Polymeric insert with crush-formed threads for mating with threaded surface

A threaded connection between adjoining male/female surfaces is produced by mating a plastic insert embedded in one surface with threads machined on the other surface. The plastic insert is pressed against the machined threads and deformed to provide conforming meshing threads in the insert, thereby producing a threaded engagement between the two surfaces. Preferably, the insert is sufficiently wide to overlap multiple machined threads and yield a stable, strip-resistant engagement. By providing at least three such inserts evenly distributed around the preexisting threads, an axially stable, screwable threaded connection is produced between the two surfaces that does not require precise machining or special tooling for unconventional threads. Although the inserts are preferably used in the female surface for ease of access, they can be used advantageously in equivalent fashion in the male surface of the screwable threaded connection.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is related in general to the field of optical devices and 
threaded connections between concentric structural sleeves in microscope 
objectives. In particular, the invention concerns a method and apparatus 
for creating a threaded connection between two adjoining surfaces. 
2. Description of the Related Art 
Optical devices utilize microscope objectives for focusing a beam of light 
on a sample surface. The objective includes separate optical components 
housed in fixed and movable portions of its structure such that the focal 
length of the device can be adjusted to focus the light as desired. 
Typically, the fixed and movable portions of the objective consist of 
concentric sleeves coupled by very fine threads adapted to provide very 
gradual translation of the movable portion with respect to the fixed 
portion along the optical axis of the microscope. The pitch, geometry and 
density of the threads are selected according to the degree of precision 
required for the particular application, but are always very fine and 
precisely machined in order to achieve the accuracy required for optical 
applications. 
As illustrated in the interferometric objective 10 shown in FIG. 1, a 
typical configuration involves a male, inner sleeve 12 threaded on an 
outside surface 14 and a mating female, outer sleeve 16 correspondingly 
threaded on an inside surface 18. The mating threads 20 must be machined 
precisely on both surfaces within very strict tolerances to ensure a 
smooth motion of the outer sleeve 16 along the device's optical axis A1 as 
the movable sleeve 16 is rotated with respect to the fixed sleeve 12 (or 
vice versa). In addition, because different threads may be optimal for 
different applications, non-standard pitch, pitch diameter, root diameter, 
outside diameter, and thread section are normally used at great production 
costs. Moreover, it is very difficult to fabricate such nearly-perfectly 
matching, extremely fine male and female threaded connections for optical 
devices. 
Therefore, there still is a need for a versatile and economical kind of 
threaded connection that is suitable for custom applications such as 
typically encountered in optical devices. This invention is directed at 
providing a solution to this problem. 
BRIEF SUMMARY OF THE INVENTION 
One primary object of this invention is a method of fabrication of a 
threaded connection that facilitates the problem of matching and ensuring 
nearly perfect mating of very fine male and female threads. 
Another object of the invention is a method of fabrication of universal 
application, such that it can be utilized for producing threaded 
connections of any kind of pitch, pitch diameter, root diameter, outside 
diameter, and thread section. 
Another object of the invention is a method and apparatus that are 
particularly suitable for implementation in conventional interferometric 
objectives. 
Still another goal is a method and apparatus that can be directly 
incorporation within existing instruments. 
Another important goal is a structural configuration of the invention that 
can be implemented easily and economically according to the above stated 
criteria. 
Therefore, according to these and other objects, the present invention 
consists of producing a threaded connection between adjoining male/female 
surfaces by mating a plastic insert embedded in one surface with threads 
machined on the other surface. The plastic insert is pressed against the 
machined threads and deformed to provide conforming meshing threads in the 
insert, thereby producing a threaded engagement between the two surfaces. 
Preferably, the insert is sufficiently wide to overlap multiple machined 
threads and yield a stable, strip-resistant engagement. By providing at 
least three such inserts evenly distributed around the preexisting 
threads, a screwable threaded connection is produced between the two 
surfaces that does not require precise machining or special tooling for 
unconventional threads. Although the inserts are preferably used in the 
female surface for ease of access, they can be used advantageously in 
equivalent fashion in the male surface of the screwable threaded 
connection. 
