Craftsman Cribsheet No. 98: Thread Milling

This cribsheet explains some reasons why to consider thread milling using three-axis CNC vertical or horizontal machining center. The process uses helical interpolation, so the machining center controller must have the ability to control all three axes simultaneously. 

Some Advantages

• The cutter runs at the optimum speed for the material.
• The spindle runs at a constant speed with no reversal necessary. 
• The thread size can be adjusted in tenths by offsetting the path of the cutter. 
• A thread can be produced to full depth ½ pitch from the bottom of a blind hole in production. 
• The chips produced by the cutter are very small, easily washed away with coolant. 
• The process is very fast, usually faster than tapping. 
• The same cutter can be used to produce many different thread sizes. Example: 3/4"-16, 7/8"-16, 13/16"-16, 1"-16; with the same tool. 
• A small machine can produce a very large thread. Example: 4"-16. This thread is too large to tap in most machining centers. 
• The cutter can be made to control the major, pitch and minor diameters. 
• The process works on internal or external threads. 
• The thread start can be controlled. 
• Cutters can be resharpened. 
• A right- or left-hand thread can be done with the same cutter. 
• Some threads can be run dry.
   

Two Primary Disadvantages are Costs and Maximum Depth on Internal Threads.

• Cutter cost can be an issue. The cost of the tool itself runs into the hundreds of dollars (It costs about $200 for a thread mill to produce a 3/8"-24 internal thread. A tapered thread mill to produce a 1/2"-14 NPTF thread costs about $390.)
• The maximum effective thread depth that can be milled can be an issue. The maximum practical depth is about 2 diameters, because the cutter geometry must be smaller than the minor diameter of the internal thread. 

External threads can often be milled using an insert-type cutter. While the cutter body runs into the hundreds of dollars and inserts can be in the $40-50 range, in the long run the titanium nitride-coated carbide inserts cost less, while operating at faster speeds and longer tool life.