If you make torque-transmitting shafts, the words “spline” and “serration” get used interchangeably on the shop floor, and that loose usage causes real problems when it is time to specify the cutting tool. The two are related, but the tooth form, the pressure angle, and the governing standard are different, and getting them wrong means a shaft that will not mate, will not centre, or will not carry the load. This is a specifier’s guide: what actually separates a spline from a serration, the pressure angles you will meet, and how each profile is produced.
The short version
A spline and a serration are both a set of ridges machined onto a shaft (external) with matching grooves in a mating hub (internal), acting like a row of integral keys to transmit torque. The difference is scale and form. Splines are fewer, larger, deeper teeth. Serrations are many small, shallow, V-shaped teeth. Historically a 45 degree pressure angle involute spline was itself called an “involute serration,” which is exactly where the confusion comes from.
What a spline is
Splines carry high torque and, in many designs, also allow the hub to slide axially along the shaft while still transmitting drive (think of a gearbox output shaft). There are two families:
Straight-sided (parallel-key) splines. The tooth sides are parallel and flat, giving a square-ridge profile. These carry very high torque and are common on machine tools and older drivetrains. Governed by SAE J499, ISO 14, and in India by IS 2610.
Involute splines. The tooth flank follows an involute curve, the same geometry as a gear tooth. This is the dominant modern choice because the involute form is self-centring, has maximum strength at the tooth root, can be measured accurately, and can be produced on standard gear cutting equipment such as gear hobs. Governed by ANSI B92.1 (United States), ISO 4156, and DIN 5480 (metric). Involute splines are specified with a pressure angle of 30, 37.5, or 45 degrees, and typically run from around 60 up to 100 teeth.
What a serration is
A serration is effectively a fine spline: many small teeth with straight but angled flanks, forming a V. The large number of shallow teeth gives excellent self-centring and lets you make fine angular adjustments between shaft and hub, which is why serrations show up on steering columns, wiper and control linkages, adjustable levers, and instrument shafts. The trade-off is that the small teeth limit torque capacity, and serrated joints are meant for fixed (non-sliding) connections. Flank angles are larger than on splines, commonly described around the 45 to 60 degree range.
The practical rule: reach for a serration when you want fine indexing, self-centring, and a compact joint at moderate torque. Reach for a spline when you need higher torque or axial sliding.
Pressure angle: the number that defines the tooth
Pressure angle is the flank angle of the tooth, and it is the single spec that most often gets mis-stated. On the cutters and drawings you will see three values:
- 30 degrees: the most common involute spline pressure angle. Larger teeth, higher torque capacity, general power transmission. Common on GM and Dana-style automotive axles.
- 37.5 degrees: a compromise between torque capacity and tooth count, used where 30 is too coarse and 45 too fine.
- 45 degrees: smaller teeth, more of them, superior self-centring, and better suited to thin-walled or small-diameter parts. This is the classic “involute serration,” common on Ford and many imported axle shafts.
Because the involute form changes slightly with the number of teeth, a form (non-generating) cutter is ground correct for a specific tooth count. That is what a marking such as “for 63 teeth” or “for 101 teeth” means on a form-relieved cutter: the profile is accurate for a spline of that count, exactly as with a numbered gear cutter.
Spline vs serration at a glance
| Feature | Spline | Serration |
| Tooth size and count | Larger, fewer | Small, many (often 60 to 100+) |
| Tooth form | Involute curve or straight-sided | Straight angled flanks (V) |
| Typical pressure/flank angle | 30, 37.5, 45 deg (involute) | ~45 to 60 deg |
| Torque capacity | High | Moderate |
| Self-centring | Good (involute) | Excellent |
| Axial sliding | Common (spline joints) | Rare (fixed joints) |
| Typical use | Drivetrain, PTO, gearbox shafts | Steering, linkages, adjustment |
| Standards | ANSI B92.1, ISO 4156, DIN 5480, SAE J499, IS 2610 | Same families, fine-pitch end |
How each profile is actually cut
This is where the tooling, not the print, decides whether the part is right.
External splines and serrations are produced three main ways:
- Hobbing. A spline hob generates the tooth form as the blank and hob roll together. Fast and accurate for medium and high volume.
- Form milling. A form-relieved disc or shell milling cutter, ground to the exact tooth-space profile for a given pitch, pressure angle, and tooth count, cuts one space at a time while a dividing head or CNC fourth axis indexes the blank. This is the flexible route for lower volumes, repairs, and one-offs, and it is where cutter profile accuracy is everything.
- Rolling. Cold forming with no chips, for high volume where the material suits it.
Internal splines and serrations are usually broached or produced on a gear shaper.
Whichever route, the cutting tool must hold the pressure angle, the pitch, and the tooth profile to the tolerance class you are targeting (ANSI B92.1 defines several fit classes). Drift of a degree or two in pressure angle, or a rounded or worn profile, and the spline will not meet its class, will fit loose, or will concentrate stress at the root. Tool material sets how long that accuracy survives in production: plain M2 high speed steel for light duty, cobalt M35 for hot hardness and edge life on harder shaft stock, and powder metallurgy grades such as ASP2030 for the longest runs or the hardest materials. If you want the tool side of this, see the serration and spline milling cutters and the involute gear and spline cutters we manufacture to standard and to drawing.
Read more: Types of Milling Cutters Used in CNC Machining
How to identify an existing spline so you can specify the cutter
A frequent real-world problem is reproducing a spline from a sample with no drawing. Work through it in this order:
- Count the teeth. This is your N.
- Measure the major (outside) diameter of the external spline over the tooth crests.
- Establish the pitch or module from the diameter and tooth count. For involute splines the diametral pitch relates outside diameter and tooth count directly.
- Determine the pressure angle with a gauge or an optical comparator: 30, 37.5, or 45 degrees for involute, or measure the flank angle for a straight serration.
- Note the root form, fillet root or flat root, since it changes the minor diameter and the cutter form.
With N, pitch or module, pressure angle, and root form, a cutter can be specified or ground exactly. If you can supply the mating part, so much the better.
FAQ
What is the difference between a spline and a serration?
Both are integral tooth profiles on a shaft and hub that transmit torque. Splines have fewer, larger teeth (involute or straight-sided) and carry higher torque, often allowing axial sliding. Serrations have many small V-shaped teeth with larger flank angles, giving excellent self-centring and fine adjustment at moderate torque in fixed joints. A 45 degree involute spline was historically called an involute serration.
What pressure angles are used for splines and serrations?
Involute splines use 30, 37.5, or 45 degree pressure angles per ANSI B92.1. Thirty degrees is the most common for general power transmission; 45 degrees (the classic serration) gives finer teeth and better self-centring for small or thin-walled parts.
How are external splines and serrations cut?
By hobbing, by form milling with a profile-ground cutter indexed on a dividing head or CNC axis, or by cold rolling. Internal splines are usually broached or shaped.
Which is better, a spline or a serration?
Neither universally. Choose a spline for high torque or a sliding connection, and a serration for fine indexing, self-centring, and compact fixed joints at moderate torque.
What standards govern splines and serrations?
Involute splines: ANSI B92.1, ISO 4156, DIN 5480. Straight-sided splines: SAE J499, ISO 14, IS 2610. These define profiles, pressure angles, and fit classes.
The takeaway
Spline and serration are not loose synonyms. The tooth form (involute or straight), the pressure angle (30, 37.5, or 45 degrees), the tooth count, and the root form together define the part, and every one of them has to be carried by the cutter that makes it. Specify those five things and the joint will fit its class; guess at them and it will not.