November 2013 Machining Tip

Material Matters: Steel, Stainless and Otherwise

There are a variety of reasons that one might want to use Firstcut to machine steel parts in small volumes, and they all revolve around speed. One is to produce low-volume runs of steel parts when it can be done faster than traditional machine shops (which is usually the case). As volume rises into the hundreds or thousands of parts, the delay and high cost of traditional toolpath development is less of a factor, but Firstcut’s automated toolpath development is faster and less expensive at low volumes. Such low production volumes appear occasionally in industries like aerospace.

Besides aerospace, steel is also widely used in computer electronics, automotive, medical and other applications, so a second application for rapid machining is prototyping steel parts that will be manufactured by high-volume machining or metal injection moulding (MIM). Machined prototypes will be functionally identical to machined production parts, but they also share many, if not all, of the characteristics of MIM parts as long as the material is the same. Being able to produce prototypes from 3D CAD models in the same materials you will use in manufacturing lets you begin functional testing earlier, feed results back into the development process, and go to market quickly with fully tested products.

And finally, there are steel items that are neither products nor prototypes such as jigs, and fixtures, often produced singly or in very small lots. Rapid machining is ideal for these applications since the high cost of setup is virtually eliminated.

Figure 1: Sample of Steel Part

Figure 1: Sample of Steel Part

Among metals, steel is often chosen for its strength and hardness, its ability to withstand friction and heat and, in the case of stainless, its resistance to corrosion. These characteristics make it ideal for many automotive and medical uses and for high-heat applications. Steel and stainless can take longer to machine than the softer metals—aluminium, brass, and copper,—that Firstcut machines. While the softer metals typically require lead times of one to three business days, the steels require four to five. And where some of the softer metals can be used to produce parts from stock as large as 558x355mm, Firstcut produces parts that fit within an approximate envelope of 250x175x44mm for s275, en8/c45, 304/304L and 316/316L stainless and steel stock.

Steel’s hardness also limits the minimum size of cutters and maximum depth of straight-sided cuts. As in other materials, deeper cuts must be made with thicker cutters. In the Firstcut process, holes machined into steel must be at least 0.80mm in diameter. The smallest radius that can be machined must be at least 0.40mm. At these minimum sizes, the thickness should be at least 0.75mm

Thin walls can also be an issue. While the absolute minimum thickness of a machined steel wall is 0.50mm, we strongly recommend that walls be at least 1mm. This is because the production of steel stock can produce internal stress that may show up as warping if the material is machined too thin. If you need a further reason to steer clear of extreme minimums, be aware that they can slow down machining and increase your cost.

As with all designs submitted to Firstcut, Firstquote design analysis will let you know if your 3D CAD model presents any machinability issues—holes, radii, depths, wall thickness, etc.—requiring your attention.

Firstcut machines the following steels:

  • S275, low carbon steel, better machinability, slightly cheaper
  • En8/c45, common higher-strength steel alloy
  • 304/304L* stainless is versatile, widely used, and somewhat less expensive than 316
  • 316/316L* offers better overall corrosion resistance

* The stock we use for 304/304L and 316/316L have “dual certification” and meet the qualifications for both forms of these materials.

Standard finish for all steel parts is de-burred, but parts can optionally be left as-milled or lightly bead blasted. Be aware however that if parts are left as-milled they can have sharp burrs.

If you have any materials or capabilities questions, a Proto Labs Customer Service Engineer is available on +44 (0)1952 683047. For a full list of material options head over to our Firstcut materials page.