Lapping internal surfaces

In improved equipment for lapping internal surfaces, use is made of a composite lap, the rod of which is held by the tool spindle and inserted with a marginal degree of clearance into a cone that carries the abrasive shell, and a chucking mechanism located beneath and coaxial with the spindle and lap; errors are avoided, particularly with smallbore work, by embodying the lap rod in three rigidly associated sections, namely, a polygonal tip, insertable and fastened with a transverse pin in a matching polygonal socket offered by the cone, a flexible coupling section, and a shank uppermost that is screwed into or otherwise clamped to the spindle. Work to be lapped is held in a chuck suspended from at least three cables threaded through relative peripheral mountings afforded by the chuck itself.

BACKGROUND OF THE INVENTION 
The invention relates to improved equipment for lapping internal surfaces. 
Conventionally, use is made of lapping machines, or lapping equipment, to 
effect polishing and truing operations that call for a high quality 
precision finish, for example, such as presented by the bores of parts 
manufactured to prescribed fit tolerances; a typical case in point is that 
of the barrels of hydraulic and pneumatic cylinders. 
The operation effected with lapping equipment is a finishing process; work 
to be lapped may be either heat-treated or untreated, heat treatment being 
adopted where the work is to be guaranteed a given degree of surface 
hardness. An initial machining or grinding pass may be made, whereupon the 
lapping operation will constitute the final step. 
Considerable accuracy is obtainable with lapping machines, down even to 
fractions of a micron, and it is therefore essential to avoid any 
contingency that may lead to dimensional inaccuracies, such as errors in 
shape. 
One such contingency, when operating with a machine designed for lapping 
internal surfaces, is created at the moment when the lap is introduced 
into the bore to be finished. 
The main difference between this type of machine and machines for external 
lapping, which can also finish plane surfaces, is that it is structured 
principally for finishing cylindrical surfaces of circular cross section, 
and will normally operate in the vertical axis with the lap entering the 
work from above. 
In order to avoid the occurrence of errors, that is, unintentional widening 
of the bore occasioned on introduction of the lap, as mentioned above, the 
work is chucked in a floating fixture, and the lap held in such a way as 
to allow a barely perceptible transverse oscillation. 
This carefully calculated freedom of movement is invisible to the naked eye 
(though verifiable by measuring the work, given the tolerances that are 
generally prescribed), but sufficient to enable the bore and the lap to 
adapt gently to one another as the lap enters the work and the pass 
commences. Notwithstanding the close tolerances characteristic of lapping 
machine construction, it will rarely occur that one has the necessary 
faultless coaxial alignment when the lap is inserted into the bore of the 
work; moreover, the lack of any self-alignment facility results in a 
conical or `banana` defect that becomes the more pronounced as the 
departure from coaxial alignment becomes greater. 
Given that internal lapping generates a profile referred to the lapped bore 
itself, rather than to any external datum (e.g. perpendicular or 
concentric alignment with other surfaces etc.), care must be taken to 
achieve coaxial alignment between the lap and the bore of the work without 
in any way forcing either component. 
Currently, self-alignment is achieved by supporting the floating chuck with 
a system of slides set at right angles to one another, in such a way that 
the work can move freely in the appropriate direction, and using a lapping 
tool of composite type design. In one such composite embodiment, the lap 
comprises: a support rod, the top end of which is associated with a 
floating spindle; a cone, ensheathing the rod and allowed a certain 
freedom of transverse movement relative thereto; and an abrasive diamond 
shell that is fitted rigidly over the cone. 
The freedom of the cone to shift in relation to the rod is obtained in this 
instance by leaving a small clearance between the two parts and supporting 
the cone on two moderately loose pins; these are fitted one at either 
side, at dissimilar distances from the end of the rod, and thus 
interconnect the two parts. Accordingly, the cone and the rod are 
prevented from separating, and a marginal float is permitted, the extent 
of which being dependent upon the clearance allowed. 