Various other purposes and advantages of the invention will become clear 
from its description in the specification that follows and from the novel 
features particularly pointed out in the appended claims. Therefore, to 
the accomplishment of the objectives described above, this invention 
consists of the features hereinafter illustrated in the drawings, fully 
described in the detailed description of the preferred embodiment and 
particularly pointed out in the claims. However, such drawings and 
description disclose but one of the various ways in which the invention 
may be practiced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The heart of the present invention lies in the concept of producing 
perfectly matching male/female threaded connections by first machining one 
and then forming the other by compressing a deformable structure against 
it, thereby avoiding the problem of producing mating threads 
independently. Based on this principle, the invention is described in 
terms of a typical threaded connection between an inner cylindrical male 
sleeve and an outer cylindrical female sleeve, such as members 12 and 16 
in FIG. 1. 
Referring to the drawings, wherein like parts are designated throughout 
with like numerals and symbols, FIG. 2 illustrates in schematic 
partially-sectioned view a threaded connection between two adjoining 
surfaces according to the present invention. The outer surface 14 of the 
male sleeve 12 includes threads 20 machined or otherwise formed thereon by 
conventional fabrication processes. The terms "machined" and "formed", as 
used herein with respect to preexisting threads 20, are intended to refer 
to preexisting threads produced on a surface by any known process, such as 
casting, molding, pressing, cutting, etching, etcetera. According to one 
aspect of the invention, the inner surface 18 of the female sleeve 16 is 
not threaded, thereby simplifying its process of manufacture. Rather, the 
diameter of the cylindrical cavity defined by the surface 18 is sized so 
that the gap T between the crest 22 of the threads 20 and the surface 18 
(the annular clearance between the two) is very small when the two sleeves 
12 and 16 are disposed concentrically. Ideally, the crest-to-crest 
diameter of the sleeve 12 should be only sufficiently smaller than the 
diameter of the surface 18 to permit its play-free but easy coupling. In 
practice, for example, a maximum tolerance T of 2-6 thousands of an inch 
for surfaces 14,18 having a diameter of about 1.0-1.5 inches is desirable 
to account for imperfections in the adjoining structures. 
According to another aspect of the invention better seen in FIG. 3, the 
female sleeve 16 is provided with substantially-cylindrical radial 
openings or bores 24 (only one is shown in the figure) adapted to receive 
a plastic insert 26 and a set screw 28. A conventional screwable 
connection between female threads 30 in the surface of the opening 24 and 
mating male threads 32 in the set screw 28 permits the adjustment of its 
position along the longitudinal axis A2 of the opening 24 (radially with 
respect to the sleeves 12 and 16). As illustrated in FIG. 4, the insert 26 
consists of a ball or pellet adapted to fit snugly within the radial 
opening 24 of the female sleeve 16. The insert 26 includes a deformable 
inner surface 34, intended for engagement with the male threads 20 of the 
sleeve 12, and an outer surface 36, preferably also deformable, for 
engagement with the tip 38 of the set screw 28. The surface 34 must be 
sufficiently plastic to make it possible to mold it to conform to the 
threads 20 and form correspondingly mating threads 40 (seen in FIGS. 2 and 
5) when compressed by the set screw 28. At the same time, the material of 
the insert 26 must be sufficiently stiff and rigid to provide the axial 
support required for the application and to retain the threads so formed 
after the pressure from the set screw 28 is released to allow sliding 
between the two sets of threads. Additionally, the material must have as 
low a coefficient of friction as possible to ensure the smooth sliding of 
the female threads 40 with respect to the male threads 20 as one sleeve is 
rotated (screwed or unscrewed) to shift its axial position with respect to 
the other. I found that polymeric material such as polytetrafluoroethane 
(PTFE, also known commercially under the registered trademark 
TEFLON.RTM.), is very good and therefore preferred for optical instrument 
applications where the preexisting threads 20 are normally made of brass 
or aluminum material. 
