Designing for Accurate Threads

Adding threaded features to machined parts can also be a challenge. Internal threads can be created with a tap, a cutting tool that closely resembles the bolt or fastener that will at some point be screwed into the workpiece, or a special cutter known as a thread mill. Either way, deep threads are difficult for the same reason that deep holes are difficult, namely chip evacuation, and in the case of thread milling, radial cutting pressures. In most cases, a thread depth of twice the diameter provides plenty of strength, and costs less to produce. If this is insufficient, consider using inserts such as Heli-Coils or key inserts to bolster thread integrity, especially on plastic CNC parts. Finally, thread tolerances are specified with an “H” limit, the most common being H2 or H3. The latter has a tighter tolerance, however, and is therefore a bit costlier to produce, so the best bet is to call out an H2 for all but critical applications.