First 5 axes CNC machining: what the two rotary axes actually change

• 1
  How the first 5 axes CNC machining axes are counted
• 2
  Why setup count drops and position error stacks less
• 3
  Holding ±0.005 mm with two axes in motion
• 4
  Tool orientation is a surface finish parameter
• 5
  Limits: when 3-axis or 3+2 is the better call
• 6
  What changes in CAM and in inspection
• 7
  Which setup type fits the part

A 3-axis vertical mill moves the tool in X, Y and Z. The workpiece stays clamped to the table. Every surface you cut has to be reachable from the spindle direction, so the part is repositioned by hand whenever the geometry turns away from the tool.

Add two rotary axes and the count becomes five. On a trunnion machine the A axis tilts about X and the C axis rotates about Z, sitting under the part. On a swivel-head machine the two rotaries live in the spindle head and the table only translates. Either way the controller now describes tool orientation as a vector, not just a position.

That vector is the whole story. A ball nose cutter can be held normal to a curved surface, or leaned a few degrees off normal so the tip of the tool does the cutting instead of the center. Tool center velocity drops to zero at the tip, which is why a cutter held exactly normal to the surface rubs rather than shears.

Five axes does not mean five directions of travel at once in a simple sense. It means the machine can place the tool tip at a point and hold a chosen angle there while it moves. Simultaneous control of all five is what separates true 5-axis work from indexed 3+2 work.

Every time a part is unclamped and turned, the new position depends on the fixture, the operator and the surface it sits on. That is a stack of error on top of the machine's own accuracy. A part with features on four faces might need four setups on a 3-axis mill, and each face carries its own position tolerance.

With a trunnion machine, holes on the front face and the side face can be drilled in one clamping. The A axis rotates to the required angle, the controller compensates for the pivot distance, and the relationship between the two features comes from the rotary encoder rather than from a re-clamped fixture.

This is where first 5 axes CNC machining pays for itself on parts that are not especially complex. A bracket with a bore on one face and a slot on the perpendicular face benefits more from one setup than from any amount of spindle speed. Position tolerance between features is the number that improves.

There is a cost. Rotary tables add stack height under the part, so Z travel is reduced compared with the same machine in 3-axis mode. Deep parts that need long tools may not fit once the trunnion is in place. Check the travel envelope before quoting a tall part.

A rotary axis is a positioning device with its own error budget. Backlash, encoder resolution and thermal drift in the table all show up in the part, and they are amplified by the distance from the pivot to the cutting edge. A 0.01° error at 200 mm from the pivot moves the tool about 0.035 mm.

That is why the tolerance you can hold depends on part size. Small parts near the rotary center hold tight tolerances comfortably; long parts cantilevered away from the pivot lose accuracy fast. GreatLight machines to ±0.005 mm on parts that suit the work envelope, and we tell you when your geometry sits outside it.

Thermal behavior matters too. The rotary table is a heat source. Running a trunnion at high feed for hours warms the casting, and the pivot height creeps. On long runs we let the machine reach stable temperature before cutting the final features, and monitoring during the run catches drift.

Tool deflection is the other half of the picture. A long reach tool that clears the trunnion flexes under load. A 3 mm diameter end mill at 50 mm of reach will deflect far more than the machine error, so we change the approach rather than chase the machine.

On a 3-axis machine the tool axis is fixed, so the surface finish on a sloped wall is whatever the stepover and the tool geometry produce. You can reduce the stepover until the cycle time is unacceptable, or accept visible scallops.

Tilt the tool and the effective contact point changes. Leaning a ball nose cutter 10–20° off normal moves the cut from the tip toward the flank, where surface speed is higher and the chip is thinner. The result is a smoother wall with a coarser stepover.

Finish values we hold on suitable geometry are Ra 0.8–1.6 μm as a normal machined finish, and Ra 0.2–0.8 μm where the specification calls for it and the tool can reach. An Ra 1.6–3.2 μm as-machined surface is realistic on non-critical faces and keeps the cycle short.

Do not treat this as a substitute for polishing on a sealing face. Tool marks follow the tool path, and a slight change in tool orientation mid-pass shows as a visible band. On optical and sealing surfaces we plan the path so the orientation changes happen off the functional area.

Flat prismatic parts with all features on one face do not need rotary axes. A 3-axis machine with a vise is faster to set, easier to inspect and usually cheaper. Adding rotary motion only adds error sources and reduces the usable Z envelope.

Indexed 3+2 work is often mistaken for simultaneous 5-axis work. The machine tilts to an angle, locks the rotaries, and cuts a pocket in that orientation. It gets you multiple faces in one setup, which is most of the benefit, without the dynamic accuracy burden of continuous rotary motion.

Simultaneous motion is needed when the tool must stay normal to a curved surface, when an undercut has to be reached in one continuous pass, or when a contoured blend between two faces has to be cut without a witness line. Those are geometry-driven reasons, not prestige reasons.

Very large parts are a third limit. Our largest travel is 4,000 × 400 × 150 mm, and the medium envelope is 750 × 1,150 × 550 mm or 600 × 600 × 600 mm. Parts beyond the envelope are split, or machined on a 3-axis machine with repositioning.

Five-axis toolpaths are generated from the same solid model, but the post processor has to know the pivot distance, the machine's rotary limits and its singularity behavior. A good post avoids the position where the C axis has to spin instantly to follow the tool.

Collision checking is not optional. The holder, the tool and the trunnion all occupy space, and a path that looks clean in the software can drive the holder into the table. We check the full assembly, not just the cutter.

Inspection follows the same logic. Features machined in one setup are checked against each other on a CMM in a single alignment. If a part was cut in three setups, inspection has to reference the same datums the machine used, or the numbers will not agree with the drawing.

We record raw material check, in-process monitoring and final inspection, and 100% inspection before shipment. Reports are available on request. For a first article, ask for the features that carry the function rather than every dimension on the print.