Cutting Large-Lead Helical Flutes on a Milling Machine

In our Machine Education pages, this article in the milling section covers cutting large-lead helical flutes. It also covers calculating the change gears to be fitted when cutting large-lead helical flutes.

Cutting Large-Lead Helical Flutes

To cut large-lead helices, the workpiece is mounted on the dividing head, and the dividing head’s worm is meshed with its worm wheel. Gear Z is fitted to the machine’s table screw shaft, and gear Z1 is fitted to the tailstock mandrel shaft with the plate.

When the table screw shaft is turned, motion is transmitted from gear Z through the intermediate gears in sequence to gear Z1, from gear Z1 to the bevel gears of the plate, and from there to the plate. From the plate, it passes through the spring pin and lever arm to the worm, then to the worm wheel, and finally to the workpiece fitted to the worm wheel’s shaft.

Calculating the Change Gears for Cutting Large-Lead Helices

If the number of teeth on the worm wheel is K = 40, then when Z = Z1, for every 40 turns of the table screw shaft, the workpiece makes one revolution. The distance traveled by the table when the table screw shaft turns 40 times is 40 . H1, which is the lead H = 40 . H1 of the helix curve on the workpiece.

If gear Z is set to twice the size of gear Z1 (Z = 2 Z1), then when gear Z makes one revolution, gear Z1 makes two revolutions. Thus, when gear Z turns 40/2 = 20 revolutions, the workpiece makes one revolution.

Example: Calculating Change Gears for Cutting Large-Lead Helices

A helical flute with a helix angle of 40° is to be cut on a cylinder with a diameter of 220 mm. Calculate the helix lead and the gears to be fitted. The table screw pitch is 8 mm.

Solution

If the helix is thought of as formed by a right triangle, the angle between the hypotenuse (helix) and the base, or the angle opposite the helix lead, is called the helix angle.

According to the figure above

we get tan α = H / π . D. From this: H = π . D . tan α = 3.14 . 220 . tan 40°
H = 690.8 . 0.8391
H = 580 mm.

Z (the driver gear. Fitted to the table screw shaft)
Z1 (the driven gear. Fitted to the tailstock mandrel.)

Intermediate gears are used to mesh these gears together.

Swiveling the Table When Cutting Large-Lead Helices:

When the helix setting angle is less than 45°, the milling cutter is mounted on the arbor or the universal head. As with small-lead helices, the table is swiveled by pushing with the left hand for a right-hand helix and with the right hand for a left-hand helix.

However, when the helix setting angle is greater than 45°, since the table cannot be swiveled more than 45°, the milling cutter is instead mounted on the universal head. In this case, the head is swiveled by the helix setting angle. Unlike with small-lead helices, the head is swiveled in the opposite direction — pushed with the right hand for right-hand helices and with the left hand for left-hand helices.

NOTE: The purpose of these rotations is to align the milling cutter with the direction of the helix.

When the table swivel angle exceeds 35°-40°, on small-diameter workpieces the tailstock and dividing head chuck can collide with the arbor or the universal head. In such cases, it becomes impossible to do the work. In these situations, the milling cutter is mounted on the universal head. Cutting is then done from the side of the workpiece rather than from above. In this operation, the milling cutter’s axis is set relative to the workpiece’s horizontal axis.