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CNC Machining Tips for Complex Parts

Design highly complex machined parts faster and more efficiently with some tips

The capabilities of CNC machine tools are increasing year by year. Power tool lathes can mill a variety of shapes and drill off-axis or radial holes, operations that once required separate trips to the milling department. The machining center is equipped with an indexing head and supports 3+2 processing, which can complete multiple sides of the part at one time. This is good news for designers and engineers. Not only can extremely complex parts be produced now, but they can be produced with higher quality, lower costs and shorter lead times.

But that doesn’t mean everything will happen — certain machining rules still apply — and failure to adhere to them can lead to costly rework and project delays. This design tip explores some key considerations that any part designer should know, including: hole locations, mill depth features, threads and inserts, text, part radius

Xavier features 5-axis milling or 3+2 milling. This feature enables the machine to clamp the bottom of a workpiece and machine its top and sides in one go. This type of milling can produce more complex parts, such as brackets with undercuts on the sides, or similar deep irregular holes.

Like Xavier’s milling centers, CNC turn-milling on high-speed lathes is capable of completing many complex parts in a single operation. Power tools and Y-axis capabilities mean the bolt can be turned, the wrench flats milled, and a cross hole drilled for the safety wire. More complex examples might include a hydraulic piston with an alignment groove on one end, a fitting with an adjustable wrench hole in the surface, or a shaft with an external keyway. In some cases it is even possible to “turn” parts that are more orthogonal than circular.

With a background in milling and power tools, here are five elements to consider when designing complex parts:

1.Punch location
On-axis and axial holes on Xavier CNC lathes have a minimum size of 0.04 in. (1 mm) and a maximum depth of 6 times the diameter. Radial holes (holes drilled out of the side of a part) should be at least 0.08 in. (2 mm) in diameter. Holes that go all the way through turned or milled parts are usually OK (especially on hollow or tubular parts), but depending on part size, hole diameter, and material, the cutting tool may not have adequate reach. Xavier will machine from each side if possible, but be sure to check your design analysis for potential limitations.

2.Deep Features
External grooves on turned parts cannot be deeper than 0.95 in. (24.1 mm), or narrower than 0.047 in. (1.2 mm). All other slotted milled features are generally the same size as drilled holes, but a good rule of thumb is to keep the depth less than 6 times the width of the feature. Also, be sure to leave at least 0.020 in. (0.5 mm) of wall thickness on adjacent material. Large flat and other milled surfaces (mill or lathe) depend entirely on part geometry in relation to available tool size. However, deep ribs and grooves can be challenging, regardless of where they are made. Radiator-like features can be cut on turned or milled parts, but this depends on the actual part geometry and available tooling. Again, please double check your DFM analysis and don’t be afraid to send us a drawing and we can test it, or contact an application engineer.

3.Thread machining
There is a lot of overlap in threading capabilities between Xavier’s turning centers and milling centers. Generally speaking, Xavier can use threads from #4-40 (M3 x 0.5) to approximately 1/2-20 (M10 x 1.25), depending on machine type and functional layout, but there are some exceptions. Check the thread guide for exact measurements and details. While you’re there, be sure to read the section on the proper way to model threads, and how this relates to internal vs. external and milled vs. turned part features. You might also consider using inserts. Coil and key inserts have a longer life than bare threads, especially in soft materials like aluminum or plastic, and are easy to install.

4.Permanent Marking
Complex aerospace and medical parts often require permanent marking with the part number and company name. Recessed text may look nice, but it’s also one of the most time-consuming of all machining operations, and becomes completely prohibitive as production volumes increase. It’s usually best to electrochemically etch or laser mark the part, but if you must engrave text, keep it short and sweet with a simple, clean font. We recommend Arial Rounded MT font 14 points 0.3 mm deep for soft metals and plastics and Arial Rounded MT 22 points 0.3 mm deep for hard metals.

A common mistake on any machined part is the appearance of sharp internal corners. For example, Xavier’s turning tools typically used for finishing operations have a nose radius of 0.016 inches (0.032 mm), so any mating parts should be designed with this in mind. The diameter of the milling cutter is reduced to 0.040 inches (1 mm), which means that the inside corner radius of any cavity will be slightly larger than half that. This is very sharp, but keep in mind that milling with such a small tool takes a long time and is limited to cavities no deeper than 0.375 inches (9.52 mm). The best approach is to eliminate inside corners or allow the largest possible inside radii in the mating part design.

A final note: Failure to apply good design to manufacturing practices can make challenging machining operations more challenging, and therefore costly. Paying a little more may not be that important for a prototype, but when demand increases and the part goes into production, it can be a huge waste of cost. As always, when you have any questions about any complex part or part functionality, please feel free to contact Xavier by emailing sales@xavier-parts.com.