Cryogenic tempering process for PCB drill bits

A process for treating carbide tool bits used by the electronics industry for printed circuit board ("PCB") fabrication combines a cryogenic cycle with two or more tempering cycles. The tool bits are subjected to a cryogenic cycle having a ramp down phase during which the tool bits are ramped down in a dry cryogenic environment to about -300.degree. F. over between about six (6) and eight (8) hours, followed by a cryogenic hold phase during which the tool bits are held at about -300.degree. F. over between about twenty-four (24) and thirty-six (36) hours, followed by a cryogenic ramp up phase during which the tool bits are ramped up to about -100.degree. F. over between about six (6) and eight (8) hours. That is followed by a first tempering cycle having a ramp up phase during which the tool bits are ramped up in a dry tempering environment to about 350.degree. F. over about one-half (1/2) hour, followed by a hold phase during which the tool bits are held at about 350.degree. F. over about two (2) hours, followed by a ramp down phase during which the tool bits are ramped down to below about 120.degree. F. but not generally all the way to the ambient temperature over between about two (2) and three-and-half (31/2) hours. A second tempering cycle follows that and it has a time-temperature profile fairly comparable to the first tempering cycle.

BACKGROUND AND SUMMARY OF THE INVENTION
 The invention generally relates to carbide bits used in rotary tools by the
 electronics industry in printed circuit board (hereinafter "PCB")
 fabrication and, more particularly, to a cryogenic tempering process for
 extending the useful life of such PCB tool bits.
 The representative tool bit of this class is a true drill bit, as used
 exclusively for axial boring. PCB drill bits range in diameter between
 about 20/10,000-ths of an inch (0.0020 inches) and 1/4-th of an inch
 (0.250 inches). However, two other members of this class of tool bits for
 rotary tools of PCB fabrication include: end mills and router bits. None
 of these three kinds of rotary-tool bits--ie., true drill bits, end mills
 or router bits--is generally ever any larger than 1/4-th of an inch (0.250
 inches) in diameter in the PCB fabrication field. Also, they are fairly
 similar in configuration. For convenience in this description, the
 phraseology "drill bit" predominantly is used to designate the general
 class of these tool bits for rotary tools.
 Unless the context makes it clear otherwise, there will be only few
 occasions where the "drill bit" tool bit under discussion is only
 specifically a true drill bit:--eg., a tool bit used for axial boring
 only. Again, generally, the phrase "drill bit" as used herein is
 predominantly non-limiting in that it applies equally as well among true
 drill bits, end mills and router bits, as used in the electronics industry
 for PCB fabrication. Thus "drill bit" and "tool bit" are often used
 interchangeably.
 The cryogenic tempering process in accordance with the invention is
 performed with equipment and machinery which is conventional in the
 thermal cycling treatment field. First, the articles-under-treatment are
 placed in a treatment chamber which is connected to a supply of cryogenic
 fluid, such as liquid nitrogen or a similar low temperature fluid.
 Exposure of the chamber to the influence of the cryogenic fluid lowers the
 temperature until the desired level is reached. In the case of liquid
 nitrogen, this is about -300.degree. F. (ie., 300.degree. F. below zero).
 PCB's typically but not exclusively are panels of "fiberglass" which more
 particularly is a composition of glass and phenolic. Fiberglass as well as
 other typical compositions used in PCB manufacture simply place high
 demands on drill bits. PCB material, fiberglass or otherwise, is generally
 always very abrasive. It dulls drill bits relatively quickly. A drill bit
 that is dulled until it fails to meet tolerance standards must be
 immediately replaced. Briefly, as background, the machining operations on
 PCB'S must be precise and match very close tolerances. For true drill bits
 or end mills, to give an example, the tolerances are measured in respect
 of bore diameter, axial straightness, and depth of bore. The PCB's are
 typically stacked for drilling operations. That way many boards or layers
 are drilled at once. The stop means provided to stop the depth of the bore
 is usually formed directly on a true drill bit; it may be a collar that
 provides a stop shoulder. Such stop collars are located on the drill bits
 with likewise very exacting tolerances. Typically the span between tip and
 the shoulder is measured and originally set by a laser device. It is that
 precise.
 Hence, this drilling/fabricating environment not only requires very close
 precision or tight tolerances, but it is also carried out on a material
 which is highly abrasive. Accordingly, the majority of tool bits used in
 this environment are hardened carbide steel so as not to dull as quickly.
 With conventional carbide PCB drill bits, users are getting between about
 500 and 2,000 cycles out of each drill bit before it is so dull it is
 spent. Spent true drill bits are typically replaced with fresh ones and
 discarded after being sharpened three times. Re-sharpening router bits and
 end mills has never proven practical because of cost of sharpening while
 maintaining tolerances.
