Cutting Short-Lead Helical Flutes

The topic of this article, in our Machine Education Milling category, will be cutting short-lead helical flutes on a universal milling machine. We’ll explain how to cut short-lead helical flutes and provide the required calculations.

Cutting Short-Lead Helical Flutes

The dividing head’s worm is disengaged from the worm wheel and the workpiece is connected to the dividing head. Gear Z is mounted on the table’s lead screw, and gear Z1 is mounted on the dividing head’s tailstock spindle; turning the table’s lead screw feeds the workpiece on the table. The Z and Z1 gears also give it rotational motion about its axis. The distance the workpiece travels in one revolution equals the lead of the helix.

The meanings of the letters in the figure for the process of cutting short-lead helical flutes:

H = Lead of the helix.
H1 = Lead of the table lead screw.
Z = driving gear (mounted on the table lead screw).
Z1 = driven gear (mounted on the dividing head’s tailstock spindle).

Finding Gears Z and Z1 to Cut Short-Lead Helical Flutes

 

Example: Finding Gears Z and Z1 (Driving and Driven)

On a milling machine with a table lead screw pitch of 8 mm, a helical flute with a lead of 12 mm is to be cut. What should the gears be?

Aligning the Direction of the Helical Flute Parallel to the Milling Cutter’s Sides:

— When the helix setting angle is less than 45°, the milling cutter is mounted on the arbor or the universal head. With the operator facing the machine, the table should be swiveled by pushing with the left hand for right-hand helices and with the right hand for left-hand helices.
— The direction of the helical flute can also be aligned with the direction of the milling cutter by swiveling the universal head by the same angle.

Direction of Rotation of the Workpiece When Cutting Short-Lead Helical Flutes:

As the machinist pulls the table toward themselves, the direction of rotation of the dividing head should be counterclockwise for a right-hand helix, and clockwise for a left-hand helix.
Whether any given helix is right- or left-handed is determined as follows: if the helix winds in the clockwise direction, it’s called a Right-Hand Helix; otherwise it’s a Left-Hand Helix.

Thread Milling

On milling machines, any thread for which a milling cutter matching the thread profile is available can be cut in series (mass production).
Since lathes often require re-grinding the tool bit and, as a result, carry a higher chance of error with each new setup, thread cutting on milling machines is much more precise and economical.

On coarse-lead threads with no incline angle on the thread surface, such as square threads, the profile comes out distorted because the top width of the helical flute becomes larger than it should be. In such cases, to reduce this error, a smaller-diameter milling cutter should be used, or an end mill should be used instead.
Short threads can be produced very quickly on milling machines. When cutting these, the workpiece is mounted on the dividing head, and a special milling cutter is mounted on the universal head. The milling cutter is comb-shaped, and the helix angle of the teeth is zero. In other words, the teeth of the comb are ring-shaped. The tooth backs are relieved eccentrically. When cutting the thread, once the workpiece completes one full revolution, the table has advanced by the amount of the thread’s lead, and the thread has been cut around the entire circumference.