Thread milling is a process for making internal threads that can be the difference between success and failure. In general, tapping is a simpler and often faster way to make threads, but if you have a problem with tap breakage or your major diameter is over 3/4", you owe it to yourself to learn about thread milling. Just be aware that some thread mills cut threads with slightly concave flanks. EMUGE-FRANKEN USA, West Boylston, Massachusetts, makes thread mills that cut 60-degree V threads with flat flanks. This is an important improvement because thread assemblies with concave thread flanks have a greater tendency to gall.
A thread mill has a smaller diameter than the inside diameter of the unthreaded hole, and with thread milling the major diameter of the thread can be adjusted with the milling machine’s cutter compensation.
A thread mill has V-shaped cutting edges arranged inside a circle on a plane perpendicular to the center line of the mill’s shank. (See image closeup inset.) I call this the “including circle,” though this is also known as the cutter diameter.
Cutting a thread with a thread mill is a three-axis operation. My example here is a hole with a center line that is parallel to the Z axis. The rotating tool is moved to the center line of the thread. Then an X or Y move positions the tool so its including circle is touching the major diameter of the hole to be threaded. Now the machine uses helical interpolation to cut the thread. At the bottom of the thread, the tool is moved back to the thread’s center line and retracted from the hole. The thread mill makes a lot of small chips so torque on the tool is a fraction of what it would be with a tap. And one huge plus, if the thread mill breaks, just take the junk out of the hole and start over. The only scrap is the broken tool. If the thread has a large pitch, it is a good idea to cut the thread with two or three progressively larger diameter passes.
Three styles of thread mills made by EMUGEFRANKEN(from left): the EMUGE THREADSALL (single-plane), the EMUGE MultiTHREAD (multi-plane) and the EMUGE THRILLER (threeplane). EMUGE
Thread mills are made in a variety of styles (see image). I’ve already explained the operation of single-plane tools above. With this type of thread mill, each tool can cut a variety of thread pitches and diameters. There are also multiplane tools where each plane, or cutting tooth, has the same cutting diameter. These tools will cut multiple pitches of the thread in one rotation of helical interpolation. Plus, there are tools with three planes that each have different cutting diameters. So, the first plane roughs, the next plane increases the thread diameter and the last plane finishes. Seems to me that such a thread mill would be great in really ornery material like nickel alloys.
Some EMUGE tools have a central port for coolant through the tool, and are made for through the spindle coolant flow. That is a good idea in a blind hole. My shop doesn’t have that capability. If you’re in a similar situation, here is a trick I have used with blind holes. Move the tool to the center of the hole, then to the bottom. Now, perform the X or Y move and cut the thread while retracting the tool from the hole. This way you won’t have trouble with chips packing in the hole under the tool, and you can thread to the bottom of the hole in one pass with one tool.
Thread mills can also be handy for making external threads. I once made some electrical connector bodies from aluminum. The male and female parts went together with a 1 1/2-20 thread. The male part was made from 2"-square stock, and I made it on a bed mill using a thread mill.
Next month I’ll go through the machining parameters and programming for thread milling.
Related Glossary Terms
- alloys
alloys
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
- coolant
coolant
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- cutter compensation
cutter compensation
Feature that allows the operator to compensate for tool diameter, length, deflection and radius during a programmed machining cycle.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- interpolation
interpolation
Process of generating a sufficient number of positioning commands for the servomotors driving the machine tool so the path of the tool closely approximates the ideal path. See CNC, computer numerical control; NC, numerical control.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- parallel
parallel
Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.
- pitch
pitch
1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.
- shank
shank
Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.
- tap
tap
Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.
- tapping
tapping
Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.
