October 2010 Machining Tip

Threaded Holes in Machined Prototype Parts

A wise man once said, "The nice thing about standards is, there are so many of them." He wasn't talking about fastener threads, but he might as well have been. There are many different standards for threaded fasteners: Inch, metric, fine and coarse. Those are the common differentiators; a thread connoisseur will also be aware of, and use, standards for left hand threads, tapered threads, double-helix threads, acme threads, pipe threads, and so on. Fortunately, in prototyping we can boil it down to three popular thread families: UNC, UNF, and metric.

UNF (unified national fine) is an inch standard for fastener threads. UNF threads are specified by an inch diameter (either a fraction such as 1/4 or a gauge number such as #4) and a thread pitch. The diameter is the major diameter (the outer diameter of the threads). The pitch of the thread is a number telling how many threads are in one linear inch of the fastener. So, a #6-40 thread is a 6-gauge with a thread pitch of 40 threads per inch. One turn of a 40-pitch screw advances it 1/40th of an inch along the screw's axis.

UNC (unified national coarse) is also an inch standard. UNC threads are specified using the same conventions as UNF threads, with equivalent diameters but coarser thread pitches.

Compared to coarse thread fasteners of equivalent diameters, a fine thread fastener is stronger. Fine thread screws generally will have less play. It takes less torque to preload a fine thread screw to a given clamping force. On the other hand, since there are more threads per unit length it takes longer to screw in a fine thread fastener. Fine threaded screws are also easier to accidentally cross-thread, and are more susceptible to grit and other contamination.

Metric fasteners are specified by giving the major diameter in mm and a thread pitch. The pitch of a metric fastener is in mm per thread. A metric fastener specified as "M6 x 1.0" is 6mm in diameter and each turn of the screw advances it 1mm along the axis. There are coarse and fine pitch standards for metric threads, but most designers are not aware of this. The vast majority of metric fasteners are fine pitch, and most often, designers don't even add the pitch spec, so "M6" is understood to mean "M6 x 1.0".

Unless you are designing near the limit of strength of a fastener, the choice of UNC, UNF, or metric is usually one of convenience (for example, what can you get at the hardware store?). Outside the US, metric is the no-brainer choice unless you have a compelling reason to do otherwise. Within the US, coarse threads are slightly more common than fine, especially in the gauge sizes of #2 to #8. Generally, it's best to pick one family and stick with it. Using a smaller number of different screw sizes across a product line is often a good goal (note: mixing UNC and UNF fasteners of the same diameter in one product will cause your manufacturing department to put a curse upon your head, and upon your family unto the seventh generation).

Internal threads are machined into your part in one of two ways. The traditional method is to drill a pilot hole, and then use a tap to cut threads into the walls of the pilot hole. Common taps are tapered to help them engage the hole and cut the threads in several passes. Because of this, full threads don't go all the way to the bottom of a blind hole. A second tap, called a "bottoming tap," which has a faster taper, can be used to cut threads nearer to the bottom of a blind hole.

Figure 1 – Cutting tools, or “taps,” used to cut threads into the walls of a pilot hole.

Figure 1 – Cutting tools, or “taps,” used to cut threads into the walls of a pilot hole.

The second way to machine threads is to use a "thread mill." As with a tap, the first step is to cut a pilot hole. The thread mill has sideways teeth on the end that cut the thread profile on the inside of the hole. Unlike a tap, which can be used either by hand or in a machine tool, a thread mill requires a CNC mill to generate the helical profile to cut the threads.

Figure 2 – Thread mills can cut very near the bottom of a blind hole, and generally produce a better overall thread than a tap.

Figure 2 – Thread mills can cut very near the bottom of a blind hole, and generally produce a better overall thread than a tap.

Designers sometimes specify thread engagement percentage. This controls how large a pilot hole is cut. Since the outer diameter of the threads on the fastener is fixed, a larger pilot hole means the threads are not as deep and the threads of the fastener don't have as much to grab onto. The higher the thread engagement percentage, the more difficult it is to cut the threads. If not specified, thread engagement is commonly 75%.

CAD packages vary in how they help you design threaded fasteners into your part. A fully-featured CAD package like SolidWorks will have a hole wizard to help you pick thread pitch, size, a lead-in treatment, and so on. Lead-ins are features at the entrance of the hole such as chamfers that help guide the fastener into the hole or sink the head of a flat-screw. Not all CAD packages have hole wizards, and those that do represent holes internally in different ways in the CAD file. Converting a model to a standard file format such as IGES will strip the metadata describing the hole out of the model. Thus, there is no universal way to designate which holes in a model get what kind of thread.

Firstcut provides the ability to easily add threaded holes to your plastic, aluminium and brass parts. Firstcut supports UNC and UNF threads from #2 up to 1/2", and metric threads from M2 to M12. For a detailed table of thread options, see our Threaded Holes page. Because of the lack of standards between CAD packages for specifying threads, Firstcut decides where to put threads by looking for holes that could be pilot holes that are oriented suitably. Don't model internal threads; it will confuse Firstcut's pilot hole finder. The Firstcut process uses 3-axis milling, so holes can only be milled and threaded if they align with one of the three cardinal axes.

When you receive your FirstQuote® the interactive quote’s 3D display will show a graphic of your part and a list of threadable holes. Clicking on a listed hole in the table will prompt the display to zoom in on that hole and drop down a menu of threading options. You can also move from hole to hole using “previous” or “next” buttons. (See a FirstQuote part sample with threaded holes.) When finished making selections, click the “save current selections” button to complete the process. If you are unable to install the ProtoView 3D viewer, a 3D PDF is also available. The PDF is not interactive, but selections can be made in FirstQuote. (This approach is as effective as interactive ProtoView, but not as much fun.)

Once you have placed your order you will receive, by email, an order confirmation which includes your selected thread sizes. Be sure to review the confirmation for accuracy.

For detailed information on modelling threaded holes, see our Threaded Holes page. If you have additional questions or need assistance, contact Proto Labs at +44 (0) 1952 683047.