What exactly is computer-numerical-controlled (CNC) machining? It’s a means to make parts by removing material via high-speed, precision robotic machines that use an array of cutting tools to create the final design. CNC machines commonly used to create the geometric shapes required by customers are vertical milling machines, horizontal milling machines, and lathes.
To successfully make 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 computer-aided-design (CAD) model provided by the customer. The CAD model is loaded into the CAM software and tool paths are created based on the required geometry of the manufactured part. After determining the tool paths, the CAM software creates machine code (G-code) that instructs the machine on how fast it should move, how fast to turn the stock and/or tool, and the location to move in a 5-axis coordinate system.
Complex cylindrical shapes can be manufactured more cost-effectively using a CNC lathe versus a 3- or 5-axis CNC milling machine. With a CNC lathe, cutting tools are stationary and the part stock is turning, whereas on a CNC mill, the tool turns and the stock is fixed. To create the geometry, the CNC computer controls the rotational speed of the stock as well as the movement and feed rates of the stationary tools required to manufacture the part. If square features need to be created on a round part, the round geometry is first created on the CNC lathe and then the square features would be made on a CNC mill.
Because the computer controls the machine movement, the X, Y, and Z axes can all move simultaneously to produce a range of features, from simple straight lines to complex geometric shapes. Some limitations do exist in CNC machining, and not all shapes and features can be created even with the advances made in tooling and CNC controls. The limitations will be discussed later.
If a drawing or specification sheet has not been provided by the customer, a company may provide general specifications to follow to manufacture a model. These specifications may change from one company to another. In addition, some companies do not have default tolerances and will require the customer to provide the specifications.
Listed below are the specifications Xometry follows when a customer has not provided any.
- Tolerance for all dimensions will be ±.005 in. for all metal parts and ±.010 in. for all plastic parts.
- The finish will be an as milled finish to a maximum 125 microinches RMS.
- If tapped holes are not added on the quote and a provided drawing, they will not be added to the part and will be machined to the diameter specified in the model.
- No surface treatment (bead blast, anodize, powder coat, etc.) will be applied unless specifically requested by the customer.
- For metal parts, walls should be a minimum 0.030 in. (~0.75 mm) thick.
- For plastic parts, walls should be a minimum of 0.060 in. (~1.5 mm) thick.
Tolerance is the acceptable range for a dimension, which is determined by the designer based on the form, fit, and function of a part. It is important to keep in mind that a tighter tolerance can result in additional cost due to increased scrap, additional fixturing, and/or special measurement tools.
Longer cycle times can also add to the cost if the machine needs to slow down to hold tighter tolerances. Depending on the tolerance call out and the geometry associated with it, costs can be more than double of what it would be to hold the standard tolerance. Tighter tolerances should only be used when it is necessary to meet the design criteria for the part.
Furthermore, overall geometric tolerances can be applied to the drawing for the part. Based on the geometric tolerance and type of tolerance applied, cost may rise due to increased inspection times.
The best way to apply tolerances is to only apply tight and/or geometric tolerances to critical areas, which will help minimize costs.