The lap and the chucking fixture thus described are suitable for bores of a 
given diameter, but tend to create problems when lapping notably small 
bores. More exactly, with a small diameter lapping tool of the composite 
type in question, the walls of the cone and the diamond shell are 
necessarily thin, so that if an obstacle should be encountered, or should 
the tool's speed of rotation drop for any reason, the rod immediately 
becomes subject to increased torque. This higher torque is transmitted 
through the rod to the cone and shell by way of the two pins, with 
stresses concentrated particularly on the opposite ends of the pins. 
However, the cone and the abrasive shell will often not be thick enough to 
sustain the thrust with which they are invested by the ends of the pins, 
and are forced outwards and distorted as a result. 
In such a situation, bulges are produced in the outer surface of the 
diamond shell which, though not large, are sufficient to enlarge the cross 
section of the abrasive surface and its action at a given point, with the 
result that the lapped surface will be inaccurate; it can also happen that 
the lap will bind against the bore of the work and break. 
When lapping a small-bore work, and/or where especially great precision is 
required, problems can also arise with movement of the chucking fixture; 
more exactly, the chuck unit is not replaced with a lighter one to suit 
the smaller work, and it happens that the weight of the smaller lap 
required for the small bore is often insufficient to shift the chuck and 
the work, in order to bring them into coaxial alignment. 
Moreover, it is not always effectively possible to replace the chuck with 
another of different size to suit heavier or lighter work. Accordingly, 
the object of the invention is one of overcoming the drawbacks mentioned 
above, both from the standpoint of the lapping tool and from that of the 
work chucking fixture. 
The stated object is achieved with lapping equipment as disclosed and 
claimed herein. 
SUMMARY OF THE INVENTION 
Improved equipment according to the invention is of the type utilizing a 
chuck to hold the work, and a composite lapping tool that consists in a 
vertical rod ensheathed by a marginally loose fitting cone supporting a 
diamond shell. 
The composite lap disclosed features a rod embodied in three rigidly 
associated sections: a bottom section of polygonal transverse profile 
engaging to an exact fit in a corresponding axial socket offered by the 
cone and locked in position axially be means of a diametral pin; a middle 
section, possessing a degree of flexibility commensurate with the 
tolerances prescribed for the work, whilst ensuring sufficient torsional 
rigidity to function as a flexible coupling; and a top section connected 
to the spindle. 
The improved work chucking fixture according to the invention is suspended 
from at least three cables which pass through respective peripheral 
mountings integral with the chuck; thus, chuck and work can shift bodily 
in any given plane, and are provided with a cushioning system at one and 
the same time. 
One of the advantages of the invention is that of its adaptability to 
different sizes of bore to be lapped, and to varying sizes of workpiece, 
inasmuch as torque is transmitted from the rod to the cone through 
polygonal mating surfaces; accordingly, the area through which these two 
parts make contact is generous, and distributed uniformly about the axis 
of the abrasive shell, however small the overall diameter. Also, the 
mobility of the chuck can be adjusted by tautening or slackening the 
cables to suit the weight of the work, and offset mechanical resistance, 
if any, generated by the lapping action. Moreover, the three-stage 
embodiment of the lap rod is instrumental in obtaining a more accurate 
seating of the lap internally of the bore to be finished, thanks to the 
combined elasticity and resilience of the middle section and its ability 
to function as a flexible coupling. 
Another advantage of the invention is that it is simple in construction, 
and therefore advantageous from the cost standpoint; in effect, the only 
parts requiring any significant degree of precision are the polygonal 
bottom section of the lap rod and the matching socket of the cone. 
Yet another advantage of the invention is that the stresses transmitted 
through the lap rod to the cone and shell can be reduced practically at 
will, since mating contact occurs across the full expanse of the polygonal 
profile offered by the bottom section of the rod, and accordingly, this 
same polygonal profile can be lengthened or shortened axially so as to 
reduce or increase mechanical pressure on the cone during the lapping 
process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The improved equipment disclosed, which is used for lapping internal 
surfaces, consists essentially in a vertically disposed composite lapping 
tool 12, fastened directly or indirectly in conventional fashion to an 
overhead spindle 11, and a chucking fixture 1 (FIGS. 4 and 5) mounted to 
the bed of the lapping machine (not illustrated). 
