rapid failure. |
Uncoated Carbide
| Attribute | Details |
|---|---|
| Best for | Certain plastics, composites, applications requiring extremely sharp edges |
| Why it works | No coating means the sharpest possible edge. |
| Trade-off | Significantly shorter tool life in metal-cutting applications. No thermal protection. |
Choosing the right coating can double or triple insert life compared to an incorrect choice.
How Do You Fix Broach Milling Chatter?
Understanding Chatter
Chatter is self-excited vibration. It shows up as a loud, high-pitched noise and leaves a wavy pattern on the surface. It ruins surface finish, dimensional accuracy, and leads to insert chipping or failure.
Symptom: High-Pitched Whine and Poor Wavy Finish
Likely cause: Lack of rigidity in the tool, workpiece, or machine setup.
Solutions:
- Use the shortest possible broach milling holder. Every extra inch of overhang multiplies deflection.
- Ensure the workpiece is clamped as securely as possible, close to the chuck jaws or vise.
- For long parts on a lathe, use a tailstock or sub-spindle for support.
- Check machine condition. Worn spindle bearings or loose gibs can cause vibration under load.
Symptom: Chipping on the Insert Cutting Edge
Likely cause: Cutting parameters too aggressive for setup rigidity, or incorrect insert geometry.
Solutions:
- Reduce depth of cut per pass by 30–50% . This is the most effective way to reduce tool pressure.
- Verify feed rate against tool manufacturer recommendations.
- Consider inserts with reinforced cutting edges (small T-land or hone) for hard materials or interrupted cuts.
Symptom: Entire Tool Vibrating Violently
Likely cause: Spindle locking issue on a live-tool lathe. The C-axis is not being held rigidly enough.
Solutions:
- Verify the C-axis lock or spindle brake is fully engaged before the tool contacts the material.
- Check hydraulic pressure if the spindle lock is hydraulic.
- Ensure the M-code to engage the brake is active before the cut begins.
Conclusion
Broach milling has transformed how shops create internal keyways and splines. By bringing this capability to standard CNC machines, it eliminates the need for dedicated broaching equipment. It enables blind hole machining that traditional methods cannot handle.
The key to success lies in understanding the process fundamentals:
- Use the right tool holder and inserts for your material
- Set up rigid workholding and proper spindle locking
- Apply pecking toolpaths for blind holes with relief grooves
- Match coatings to materials—TiAlN for steels, DLC for aluminum
- Diagnose chatter systematically
With these practices, broach milling becomes a routine, reliable, and profitable capability for any machine shop.
FAQ
What is the main advantage of broach milling over traditional broaching?
Broach milling works on standard CNC machines, eliminating the need for dedicated broaching equipment. It also allows machining of blind holes, which is impossible with traditional through-broaching. One tool holder can machine multiple feature types by changing inserts.
Can broach milling be done on a CNC lathe?
Yes. With live tooling and an axial or radial broach holder, you can machine keyways and splines directly on a lathe. The process uses the C-axis to orient the part and a spindle lock to hold it stationary during the cut.
What coating is best for broach milling aluminum?
DLC (Diamond-Like Carbon) coating is ideal for aluminum. Its extremely low coefficient of friction prevents built-up edge, where soft aluminum welds to the cutting edge. Do not use DLC on steels or ferrous materials.
How do you prevent chatter in broach milling?
Start by checking rigidity. Use the shortest possible tool holder. Ensure the workpiece is clamped securely. Reduce depth of cut if chatter persists. On a lathe, verify the C-axis lock is fully engaged before cutting.
How deep should each pass be for stainless steel?
For tough materials like 316 stainless steel or Inconel, start with a depth of cut of 0.05–0.08 mm per pass. This balances material removal with tool life and prevents excessive tool pressure that can cause chatter or insert failure.
Contact Yigu Technology for Custom Manufacturing
At Yigu Technology, we specialize in precision broach milling for internal keyways, splines, and custom grooves. Our expertise spans both CNC mills and live-tool lathes. We use TiAlN-coated and DLC-coated inserts matched to your specific material for optimal tool life and surface finish.
Our programming includes pecking toolpaths, relief grooves, and active chip management strategies that ensure reliable results even in blind hole applications. We maintain rigid workholding standards to eliminate chatter and achieve tight tolerances.
Contact us today to discuss your broach milling project. Let our engineering team help you integrate this efficient, flexible capability into your manufacturing process.
