Tombstone
A tombstone is a tall, multi-faced workholding column that stands on the pallet of a machining centre — most characteristically a horizontal machining centre — so that parts can be mounted on several faces at once and machined in a single cycle. Its name comes from its resemblance to a grave marker: a rectangular column rising from its base, its faces machined square to one another and drilled and tapped in regular patterns to carry fixtures and parts. Where a simple fixture holds one part, or a vise holds a handful, a tombstone multiplies the work a single setup can hold — a part mounted on each of its four faces, several parts deep per face, the machine indexing the column round to reach them all without the spindle ever stopping for a changeover. It is one of the most productive pieces of workholding in production machining, and it exists to answer one question: how much useful cutting can happen while the machine is running.
Why a column?
The tombstone answers the productivity question by making the fixture itself a many-sided volume rather than a flat plate. Because it is mounted upright on the pallet, its vertical faces stand square to the spindle of a horizontal machine, whose cutter reaches them straight on, and the machine’s rotary axis simply turns the column to present each face in turn. The same part can be machined on several of its own sides across successive faces, or — more often — dozens of identical parts are loaded all over the column, one program walking the spindle face by face and part by part. The geometry of the tombstone is what makes this possible: four, six, or eight faces arranged round the column give the machine access to a large number of parts, each mounted on its own small sub-plate or vise, all within a single loading of the pallet. The result is that the machine can run for a long stretch on one setup, cutting part after part with no operator in the loop, which is exactly the property that makes tombstones the standard workholding for production, for cells with pallet changers, and for the long unattended runs of lights-out machining.
Built for rigidity and flexibility
A tombstone carries its parts while they are being cut, so its own stiffness and stability are part of the cutting stiffness of the job, and its material is chosen accordingly. Cast iron is the classic tombstone material because grey iron damps vibration well — a property machinists value in a column that stands directly in the cutting-force loop, where a ringing or flexing fixture would pass its vibration into every part it holds. Aluminium tombstones are lighter, suiting machines with fast pallet shuttling where weight is a cost, at the price of less damping; hybrids and hollow “caged” designs cut weight further and let air or hydraulic lines be routed internally for powered clamping. Whatever the body, the faces are the working surface and are machined square to one another and to the base, then drilled and tapped in a regular grid — a standard hole pattern repeated over every face. That grid is what gives the tombstone its flexibility: standard sub-plates, machine vises and clamps bolt anywhere on the pattern, so the same column is reconfigured in minutes to hold a different part, and the tombstone itself outlives any one job. Because the column is a permanent fixture on its pallet, its faces are located and re-located the same way every time, following the locating discipline that every fixture shares.
Tombstones in production
The tombstone shows its worth in the economics of the machine cycle. With many parts mounted at once, one program produces many finished parts per pallet load, and the operator’s time is spent loading and unloading the whole column between cycles rather than babysitting single parts — so spindle utilisation rises, and the hours of cutting per shift rise with it. On a machine with a pallet changer, the effect compounds: while one tombstone is in the machine being cut, the operator is loading the next one outside, and the changeover is a pallet swap measured in moments rather than a re-fixturing job measured in minutes. The same arrangement is what lets a machining cell run unattended through the night, the machine working through pallet after pallet of loaded tombstones. For all these reasons the tombstone is the production workhorse of the horizontal machining centre, and it appears wherever the same part, or the same family of parts, is made in enough volume to justify loading the column — the classic territory of the machine that runs part after part for hours on end.
Tombstones and the wider picture
A tombstone is best understood as the column-shaped member of the larger family of workholding: it locates, supports and holds, exactly as a vise or a fixture plate does, but in three dimensions and at production scale. Its faces are grids of locating and mounting points; the parts on them are located by sub-plates and vises that obey the same principles as any fixture; and the whole column stands or falls on the same fundamentals — clean, true faces, rigid mounting, and parts held so they do not move under the cut. On the shop floor it is inseparable from the machining centre and the rotary and pallet systems that let it work unattended, which is why it is described alongside them in the machine entries of this wiki and practised as part of the craft of CNC machining. Few workholding devices say more about what production machining has become: not one part per setup, but a column of parts, cut round after round while the machine does what it is paid to do.