"Method for machining an ""O"" ring retention groove into a curved surface"

A method for cutting an "O" ring retention groove around the cutting edge of a hydropiercing die button. A groove machining tool is given a convex curved end surface the radius of which is slightly less than or, at most, substantially equal to the tightest concave radius that the end surface will have to sweep through as the groove is cut. Consequently, an accurate, slightly curved groove bottom surface is created for the "O" ring, while the machining tool will not bind as it moves through the tightest concave portion of the cutting path.

TECHNICAL FIELD 
This invention relates to a groove machining method in general, and 
specifically to a method for machining an "O" ring retention groove into a 
curved seal surface. 
BACKGROUND OF THE INVENTION 
Hydroforming, a process in which single piece, generally cylindrical steel 
blanks are expanded within a die cavity under great internal pressure to 
produce non cylindrical frame rails and the like, is finding greater and 
greater production use. A recent development which has greatly increased 
the utility of the process is so called hydropiercing, in which holes and 
slots can be cut through the surfaces of the pressure formed part right in 
the die, so as to avoid the necessity of later hole cutting steps. An 
example of hydropiercing can be seen in co assigned U.S. Pat. No. 
5,0398,533 issued Mar. 21, 1995 to Shimanovski et al., where a flat 
surface on a hydroformed part is pierced by allowing the highly 
pressurized internal fluid to blow out through a sharp edged die button, 
removing a slug of metal as it escapes to leave behind a hole shaped like 
the die button edge. It is necessary that the perimeter of the cutting 
edge of the die button be surrounded by an "O" ring, which is inset into a 
retention groove. The "O" ring seal is firmly pressed into the part 
surface, surrounding the area to be cut through. The "O" ring acts as a 
face seal to prevent the escape of pressurized fluid as the hole is cut. 
Production of the die button itself, including the machining of the "O" 
ring retention groove, is a simple process when the part surface 
surrounding the hole to be cut is flat. In that case, the die button 
surface and groove are also correspondingly flat. When the hole is to be 
cut through a non flat, trough like surface, manufacture of the die button 
is more difficult. While it is relatively simple to machine the basic 
surface of the die button to match the part surface, there is no known way 
to easily machine the "O" ring retention groove down into that complex, 
non flat surface, especially where the groove must pass through concave 
curved transition areas or "valleys". The machining process is complicated 
by the fact that the ideal groove cross section should have undercut 
shoulders on each side so as to retain the round cross sectioned "O" ring 
in the groove with a "snap" fit around the sides of the "O" ring. 
One known U.S. Pat. 4,786,219 issued Nov. 22, 1988 to Oberlin et al., does 
disclose a method for machining a continuous groove into the outer surface 
of an elliptical tube. Such an exterior surface is everywhere convex, 
however, with no concave transition areas. A flat bottomed machining tool 
is disclosed, which is moved around the cutting path, and maintained at 
both a constant cutting depth relative to the surface and at substantially 
a perpendicular orientation relative to the surrounding surface. Those 
tool conditions would be both givens for any such machining process, of 
course. The primary focus of the patent is maintaining the tool at a 
constant cutting depth. However, the flat bottomed tool disclosed would 
simply not work if used in a curved surface like that disclosed in the 
subject invention, as it would interfere or bind drag when moved through 
the concave, sharply radiused transition portions of the cutting path. 
SUMMARY OF THE INVENTION 
The invention provides a method and tool which can successfully cut a 
circumferentially complete groove of the desired cross sectional shape 
into a complex, non flat seal surface that does have such concave 
transition areas. 
In the preferred embodiment disclosed, a desired retention groove cutting 
path is first established, which is a path that completely surrounds the 
perimeter of the die button's cutting edge. The smallest radius of any 
concave transition portion along that cutting path is determined, as well. 
The desired cross section for the groove is established, which has 
concave, undercut sides or shoulders that are spaced apart by less than 
the diameter of the round cross section of the annular "O" ring. 
Therefore, the "O" ring can be resiliently "snapped" into the groove and 
retained therein. The desired groove cross section also has a smooth 
bottom surface, against which the "O" ring will be held by the snap 
shoulders, and against which the undersurface of the "O" ring will be 
compressed when the upper surface is pressed against the outer surface of 
the part to be hydroformed. While the groove bottom surface need not be 
flat in cross section, and is not as disclosed, it should be smooth and 
continuous at all points, including the areas where it passes through one 
of the concave transitions of the cutting path. 
