Vertical fixturing, or “tombstone” workholding, reorients a part’s primary datum surface from horizontal to vertical, dramatically increasing the machine’s ability to access multiple faces in a single setup. This method transforms a standard three-axis VMC into a pseudo-four-axis system, enabling the machining of features on perpendicular planes without reclamping. The core challenge shifts from simple down-holding to managing gravity, ensuring rigidity against lateral cutting forces, and maintaining precise perpendicularity across all mounted components.
Establishing and Verifying Perpendicularity
The absolute prerequisite for vertical fixturing is a perfectly square fixture base. Whether using a commercial tombstone, a custom-built angle plate, or a modular grid system, the mounting face must be verified perpendicular to the machine table within a tight tolerance, typically less than 0.01mm over 300mm. Do not assume a new fixture is square. Mount it to the machine table and indicate the vertical face along its full height using a precision square or a dial test indicator mounted on the spindle. Any deviation must be corrected by shimming between the fixture base and the machine table before final tightening.
Once the primary fixture is square, the same principle applies to any sub-plates or vise jaws mounted onto it. Treat the vertical face as the new “table.” Use a square to ensure that any vise clamped to this face has its fixed jaw running perfectly vertical. A small error at the base compounds into a large error at the top of a tall part. For the highest accuracy, machine the mounting surfaces of accessory plates in-place on the fixture after it is bolted to the machine. This “self-truing” process guarantees that all subsequent mounting surfaces are parallel to the machine’s axes.
For multi-part setups, consistency is key. Use a master part or a precision ground block to set all vises or locators on the tombstone face. Clamp the master block in the first vise, indicate it true, then use it as a physical reference to align the fixed jaw of the next vise, and so on. This ensures all workholding points are co-planar, so parts machined in different locations will have features in the same relative orientation.
Counteracting Gravity and Lateral Forces
Gravity is now a lateral force trying to pull the part away from the fixture face. Clamping must actively pull the part into the vertical datum, not just down. Use step jaws or custom soft jaws with a vertical lip that captures the bottom edge of the part, providing positive location against both the vertical and horizontal planes. The primary clamping force should then be applied horizontally, pushing the part back against the vertical face of the fixture or vise.
For tall or slender parts, support against deflection is critical. Use adjustable heel blocks or live (spring-loaded) supports at the unsupported top end of the part. These supports take up any slack and prevent the part from vibrating or bending away from the tool during cutting. They should be adjusted to just make contact with the part—tight enough to eliminate play, but not so tight that they induce a bending stress. For long parts, multiple intermediate supports may be necessary.
The direction of toolpath engagement is paramount. Program toolpaths so that the predominant cutting force pushes the part against its vertical stops and downward onto its bottom support. For example, when facing the vertical surface, use a conventional (up) milling strategy where the cutter rotation tends to lift the part slightly; this force is safely countered by the downward-pulling step jaw. Avoid toolpaths that create a consistent outward pull, like drilling with a large pecking force, unless substantial mechanical back-stops are in place behind the part.
Chip Management and Process Sequencing
Chip evacuation becomes a major concern in vertical setups. Chips no longer fall clear of the part; they accumulate in pockets, on top of clamps, and against the vertical face of the fixture. This can lead to recutting, poor surface finish, and even part displacement if a large chip gets trapped under a locating surface. Incorporate aggressive coolant flow with nozzles aimed to flush chips downward and out of the work area. Design fixtures with open skeletons and avoid flat ledges where chips can pile up.
The order of operations must account for stability. Always machine the most critical features and tightest tolerances first, while the part is in its most rigid state—typically when it has maximum stock material and support. Perform any heavy roughing on the side facing away from the fixture early on; the resulting stresses and heat will cause the part to warp away from the fixture, and subsequent finishing passes on the fixtured side will bring it back to true. Leave light finishing passes on fragile or unsupported areas for the very end.
After completing operations on one vertical face, the tombstone can often be rotated 90 or 180 degrees (manually or via an indexer) to present a new face to the spindle. When doing this, use a probe to verify the new position of a known datum. Do not assume the rotation is perfectly accurate; small errors in the indexer or fixture squareness will accumulate. Probing allows for work coordinate rotation or translation in the CNC control to compensate for these minor deviations, ensuring features on different faces maintain their positional relationship.
