The complete guide to custom machined parts

The complete guide to custom machined parts

Machined parts are ubiquitous, and it is easy to see why: CNC machining works on a wide variety of metals and plastics, and machined components can be manufactured quickly and cost-effectively, without the need for tools.

With Tik Precision, you can get machined parts and prototypes at reasonable prices with short lead times, and we have experience working with customers in various industries. However, why choose machined parts over molded or 3D-printed parts? Sometimes the choice is obvious, but sometimes it is hard to decide. How do you design the machining part, whether in-house or outsourced to a manufacturer?

This guide covers the basics of machined parts: what they are, why company’s need them, the best-machined materials, typical tolerances for machined parts, how to design machined parts, and more.

What are machined parts?

Machined parts are everywhere. Everything from tiny metal fasteners to aircraft engine components is machined. However, what exactly is machining, and therefore what is a machined part?

When we talk about machined parts, we mean something more specific than objects made using machinery. We refer to specific parts manufactured using cutting machines such as milling machines, lathes and milling machines. These machines all work in different ways, but their basic purpose is the same: use a sharp cutting tool to cut sections from a piece of material called a work piece.

Even within this definition, machined parts can be formed in different ways. The machining process can be manual, with a machinist (skilled professional operator of machining equipment) operating a machine, such as a milling machine, to manually cut the workpiece into the desired shape. Alternatively, it can be digital, in which case an electric CNC machine automatically cuts the machined part according to computer instructions.

Today, most complex or custom-machined parts are made with CNC machines, but machinists still do manual machining for some jobs because it is faster than creating digital designs and programming digital machinery.

Machined parts can be metal or plastic (and sometimes other materials), but they must be made of materials that can be cut without violent deformation.

Sometimes parts are machined after being made with another manufacturing process. For example, cast or molded items may be machined to certain details or features at a later stage. These can be described as partially machined parts or post-machined parts.

Why use machined parts?

There are many reasons why companies, product designers, R&D departments, and other professionals might use machined parts, and the next section will cover many of the specific advantages of precision-machined parts.

In short, finished parts have excellent strength because they are made of solid block material and can be made in a variety of shapes and thicknesses. They can have very detailed characteristics and can be created from a variety of components. A small number of machined parts can be manufactured quickly because they do not require tooling and tolerances can be very tight if the machining speed is reduced.

Companies can also use machined parts because machining is a proven production technique that has been the industry standard for decades. As a result, manufactured parts are likely to meet industry-specific standards and certifications.

Advantages of machined parts

Machined parts offer certain advantages that may not be possible with injection-molded or 3D-printed parts. Some of the key benefits of machined parts are listed here.

1. No MOQ
One of the main advantages of machined parts is the ability to purchase them without a minimum order quantity.

For molded parts, metal molds must be made—a process that takes a long time, often costing tens of thousands of dollars. However, machined parts are cut directly from blank work pieces, making it cost-effective to order very small quantities or even one-off parts.

Of course, needing many (plastic) parts may mean molding is the better option. However, machining is almost unique in delivering high-quality parts with no MOQ, making it suitable for small companies, low-volume production, and prototyping.

2. Good prototypes
Some companies choose to order injection-molded prototypes, but usually only larger companies have the ability to do so. The cost of tools can make prototyping prohibitively expensive.

Machined parts are suitable and affordable as prototypes because they can be manufactured in one go. Machining is also much faster than forming, which means R&D can quickly iterate over multiple versions of a part and then do any testing or evaluation it needs before it goes into production.

Machined material versatility also means companies can order machined parts from several different metal alloys or composite plastics to see which performs best under test conditions.

3. Design freedom
Machined parts can come in many shapes and sizes. This is because CNC machining is not limited by extreme forming designs like thin walls and tapers; machined parts can be thick and strong, but their features can also be fine and detailed.

Although machined parts have some limitations in terms of internal sections and deep grooves, machining remains one of the most geometrically flexible manufacturing processes.

Molded parts, on the other hand, must have thin walls and generally meet more stringent design criteria.

Even for 3D printing, often regarded as one of the best manufacturing techniques, has limitations such as avoiding overhangs in terms of design freedom. (For more complex and bulky designs, extensive support structures may be required, which must be removed through expensive post-processing steps.)

4. Quality
Machined parts could be produced with extremely high precision. Perhaps more importantly, customers may specify tolerances that need to be met by the machinist. This means the machinist or equipment operator can take extra time on tight-tolerance parts and individual features.

While injection molds may also be made to tight tolerances, each individual molding cannot be held to such a high standard. Moldings produced toward the end of the mold lifespan might lack the definition of earlier units.

5. Lead times
Machined parts can be created faster than parts made via additional production procedures like molding.

This is partly due to the absence of labor-intensive tooling, but the production process itself can be highly efficient: some of the faster machining centers equipped with linear guide rails have got rapid rates of around 4, 000 centimeters per minute (though parts should not actually be machined at those speeds).

The one-step nature of machining and the speed of a CNC engineering center combine to make precision machined parts the fastest to manufacture (small batches), reducing lead times for faster time-to-market and practical rapid prototyping.
6. Alterations
Because CNC machined parts are made from digital CAD files, changes can be made to this digital design prior to manufacture.

This is useful during R&D and prototyping, where engineers may want to make partial adjustments to machined parts or create multiple versions. It also reduces the potential for waste, as defective parts are less likely to be manufactured.

This is a significant advantage of machined parts over molded parts: tools are not easy to change, and if last minute changes need to be made, creating a new mold would be a huge waste of money.

7. Strength
Machined parts are cut from a solid material called a billet, usually cast or extruded. This makes them very strong compared to, for example, 3D printed parts, which can be much weaker along one axis, where one layer is built on top of the next.

Many machined parts are also stronger than their molded parts, which are limited in mechanical properties because molded parts must have thin walls.

8. Surface finish
Made parts avoid quality issues on the surface associated with molding such as flow lines, jetting, and flash at the parting line. With a moderate amount of post-processing, machined parts could be brought to a very high common with regard to surface finish.

Engineering also gives a far superior surface finish to 3-DIMENSIONAL printing, actually, before any post-digesting has been carried out. 3D stamping, especially FDM printing, can easily leave visible coating lines on the surface of the part that should be smoothed over via sanding or chemical treatment. Machined parts do not have these level lines.