Dainsta CNC Guide I – Best Design Practices for Custom Machined Parts

Dainsta-CNC-Guide-I-–-Design-Best-Practices-for-Custom-Machined-Parts

We are Dainsta are obsessed about manufacturing and to share our knowledge with every manufacturing enthusiast, we decided to create a series of informative Dainsta Guide blogs where we talk about everything from manufacturing, CNC machining/3D printing best practices to cost savings methodologies and emerging technologies and trends. This is the first one of the many such articles.

Before we begin with the best practices, we need to understand what exactly does it mean by computer-numerical-controlled (CNC) machine. It means to create parts by removing material via high speed, accuracy machines that use a collection of cutting tools to create the ultimate design.

Programs instruct the machine on movement to successfully create a part in the desired style. The instructions are encoded using computer-aided-manufacturing (CAM) software in combination with the computer-aided-design (CAD) model given by the customer.

The CAD model is then loaded in the CAM software and tool parts are created based on the essential geometry of the manufactured part. After determining tool paths, CAM software creates machine code (also known as G-code) that directs the machine how fast it should move, how fast to turn, and the location to move in the 5-axis coordination system.

Since the computer controls the machine movements, the X, Y, and Z axes can all move in unison to produce a range of features, from simple straight lines to complex geometric shapes.

 

General Tolerance

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If a customer does not provide a drawing or specification sheet, a company may provide general specifications to follow to produce a model.

Listed below are some specifications that Dainsta follow when they are not provided:

· For metal parts, walls should be a minimum of 0.030 in. (~0.75 mm) thick.
· For plastic parts, walls should be a minimum of 0.060 in. (~1.5 mm) thick.
· No surface treatment (bead blast, anodize, powder coat, etc.) will be implemented unless specifically requested by the customer.

Part Tolerances

Tolerance is the amount of acceptable variance in the dimension of a part. defined by the designer based on the form, fit, and function of a part. A tighter tolerance can result in extra cost due to increased scrap, additional fixturing, and/or special measurement tools.

Tighter tolerances should only be used when it is essential to meet the design criteria for the part. Extended cycle times can also add to the cost if the machine needs to slow down to hold tighter tolerances. Depending on the tolerance call and geometry associated with it, costs can be more than double of what it would be to hold the standard tolerance.

The most reliable way to apply tolerances is to only apply tight and/or geometric tolerances to critical areas, which will help minimize costs.

 

Size Limitations

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Milling

Part size is restricted to the machine’s capacities and depth of cut required by a feature in the part. Keep in mind that a build space’s dimensions don’t relate to part size. For example, a Z movement of 38 inches doesn’t mean that a part can be machined to that depth or height.

Lathe

Lathe capacities will depend on the build space, or the diameter and length. You may also offer a live tooling lathe, which dramatically reduces lead times and improves the number of features that can be manufactured by coupling additional CNC milling functions within the lathe.

Material Selection

Material selection is important in defining the overall functionality and cost of the part. The designer must define important material characteristics such as—hardness, rigidity, chemical resistance, heat treatability, and thermal stability.

Metals

As a general rule, for softer metals like aluminium and brass, as well as plastics, the machine will take less time to remove material, which in turn decreases time and cost.
Tougher materials like stainless steel and carbon steel must be manufactured with slower axis RPMs and machine feed rates, which would improve the cycle times as compared to the softer materials.

As a common rule, aluminium will machine about four times faster than carbon steel, and eight times faster than stainless steel.

Plastics

Plastic material can be a less costly choice to metals if the design doesn’t require the rigidity of metal. It is easy to machine and costs about 1/3 that of 6061 aluminium.
We need to keep in mind that depending on the geometry, tight tolerances can be harder to hold with plastics, and the parts could warp after machining because of the stress created when the material is removed.

Complexity and Limitations

The more complex the part with moulded geometry or multiple faces that need to be cut, the more costly it is due to additional setup time and time to cut the part. When a part can be cut in two axes, the setup and machining can be achieved faster, thus minimizing the cost.
For simple two-axis parts, more material will be removed as the tool moves around the part than with a modeled part. With a more complicated part, some areas will need to be cut with X, Y, and Z axes moving together.

Five-Axis Machining

It allows for more complex parts to be manufactured most cost-effectively. Five-axis machining means that the machine and the part can run in up to five ways simultaneously around various axes. The synchronized movement allows for very complex parts to be manufactured more efficiently.
It minimizes setups, attains rapid cutting speeds, creates more efficient tool paths, and achieves better surface finishes.

 

In Part 2 of this guide, we will learn how to choose a perfect 3D software for your business. In this article, we learnt about general tolerance, part tolerance and size limitations. If you are interested to read more on similar manufacturing topics, you can check out our other fascinating blogs here

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