Basic Information on the Four-Axis Rotary Table of a Horizontal Machining Center
This page covers the basic information four axis buyers and process engineers need before quoting: how the B-axis table is arranged on a horizontal machining center, what it can and cannot hold, and how to choose between 3-axis, 4-axis and 5-axis work. It is written for engineers who read drawings and need a straight answer on machine selection.
What this page covers
Axis layout first, then workholding, then the decision rules.
Where the fourth axis sits on a horizontal machine
A horizontal machining center holds the spindle level, and the work sits on a table that faces it. A three-axis machine moves X, Y and Z only. Add a rotary table and you get a fourth axis, usually called B, that rotates the work about a vertical axis. That single rotation is what turns a one-face job into a four-face job.
The usual arrangement is a B-axis table mounted on the X saddle, with the pallet or fixture clamped to the table face. Rotation runs 360° in indexing mode or continuous when the control and drive support it.
Indexing tables stop at fixed angles, commonly every 1° or 0.001°, and lock before the cut. Continuous tables interpolate while cutting, which allows contouring on a cylinder or a curved slot. The two behave differently in a process plan, so check which one the shop is quoting.
A rotary table adds a rotating mass to the axis stack. Table size, clamping torque and allowable workpiece weight all set the limit. Our own four-axis mills carry a Ø400 mm rotary table, so parts that swing past that envelope belong on a larger platform.
Fixtures, tombstones and how many faces you get
A four-axis horizontal machine pays off when one setup exposes several faces. The standard tool is a tombstone: a vertical plate or box clamped to the rotary table, with a fixture on each side. Index 90° and the next face comes into the spindle.
Face count follows from the tombstone layout. A square tombstone gives four sides, so four faces in one program. Add a top plate and you get a fifth direction, though the spindle has to reach it without the table fouling the tool.
Tombstones also let you run several small parts at once on the same side. Cycle time per part drops because the tool changes and the index move are shared across the load. That is often the real reason a shop moves a family of parts from a vertical mill to a horizontal four-axis machine.
Workholding has to survive the index. Anything loose will shift when the table rotates. We clamp, indicate and, where the drawing allows, add a witness mark so the operator can confirm position after the first index.
Four-axis rotary table at a glance
Typical values on a horizontal machining center. Confirm against the specific machine before quoting.
| Item | Typical range | Why it matters |
|---|---|---|
| Axis name | B, rotation about vertical | Sets which faces one setup can reach |
| Table diameter | Ø400 mm class on our 4-axis mills | Caps workpiece swing and fixture size |
| Index increment | 1° standard, 0.001° on some tables | Decides angular position accuracy |
| Rotation mode | Indexing or continuous | Indexing for faces, continuous for contouring |
| Positioning accuracy | Minutes of arc, not linear mm | Angular error grows with part radius |
| Clamp torque | Set by table and drive | Must hold the part during heavy cuts |
| Load limit | Machine-specific, includes fixture | Overloading stalls the axis or trips alarms |
| Control axes | X, Y, Z plus B | Fourth axis is programmed as a rotary move |
When four axes are enough, and when they are not
Choose four axes when the part has features on several flat faces that are parallel to the spindle axis, or when features repeat at fixed angles. Pump housings, manifold blocks, gearbox covers and valve bodies fit this pattern well. One setup, four faces, no re-fixturing.
Do not force four axes when the geometry needs the tool to tilt. Undercuts, compound angles and blended surfaces that wrap in two directions at once need the tool vector to change, which is a fifth-axis job. Trying to reach them on a four-axis table usually means special cutters, extra setups or hand blending.
Also check the rotation itself. If the part is long, the swing radius at the corners may exceed the table envelope before the part looks large. Long shafts and tall housings are the usual offenders here.
Cost is not the only factor, but it is real. Four-axis work is simpler to program and cheaper to fixture than five-axis work. If the part can be reached in four axes without hand work, that is normally the better route.
Accuracy, datums and inspection on a rotary table
Angular error on a rotary table is not the same as linear error. A small angular deviation becomes a larger linear miss the further the feature sits from the center of rotation. A part 200 mm from center sees roughly 0.0035 mm of linear shift for every 1 arc second of table error.
That is why datums matter on four-axis work. Establish the rotary center in the program and keep the part datum tied to it. If the fixture is offset from center, every indexed face inherits the offset unless the control is told about it.
We set the rotary center, probe the fixture, and verify the first part on all indexed faces before running the batch. Our general machining tolerance is ±0.005 mm and fine surfaces can reach Ra 0.2–0.8 μm, so the table setup has to be tight enough not to eat that budget.
Inspection follows the same logic. Measure features in the same orientation they were cut, or account for the rotation mathematically. A CMM report that ignores the table angle will disagree with the machine even when both are right.
Common questions on four-axis rotary tables
Is the fourth axis always a rotary table?
On a horizontal machining center, yes in most cases. The fourth axis is a B rotary table that turns the work about a vertical axis.
Some machines instead fit a trunnion or a rotary unit on the spindle side. The naming varies by builder, so confirm the actual configuration before you plan a process.
How many faces can one four-axis setup machine?
A square tombstone gives four sides, so four faces in one program. Add fixturing on top and you can reach a fifth direction.
The practical limit is tool reach and clearance. The spindle must clear the tombstone at every index angle, and long tools may not fit between faces.
What decides the maximum part size on a rotary table?
Three things together: table diameter, allowable load, and the swing radius of the part plus fixture.
A part can pass the load check and still foul the table at the corners when it rotates. Check the swing envelope, not just the weight.
Does a rotary table hurt accuracy?
It adds a source of angular error, and that error grows with distance from the center of rotation.
Kept in good condition and set up on the rotary center, a four-axis table holds normal machining tolerances. We verify the first part on every indexed face.
When should a job move to five axes instead?
When features need the tool to tilt, or when surfaces blend in two directions and cannot be reached from fixed angles.
If a four-axis plan needs special cutters, extra setups or hand blending to finish the part, five-axis is usually cheaper overall.
Can four-axis and three-axis work run on the same machine?
Yes. With the table locked at one angle, a four-axis machine behaves like a three-axis machine for that setup.
The reverse is not true. A three-axis machine cannot index the work, so multi-face parts need separate setups or a different machine.
Send the drawing, get a process answer
Tell us the part and the faces you need. We will say whether four axes cover it or whether it needs five.
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