The composite lap 12, as illustrated in FIG. 1, comprises a rod 2, located 
uppermost and connected by its top end to the spindle 11, a cone 3, 
located below and carried by the rod 2, and an abrasive diamond shell 4 
that ensheaths the cone 3 to an exact fit. 
The cone 3 has an axial hole 13 with an open top end in which the bottom 
end of the rod 2 is freely inserted, the degree of clearance allowed being 
compatible with the tolerances prescribed for the lapping operation. 
The diamond shell 4, which provides the abrasive part proper to the 
composite lap 12, is fitted to and removable from the cone 3 by 
conventional means such as lock nuts 23 and 24, for example, that screw 
onto corresponding threads offered by the ends of the cone 3 and clamp the 
shell 4 from either end, as illustrated in FIG. 1. 
According to the invention, the lap rod 2 comprises three sections, 7, 8 
and 9, which are associated rigidly and coaxially one with the next. The 
bottom section or tip, denoted 7, exhibits a polygonal, preferably 
hexagonal transverse profile (discernable in FIG. 2), and has a 
diametrically disposed hole 25 serving to accommodate a pin. The polygonal 
tip 7 is insertable to an exact fit in a matching polygonal socket 13a 
afforded by the axial hole 13 of the cone 3, which also presents a 
diametrically disposed hole 26 positioned to coincide coaxially with the 
hole 25 of the tip 7. 
Numeral 10 denotes the aforementioned pin, which inserts to an exact fit in 
the holes 25 and 26 offered by the hex tip 7 and socket 13a, thereby 
furnishing the means by which the rod and cone parts 2 and 3 of the 
composite tool 12 are connected, and constituting the sole means by which 
the cone 3 is supported. 
The length of the hex tip 7 will depend first and foremost on the outer 
diameter of the abrasive shell 4: the smaller the diameter of the shell 4, 
the greater the length of the hex tip 7. Given, in fact, that torque 
generated by the spindle motor is transmitted from the rod 2 to the cone 3 
by way of the hex tip 7, the pressure with which the cone 3 is invested by 
the tip 7 will be proportionally less (transverse dimensions remaining 
equal) the greater the lateral surface area of the tip 7 that enters into 
contact with the cone 3, i.e. the longer the axial dimensions of the tip 
7. 
More exactly, the hex tip 7 coincides with the center of balance of the 
composite tool, so as to avoid overloading the bottom end and ensure 
better balance overall. 
The middle section of the rod 2, denoted 8 and referred to hereinafter as a 
coupling (the reasons for which will shortly become clear), is formed of 
material possessing a given degree of flexibility that will be 
commensurate with tolerances prescribed for the lapped work, though 
sufficient at all events to ensure good torsional rigidity. 
In the embodiment illustrated, the coupling 8 consists in a longitudinal 
element embodied as a tightly coiled helicoid, i.e. with no gaps between 
the turns of the helix, generated in the opposite direction to that in 
which the lap 12 turns with the spindle 11; the coupling 8 is accommodated 
freely by the axial hole 13 of the cone 3, the actual degree of clearance 
being compatible with the prescribed lapping tolerances. 
The top section, or shank 9, exhibits a stretch of diameter substantially 
identical to that of the coupling 8, an intermediate boss 27 of wider 
transverse dimensions, and a screw thread 28 at the very top end. The boss 
27 affords two flat parallel faces that can be engaged by a key or wrench 
to the end of tightening the screw thread 28 in the female thread of a 
holder, either integral with or attached to the spindle 11. 
The method of assembling an improved lapping tool 12 according to the 
invention is substantially the same as for a conventional composite tool, 
and therefore will not be described. 
