Cast Iron vs. Aluminum Tombstones in Machining

Tooling columns, or tombstones, are modular jig and fixture systems used in CNC and turning machines. These have two to four flat faces, potentially more for specialized work, with precisely machined lengths and widths. These faces are perforated with holes machined with equal precision that allow workpieces to be clamped to them. By flipping these tombstones from face-to-face and screwing or clamping the workpiece to the tombstone, a machinist can swiftly move the workpiece through a preset series of steps, maintaining precision without needing to stop and do measurements for every movement. These specialized spacers can vastly speed up the machining of routinely produced components or the production of large batch jobs.

The usual materials for machinist tombstones are cast iron and aluminum. Steel and epoxy-mineral materials are also used, but they’re rare. Steel is more difficult to machine than aluminum or cast iron, and it can be hard to justify the time and tool wear needed to machine it for a jig. Epoxy-mineral materials are much newer, but their properties aren’t widely understood. Since CNC tombstones require tight tolerances for flatness, parallelism, surface finish, and perpendicularity to be suitable for use, many machinists may not be comfortable working with them. That leaves cast iron vs. aluminum tombstones as the only two options in the minds of many machinists.

Understanding Cast Iron vs. Aluminum Tombstones

Factors such as strength, rigidity, weight, machinability, set-up time, configurations, and the fixtures needed to create that configuration are the critical factors for a tombstone’s performance in the shop. The stiffness and damping characteristics of a tombstone directly impact the quality of output. It will be difficult to do precision machining if the jig the workpiece is clamped to is vibrating. Cast iron and aluminum tooling columns have advantages when it comes to damping vibration compared to steel.

Material cost, though, is probably the leading consideration for many machine shops. Aluminum costs significantly more than steel, and steel costs significantly more than cast iron. However, aluminum is considerably easier to work with than cast iron and steel, and the time savings, along with the longevity of aluminum—both steel and iron corrode more easily—make aluminum a preferred choice for many applications despite its additional upfront expense.

Cast Iron Tombstones

Cast iron tombstones maximize rigidity and offer a solid base for custom work holding. Compared to steel tombstones, cast iron tombstones provide significantly more damping of vibration. Cast iron is also significantly easier to machine than steel, as graphite distribution through the structure allows for the formation of very fine chips, and the carbon distribution makes it much softer than steel and allows for it to be machined easily without a lot of wear on bits or tooling.

This carbon distribution is also cast iron’s chief disadvantage as material for tombstones. It is not always equally distributed throughout the metal. This means that producing a surface with the needed precision can be challenging, as a bit can carve off a larger chip than intended. Similar issues can occur when a cast iron tombstone is placed into use as a jig, as threads machined into anchoring holes can fail when a fastener is tightened into them, leaving workpieces improperly anchored or unable to be easily repositioned for the next step in the processes. These issues mean that while cast iron is a good material for a tombstone, for most applications, it can be less reliable than an aluminum tombstone in the same role.

Aluminum Tombstones

Aluminum properties can vary depending on which aluminum alloy is used. Generally, however, it is approximately as soft as cast iron while offering tensile strength that, while less than steel, is still comparable. These factors combined mean that aluminum is more easily machined than cast iron and less easily damaged during the process. It is also more impact-resistant and less brittle than cast iron while being significantly lighter, meaning that it’s a lot more easily moved through the positions required by a machinist rushing to meet production deadlines and far less likely to be damaged while they’re doing it. The combination of workability, strength, reliability, and light weight means that aluminum tombstones can significantly reduce production cycle times for machining components compared to cast iron tombstones.

However, aluminum is not without its disadvantages. Vibration in the workpiece can be higher with aluminum than with cast iron. The lighter weight means it is easier to move during machining between different positions. This vibration increases as a user goes higher up the tombstone away from the anchor point. The vibrations can be more manageable by keeping the workpiece anchor points low on the tombstone near the vice. This issue isn’t likely to arise during horizontal machining processes. The other disadvantages are a little more difficult to compensate for and include:

• Wear: The softness that makes aluminum easy to work with also means that it can be vulnerable to wear. Friction against the workpiece, the fasteners, and between an aluminum tombstone and the vice holding will remove some surface material every time. Eventually, this wear will leave the tombstone out of specification. The more precision that is required in the end output, the sooner this will happen.
• Cost: Aluminum is substantially more expensive than either cast iron or steel on a price-per-weight and volume basis. This additional expense can be difficult to justify for what is essentially a very precisely machined shim that will eventually need replacement. However, the time savings and the cost reduction that reducing cycle times and increasing output offer may more than justify the initial expense of aluminum tombstones vs. cast iron ones.

One option for reducing the expense of aluminum tombstones while maximizing the benefits is for a manufacturer to produce their own rather than buying pre-machined ones that can cost several thousand dollars per tombstone. Producing aluminum tombstones is easily within the capability of most machine shops, given they are supplied with the right materials, which are simply blocks of aluminum cut to precise lengths and widths with precision angles between their faces. From there, creating aluminum tombstones is a simple matter of drilling and threading holes at the needed anchor points.

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Building Aluminum Tombstones With a Trusted Metal Supplier

At Industrial Metal Service, we have an extensive inventory of a wide range of aluminum and alloys, including the grades most often demanded by manufacturers and machinists, and aluminum plate cut to order with precision.

Whether you need certified plates, bar stock, and round bars sourced straight from the mill or verified remnants to design cost-effective aluminum tombstones, we assure you that you receive quality materials to match your manufacturing standards. The composition of all our remnant aluminum is verified using x-ray fluorescent scanners. We also offer in-house precision-sawing services with state-of-art sawing equipment to achieve tolerances to the thousandths of an inch. With very fast turnarounds on custom cut-to-orders, we help you engineer aluminum tombstones as per your design specifications.