If the lands on a valve spool aren’t perfectly round and mirror-smooth the valve isn’t going to perform as expected. The same applies to the journals on a crankshaft or camshaft, to control rods, turbocharger rotors and a host of other high-precision machined parts. Cylindricity, concentricity and surface finish are vitally important to smooth and dependable operation, and that’s what centerless and cylindrical grinding provide.

These two machining processes both produce precise cylindrical shapes but they differ in several ways. Each has strengths and limitations and it’s important to understand these during product design and when seeking precision machining services. Here’s an explanation.

Centerless and Between Centers

The fundamental difference between these two ways of grinding cylindrical shapes is how the workpiece is held. In cylindrical grinding it’s held on its axis of rotation, “between centers” while in centerless grinding it’s supported along its axial length.

Centers are precisely ground cones that fit into mirror image recesses drilled in each end of the workpiece. The part is loaded into the machine by clamping the workpiece between a pair or opposed centers.

As the phrase implies, centerless grinding doesn’t use centers. Instead, the workpiece is held on a long, thin by a regulating wheel that pushes it against the grinding wheel. So in centerless grinding the workpiece is located by its outside diameter while grinding between centers holds it on its centerline.

Note that a cylindrical grinding doesn’t have to hold the workpiece between centers. It could use chuck jaws or a collet instead.

Workpiece Rotation

In centerless grinding the grinding wheel turns the workpiece with the regulating wheel controlling rotation speed. This provides the differential motion needed for material removal.

In cylindrical grinding the workpiece turns in the opposite direction to the wheel. This is achieved by driving workpiece rotation from a chuck that usually holds one of the centers. To make the workpiece turn, a driving “dog” is fitted to it, and this is pushed around in a circle by a pin on the chuck.

Types of Cylindrical Grinding

If the workpiece is longer than the width of the grinding wheel, the wheel is traversed along its length as both turn. If the workpiece is shorter the grinding wheel need only move forwards in what’s called “plunge” grinding. If the wheel used in plunge grinding is profiled, this shape is transferred to the workpiece.

A cylindrical grinding machine may be fitted with an additional axis to grind internal diameters. The workpiece is held in the chuck with an open end exposed to a small grinding wheel. This wheel is moved into the workpiece to grind the inner surface.

Creep-feed grinding is a specialized form of plunge grinding. It’s capable of high metal removal rates but does need specialized CNC grinding equipment.

Centerless Grinding Options

Centerless grinders are often set up for through-feed grinding. This is where a slight angle on the regulating wheel pushes the workpiece along the rest and past the grinding wheel. In this arrangement the machine can be kept constantly supplied with material.

A centerless grinding machine can also work like a plunge grinder. As with plunge grinding on a cylindrical grinder, the grinding wheel is given the required profile and the workpiece brought into contact with it to create the shape.

Comparing and Contrasting Precision Cylindrical Grinding Methods

Both of these grinding processes are capable of producing very smooth surfaces. The main differences are accuracy and throughput.

Centerless grinding is capable of high throughput rates. However, the machines take time to set up. In contrast, a changeover or setup on a CNC cylindrical grinder can be accomplished in a few minutes. The downside is that part loading and unloading is slower than a centerless grinder running in through-grinding mode.

For these reasons centerless is preferred for high volume machining, providing it can maintain the geometric precision needed. In general, cylindrical grinding is more accurate and achieves superior roundness. The difference is small – a matter of tenths of a thousandth of an inch – but it’s often worth paying the additional cost when function depends on precision.

Precision Machining Technology at Impro

Impro performs both centerless and cylindrical precision machining on modern CNC grinders. The largest parts that can be centerless ground are 2” diameter while our cylindrical grinders can handle parts up to 14” diameter. Contact us to discuss your precision machining needs.