Machining splines requires choosing the right cutting method for your part geometry, production volume, and whether you’re cutting external or internal teeth. The most common approaches are hobbing, shaping, broaching, milling with disc cutters, and wire EDM, each suited to different situations. Here’s how each process works and when to use it.
External vs. Internal Splines
The first decision is whether you’re cutting teeth on the outside of a shaft (external spline) or inside a bore (internal spline). This distinction narrows your options immediately. Hobbing and milling work well for external splines but can’t reach inside a bore. Broaching and wire EDM are the go-to methods for internal splines, though shaping can handle both.
Most splines you’ll encounter use an involute tooth profile, similar to gear teeth but much shorter. Involute splines don’t roll against each other the way gear teeth do. They simply slide together to transmit torque. The standard pressure angles are 30, 37.5, and 45 degrees, with metric modules ranging from 0.25 to 10 under the ANSI B92.2M standard.
Hobbing: The Workhorse for External Splines
Hobbing is the most widely used method for cutting external splines. A hob is a helical cutting tool that looks like a worm gear with cutting edges ground into it. As the hob rotates and feeds across the workpiece, the workpiece rotates in sync, and the hob progressively generates the tooth profile. The process is fast, accurate, and ideal for medium to large production runs.
Hobbing works particularly well for spur and helical tooth forms because the generating action naturally produces the involute curve. You don’t need a separate cutter for each tooth count. One hob of the correct module can cut any number of teeth, which makes it flexible and cost-effective. The tradeoff is that hobbing requires a dedicated hobbing machine with synchronized spindle axes, so it’s not something you’d set up on a standard mill.
Broaching for High-Volume Internal Splines
Broaching is the fastest way to cut internal splines when you’re making a lot of parts. A broach is a long bar with progressively larger teeth along its length. It’s pushed or pulled through a pre-drilled bore in a single pass, and each tooth removes a small amount of material until the final teeth leave the finished spline profile. The entire cut can take just seconds.
The catch is cost. Broaches are expensive custom tools, often costing thousands of dollars, and each one is made for a specific spline size and tooth count. If you damage a broach or need a different configuration, you’re buying a new tool. This makes broaching best suited for production runs large enough to justify the tooling investment. For a one-off prototype or short run, it’s rarely practical.
Broaching also requires a through-hole. If your internal spline is a blind bore (closed at one end), you’ll need a different method. The workpiece needs to allow the broach to pass completely through.
Shaping: Versatile for Internal and External Cuts
Gear shaping uses a cutter that looks like a small gear or spline itself. The cutter reciprocates up and down while slowly rotating in mesh with the workpiece, generating the tooth profile through a series of small cuts. It’s one of the most convenient methods for cutting splines because it handles both internal and external profiles and works well for parts where a shoulder or obstruction prevents a hob from completing its pass.
Shaping is slower than hobbing for external splines and slower than broaching for internal ones. But its flexibility makes it valuable in job shops and for parts with unusual geometry. If you need to cut an internal spline that stops before a shoulder, shaping can do it where broaching cannot.
Milling With Disc Cutters
For shorter production runs or prototyping, milling splines with disc cutters on a CNC machine is a practical option. Tooling manufacturers offer disc-style cutters designed specifically for spline profiles. Sandvik Coromant, for example, makes disc cutters covering module ranges from 0.8 to 10, suitable for most common spline sizes. These cutters index through each tooth slot one at a time, cutting one gap, then rotating the workpiece to cut the next.
This approach is slower than hobbing or broaching, but it doesn’t require a dedicated gear-cutting machine. If you already have a CNC mill or machining center with an indexing head, you can cut splines without investing in specialized equipment. It’s the best option when you need a handful of splined shafts and can’t justify the setup cost of hobbing or a custom broach.
Wire EDM for Hardened and Complex Parts
Wire EDM (electrical discharge machining) uses a thin electrically charged wire, typically 0.25 mm in diameter, to erode material with sparks rather than cutting it mechanically. This makes it ideal for machining internal splines in hardened steel, tool steel, Inconel, titanium, and other tough materials that would destroy conventional cutting tools.
The process achieves precise tolerances and smooth surfaces without deforming the workpiece, which matters for parts that have already been heat-treated. It also handles complex geometry with sharp corners and small radii that would be difficult or impossible with a broach or shaper.
Wire EDM does require some preparation. The workpiece needs a starting hole for the wire to thread through, and that hole must be precisely centered relative to the part. The bore is often pre-machined slightly undersized or oversized before EDM work begins, with the exact approach agreed upon beforehand. Parts should arrive clean, free of burrs, oil, and chips. Wire EDM is slow compared to broaching or hobbing, so it’s best suited for low-volume work, prototypes, or parts made from materials that rule out other methods.
Choosing a Fit Type
Before you machine a spline, you need to know what type of fit the design calls for. The two main options are major-diameter fit and side fit, and they affect how tight the centering is between the mating parts.
In a major-diameter fit, the external spline’s outer diameter contacts the internal spline’s bore to center the two parts. The teeth themselves carry torque but don’t do much centering. This is the better choice when precise alignment between the shaft and hub matters, such as in high-speed rotating assemblies where runout must be minimized.
In a side fit, the tooth flanks do the centering work under load, thanks to the pressure angle of the involute profile. The major diameters have clearance and don’t contact each other. Side-fit splines are widely used across automotive and aerospace applications because they self-center when torque is applied, which simplifies manufacturing tolerances on the diameters.
The fit type determines which surfaces need to be held to tight tolerances and which can have more clearance, so it directly affects how you set up the machining operation and what you inspect afterward.
Checking Your Work
The standard shop-floor method for verifying spline tooth thickness is measurement over pins (for external splines) or between pins (for internal splines). You place precision ground pins in opposite tooth spaces and measure across them with a micrometer. The resulting dimension tells you whether the tooth thickness is within spec.
The calculation behind this measurement involves the module, number of teeth, pressure angle, and pin diameter. For external splines with an even number of teeth, the measurement is straightforward: you measure directly across two opposite pins. With an odd number of teeth, the pins aren’t directly opposite, so a cosine correction factor adjusts for the offset. Most engineers use spreadsheets or dedicated software to compute the target dimension rather than working through the formulas by hand, since one of the intermediate values (the transverse pressure angle at the pin contact point) historically required looking up involute function tables.
Beyond pin measurement, gear-checking machines can evaluate tooth profile, spacing, and runout across all teeth simultaneously. For production work, this level of inspection catches systematic errors like hob wear or indexing drift that pin measurements alone might miss.