What are the complex features of machined parts?

CNC machine tools get greater development every year. Active tool lathes can mill a variety of shapes and drill off-axis or radial holes, operations that once required a separate trip to the milling department. The machining center is equipped with an indexing head that supports 3+2 machining, and multiple sides of a part can be done in one operation. This is great news for designers and engineers. Not only extremely complex parts can now be produced, but also with higher quality, lower cost and shorter lead times.

But not following these rules can lead to expensive rework and project delays. This article explores some key considerations any part designer should be aware of.

True 5-axis machining?

CNC turned parts on high-speed lathes is capable of completing many complex parts in a single operation. Active tooling and Y-axis functionality means it's possible to turn a bolt, mill a wrench flat, and drill a cross hole for a safety line. More complex examples might include hydraulic pistons with aligned grooves on one end, fittings with wrench holes on the surface, or shafts with external keyways. In some cases, it is even possible to "turn" parts that are more orthogonal than circles.

With this milling and live tool background in mind, there are five elements to consider when designing complex parts:

1. Hole placement

On-axis and axial bores on CNC lathes have a minimum size of 0.04" (1 mm) and a maximum depth of 6 times the diameter. Radial holes (holes drilled from the side of the part) should be at least 0.08" (2mm) diameter. Holes through turned or milled parts are usually OK (especially on hollow or tubular parts), but depending on part size, hole diameter and material, the tool may not have enough reach

2. Deep Features

External grooves on turned parts shall not be more than 0.95" (24.1mm) deep, or less than 0.047" (1.2mm) deep. In terms of size, all other groove milling features are generally the same as drilling, but a good rule of thumb is to keep the depth less than 6 times the feature width. Also, make sure to leave at least 0.020 inches (0.5mm) of wall thickness on adjacent material. Milling or turning of large planes and other milled surfaces is entirely dependent on the part geometry relative to the available tool size. However, deep ribs and grooves can be a challenge no matter where it is made. Heat sink-like features can be cut on turned or milled parts, but this depends on the actual part geometry and available tools.

3. Better threads

There is a lot of overlap in threading capabilities between turning and milling centers. Generally, threads from #4-40 (M3 x 0.5) to about 1/2-20 (M10 x 1.25) can be machined, depending on machine type and feature location, although some exceptions exist. Here, be sure to consider the section on the proper way to model threads and how this relates to internal vs external, milled vs turned part features. You might also consider inserts, coil and key inserts have a longer life than bare threads, especially in soft materials like aluminum or plastic, and are easier to install.

4. The cost of text marking is high

Complex aerospace and medical parts often require permanent marking of part numbers and company names. Indented text looks good, but it's also one of the most time-consuming of all machining operations and prohibitive as production volumes increase. Often it is better to use electrochemical etching or laser marking parts, but if you must engrave text, keep it short and beautiful with a simple, clean font. We recommend the soft metal and plastic ArialRounded MT fonts at 14 points 0.3mm deep and the hard metal ArialRounded MT fonts at 22 points 0.3mm deep.

5. Radius: Observe the corners

A common mistake on any machined part is the protrusion of sharp interior corners. For example, turning tools typically used for finishing have a nose radius of 0.016 inches (0.032 mm), so any mating part design should take this into account. The cutter goes down to 0.040" (1mm), which means that the inside corner radius of any groove will be slightly more than half that radius. This is very sharp, but keep in mind that milling with such a small tool will take a long time and will be limited to grooves no more than 0.375" (9.52mm) deep. The best solution is to lighten the inner corners, or allow the largest possible inner radius on the mating part design.