CNC Machining Design Guide

CNC machining is the most popular manufacturing technology for making high precision parts for functional analysis, prototyping, and regular production, especially for metal parts. While designing the parts for CNC machining, a designer needs to remember few important tips to generate a successful design from the manufacturing point of view. Poorly designed parts may lead to cost overruns, longer lead time, design rectifications or simply cannot be machined.

Here are a few important design rules to which can improve the CAD part for CNC machining.

 

Avoid too small features

Features smaller than 2.5mm cannot be machined since most CNC machines tools can be machined up to 2.5mm. Features below 2.5mm need special tools and processes which will lead to additional cost impact.

Use standard hole size

Nonstandard hole sizes are sometimes difficult to machine and may need special tools to create them with an extra cost overrun and time. Holes matching with the drill bit size are easily drilled and save machine time. if a certain hole size is close to some drill bit size, then it is recommended to match the hole with the size of the bit. For multiple holes try to keep the same hole sizes to avoid multiple tool changes.

Limit thread length

It is recommended to keep thread length up to 2.5 to 3 times of the thread diameter while designing the tapped hole. Thread length beyond three times the diameter is sometimes not needed and just unnecessary. As designer should avoid deep taps because of chances of vibration and tool deviation which results in the defective tap. For blind holes,  it is recommended to add extra unthreaded length towards the hole bottom to let the tapping tool fully create the required thread length.

Avoid too thin walls

Extremely thin walls should be avoided while designing parts for CNC machining. Lowering the wall thickness disproportionately affects the stiffness negatively. According to studies, stiffness is directly proportional to the thickness of the material. Lower thickness may compromise the part accuracy due to unavoidable vibrations during the machining process. Recommended minimum wall thickness for metals is 0.8mm and for plastics 1.5mm. If the thin wall design is a necessity, then the designer should opt for other manufacturing processes like sheet metal fabrication.

Avoid partial holes

Partial holes are those where the hole axis is outside the periphery. These types of holes should be avoided by the designer because this may make the tooltip deviate from its axis. If the partial hole is a must, then it is advised to keep the tool axis within the part periphery so that the max hole lies on the part.

 

Avoid too deep cavities

Deep cavities are those which are more than five to six times deep of the tool diameter. It is advisable to keep the width to depth ratio of any cavity, maximum depth should be up to four times the cavity width. Too deep cavities result in chip accumulation, chances of tool deviation, and tool damage.