As regards operation of he composite lap 12, it will be seen that the 
abrasive shell 4 is capable of movement in relation to the rod 2 by virtue 
both of the clearance existing between rod and cone 3, and of the elastic 
properties of the coupling 8, which are such as to make the rod 
advantageously flexible while ensuring that it retains a sufficient degree 
of torsional rigidity. 
The inclusion of the coupling 8 provides another marked advantage, namely, 
the capacity to function as a safety device in the event of some 
unforeseen obstacle slowing down or jamming the shell 4. More exactly, the 
coupling 8 can be proportioned in the manner of a shear pin, i.e. capable 
of transmitting torque only up to a given maximum value, in excess of 
which it will break, preventing damage either to the work or to other 
components of the equipment. 
Turning now to the chucking fixture 1 which, for example, might be a 
conventional type self-aligning chucking mechanism, this is suspended from 
at least three longitudinal elements, embodied as cables 5 and 6, anchored 
to fixed supports provided by the side walls of a vertically disposed 
hollow frame 16 (see FIG. 4) that is made fast to the machine bed in 
conventional manner. In the embodiment illustrated, the cables 5 and 6 are 
arranged in two pairs, set at different heights in order to avoid mutual 
contact; it will be seen, in fact, that the two cables of each pair are 
parallel and disposed at right angles to those of the remaining pair. 
The cables 5 and 6 associate with the chuck 1 by way of four peripheral 
hollow mountings 14 fitted in pairs and occupying positions that are 
diametrically opposed in relation to the vertical axis of the chuck 1. 
It will be observed from FIG. 4 that the diameter of the passage 15 through 
each mounting 14 decreases from the ends toward the middle, creating what 
is a substantially biconical, hourglass type profile the waisted portion 
of which is rounded so as to avoid any excessive build-up of tension at 
that point. The minimum diameter of the passage 15 substantially matches 
the diameter of the respective cable 5 or 6, in such a way as to prevent 
any unwanted sideways movement of the chuck 1 in relation to the cables 5 
and 6. 
The two ends of each cable 5 and 6 pass through the side walls of the frame 
16, and are prevented from working free by relative fixed and removable 
locking elements 17 located externally of the frame. In the embodiment 
illustrated in FIGS. 4 and 5, each cable 5 and 6 is secured by one fixed 
element 17, and one removable element, or clamp. 
Numeral 18 denotes a device encircling each cable 5 and 6 at one end, 
located between the relative clamp 17 and the frame 16, which serves to 
adjust the tension of the cable with which it is associated. Each such 
device 18 consists in an externally threaded hollow member 19 associated 
rigidly with the cable 5 or 6, an adjuster nut 20 screwed onto the 
threaded member 19, and a set of belleville disc springs 21 through which 
the threaded member 19 is inserted. Where the device is assembled such 
that the springs 21 impinge on the clamp 17, a distance collar 22 may also 
be added, as illustrated in FIGS. 4 and 5. 
By moving the nut 20 one way or the other along the threaded member 19, the 
relative cable 5 or 6 can be tensioned or slackened to suit the size of 
the work, and more important, to suit its weight. The cables 5 and 6 will 
need to be well tightened for large and bulky items, so as to avoid too 
loose a suspension, whereas a slacker setting is needed for lighter work 
in order to permit a reasonable freedom of movement as the lap 12 enters 
the bore. At all times, the cables 5 and 6 are always sure of a certain 
degree of elasticity thanks to the action of the springs 21, which will 
rarely be compressed to the limit. 
It will be clear then, from FIGS. 4 and 5, that the chuck 1 is capable of 
movement in practically any direction; the arrangement of the cables 5 and 
6 permits a marginal degree of rotation not only about the vertical axis, 
but about any given horizontal or inclined axis too. 
The embodiment described and illustrated is by no means limitative; for 
example, the middle section 8 of the lap rod 2 might be fashioned solidly 
in a material with the appropriate flexible and torsional properties, or a 
small universal joint could even be incorporated.