Our Insight video series will help you master digital manufacturing.
Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.
So join us and don’t miss out.
Insight: CNC Turning
Hello and welcome to this week’s Insight.
In this video, the topic we’re tackling is - when it’s best to make cylindrical parts using a lathe.
When CNC machining centres came along, the line between milling and turning became a little blurred. Then we started to introduce live-tool capability for CNC lathes, and the blurring increased even more.
Operations that were once the exclusive domain of the milling department were now coming off the lathe complete. As a result, deciding which machine is the best fit for producing any given part has become far more complicated than it once was.
Sometimes it’s pretty obvious, of course. Consider the piston for a spool valve, or a hydraulic fitting. They have a cylindrical symmetry, complex external geometry and challenging internal features. All of this means you’re going to want to keep them confined to your turning department.
On the other side of the coin, the rectangular valve body that mates with those turned parts, will never be spun on a lathe, no matter how sophisticated the machines get. Or at least not in the near future.
In the middle of these two extremes, however, are a whole range of edge cases.
Why does this matter? Well, we’ve found that CNC turning can generally produce better surface finish on cylindrical features and at a typically lower price for customers. Lathes also help to make parts that may skate on the edge of our milling capabilities more efficient to produce, and if the goal is eventual low-volume production, turned parts are good candidates.
Taking advantage of this means being able to spot which items and designs might be lathe-worthy, though. There are a fair few factors at play here, but let’s get the basics down by considering a few household objects.
A regular old pint glass, for example, with its smooth, regular shape and length several times greater than the outside diameter is a nice and easy to produce on a lathe. A coffee cup with a jutting handle and finger-ready hole, however, is impossible to turn. That’s a job for the milling machine.
A small teacup saucer? Well, that could go either way. Those concentric ridges and curved surfaces are equally possible on a mill or a lathe, requiring nothing more than accurate code and a suitable cutter. However, it would almost certainly be faster and more efficient to turn the saucer rather than machine it.
As well as shape, though, we do also have to consider size when it comes up to deciding whether to stick something on the lathe. The exact details of this will obviously depend on your manufacturer and the kit they have, but here at Protolabs we’ll tackle parts with max dimensions of about 73mm in diameter and 228mm long. On the other side of the scale, we’re going to struggle with anything with a diameter of less than 4mm or shorter than about 1.5mm.
There can be pieces of the design even smaller than this, like conical points, but the angle can’t get too shallow or things don’t really work.
Once upon a time having a side-hole or milling a flat would have made it impossible to turn a part, but many new lathes have milling capability so this is well within their means - as long as that feature is parallel or perpendicular to the long axis, anyway. No angled holes yet, sorry.
Similarly, if you need your company’s name engraved on the parts… well, indented text is probably best with milling but raised text can work wonderfully on a lathe these days.
Hopefully this has helped you to work out what’s best for you and your parts, and has given you a clearer idea of when you can – and should – turn to the lathe. As ever, though, if you have doubts just speak to your manufacturer.
That’s it for this week. I look forward to seeing you again next Friday.
With special thanks to Natalie Constable.