Four Face Tombstone Fixture Cast Aluminum Horizontal Machining Center 0.01mm Squareness

Dongguan Changxinda Precision Technology Co., Ltd. has machined custom parts to drawing since 2008. The plant runs 4-axis and 5-axis machining centers, turn-mill combined lathes and precision CNC lathes, employs more than 90 people, and exports 50 to 60 percent of output to industrial buyers in Europe, North America and Southeast Asia. ISO 9001:2015 certified, first article reports with ballooned drawings as standard.

A tombstone is the fixture that turns a horizontal machining center into a multi part machine. Four vertical faces carry four sets of parts, the table indexes 90 degrees, and the spindle works on one face while the operator loads the next. The gain is not only in cycle time, it is in spindle hours per part: the machine cuts while the parts are being changed. This page covers the geometry, the datum strategy and the positional accuracy that make a tombstone pay for itself.

Four Faces, One Datum

A tombstone that is square to itself lets one program run on all four faces. A tombstone that is not square forces a separate offset per face and loses the advantage.

• All four faces machined in one setup on a 5-axis machine, so squareness comes from the machine rather than from a sequence of re-chucks
• Squareness between adjacent faces 0.02 mm over 300 mm, parallelism between opposite faces 0.02 mm
• Base face flatness 0.01 mm, since the tombstone sits on the table and any error there is multiplied up the height
• Common datum: a bored and reamed dowel hole pair on the base plus a reference face, used by every fixture plate that bolts on
• Serial number and face numbering laser marked, so the setup sheet and the physical fixture always agree

Cast Iron Or Billet Aluminum

The choice changes stiffness, weight and the number of parts the operator can load without fatigue.

• Cast iron: the default for heavy cutting and long running jobs, with ribbed internal walls and stress relieved before finishing, because castings move after the first machining pass.
• Billet 6061 or 7075 aluminum: lighter and faster to produce, suitable for aluminum part families and for automation cells where the robot has a payload limit.
• Steel weldments: used for very large or oddly shaped fixtures, normalized after welding and then machined.
• Weight budget: given at quotation, because the tombstone plus fixtures must stay inside the pallet capacity of the machine.
• Lifting points: tapped holes or cast in eyes sized for the finished weight, marked on the drawing.

Position And Repeatability

• Dowel hole position 0.01 mm true position, bored and reamed rather than drilled, because a drilled hole will not repeat after a fixture is removed and refitted
• Fixture plate mounting holes on a 0.02 mm grid, so a plate can be moved from face to face without re-indicating
• Face to base squareness verified on the CMM with the tombstone standing as it will stand on the machine
• Indexing repeatability is a machine property, but the fixture must not add to it, so no face is used as a datum for another
• Inspection report with the actual squareness values per face, not a pass or fail stamp

Custom Configurations

• Two, three, four or six faces, with the number of faces set by the machine and the part mix
• Heights from 200 mm to 800 mm, widths from 200 mm to 630 mm to suit common pallet sizes
• Integrated hydraulic or pneumatic clamping manifolds, with ports routed through the base so hoses do not wrap around the fixture
• T-slots or a bolt grid on each face, or dedicated fixture plates machined and matched to the tombstone
• Chip clearance: faces angled or relieved so chips fall away instead of packing into corners
• Sub plate and zero point system interface available for quick change between jobs

Delivery And Commissioning

• Billet aluminum tombstones: 15 to 25 working days including stress relief and finishing
• Cast iron tombstones: 25 to 40 working days, including casting, normalization and machining
• Fixture plates matched to the tombstone can be supplied as a set, machined in the same batch so the dowel fit is proven
• Dimension sheet and CMM report supplied before shipment for approval
• On site or remote commissioning support for the first setup, with the program datum sheet written for the customer machine

Questions From Manufacturing Engineers

Why not just bolt four angle plates to a cube?

Because the assembly accumulates error at every joint. A machined tombstone holds squareness across all faces from one setup, so a single program runs on every face and the operator does not re-indicate between jobs.

How do you stop a casting from moving after machining?

Rough machine, stress relieve, then finish. On large castings we sometimes do two stress relief cycles. The CMM report after finishing shows whether the part is stable, and we hold the tombstone for a week before final inspection on critical jobs.

Can you match a tombstone to an existing fixture plate I already own?

Yes. Send the plate drawing or the plate itself and we will machine the mounting pattern and dowel holes to suit, so the existing plate becomes the master and the tombstone is fitted to it.