Page 673 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 23 Machining Processes: Turning and Hole Making

combining the correct relative rotation and the longitudinal feed. Special tapping
machines are available with features for multiple tapping operations. Multiple-spin-
dle tapping heads are used extensively, particularly in the automotive industry,
where 30 to 40% of machining operations involve the tapping of holes. One system
for the automatic tapping of nuts is shown in Fig. 23.27b.
With proper lubrication, tap life may be as high as 10,000 holes. Tap life can
be determined with the same technique used to measure drill life. Taps usually are
made of high-speed steels (M1, MZ, M7, and M10). Productivity in tapping opera-
tions can be improved by high-speed tapping, with surface speeds as high as
100 m/min. Self-reversing tapping systems also have been improved significantly
and are now in use with modern computer-controlled machine tools. Operating
speeds can be as high as 5000 rpm, although actual cutting speeds in most applica-
tions are considerably lower. Cycle times typically are on the order of 1 to 2 seconds.
Some tapping systems now have capabilities for directing the cutting fluid to
the cutting zone through the spindle and a hole in the tap, which also helps flush the
chips out of the hole being tapped. Chipless tapping is a process of internal thread
rolling using a forming tap (Section 13.5).

CASE STUDY 23.l Bone Screw Retainer

A cervical spine implant is shown in Fig. 23.28a. In then captured. In order to ensure the retainer’s proper
the event that a patient requires cervical bone fusion at orientation in the plate, the thread of its shank must
one or more vertebral levels, this implant can act as an start in the same axial location as point S in Fig. 23.28b.
internal stabilizer by decreasing the amount of motion The manufacturing steps followed to produce
in the region and thereby help promote a successful this part are shown in Fig. 23.28b. First, a 12-mm.
fusion. The plate affixes to the anterior aspect of the diameter Ti-6Al-4V rod is placed in a CNC lathe and
spine, with bone screws that go through the plate and faced. Then the threaded area is turned to the diameter
into the bone. The undersurface of the plate has a very necessary to machine the threads. The thread is turned
rough surface that helps hold the plate in place while on the shank-but over a longer length than is
the bone screws are being inserted. ultimately required, because of difficulties in obtaining
One concern with this type of implant is the high-quality threads at the start of machining. The cap
possibility of the bone screws loosening with time then is turned to the required diameter and the 2.5 -mm
due to normal, repetitive loading from the patient. radius is machined on the underside of the head. The
In extreme cases, this can result in a screw backing out, part is removed, inspected, and placed in another CNC
with the head of the screw no longer flush with the lathe, where it is faced to length. The spherical radius
plate-a condition that obviously is undesirable. This in the cap is machined, the center hole is drilled, and
implant uses a retainer to prevent the bone screw from the hex head is broached. The cap is removed and
backing out away from the plate. The part drawing for inspected. If the desired length has not been achieved,
the retainer is shown in the left half of Fig. 23.28b. the cap is lapped (Section 26.7) to the final dimension.
The retainer has a number of design features that At this point, the retainer is placed in a CNC
are essential for it to function correctly and without milling machine using a specially designed fixture
complicating the surgical procedure. To ease its use in that consists basically of a threaded, tapered hole. By
surgery, the plate is provided with the retainers already carefully applying a predetermined torque on the
in place, with the circular notches aligned with the bone retainer when placing it into the fixture, the starting
screw holes. This arrangement allows the surgeon to location of the threads can be controlled accurately.
insert the bone screws without interference from the Once the cap is located in the fixture, the three circular
retainer. Once the screws are inserted, the surgeon turns notches are machined as per the drawing. The retainer
the retainer a few degrees so that each screw head is is then deburred and tumbled to remove all sharp