A rotatable machining tool is provided, which spins about its center axis, 
and which has a machining head at the end which is generally bulbous or 
knob shaped in appearance. The cross section of the machining head, taken 
in any plane through the tool rotation axis, is constant and convex. In 
general, the constant, convex cross section of the machining head is made 
to match and ultimately produce the concave cross section desired of the 
"O" ring retention groove. Specifically, the convex sides of the machining 
head's cross section match the retention shoulders desired for the "O" 
ring groove. Also, the machining head end surface, is convex and curved, 
with a radius of curvature that is deliberately made equal to or slightly 
less than the smallest concave radius of curvature in the cutting path. A 
plunge point at some convenient single point along the cutting path is 
established where the machining head can be both inserted into the cutting 
path to a cutting depth, and later withdrawn from the groove produced. 
The rotating tool is then inserted into the surface to the required depth, 
and moved completely around the cutting path, while always being 
maintained substantially perpendicular to the adjacent surface. As the 
machining head moves through the tightest concave transition portion in 
the cutting path, the deliberate radius limitation on the machining head 
end surface assures that the machining head will sweep through without 
binding or gouging at the bottom surface of the groove. Finally, the tool 
is withdrawn at the same, single point.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, a hydroformed part, in this case, a hollow steel 
beam of generally rectangular cross section, is indicated generally at 10. 
Beam 10 is distinctive in that at least one side thereof contains a 
lengthwise, trough like indentation at 12, through which a generally 
elliptical hole 14 is to be pierced. The outer surface of beam 10 
surrounding hole 14 transitions from flat to sloped across a pair of 
convex, parallel corner edges indicated at E. The edges E are not acutely 
pointed, of course, but have a fairly sharp radius of curvature, about 
0.390 inches as disclosed. This is a transition that occurs at four points 
while moving a full 360 degrees around hole 14. Of course, the beam 
surface is concave at the very bottom of the indentation 12, also. The 
complex, non flat shape of beam 10 surrounding hole 14 is significant to 
the apparatus that produces the hole 14, and to the method by which that 
apparatus is manufactured, which is the subject matter of the instant 
invention. 
Referring next to FIG. 2, a single hydroforming and piercing apparatus 
performs both the basic beam shape forming function and the piercing of 
hole 14. A pair of heavy, solid hydroforming dies 16 and 18 clamp around a 
generally cylindrical tube blank, not illustrated in its initial shape, 
which is sealed at the ends and highly internally pressurized to take on 
the final shape shown. The lower die 16 supports the three flat sides of 
beam 10, while the upper die 18 supports and forms the other side, 
including the indentation 12 where hole 14 is to be ultimately cut. Hole 
14 is formed as a final step, while the interior of beam 10 is still 
highly pressurized. A die button, indicated generally at 20, is mounted 
flush to the inner surface of upper die 18, and held close against the 
outer surface of beam 10. Die button 20 is generally hollow or sleeve 
shaped, formed with a sharp cutting edge 22, the perimeter of which 
matches the size and shape of the hole 14 to be cut. The interior of die 
button 20 includes a backing plunger 24, which is initially held solidly 
flush to the cutting edge 22, but which can be backed off once the basic 
shape of beam 10 has been formed. Initially, the flush backing plunger 24 
rigidly supports the beam wall material interior to the die button cutting 
edge 22, just as the inner surface of the upper die 18 itself would. When 
the plunger 24 is backed up, the beam wall material is no longer supported 
inside of the cutting edge 22, and the still highly pressurized fluid 
inside of beam 10 blows out a slug of material 26 through the cutting edge 
22, leaving behind the desired shape hole 14. The beam 10 is then 
depressurized, drained, slug 26 removed, and the forming process is 
complete. In order to prevent the loss of pressurized fluid past the 
cutting edge 22 as the slug 26 is blown through, an "O" ring type 
compressible face seal 28 is inset into the surface of die button 20, 
surrounding the perimeter of cutting edge 22. The "O" ring seal 28 is 
circular in cross section, and its upper surface is compressed and 
flattened slightly against the outer surface of beam 10, surrounding the 
hole 14. Other seals surrounding plunger 24 prevent the loss of fluid 
through the center of die button 20. It is critical that seal 28 be 
accurately inset into the surface of die button 20 in order to be 
continuously compressed against the surface of beam 10. The method by 
which seal 28 is retained to the die button 20 is described next. 
