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Understanding Materials and Tolerances

By Rapid Enterprises Inc.

Understanding Materials and Tolerances

08/20/2021 The most important aspect of quality machining is tolerances. Parts for automotive, medical, or aviation require components that fit seamlessly together without burrs or distortion. Safety is paramount in these industries making tolerances critical. Even in other machining projects understanding materials and tolerances is important to efficiency, cost, and safety in your shop.

Specifying Tolerances
Specifying tolerances correctly is an incredibly important step in ensuring the finished part will work as intended. Tolerance is simply a range of measurements the workpiece will adhere to. Not all measurements are critical; those that connect with other components require the highest tolerance. Working with your client in the design process tolerances must be the main consideration. Some pieces that are more artistic, or that don’t require a critical fit will easily fall under standard tolerances of 0.005” or 0.13 mm. Some can be even more lenient. Some require even higher precision up 0.0005”.
The tighter the tolerance the more important the fixture is to hold the workpiece and the tool. Sometimes special jigs or clamps are needed for that precise fit. This is well worth the investment if it provides the quality needed in an efficient time. Reworking pieces or breaking tools make a job more expensive and more time-consuming in the long run.
Setting tight tolerances that were not needed can also be a costly endeavour. More time to program and run the operation is required for tighter tolerances. Understanding where to apply a rough finish or precise tooling is machining fundamental.

Choosing the Right Material
There are hundreds of metal alloys available for CNC milling and turning. And within each category of alloy, there are several grades. Which one is best for your next project depends on a number of factors. Consider price, workability, corrosion resistance, strength, weight, and cosmetic appearance. Tolerances also vary by material. Soft workpieces can bend or warp. A coarse or hard material can wear out the cutting tool quickly. Titanium and stainless steel, for example, present more difficulties for thermal warp and tool wear. To help you choose, here’s a closer look at the ones we use every day and what benefits they may have for your next project.

Aluminum 6061 – This is the most common general-purpose aluminum grade. The main alloying elements are magnesium, silicon, and iron. Like all aluminum alloys, it has a good strength-to-weight ratio and is naturally resistant to atmospheric corrosion. Other advantages are that it has good workability and machinability, can be welded and anodized, and is widely available which translates to cost savings. It has applications in aerospace, architecture, and consumer goods. It can be used for structural parts, valves, pipes, truck bodies, and computer parts.

Aluminum 7075 – 7075 is a higher grade of aluminum, alloyed mainly with zinc. It’s one of the strongest aluminum alloys, with excellent strength-to-weight characteristics. It can become brittle but is ductile, strong, and tough. It has application in aerospace and sporting goods for aircraft parts, weapons, bicycles, and golf clubs.

Brass – Brass is an alloy of copper and zinc. It’s a very soft metal and can often be machined without lubrication. It’s also highly workable at room temperature, so it often finds applications that do not require great strength. There are many kinds of brass, largely depending on the percentage of zinc. As this percentage increases, corrosion resistance decreases. Brass is suited for corrosion-resistant and high heat or friction applications, lending it well to automotive parts, gas fittings, and telecom.

Stainless Steel 303 – The addition of chromium to iron helps deter oxidation to make stainless steel. The amount of chromium affects the resistance to rust and forms a number of stainless steel grades. Because all stainless steels look-alike, great care must be taken to test incoming raw material with modern metrology equipment like OES detectors to confirm the characteristics of the steel you’re using. Uses include hardware like nuts, bolts, and screws. It is also suited to gears.

Stainless Steel 304 – This is the most common form of stainless steel found in a wide variety of consumer and industrial products. Often called 18/8, this refers to the addition of 18% chromium and 8% nickel to the alloy. These two elements also make the material especially tough and non-magnetic. Industrial applications and kitchen equipment are perfect candidates for 304 where heat and chemical corrosion resistance are important.

Titanium – Titanium is well-known for possessing high strength, lightweight, toughness, and corrosion resistance. It can be welded, passivated, and anodized for increased protection and to improve its appearance. Titanium doesn’t polish especially well and is a poor conductor of electricity but a good conductor of heat. It’s a tough material to machine and only specialty cutters should be used. It is the most expensive but the highest performing. Automotive, aerospace, medical, and weapon industries all utilize titanium for precise, strong parts.

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About This Author

Rapid Enterprises Inc.

Rapid Enterprises Inc.

CNC machines are finely tuned pieces of equipment. The work they produce is robust and tough, but they are sensitive and require delicate care and attention to keep them running their best…

Read More »

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