Threading Design in CNC Machining

Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.

Threads in your parts can sometime eliminate the final assembly process, in order to manufacture precise threads, we need to follow a lot of threading design rules. In addition, these features extra considerations and careful work to guarantee final production as designed. Successful threads can make assembly last for years.

External & Internal Threading

This is the basic threads types, but also important. The main difference is external threading on screw and bolt and external to hardware. While internal threading is inside the main parts and lock in screw and bolt threads.

Thread Pitch

Once we consider threads on bolts and screws, there is not one size fit for all kinds. Except metric threads, there are primary kind of imperial measure threads of Unified Thread Series, such as UNC (coarse pitch), UNF (fine pitch), UNEF (extra fine pitch). As you apply our system to assign a particular thread hole, the desired thread pitch will follow the ride.

Thread Position

Normally, a thread can be placed anywhere as your assembly requirement in turned or milled parts. As long as the area is accessible to our equipment. But if there are obstructions, our design analysis system will notice you which location is impossible for threading. In order to save your design time and cost, it is always a good idea to follow our design guidelines.

Even there are fewer restrictions of threads placement, the depth of internal threads are critical to consider. Once the thread depth exceeds the maximum tooling depth, the only way is drill through both side of holes and finish this process. There is an additional issue in this case, the threads will be not continuous form one end to other.

For turning parts, we provide three type of holes to accommodate internal threads:

On-axis: holes are straight through parts center from one end.

Axial: holes are through parts form one end without center area.

Radial;holes are through parts exterior act.

Internal Threads

We normally apply a single-lip threading tool to machine internal threads, not a traditional threading tap. You need to leave the pilot diameter on your CAD model and remove actual threads. We will analysis the hole for threading on follow situations:

  1. The hole diameters are in range of desired thread.
  2. The holes are on one of three cardinal axes for milling.
  3. The holes are perpendicular to axis of turning revolution.

Runsom can provide right-hand threaded holes on parts for UNC,UNF, and metric threads. Holes location and manufacturing methods will limit the eligibility of threads.

In internal machining process, the hole depth will be longer than our threading tools maximum length, in this case, we recommend some option for your special requirement.

  1. For a long through hole exceed the tooling maximum reach, choose the hole form the side where you anticipate screw to be start from. Once your screw need to pass the entire part, you should apply a tap through the hole to complete it as secondary process.
  2. You can also thread both side of the features, but notice the the maximum thread depth to avoid overlap with each other in the hole. There are concerns with threading form both side, in reason of cross-threading risk and screw cannot pass through the parts cleanly. Selecting threads form both side is typical fine, as long as avoid threads intersection.

There is an important consideration in threads creating for various diameters. Normally, three measurement we need to think about: major thread, minor thread, pilot hole diameter. Once you instruct your manufacturer to mill out a pilot hole with same diameter as the major thread, the hardware will never fit properly in the hole. It will drop in and the screw will spin around endlessly in the hole. There is nothing for its thread to grab. In this case, you need to chose larger hardware, but which may not be functional acceptable or consistent with your design. So you should make sure any threaded feature of pilot holes reflect the minor thread diameter, and most CAD programs will help you with this process.

External Threads

External thread on turned parts can extend the part length as long as parts are qualify for turning. We normally apply custom threading tools with selections of thread sizes, depths and placement on parts geometry. Our advanced turning process provides external threads on parts center-line.

Runsom offer external threading for on-axis, axial, and radial holes. Milling external threads is in two stages. The first set of threads go halfway on turned parts, then thread milling on the other hand. These two sides meet along parts center-line. In addition, we advise to remove excess materials and smooth out mismatches.

Small external threads are difficult to produce by ball or flat endmills because the pitch is too tight and larger radius will be left in the thread root. You need to use a thread cutting die for materials removing.

Inserts Alternation

Threading is a perfect way of strong bond in elements assembly for most metal parts. But for weak materials like plastics and aluminum, this band is not enough. This is where inserts come to apply. Incorporate special coil inserts in plastic parts will guarantee long application life. This allow your to get strong threads on weaker material. Basically, you can design a hole on desired location, and add inserts later. We will mill out the hole for your insert installation.

Runsom provides threading features for turned and milled parts in both plastic and metal. If you have any question on threading, feel free to contact our engineering team.