Production Machining 101: the Good, the Bad, and the Pretty

by | Jan 24, 2020

As a former machinist and CNC programmer, I have an affinity for machined parts in products. The repeatability and precision, material flexibility, and ability to make quick changes make machining an attractive (yet expensive) manufacturing option. Scott Miller of Dragon Innovation does a great job overviewing some of the pros and cons, design considerations, and many common part applications in his DFM series. I have summarized many of the highlights from his article on machining here.

The only difference between computer numeric control (CNC) milling and manual milling is that a computer drives the movement of the cutting tool, rather than a manual operator, which allows for many parts to machined in exactly the same way. Typically, this is good for production volumes and is often utilized for parts that require the extra strength of metal over plastic such as precision gears, fittings, and drive components.

Advantages of machining:
  • There is a wide range of materials available and you can select materials that have a variety of strength and thermal characteristics (common examples are steel, stainless, aluminum, titanium, and copper).
  • Unlike injection molding, machining does not require a uniform wall thickness and also can accommodate complex part geometries such as undercuts
  • Machining does not require tooling and can shorten your time from design to parts-in-hand
  • The process is very repeatable and precise

Disadvantages of machining:
  • Depending on the geometry, the cycle times can be relatively long
  • There are limitations on what you can machine, ie - not good for very deep features in parts
  • It’s a subtractive process which results in waste that adds to the cost
  • You’ll have to consider clamping and cutting head geometries

There are four main processes associated with machining: milling (tool moves, material stationary), turning (material moves, tool stationary), drilling, and grinding. Lathes are primarily used for round features and parts with processes such as turning, facing, boring, external threading, and cut-offs. Mills are typically utilized to machine rectangular parts with operations such as horizontal surfacing, form cutting / pocketing, and slotting. Milling and turning machines are also utilized to perform drilling operations such as center drilling, drilling, reaming, spot facing and tapping. Grinding is utilized to very carefully control the thickness and precision of parts with operations such as surfacing and plunge grinding.

The cost of machining is generally driven by material, time, tools, overhead and maintenance. You can remove a lot of material quickly with rough cuts, and then use a finishing cut with a more rigid tool to get a really nice finish and precise tolerances:
  • Easy: > 0.01”
  • Medium: >0.002”
  • Expensive: >0.0005”
Design guidelines:
  • Start with material that is as close to the final shape as possible to save time
  • Look for opportunities to incorporate off-the-shelf components when possible
  • Design parts that can ideally be made on 1 machine
  • Avoid drilling holes in long, slender components
  • Keep in mind how the part is going to be gripped as it is machined

In general, machining is an invaluable subtractive manufacturing process that is a cornerstone of prototyping, tool development, low volume, and high volume production today.
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