How to Design Machined Parts for Manufacturability

A discussion of the manufacturing industry is never complete if Henry Ford and his assembly line are not mentioned. Henry Ford had once said, “Any customer can have a car painted any color that he wants, so long as it is black.” One driving factor behind the ability of the Ford Motor Company to produce an affordable vehicle for the masses at that time, was their pursuit of simplicity and standardization.

As processes, materials, tools, and parts are standardized or simplified in a manufacturing environment, the total production time of a finished product is typically reduced. In the modern era of manufacturing the need for, and execution of, instant access to information, data, and precision parts is vastly different from Henry Ford’s time. Although this may be true, the benefits of planning the design of a product around it’s manufacturing processes is an age old practice that is still valid to this day.

Most products in the manufacturing industry can, in some way, be tied to a machining operation. Whether the product requires machined parts to complete its’ assembly, or it requires precision machined tooling for subassembly fabrication, machining operations are often the core of product development and manufacturing. With the spotlight on machining, what are the essential items that must be considered when designing a part for manufacturability? How does a machined part go from concept to reality? The following information should assist in the effort to achieve a manufacturable product.

Machining Processes

One of the first things to consider when designing a machined part for manufacture is to assess what manufacturing processes will have to occur and in what order they will need to occur in.

● Saw cutting – Completed using vertical or horizontal bandsaws. Typically used as an initial operation to bring bar and round stock to size for further machining operations. When batches of parts must be saw cut, material stops should be utilized to provide repeatability of the saw cut length. When selecting tolerances for saw cut material it is important to note that a unilateral tolerance should be assigned to ensure that enough material is present for the proceeding machining operations. (ex. 3.00 + .03/- 0)

● Milling – A machining process that utilizes a multi axis machine tool with a rotary cutting bit to subtractively remove material from the saw cut stock. Traditional manually operated milling machines come in vertical or horizontal spindle orientations, and will include three axis of movement (X,Y, Z).

Accommodating for the world of complex shape machined parts is easily achieved by the way of a CNC (Computer Numerical Control) milling center. These automated machine tools exponentially increase the capability of more complex and precision parts to be manufactured.

The simultaneous, and positional capabilities of 5-axis CNC machine tools are currently rising in popularity due to the reduction in setup time as compared to more traditional 3-axis work.

Horizontal CNC milling machines offer great chip evacuation that is a direct result from the orientation of the part, and the spindle inside of the enclosure. Increasing chip evacuation will improve surface finishes and tool life.

● Turning – A machining process that utilizes a multi axis machine with a non-rotating cutting tool that subractively removes material from rotating round stock. Traditional manually operated lathes move in two axis.

CNC turning centers offer increased automation as compared to their traditional manual predecessors. CNC lathes will often feature live tooling options that allow milling operations to occur in one machine tool.

The use of sub spindles in a CNC lathe can provide an automated solution to second operation setups.

CNC lathes may also have bar feeders that can provide “lights out” machining for increased part throughput.

● Additional Machining Processes – Surface grinders, and EDM (Electrical Discharge Machining) are additional machine tools that are proven to be capable of extremely tight tolerances. These machine tools are most prevalent in the tool and die industry, but can be found in most capable machine shops.

Material Considerations

Choosing the right material for a machined part may seem to be a daunting task, but the following materials are good starting points to narrow your search:

● 6061, 7075 Aluminum – Great machinability, light weight, available in a variety of sizes and shapes, corrosion resistant.
● 1018 Steel – Inexpensive, increased machinability as compared to other carbon steel grades, great weldability.
● 4140 Alloy Steel –
● 304, 316 Stainless Steel – Corrosion resistant, increased machinability as compared to other stainless steels, great weldability,
● Delrin – Great machinability, moisture and chemical resistant, low coefficient of friction.

There are plenty of other materials to select from including brass, bronze, inconel, nylon, ABS, and UHMW. The process of selecting the correct material for a machined part should really begin with assessing the needs of the end product, and a clear line of communication between the client, engineer, and machinist.

Machined Part Feature Considerations

The real work in designing a machined part for ease of manufacture lies within the decisions that are made when designing the features of the part. A design is only a concept unless it can actually be machined. All too often, conceptual 3D models include features that are not possible or not feasible to be machined. The following list contains some of the general design considerations regarding the features of a machined part:

● Design holes to use industry recognized drill bit sizes
● Hole depth should be less than 4 or 5 times the diameter of the drill
● Holes should not be placed on angles. Drilling a hole on an angled surface will result in the drill bit walking.
● Design pocket radii to accomodate for standard end mills (ex. .25” internal radius for a .5” dia end mill)
● Avoid sharp internal corners.
● If sharp internal corners are necessary they can be created with an EDM or by drilling “mickey mouse corners.”
● Choose chamfers as an external edge break as compared to a fillet.
● Design internal threads to include a relief at the bottom of the hole for tap and chip clearance.
● Avoid deep pocket features. Depth of the pocket four times larger than its width.
● Design parts to fit within standard stock material sizes.
● Avoid tight tolerances when they are not crucial to the function of the end product.

Fixturing

Although the design considerations that go into the features of a machined part are among the most important, what may be the most overlooked aspect of designing parts for ease of manufacture is how the part will be held while it is being machined. Any part can be designed, but the real question is can it be made?

The answer to the question above comes from what is known as a fixture. A fixture in machining is simply a device that will hold onto the stock material as it is being machined. This can be accomplished by a variety of simple and complex methods, but when designing the finished part it is imperative to keep in mind how the part will be fixtured inside of the machine.

When planning and designing the fixture for a part it is important to reduce the amount of different setups or operations it takes to machine a single part. The more times a part needs to be relocated inside of the machine, the greater the time it takes to make that part, the greater the risk will be for inaccuracies, and ultimately the greater the final cost of the product will be.

CAD/CAM Considerations

For technically minded people, part of the beauty of living in the 21st century is the efficiency of designing a product and bringing that product to life. This is due, in part, to the instant access to CAD software (computer aided design) that is available currently.

Whether you are an engineer, a machinist, or a hobbyist there is a CAD program that suits the needs of your work. The industry standard choices in CAD and CAM (Computer Aided Manufacturing) software are typically Mastercam and Solidworks. Software packages such as Autodesk Fusion 360 are gaining momentum in the industry due to their low cost and large versatility by the way of a complete CAD/CAM/Simulation program that allows the end user to see a part go from idea through manufacture all within one program.

Final Thoughts

There are many more factors to consider than what are listed above when designing a part that will be machined. Although this may be true, if the considerations mentioned are adhered to the manufacturability of the part will certainly increase. If you are an engineer, an industrial designer, a machinist, or a hobbyist, make an effort to standardize the tools, materials, and processes that you use to bring a product to market. This effort will pay back in the form of reduced production time, reduced model revisions, and in the end increased revenue.

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