In a one-person shop, you can get by with a jointer and a thickness planer — joint one face and one edge, then plane the opposite face parallel, and you have a flat, square board ready for machining. But if you need to process hundreds or thousands of boards per week, running each piece through two separate machines four times is a serious bottleneck. A four sided planer (also called a four-side moulder or moulder-planer) solves this by surfacing all four sides of a board in a single pass.
This guide explains how four sided planers work, what they’re used for, and what to consider if you’re thinking about adding one to your shop.
What Is a Four Sided Planer?
A four sided planer is a woodworking machine that planes the top, bottom, and both edges of a board simultaneously as it passes through the machine on feed rollers. In a single pass, it takes a rough board and produces a dimensioned piece with flat, smooth surfaces on all four sides — ready for additional machining, assembly, or finishing.
The concept has been around for over a century, and while the basic principle hasn’t changed, modern machines add CNC controls, automatic settings, and sophisticated dust extraction that make them faster, more accurate, and easier to operate than their predecessors.
How It Works
Inside the machine, four cutter heads are arranged to cut the four sides of the board:
- Top head: Planes the top face of the board to the target thickness
- Bottom head: Planes the bottom face, establishing a reference surface and controlling overall thickness
- Left side head: Planes the left edge to a specific width and profile
- Right side head: Planes the right edge, establishing the final board width and profile
The board is fed through the machine by powered feed rollers (typically rubber or steel) that grip the stock and push it past the cutter heads at a consistent speed. The feed speed and cutter head rotations are coordinated to produce a smooth, consistent surface finish.
Each cutter head is mounted on its own spindle, and the position of each head can be adjusted independently to control how much material is removed from each side. On CNC-equipped machines, these adjustments are motorized and can be saved as presets for common profile dimensions.
What Can You Do With a Four Sided Planer?
S4S Lumber Production
The most common use is producing S4S (surfaced four sides) lumber — boards that are planed flat and square on all four sides to a specific dimension. This is the standard stock that furniture makers, cabinet shops, and builders purchase. If you mill your own lumber, a four sided planer produces S4S stock far faster than a separate jointer and planer.
Profiling and Moulding
Because each cutter head spins its own profiled cutter, a four sided planer can produce moulded profiles on any or all sides of the board in a single pass. Common applications include tongue-and-groove flooring, shiplap siding, window and door frame stock, crown moulding, and decorative profiled trim.
To change the profile, you swap the cutter heads or knives. On machines with quick-change cutter heads, this can be done in a matter of minutes. Some modern machines use insert knives that can be replaced individually without removing the entire cutter head.
Matched Joinery
Tongue-and-groove, shiplap, and other interlocking profiles can be cut in a single pass — one side head cuts the tongue, the opposite side head cuts the groove. This is much faster and more consistent than routing these profiles separately.
Key Specifications
Maximum Working Width
This is the widest board the machine can process. Common capacities range from 4 inches (100mm) for small machines up to 12 inches (300mm) or more for industrial models. Choose based on the widest stock you’ll regularly need to run.
Maximum Working Height
This determines the thickest board the machine can handle. Most four sided planers accommodate stock from about 1/2 inch up to 4-6 inches thick, depending on the model.
Number of Spindles
Basic machines have four spindles (top, bottom, left, right). More capable machines add a fifth or sixth spindle — typically for pre-cutting rough stock, sizing the width before the final profile cut, or adding a bottom groove or additional profile detail.
Feed Speed
Feed speed determines how quickly boards pass through the machine and therefore your production rate. Variable feed speed is desirable because it lets you slow down for difficult profiles or dense hardwoods and speed up for simple S4S work in softwoods. Typical feed speeds range from 20 to 100 feet per minute.
Cutter Head Type
Most modern four sided planers use one of three cutter head types:
- Straight knife (HSS): The traditional type. Knives are sharpened or replaced when dull. Good surface finish but requires regular knife maintenance.
- Spiral insert cutter: Small square carbide inserts arranged in a spiral pattern. When an insert dulls, you rotate it to a fresh edge. Better finish on difficult grain and much less maintenance.
- Helical cutter: Similar to spiral but with a continuous helical cutting edge. Produces the smoothest finish and is the quietest option, but also the most expensive.
Maintenance and Safety
- Keep knives sharp. Dull knives produce poor surface finish and put excessive load on the feed system and motors.
- Check feed roller condition. Worn or hardened rollers don’t grip the stock properly, causing inconsistent feeding and variable thickness.
- Verify fence and guide alignment. Misaligned guides cause boards to feed at an angle, resulting in tapered or non-square output.
- Never reach into the machine while it’s running. The feed rollers and cutter heads will pull your hand in before you can react. Always use push sticks and guards.
- Maintain dust extraction. Four sided planers generate chips from all four sides simultaneously. An adequate dust collection system is essential for visibility, cleanliness, and fire prevention.
Conclusion
A four sided planer is a production machine that pays for itself quickly in shops running significant volumes of dimensioned lumber or profiled moulding. The ability to surface and profile all four sides of a board in one pass — at speeds measured in feet per minute rather than minutes per board — transforms the efficiency of a milling operation. If your shop processes enough volume to justify the investment, a four sided planer is one of the most impactful machines you can add.