What a tombstone is actually buying you
A tombstone on a horizontal machining centre converts one pallet position into several faces of work. Every face that can be machined without a re-clamp removes a set-up, and every set-up removed is both time and an opportunity for error. That is the whole value proposition — it is not about fixture elegance.
So the design question is narrow: how many parts can be presented per load while still allowing the spindle to reach every feature, chips to leave the cut, and the operator to load the next pallet in less time than the machine takes to finish the current one.
The constraint that usually binds: spindle reach
Horizontal machines have a reachable envelope in front of the spindle, and a tombstone pushes parts away from the column. On a 400 mm pallet machine, a cube tombstone of 400 mm per side puts the far corners close to the limit for short tools. If a face requires a long, slender tool to reach a deep pocket, that tool becomes the weakest link in the whole set-up — chatter, deflection and poor surface finish.
Practical rule: keep the tombstone as small as the part count allows, and check every critical feature against the longest tool you intend to use, not the shortest.
How many faces is right
| Set-up | Parts per load | Best for | Watch out for |
|---|---|---|---|
| Single-face plate | 1–4 | Large parts, deep pockets, heavy cuts | Pallet changes dominate the cycle |
| Two-face (L or bridge) | 4–10 | Medium parts, mixed operations | Reach at the outer face |
| Four-face cube | 8–24 | Small to medium parts, high volume | Chip packing in the interior corner |
| Four-face with top plate | 16–40 | Small parts, lights-out running | Weight and load time; coolant coverage at the top |
Weight is a hard stop
Pallet and shuttle ratings are given for the pallet with its load. A 400 mm pallet on a typical horizontal machine accepts 400 to 500 kg. A steel tombstone of 400 × 400 × 400 mm with 25 mm walls already weighs 130 to 160 kg empty, and a fully loaded aluminium fixture with 24 parts can add 60 to 100 kg. That is fine. Add hydraulic clamps, manifolds and a fifth face and you can be within 50 kg of the limit before anyone checks.
Aluminium tombstone bodies solve the weight problem but introduce thermal movement and lower stiffness; cast iron is stiffer and more stable but heavy. Both work — the arithmetic has to be done, not assumed.
Chip evacuation decides whether density works
The interior corner where two faces of a cube meet is where chips collect. In aluminium that means recutting and built-up edge; in cast iron it means a packed corner that eventually drops a slug of swarf into a precision bore. Design features that fix this: chamfer the internal corner generously, add a chip relief groove under each part nest, and orient coolant nozzles to sweep the corner rather than the part face.
If the fixture will run unattended, add an air blast timed to the pallet change, and a wash position in the programme. Lights-out failures on tombstones are almost always chip or coolant problems, not fixturing problems.
Load time is the hidden cost of density
Loading 24 parts into a tombstone by hand takes an operator four to eight minutes. If the machine cuts a pallet in twelve minutes, the operator is loading almost continuously and the pallet system provides no buffer. The density that looks good on paper becomes a bottleneck at the load station.
The fix is not fewer parts — it is load-station design: a second tombstone, quick-change coupling, pre-loaded part trays, and clamps that close in one action rather than twelve. Count the operator's minutes, not just the spindle's.
When a tombstone is the wrong answer
If the part is large, deep-pocketed, or needs a long tool to reach, a single-face fixture with fast pallet changes beats a tombstone. If the batch is under a few hundred parts, the fixture cost rarely pays back. And if the shop air and coolant are not reliable, unattended density will simply produce more scrap per event.
Liuzhou Dongben Machinery Equipment Co., Ltd. designs tombstone and multi-face workholding for horizontal machining centres, sized against the machine envelope and the pallet rating rather than against a catalogue drawing.