How to Select the Right Material for your Workholding Tombstone

By: Nick Rupprecht, Business Unit Manager, AMROK

Workholding tombstones come in a variety of materials. Here is a table guide to select the right material for your workholding tombstone:

Logarithmic Decrement, c. (damping)

TOMBSTONE MATERIALS		Cast Iron	Steel Weldment	Cast Aluminum	Aluminum Weldment	Epoxy Mineral Composite
Properties	Weight	High	High	Low	Low	Low
	Stiffness	High	High	Moderate	Moderate	Moderate
	Vibration Damping	High	Moderate	Very Low	Low	Very High
	Thermal Expansion Resistance	High	High	Low	Low	High
	Machinability	High	Moderate	Very High	Very High	n/a
	Cost	Low	Moderate	Moderate	Moderate	Moderate
Technical Data	Density, c. (lb/ft3)	425-487	490	190	170	150
	0.004	0.002	0.0004	0.0004	0.035
	Linear Coefficient of Thermal Expansion, c. (1/K)	11 x10-6	11-13 x10-6	23 x10-6	23 x10-6	12 x10-6
	Tensile Strength, c. (psi)	30,000-60,000	58,000-80,000	68,000-75,000	68,000-75,000	2,000-3,000
	Thermal Conductivity, c. (W/mK)	50	50	140	140	2

Each workholding tombstone material has various properties that may affect your application. Review the table to see which material is best for you.

Cast Workholding Tombstone Materials

Cast Iron Tombstones

Cast iron is a popular choice for workholding tombstones. It sets the standard for other workholding tombstone materials with its various favorable properties that include:

High stiffness
High vibration damping
High resistance to thermal expansion
High machinability
Low cost

Cast Aluminum Tombstones

Cast aluminum is favored when weight is critical. Compared to cast iron, it features a lower weight and higher machinability, but lacks in stiffness, vibration damping, and resistance to thermal expansion. Its main properties include:

Lightweight
Very high machinability
Very low vibration damping
Low resistance to thermal expansion
High thermal conductivity

Welded Workholding Tombstone Materials

Steel Weldment Tombstones

A steel weldment is generally used as an alternative to cast iron or unusual tombstone shapes. Cast iron can get costly with unique, startup casting patterns. Here are the main highlights of a steel weldment as a tombstone material.

High stiffness
High resistance to thermal expansion
No casting pattern required

Aluminum Weldment Tombstones

Aluminum weldments are favored over cast aluminum tombstones for unique, start-up patterns and better production lead time. Sometimes large aluminum castings can have porosity issues and have to be re-poured, which adds to the lead time. The downside: weldment does not have as much vibration damping as a casting. The main properties of an aluminum weldment include:

Lightweight
Very high machinability
Very low vibration damping
Low resistance to thermal expansion
High thermal conductivity
No casting pattern required

Special Workholding Tombstone Material

Epoxy Mineral Composite Tombstone

Epoxy mineral is generally used as an alternative to aluminum in a situation where low weight and high precision is critical. It is known for its exceptional vibration damping properties, low coefficient of thermal expansion, and very low thermal conductivity. This material is normally offered with a full grid of multi-purpose holes in the faces of the tombstone. These grid holes go directly into steel plugs that have been cast into the composite mold. Once the mold is created, tombstone customization is limited. To add machinability to an epoxy mineral tombstone, aluminum fixture plates can be attached to the tombstone faces, allowing for custom features. Its main properties include:

Lightweight
Very high vibration damping
High resistance to thermal expansion
Very low thermal conductivity
Not machinable

Now that you know what material to select for your application, what about the tombstone fixture shape? Read: How to Select the Right Tombstone Fixture Shape for Your Application.

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