Specialized Turning Blog

Why Are Difficult-to-Machine Parts So Expensive?

Posted by Chip Holm on Jan 6, 2021 1:34:23 PM

Explaining the High Cost of DTM Parts

It’s no secret that difficult-to-machine (DTM) parts are expensive. To compound that, from a customer standpoint, the number of shops willing to entertain machining Inconel, titanium, Hastelloy, Kovar, invar and other DTMs are few and far between. When it comes to pricing, material, labor, and tooling, are all very expensive in the machining process. Not only does the engineering and the machinist need to be highly skilled, the cost for tooling can be ridiculously high when compared to carbon steels, aluminum, brass, and to some extent, stainless steels.

Why Are Difficult-to-Machine Parts So Expensive?

Knowledge and Know-How

Engineers and machinists know one thing about some of these materials: they’re hard to machine in the sense that they are very gummy and abrasive and the metal removal process generates a lot of heat, breaking down tools on a frequent basis. While titanium and Hastelloy can be fairly predictable in conforming to the part’s specifications, Inconel, on the other hand, is a wild child where consecutive parts can vary a great deal due to the spring nature of the material.

Tooling Technicalities

As far as tooling goes, the number of tools required to machine a part can fill a turret or carousel. When tooling is used with tighter corner radius or smaller areas for the distribution of heat like a tap, tool changes can be as frequent as a single-digit production from a tool. Not only is the tooling expensive, but the labor involved is extensive as well. Lastly, the customer is paying for an idle spindle.

Tooling manufacturers have honed in on designing geometries and coatings that lend themselves to these materials and are typically described as “high performance.” These geometries are specific to a type of material and what works well in titanium may not work so well in Inconel, and vice versa.

Despite this tooling being made specifically for the particular DTM material, it is not uncommon to experience variability from tool to tool and from lot to lot. A tool that worked superbly in a prior job may not perform as well in the next run. Experienced engineers and machinists have come to expect this—and while it’s frustrating—have learned to cope with this variation in conservative speeds and feeds.

Gauging Tolerances

In-process gauging is also important. There will be times, especially on tight bores and threads, where gauging the part prior to removing it from the machine is necessary. This is especially true in the Inconel materials where the material is “spongy,” or as described above “springy.” Final passes or “spring” passes may be necessary to bring the specification within limits.

Yield Losses

Sometimes all the planning and care will result in yield losses. Not only is the material, lost but the tooling, tool changes, labor, and other overhead is lost with it. It all comes down to the deign of the parts and how easy or hard it is to machine. When designing a part, engineers should be mindful of the challenges that manufacturing engineers and machinists face. Unnecessary tolerances and details such a small fillet or corner radii will definitely add cost to the part.

DTM Experts

Specialized Turning is expert at machining difficult-to-machine materials such as titanium, Inconel, Hastelloy, invar, Kovar and other specialty alloys. Contact us now to see how our expertise matches up against your toughest and most challenging parts. Registered to the AS9100 quality management system, we serve customers in the aerospace, defense, medical device, high technology, and other discriminating industries.

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Topics: Insider, titanium price, inconel price