June 2015 Design Tip



Turning to Lathe For Parts with Cylindrical Features

It was once a no-brainer. Round parts were turned on lathes; non-round parts were machined on mills. With the advent of CNC machining centres, which interpolate round part features with ease, the line between the two machining processes became blurred.


The situation grew even more confusing when CNC lathes attained live-tool capability. 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.


Some parts are obvious lathe candidates. Consider the piston for a spool valve, or a hydraulic fitting. The cylindrical symmetry of these components, coupled with complex external geometry and challenging internal features, makes them permanent residents of the turning department.


Conversely, the rectangular valve body that mates with those turned parts, with its large milled surfaces, detailed pockets and intersecting bores will never be spun on a lathe, no matter how live-tool capable that machine may be.


These are some of the reasons why our CNC machining service has turned to, well, turning. By adding live-tool (end mill) equipped CNC turning centres to our already extensive three-axis milling service, we’re able to produce better surface finish on cylindrical features and at a typically lower price for customers. Lathe also makes the manufacturing of parts that skate on the edge of our milling capabilities, more efficient. And if the eventual goal is low-volume production, turned parts are good candidates.

The Metal rod stock is firmly fixtured in a high-speed CNC lathe before turning and milling begins.

The Metal rod stock is firmly fixtured in a high-speed CNC lathe before turning and milling begins.


Have a design concept for the next best camera lens housing? How about a revolutionary drive shaft for a lawnmower, one with milled flats and threaded cross-holes? Chances are, we can deliver a turned prototype for that geometry.


Turning parts such as these offers several advantages over milling. As implied previously, long length to diameter ratios on pistons and shafts give mill operators an upset stomach. Turning a set of candlestick holders for your Aunt Martha’s 60th birthday, however, is a piece of cake on a lathe.


If you’re still unsure about what parts are lathe-worthy, consider a few household objects. A pint glass, for example, with its smooth, regular shape and length several times greater than the outside diameter is a straightforward exercise on a lathe. Not so on a machining centre. A coffee cup, on the other hand, with its jutting handle and finger-ready hole, is impossible to turn.


Those 3 lb. dumbbells collecting dust in the closet could be turned fairly easily on a lathe. The wide, relatively deep recessed area where your hand grips the bar can be turned with a simple grooving routine, a feature that would be murderous to cut on a milling machine.


A small teacup saucer could go either way. Interpolating the concentric ridges and curved surfaces is equally possible on a mill or a lathe, requiring nothing more than accurate G-code and a suitable cutter. That said, it would almost certainly be faster to turn the saucer, and far more efficient in terms of material use.


How about a hose barb for the sprinkler? The v-shaped grooves would require a special cutter on a machining centre, whereas a lathe can use a standard turning tool. The same holds true with a replacement wheel for the barbecue grill, although milling those little mag wheel-like cut-outs on the face would be challenging or outright impossible on most turning machines.


Soup cans and salt shakers, water bottles and flower pots — these shapes are what lathes are all about. Milk cartons and picture frames? Not so much. That’s not to say we’ll turn anything that comes our way. Proto Labs offers turned parts in aluminium. Very soon steel and a select range of stainless steel materials will be available. We continuously expand our capabilities with brass and copper being the next additions. Followed-by the future introduction of plastic.


Size wise, we’ll tackle parts up to 75 mm in diameter by 228 mm long, and as small as 4 mm diameter by 1.5 mm long. Sharp conical points are okay, as long as the angle is greater than 30 degrees. No promises on ballpoint pen prototypes, or models of soda straws.


As mentioned earlier, our new lathes have milling capability. Drilling a side hole or milling a flat is well within their means, as long as that feature is parallel or perpendicular to the long axis of the part. No angled holes yet, sorry. Any milled grooves should be wider than 1.2 mm, but because all of our lathes are equipped with a Y-axis, we can machine slots or holes off-centre (within reason).

Proto Labs has the ability to machine on-axis and axial holes as well as radial holes.

Proto Labs has the ability to machine on-axis and axial holes as well as radial holes.


Need your company’s name engraved on the parts? Raised text is best, but we can machine most any lettering as long as the line width and character spacing measures at least 0.5 mm across. Small holes don’t scare us off, nor do threads. In fact, we’ll drill radial holes down to 2 mm and axial holes half that size. Likewise, we support a range of internal and external UNC, UNF and metric threads.


If you’ve previously used our machining service, you may find that turned surfaces are smoother and more round than their milled counterparts. Milled features may have visible tool marks, but we can bead blast these surfaces to give them a matte finish and knock down any small burrs that remain after the machining process.


Like all machined parts, the process begins with entering your CAD model into our automated quoting system. Our software will attempt to determine the best machining method (milling or turning) for each material. This decision, however, can sometimes be overridden when you configure the quote. Each process has different capabilities for creating desired features and threading; pricing also varies between the two processes. When you configure the quote, select the machining process that best meets your needs after weighing these factors. If a part does not qualify to be turned in a given material, this option will not be available in the quote for that material.


We’ve found over the years that making round parts out of square or rectangular stock can leave a few things to be desired. Milling away the square corners to get to the round part underneath takes longer than turning it from bar stock. It also creates more material waste. Turning those parts in a machine designed for such work is certain to be a win-win for everyone. To learn more about turning at Proto Labs, check our process and guidelines page or upload your part now to see if it qualifies.


You’ll find a number of helpful online resources centred on CNC machining. Go inside a HAAS CNC mill with our new short machining video, read our design tip on cutting milling costs and download our free Prototyping Processes tech brief.