Planing is a machining process traditionally used to produce large flat, straight and angular surfaces. In a conventional planer the workpiece is secured to a reciprocating table while a single-point cutting tool is supported by a substantially stationary toolhead.
Material is normally removed during the cutting stroke of the table. During the return part of the cycle the cutting tool clears the workpiece and the required feed movement is applied in preparation for the next cut.
Planing was particularly important for machining large machine beds, frames, guideways and other components which were difficult to accommodate on smaller machine tools. Conventional planers are now much less common in production because large milling, grinding and CNC machines can perform many of the same operations more productively and with greater flexibility.
The workpiece is clamped securely to the planer table. The complete table and workpiece travel beneath one or more cutting tools mounted on the machine structure.
A typical planing cycle consists of:
A planer requires a substantial machine structure because the table, workpiece and workholding equipment are reciprocated during machining. The table travels along accurately aligned guideways while the cutting tool is supported by the cross-rail and toolhead structure.
Traditional planer construction is designed to provide rigidity and to support large and heavy components. Depending on machine design, more than one toolhead may be fitted so that several surfaces can be machined during a table stroke.
| Part | Function |
|---|---|
| Bed | Provides the main structural foundation and supports and guides the reciprocating table. |
| Table | Supports the workpiece and workholding equipment and provides the principal reciprocating cutting motion. |
| Columns or housings | Provide the structural support for the cross-rail and toolheads. |
| Cross-rail | Spans the working area and supports one or more toolheads. Its position may be adjustable to suit the height of the workpiece. |
| Toolhead | Carries and positions the single-point cutting tool and provides the required feed and depth adjustment. |
| Table drive | Produces and controls the reciprocating movement of the table and workpiece. |
Traditional planing uses single-point cutting tools operating on the same basic cutting principles as turning and shaping tools. The cutting edge removes material as the workpiece passes beneath it during the cutting stroke.
Tool geometry depends on the workpiece material, cutting direction and surface being produced. The toolholder and toolhead must provide sufficient rigidity to resist the cutting forces while allowing the cutting edge to clear the workpiece appropriately during the return movement.
One advantage of the traditional planer was that relatively simple single-point tools could be prepared for different surfaces and forms without requiring large specialised rotary cutters.
Although most closely associated with large flat surfaces, planing machines can produce a range of features by controlling tool position, feed direction and tool geometry.
Typical operations include:
The traditional strength of the planer is its ability to machine large surfaces on components which can be secured directly to the reciprocating table.
The cutting tool traverses the workpiece as the table moves through the cutting stroke. A feed movement between strokes allows successive tool paths to cover the required width of surface.
Large machine beds and structural components historically suited this arrangement because the workpiece could remain rigidly clamped to a large table while accurately controlled machine motions generated the required surfaces.
Large planing machines may be equipped with several toolheads positioned on the cross-rail and machine columns. This arrangement permits more than one cutting tool to operate during a table stroke.
Depending on the component and machine arrangement, multiple toolheads can allow different surfaces to be machined during the same general operation. This partly offsets the low productivity associated with the non-cutting return movement of the table.
In planing, cutting speed is associated with the linear movement of the workpiece relative to the cutting tool. Where the cutting stroke travels a distance L in time tc, an average cutting speed can be expressed as:
Vc = L / tc
where consistent units must be used and:
The machine table normally travels beyond the actual workpiece length to provide the necessary approach and clearance. The total table movement is therefore greater than the length of surface being machined.
The feed movement positions the cutting tool relative to the workpiece between successive cutting strokes. For a large horizontal surface, the tool may be fed across the width of the component so that adjacent cutting paths progressively generate the complete surface.
Vertical or angular feed can be used where the machine and toolhead are arranged to produce other surfaces.
Depth of cut is established by the relative position of the cutting tool and workpiece. Feed and depth of cut influence cutting forces, material-removal rate, tool life and the resulting surface.
Large components must be located and clamped securely to the planer table. The workholding arrangement must resist the cutting forces while supporting the component sufficiently to limit movement, vibration and distortion.
Clamps, stops, packing pieces and purpose-designed fixtures may be used depending on the geometry of the workpiece. Care is required when clamping large or relatively flexible structures because excessive clamping force can distort the component before machining.
The size of workpiece which can be machined depends on the complete working envelope of the planer. Important dimensions include table length and width, available table travel, distance beneath the cross-rail and the available movement and position of the toolheads.
Machine power, structural rigidity, workpiece mass and the capacity of the table drive and guideways are also important when considering large components.
A well-maintained planer can produce accurate large surfaces, but the accuracy achievable by the process cannot be represented by a single universal tolerance.
The finished geometry depends on factors including:
Planing and shaping are closely related because both traditionally use single-point tools and reciprocating cutting motion. The principal difference is which part of the machine provides that reciprocating motion.
| Planing | Shaping |
|---|---|
| The table and workpiece normally provide the reciprocating cutting motion. | The cutting tool and ram normally provide the reciprocating cutting motion. |
| Traditionally suited to large and heavy workpieces. | Traditionally suited to smaller workpieces within the working envelope of the shaper. |
| May use several toolheads to machine multiple surfaces. | Conventionally operates with a single tool during an operation. |
| Requires reciprocation of the workpiece and the substantial machine table. | The workpiece normally remains substantially stationary while the lighter ram assembly reciprocates. |
See also: Shaping Machines and the Shaping Process .
Planing and milling can both generate large flat surfaces, but the cutting motions are fundamentally different.
Traditional planing uses one or more stationary single-point cutting tools while the complete workpiece reciprocates. Milling uses rotating multi-edge cutters and can provide substantially more continuous cutting action.
The development of large milling machines, CNC controls and modern cutting tools has therefore displaced conventional planing for many applications. A large component can be held stationary while the spindle and cutting tool move around it, avoiding the need to repeatedly accelerate and decelerate a massive workpiece and table.
Planer millers, also known as planer-type milling machines, combine the large workpiece capacity and substantial structure associated with planing machines with rotating milling spindles rather than relying solely on stationary single-point planing tools.
In a planer-type mill the workpiece is supported on a large table while one or more milling heads perform the cutting. Machine configurations vary: depending on the design, the table, bridge, cross-rail or milling heads may provide the required machining movements.
The development from conventional planing towards planer-type milling is significant because it combines the ability to accommodate large workpieces with the productivity and versatility of rotary cutting tools. Modern large gantry and bridge-type machining centres continue this general engineering approach using CNC-controlled axes, automatic tooling and multi-axis machining capability.
| Advantages | Limitations |
|---|---|
| Suitable for very large workpieces | The return stroke normally performs no cutting |
| Large components can be supported on a substantial machine table | Large reciprocating masses limit cycle productivity |
| Simple single-point cutting tools can be used | Requires substantial machine structure and floor space |
| Multiple toolheads can be used on suitable machines | Less flexible than modern CNC machining for complex geometry |
| Can generate large flat and related surfaces | Conventional planers are now relatively uncommon |
Although conventional metal planing is now uncommon in modern production, the process remains valuable for understanding the development of large machine tools and the relationship between workpiece mass, machine motion, tooling and productivity.
Planing also helps explain the evolution from reciprocating single-point machine tools to planer-type mills, gantry machines and modern CNC machining centres designed for large components.
Where planing is being considered for an existing machine or specialised application, important factors include:
The following external resources were included on the original RoyMech page and are retained for additional information relating to planing and manufacturing processes.