3 Things That Make Precision Metals Precise

July 15, 2019 Joshua Jablons Ph.D.

Precision Polished Metal Parts

What are “precision metals”? 

In the world of metal machining, people are often looking for what they call precision metals. But what exactly does that mean? 

Depending on the application, the term precision can pertain to a number of characteristics:

  • The precision of a metal’s dimensions
  • The precision of its composition
  • The precision of the metal’s performance.

What, then, is important to consider about precision metals for your product or manufacturing need in terms of these important characteristics?

Dimensions of Precision Metals

For our customers, dimensions are the primary focus when it comes to precision metals. In fact at Metal Cutting, where our very tight cutting tolerances can result in high Cpk/Ppk values, our methods deliver a level of dimensional precision that may actually be greater than what a customer needs.

In general, precise dimensions and cost are highly correlated, in that greater precision comes with a higher price tag. That is because attaining precise dimensions usually requires machinery that itself has the tightest tolerances, being made from components of the tightest tolerances

Moreover, to get the ultimate results from this machinery requires years of experience and the best operators. Therefore, the cost of the machinery and labor — and, ultimately, the cost of precision metals with dimensions of the tightest tolerances — is high.

Is the precision of laser worth the cost?

One great example is laser processing. Although it can be versatile and precise, producing tight tolerances and small kerfs, laser cutting is generally slow and expensive, especially for 2-axis cutting.

While large power lasers can make faster cuts, in doing so they produce rough end cut surface finishes and wide, deep heat-affected zones — probably not the results you’re looking for in the dimensions of precision metals.

For laser cutting of tubing, the inside of the tube must be coated with anti-spatter fluid and the materials must be laser cut one at a time, both of which increase production time and add to the cost.

Is 3D all it’s cracked up to be?

Another example of the trade-off between cost and dimensions is 3D manufacturing. Here, the laser sintering process relies on a number of variables, including:

  • The size of the laser spot
  • The micron size of the metal powder
  • The height intervals of the “elevator” steps presenting the fresh powder to be laser sintered

In the early days of laser sintering, these gradations — especially the height intervals of the elevator steps — were quite large compared with the state-of-the-art Swiss-style turning and milling machines of the same era.

So, although the 3D method could add metal features that could not be achieved through subtraction using conventional machining, the end product was surprisingly rough looking. This was due to the comparatively poor resolution of early 3D printing.

While the precision of the intervals, powder size, and laser size are improved greatly in today’s direct metal laser sintering, some of those basic trade-offs remain.

For instance, there have been some extraordinary advances in shrinking the laser spot size. However, along with this comes an increase in the time required to additively construct a part.

That makes for higher part prices — and another example of where greater precision correlates with higher cost. 

In addition, with the nature of sintering and the challenges of annealing, laser sintering a material such as tungsten remains a work in progress. 

While some advances have been made — for example, in testing the properties of 3D printed tungsten for use in medical radiation shielding and nuclear imaging — success remains elusive for the time being.

Composition of Precision Metals

There are a lot of engineered metals and alloys whose composition makes them “precise.” 

One famous example is nickel titanium alloy. First popularly used back in the day to make eyeglass frames, NiTi (or Nitinol) is what provided the frames with shape memory and super elasticity — enabling the rugged frames to withstand being bent and to pop back into shape.

Today, NiTi is used widely for tubing in the medical device industry. (You can learn more about NiTi and other medical device tubing materials in our free white paper.) 

Another example is tungsten element, which requires a precise composition in order to achieve its desired and very specific performance goals

For instance, for high-speed automated welding operations that require high levels of thermal and electrical conductivity and superior hardness, Metal Cutting offers high-performance electrodes made from pure tungsten.

Tungsten element and NiTi neatly illustrate the basic dichotomy in the chemical composition of precision metals: namely, purity vs. recipe. For some applications, a precision metal is a pure element, such as tungsten element. But for other applications, it is a blend requiring a specific recipe.

The recipe will vary, but the results must be precise.

Purity testing is done with both pure precision metals and those that come from a recipe, determining the number of decimal points of purity (e.g., 99.95% to 99.99%) and the exact percentages of ingredients (e.g., 55% nickel by weight).

For example, NiTi is a custom blend that varies by manufacturer and is never exactly a 50/50 blend of nickel and titanium. 

In fact, while there should be ASTM specifications for all metals, there are exceptions and NiTi is a case in point: There is no ASTM spec for it. Even “pure” metals, such as pure titanium, can have various recipes (basically alloys) and be available in different grades.

Another good example is stainless steel, which is frequently alloyed with other metals. Different manufacturers produce their own unique brand of stainless steel using a secret recipe that requires precision.

The same way that not all peanut butters are the same — even when they use the same ingredients — not all stainless steels are the same. Each recipe must meet purity testing requirements for a certain ratio of ingredients, put together consistently and repeatedly, to produce precision metals.

Purity can also determine the results.

In our business, our pure tungsten element is in demand for use in projector lamps. These are still utilized in applications such as small, portable, self-contained projectors sometimes use by businesspeople to display presentations from a laptop or notebook computer onto a screen.

These small units require a very intense lamp, and the tungsten element used must be very pure to achieve that level of performance. 

Metal Cutting has 99.999% pure tungsten powder, which when sold in rod form delivers 99.99% pure tungsten element. This slightly reduced purity (<100%) is due to some tiny amount of impurity from the processing equipment.

Performance of Precision Metals

Performance may be closely related to a metal’s composition. For instance, a new metal may be invented or an alloy specially engineered to deliver a particular performance. A good example is NiTi, the poster child for shape memory, super-elastic alloys.

An application requiring the high melting point performance of tungsten — such as, famously, the incandescent bulb — requires the purest tungsten element. Otherwise, the material would not perform correctly and would fail prematurely.

The performance of metals may also be affected by the production process. For example, the process of forming a metal into the required shape, such as wire, changes the hardness of the material.

That means if a specific tensile strength is required for the end purpose, the way the wire is produced would need to be adjusted accordingly — perhaps adding an annealing process to maintain the necessary tensile performance.

Another precision metal utilized for its performance is magnesium, which along with iron, zinc, and manganese is finding wide application in medical devices due to its bioabsorbable properties.

While we might imagine that performance relates to spacecraft, satellites, and other complex, high-energy processes and equipment, there is probably no more important application than making sure the magnesium used in bioabsorbable stents is made precisely and will degrade safely within the body as designed.

Specify what precision is for your manufacturing needs.

The most important characteristic of precision metals — the thing that makes a particular metal precise — is going to vary depending on your application and your production goals.

Whether your emphasis is on dimensions (as it is here at Metal Cutting), composition, or performance, you can help to ensure the best  results and optimize manufacturability by carefully crafting your part specifications.

 
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