Referring next to FIGS. 3 and 5, the basic challenge involved in 
successfully and accurately retaining an "O" ring seal 28 surrounding the 
perimeter of the die button cutting edge 22 can be seen. The non flat 
surface of die button 20 surrounding the edge 22 matches, but is the 
converse of, the outer surface of beam 10, described in detail below. 
Therefore, the surface of die button 20 will be concave where the beam 
surface is convex, convex where it is concave, but flat where it is flat. 
Consequently, the surface of die button 20 must also make four sharply 
curved concave transitions from flat to sloped at the corresponding four 
convex points where it crosses the beam edges E. At two points, the 
perimeter surface would be convex, corresponding to the concave bottom of 
the indentation 12. The four concave transition areas are most relevant to 
the subject invention, and are difficult to distinguish visually, but 
their general location is noted at the four bracketed areas marked "C" in 
FIG. 3. FIG. 5 is a schematic view of a section of a 360 degree perimeter 
cutting path around cutting edge 22 flattened or "rolled out" to indicate 
one such concave portion and its radius of curvature, designated Rt. The 
smallest such concave radius of curvature Rt along the cutting path would 
match the transition areas C on beam 10, which in turn match the 
transition at the edges E on the beam surface, and represents a given, 
fixed condition in any given case. A retention groove, indicated generally 
at 30, must be cut into the surface of die button 20 to retain the "O" 
ring 28, along a 360 degree cutting path that runs through all of the same 
transition areas. The two convex areas of the cutting path present no 
difficulty, and could be cut by conventional groove cutting techniques as 
described above. At the four concave transition portions of the cutting 
path, however, known techniques would not work. 
Referring next to FIG. 4, other significant details of the cross sectional 
shape of retention groove 30 are illustrated in relationship to the 
circular cross section of the "O" ring 28 itself. In general, such a 
groove 30 would be as wide as the diameter of the circular cross section 
of "O" ring 28, but not significantly wider, and slightly less deep, so as 
to leave the upper surface exposed while holding the sides of "O" ring 28 
closely. As such, the circular cross section of the retained "O" ring 28 
will be flattened out slightly to a generally elliptical shape as it is 
compressed between the outer surface of beam 10 and the retention groove 
bottom surface 32. Groove 30, in order to seal successfully, must provide 
a bottom surface 32 suitable to compress smoothly and continuously against 
the flattened bottom surface of the "O" ring 28. A suitable groove bottom 
surface 32, therefore, will be either flat or have a slight radius of 
curvature that is greater than, but not less than, the radius of the cross 
section of "O" ring 28. In addition, it is preferable that the groove 30 
do more than simply provide a suitable seal compression bottom surface 32. 
Ideally, it would also serve to solidly retain ring 28 in place, and this 
is done here by a pair of retention shoulders 34, spaced apart by a width 
W somewhat less than the diameter of the cross section of "O" ring 28. The 
shoulders 34 act as a constriction in the groove 30 to hold the "O" ring 
28 down against the bottom surface 32 by a slight "snap" fit. Details of 
the tooling and method that produce groove 30 are described next. 
Referring next to FIGS. 6 through 9, groove 30 is ground or cut by a series 
of two tools, a roughing tool indicated generally at 36, which cuts the 
basic groove 30, and a finishing tool indicated generally at 38. Roughing 
tool 36 has a generally cylindrical machining head 40 that spins about its 
central axis, with a cross section that is constant as taken in any plane 
containing the center axis. Specifically, that cross section is defined by 
a constant width substantially equal to the groove least width W defined 
above and, most importantly, by a convex, curved end surface 42 having a 
radius of curvature Rg at most equal to, or just slightly less than, the 
least concave radius of curvature Rt of the cutting path concave curved 
portions C as defined above. The purpose for this relative radius 
limitation is described below. The finishing tool 38 has a machining head 
44 that is generally bulbous, also with a cross section that is identical 
in any plane through its center axis. That cross section, as best seen in 
FIG. 8, has the same curved end surface 46 with the same radius Rg as the 
roughing tool 36, and so has the same radius limitation relative to the 
least radius Rt of the cutting path concave curved portion C. The cross 
section differs from the roughing tool machining head 40 by having convex 
sides 48 that match the concave retention shoulders desired in the 
finished groove 30. The tools 36 and 38 operate as described below. 