Using the interferometric microscope objective 10 of FIG. 1 as a vehicle to 
describe the preferred embodiment of the invention, at least three evenly 
spaced openings 24 are drilled radially in the female sleeve 16 and 
threaded to receive conforming set screws 28. A Teflon.RTM. pellet 26 is 
inserted in each opening and pushed to contact the threads 20 in the male 
sleeve 12. Then a set screw 28 is introduced in each opening 24 and 
tightened to compress the insert 26 against the preexisting male threads 
20 and force the abutting surface 34 (FIG. 4) to conform to the threads' 
geometry, as seen in FIG. 2. Initially, the insert is crushed with the 
maximum force necessary to form mating threads 40 (FIG. 5) without 
damaging the structure of the newly formed threads, which is achieved by 
preloading the set screw 28 to a desired maximum thread-interference 
preload setting previously determined by empirical methods. For example, a 
crush preload setting of 4,300 pounds/square-inch was found to be optimal 
for Teflon.RTM. pellets or balls. Once the threads 40 are formed, the set 
screw 28 is partially backed off to a working load setting, such as 54 
pounds/square-inch for Teflon.RTM., to release the pressure and facilitate 
the sliding interaction between the threads 20 and 40. The resulting 
structure is a threaded connection between male and female sleeves 12,16 
with perfectly mating threads irrespective of their geometry, density and 
fine pitch. 
As would be obvious to those skilled in the art, the concept of the 
invention can be implemented in infinite equivalent configurations of 
thread and insert locations. For example, the inserts could be located on 
a single plane normal to the longitudinal (cylindrical) axis of the 
sleeves 12,16, which is preferred for optical applications that require a 
balanced support between the inner and outer sleeves. Alternatively, 
multiple inserts could be used distributed over different planes 
perpendicular to the cylindrical axis of the structure. Two inserts placed 
on diametrically opposite sides of the sleeves suffice to provide a 
balanced threaded connection between the two sleeves, but in practice they 
were found not to be sufficiently stiff and strong for many applications, 
is such cases allowing an axial misalignment between the two structures. I 
found that at least three equally spaced inserts (120 degrees apart), 
preferably on the same plane, provide adequate support for most optical 
microscope applications. Preferably, four inserts 90 degrees apart are 
utilized for additional axial support and stability, as illustrated in the 
assembly drawing of FIG. 6 (note that only two opposite inserts are seen 
in the figure, the other two being located at 90 degrees therefrom in both 
directions). 
The invention has been described for convenience illustrating an 
interferometric objective in vertical position, which is a common 
arrangement for interferometric devices, but is obviously applicable in 
equivalent fashion to any threaded connection between coaxial cylindrical 
structures. Similarly, the invention is illustrated with preexisting male 
threads 20 and plastic inserts 26 utilized to form mating female threads 
40, but the invention can be practiced as well with an opposite 
arrangement. Preexisting threads could be machined in the inner surface 18 
of the female sleeve 16 and multiple inserts 26 could be mounted in 
openings 24 drilled radially in the male portion 12 which, of course, 
would be constructed without threads. This embodiment is not preferred 
because of the difficulty in reaching and adjusting set screws 28 from the 
inside out, but in principle it is equivalent to the embodiment 
illustrated in this disclosure. 
It is also noted that the concept of the invention is equally applicable to 
produce a threaded connection between two surfaces that are not 
concentric. For example, the insert of the invention could be used to form 
a threaded surface in a fixed structure of any shape for mating with a 
pre-threaded rotating gear. Moreover, it is obvious that the choice of 
materials must be such that it is possible to deform the plastic insert by 
compressing it against the preexisting threads without damaging them. 
Thus, for example, while Teflon.RTM. inserts have been found to work well 
with aluminum, brass and steel threads, a suitably softer material would 
have to be used with preexisting threads made of synthetic polymer or 
other relatively pliable material. 
As also well understood by one skilled in the art, the abutting, 
thread-forming surface 34 of the insert 26 must be sufficiently wide to 
overlap multiple machined threads 20 and produce a stable, strip-resistant 
engagement. For optical applications using aluminum/Teflon.RTM. or 
brass/Teflon.RTM. thread connections between sleeves with adjoining 
surfaces about 1.0 to 1.5 inches in diameter, about 10 overlapping threads 
were found to be sufficient. 
Therefore, various changes in the details, steps and components that have 
been described may be made by those skilled in the art within the 
principles and scope of the invention herein illustrated and defined in 
the appended claims. While the present invention has been shown and 
described herein in what is believed to be the most practical and 
preferred embodiments, it is recognized that departures can be made 
therefrom within the scope of the invention, which is not to be limited to 
the details disclosed herein but is to be accorded the full scope of the 
claims so as to embrace any and all equivalent processes and products.