Manufacturing CNC Parts Online – Best Practices

To take advantage of CNC machining capabilities to the fullest, a designer must follow specific Design for manufacturing rules. This can be challenging, though, as particular industry-wide standards do not exist. In this article, we’ve compiled comprehensive guidelines with the best design practices for CNC machining.

The CNC Machining Process
CNC machining is subtractive manufacturing technology. In CNC, the material is eliminated from a solid block using various cutting tools that rotate at average to high speed (thousands of RPM) to manufacture a part based on a CAD model. Both plastics and metals can be CNC machined.
CNC machined parts have tight tolerances and high dimensional accuracy. CNC is suitable for both high-volume production and one-off jobs. CNC machining is presently one of the most cost-effective ways of producing metal prototypes, even compared to 3D printing.

Design Guidelines for CNC Machining Best Practices
A machined part is just as good as its design. CNC machining equipment has some practical limitations, and designers have to understand these or continue designing un-machinable parts. Designers can enhance a design for the most efficient usage of CNC equipment and follow particular design rules to ensure the part’s manufacturability.

This section will look at all of this to aid in successfully designing a component according to the CNC machining best practices starting with some general limitations of milling and turning parts.

To successfully manufacture a part on a CNC machine, programs instruct the machine how it should move. The programmed instructions are encoded using computer-aided manufacturing (CAM) software in conjunction with the client’s CAD (computer-aided-design) model. The CAD model is transformed into the CAM software, and tool paths are designed based on the needed geometry of the manufactured part.

After deciding the tool paths, the CAM software makes machine code (G-code) that tells the machine how fast it needs to move, how fast to change the stock and/or tool, and the ideal location to move in a 5-axis coordinate system.

Complex cylindrical shapes can be made more cost-effectively utilizing a CNC lathe versus a 3- or 5-axis CNC milling machine. With a CNC lathe, the part stock turns, and cutting tools are stationary, whereas, on a CNC mill, the tool moves, and the stock is fixed.

To design the geometry, the CNC computer controls the rotational speed of the stock and the feed rates and movement of the stationary tools needed to manufacture the part. If square features have to be made on a round part, the round geometry is first designed on the CNC lathe, and then the square will be made on a CNC mill.

Since the computer controls all machine movement, the X, Y, and Z axes can all move at the same time to produce a range of features, from simple straight lines to complex geometric shapes. Some limitations exist in CNC machining, and not all shapes and features can be created even with the advances in tooling and CNC controls.

Part Tolerances
Tolerance is the permissible range for a dimension determined by the designer based on a part’s form, fit, and function. It is essential to remember that a tighter tolerance can result in extra cost due to additional fixturing, increased scrap, and/or special measurement tools.

Longer cycle times can additionally add to the cost of the machine’s need to slow down to hold tighter tolerances. Depending on the tolerance call out and its geometry, costs can be more than double what it would be to hold the average tolerance. Tighter tolerances must only be utilized when necessary to meet the design specification for the part.

Moreover, overall geometric tolerances can be applied to the drawing for the part. Due to increased inspection times, the cost may rise based on the geometric tolerance and type of tolerance applied.

The ideal way to apply tolerances is to just try tight and/or geometric tolerances to crucial areas, which will minimize expenses.

Unless especially called out by the designer, the standard tolerance utilized by Truventor is ±.005 inches for metal parts and ±.010 inches for plastic parts. If tighter tolerances are needed, the information must be communicated as to which dimensions need a more narrow range. Think, a piece of paper is about 0.003 in. thick.

General Tolerances
If the customer has not provided a drawing or specification sheet, an organization may provide general specifications to follow to make a model. These criteria may change from one organization to another. Moreover, some companies do not have default tolerances and will require the customer to provide the specifications.

  • Listed below are the specifications Truventor follows when a customer has not given any.
  • Tolerance for all dimensions will be ±.005 inches for all metal parts and ±.010 inches for all plastic parts.
  • The finish will be a milled finish to a maximum of 125 microinches RMS.
  • If tapped holes are not included on the quote and a given drawing, they will not be included in the part and will be machined to the diameter explicit in the model.
  • No surface treatment such as bead blast, powder coat, anodize, etc., will be applied unless explicitly requested by the customer.
    – o For metal parts, walls should be at least 0.030 in. (~0.75 mm) thick.
    – o For plastic parts, walls must be at least 0.060 in. (~1.5 mm) thick.


Size Limitations

Lathe
Lathe capabilities rely on the build space or the diameter and length. An organization may also offer a live tooling lathe, which dramatically decreases lead times and increases the number of features that can be machined by combining additional CNC milling functions within the lathe.

Metals
Generally, softer metals, like aluminum and brass, and plastics, the machine will take less time to remove material, which reduces time and cost. More complex materials, like carbon steel and stainless steel, must be machined with slower machine feed rates and spindle RPMs, which would increase the cycle times and the softer materials.

Generally, aluminum will machine about 4x times faster than carbon steel and 8x times faster than stainless steel.

The material type is a critical driver in deciding the overall cost of the part. For instance, 6061 aluminum bar stock is nearly half the price per pound of aluminum plate, and 7075 aluminum bar stock can be 3x the cost of 6061 bar stock. The cost for 304 stainless steel is nearly 3x that of 6061 aluminum and about twice as much as 1018 carbon steel.

Milling
Remember that a build space’s dimensions don’t equate to part size. Part size is limited to the machine’s unique capabilities and depth of cut needed by a feature in part. For instance, a Z travel of 38 inches doesn’t mean that a part can be machined to that depth or height (see below).

Based on the part size and feature that have to be machined, the Z height of the part needs to be less than 38 inches due to the tool clearance and depth of cut. The size and features of the part will decide the part’s machinable height.

Complexity and Limitations
The more complex the part, the more costly it is to manufacture. Because of additional setup time and time to cut the part, it becomes more complex. When a part can be cut in at least two axes, the setup and machining can be achieved faster, thus minimizing the cost.

More material for simple two-axis parts will be removed as the tool moves around the part than with a contoured part. Some areas will have to be cut with X, Y, and Z axes moving together with a more complex part.

To create a complex surface with a perfect surface finish, minor cuts will need to be used. This increases the time and, hence, price of a part. A general rule to help minimize the expense is to design utilizing only two axes cuts; however, this isn’t always possible if a specific look or functionality is needed. Keeping things consistent, such as tapped holes and internal corner radii, will also help save time and money on parts by decreasing the need for tool changes.

Is your supplier utilizing the best technology and machining techniques?
It is essential to find it out. Make sure to ask your supplier how they’re fully utilizing their technological and intellectual capacity to get you the high-quality parts faster and at the best price.

If your supplier is still using that old 3-axis technology to manufacture complex parts, you should encourage them to invest in multi-axis milling. If they’re unwilling to upgrade, it’s likely time to find a supplier that adheres to machining best practices.

Truventor is a cloud manufacturing platform with 100+ Manufacturing Partners from CNC machining domain.

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