Referring next to FIGS. 3 and 5, groove 30 is cut initially by first 
establishing a suitable cutting path as defined above, that is, an annular 
area (whether circular, elliptical or whatever shape) that runs at 
substantially a constant radius around the center point of the hole 14, 
for 360 degrees. Once established, the smallest radius of curvature of any 
concave transition portion along the path is determined, which was already 
noted above. Then, a single entry/exit or "plunge" point 50 is established 
on the cutting path, which is a single point where a tool can be both 
inserted and withdrawn from the cut. Conveniently, as seen in FIG. 3, that 
common point 50 is established at one of the two high, convex points along 
the path, which correspond to the low points of the beam indentation 12. 
Then, the roughing tool 36 is pushed into the surface of die button 20, at 
the point 50, and run completely around the cutting path as it spins about 
its center axis, shown by the dotted line. The center axis is maintained 
substantially perpendicular to the surface of die button 20 adjacent to 
the cutting path. FIG. 5 shows the orientation of the finishing tool 38 at 
a plurality of points. The roughing tool 36 is run through the same 
cutting path first. It establishes the basic width, depth and, most 
importantly, the basic bottom surface 32 of groove 30, all but the 
retention shoulders 34 (whose ultimate location is shown in dotted lines 
in FIG. 6). By limiting the curved machining head end surface 42 to the 
radius as defined above, the machining head 40 will sweep through the four 
concave curved portions C along the cutting path without binding or 
interference, producing a constant, smooth groove cross section. Stated 
differently, as the end surface 42 moves through a constricted area C, the 
axis of the tool 36 can effectively swing about the end surface 42 without 
catching in the curved area C, which is everywhere as large or slightly 
larger in radius than end surface 42. When the roughing tool 36 has been 
withdrawn from point 50, the finishing tool 38 is run around the same path 
in the rough cut groove, as shown in FIG. 5. Its end surface 46 maintains 
the same slightly curved groove bottom surface 32, finishing it a bit more 
finely, but holding the same shape. Its sides 48 cut the retention 
shoulders 34, and finishing groove 30. With both tools 36 and 38, the 
curved end surfaces 42 and 46 are designed not so much to match or produce 
a particular bottom surface 32 desired for the "O" ring groove 30, so much 
as they are designed to successfully cut a groove 30 without scuffing or 
binding as they move through the constricted areas C along the cutting 
path. The curved groove bottom surface 32 that is produced in that process 
also happens to be smooth, circumferentially continuous, and of a proper 
curvature (the same curvature as the end surfaces of the tools 36 and 38) 
to properly compress the undersurface of the circular cross sectioned "O" 
ring 28. The process can be conceptualized in reverse, however, as one of 
first establishing a groove cross section which, in addition to being the 
proper width and depth to receive and hold the circular cross section of 
the "O" ring 28, also has a slightly curved bottom surface 32 with the 
same "equal to or less than" radius limitation relative to the cutting 
path portion C. So conceptualized, the next step is to match the machining 
tool head cross section to that desired groove cross section, and then 
machine the groove in the same way. However conceptualized, the end result 
of the process is the same. Finally, "O" ring 28 can be snapped into the 
completed groove 30. 
Other beams to be formed could well have very different surfaces 
surrounding a hole to be pierced therethrough, but the basic technique 
disclosed for establishing the cutting path and for shaping and using the 
groove machining or cutting tools will work. Even if the surface through 
which the cutting path runs is everywhere convex, the basic process will 
still work, although it was developed to handle concave transition areas 
along the path, which were not amenable to existing techniques. It is also 
theoretically possible that just the single finishing tool 38 could be 
used to cut the final shape of groove 30 in one pass. However, the tool 
might have to be moved more slowly, and tool wear would be higher. 
Conversely, it would be possible to machine a very basic groove without 
the retention shoulders 34, and with just the curved bottom surface 32 as 
defined. Such a groove could be suitable where the deviation from a flat 
surface was not great, so that a snap fit retention of the seal into the 
groove was not so necessary. The single entry exit point 50 is preferable, 
so as not to disturb the otherwise constant cross section of groove 30 
more than needed. However, even if the tools were withdrawn at other 
points, the bottom surface 32, which is what provides the seal, would 
still be continuous.