 What is needed is an improvement which will extend the use life of such PCB
 tool bits beyond the prior art benchmark of, say, 500 to 2,000 cycles or
 so.
 Certain formats of cryogenic treatment are known for extending the
 wearability of various steel alloy articles. For instance, the U.S. patent
 to Nu-Bit, Inc., U.S. Pat. No. 5,259,200--Kamody discloses particular
 format of a cryogenic treatment for drill bits:--large drill bits.
 According to Kamody, the state of the prior art at the time of his
 invention practiced by the following convention:
 As is apparent from the above description, the time period necessary to
 complete each step in the cycle of the treatment process generally is a
 minimum of about an hour per cross-section inch of the article being
 treated. Thus, for example, treatment of a steel article having a one inch
 cross-section in the minimum dimension would require a minimum of four
 hours total to complete the treatment according to generally accepted
 practices. In a like fashion, an article having a three inch minimum
 cross-section dimension would require a minimum of twelve hours total to
 complete the treatment according to the same accepted practices. However,
 it has been fairly conventional to increase the time periods for each step
 of the process to ensure that treatment is complete. Thus, for example,
 many of those practicing the above process routinely provide a safety
 factor of two or three or more in determining the respective time periods
 for the steps and as a consequence, overall treatment time periods of up
 to 50 hours or more for an article having a cross-sectional minimum
 dimension of one inch are often used. In using such extended time periods
 for the cryogenic treatment, it is believed that possible stress cracking
 and distortion of the article are thereby minimized or even eliminated.
 U.S. Pat. No. 5,259,200.
 However, Kamody's personal inventive efforts are directed at reducing such
 process time.
 Generally, the commercial economics of metallurgical procedures dictate
 that a particular treatment should be accomplished as quickly as possible
 so as to minimize the size of the equipment necessary and thus equipment
 costs as well as requiring less space, energy and inventory in
 processing.*** Thus, for example, a tool steel article having a minimum
 cross-sectional dimension of about four inches, the maximum time for
 treatment [in accordance with Kamody's discovery] of the article in the
 bath of cryogenic fluid would be about ten minutes. U.S. Pat. No.
 5,259,200.
 Another format of a cryogenic process for extending the wearability of a
 steel article is disclosed by U.S. Pat. No. 5,865,913--Paulin et, al., for
 firearm barrels. This patent for treatment of firearm barrels can be taken
 as representative of various others still.
 In general, cryogenic process is popular for steel alloys because it
 improves the resistance of metal to normal wear and tear. It is speculated
 that cryogenic processes affect the wearability of steel by four known
 mechanisms:--conversion of austenite to martensite; precipitation
 hardening which may increase Rockwell hardness; formation of fine carbide
 particles; and residual stress relief. Whether the mechanics are truly
 known, actual trials on numerous articles bears witness to cryogenics
 efficacy. Thus, in the case of firearm barrels, "the accuracy of a firearm
 is directly tied to the heat generated by repeated firing and the wear of
 the firearm barrel. As the firearm barrels heat up from repeated firing
 they will warp off axis due to residual stresses in the metal structure.
 This movement though ever so slight when measured at the muzzle becomes
 quite significant when measured at a target 200-300 yards away. In
 addition as the firearm barrels wear, their ability to maintain accuracy
 is severely diminished. Frequent replacement of conventional firearm
 barrels and components is necessary, particularly in bench rest shooting,
 varmint hunting, shooting teams, and the military. Firearm barrels and
 components treated with the controlled thermal profiling process of this
 invention have demonstrated that they have reduced residual stresses and
 increased wear resistance. This allows the firearm barrels and components
 to be fired with greater accuracy for longer periods of time." U.S. Pat.
 No. 5.865,913.
 However, cryogenic process is laced with problems in aspects of how to best
 carry it out. For example, from the above-quoted patent on the firearms
 barrels--U.S. Pat. No. 5,865,913--it gave the warning that "sub-ambient
 treatments in the past utilized a liquid process which in some cases will
 cause thermal shock. This is detrimental as it will add stress to the
 structure." Id.
 In U.S. Pat. No. 5,442,929--Gillin, a cryogenic treatment of electrical
 contacts is disclosed in which, the contacts-under-treatment are enclosed
 within a sheath, such as a layer of aluminum foil, "to cover the
 contacting surface and protect the contact from convection currents or
 other sources of thermal irregularities and to provide a uniform
 microclimate about the contact." U.S. Pat. No. 5,442,929.
 U.S. Pat. No. 5,174,122--Levine, lists compound ways which cryogenic
 processing can go awry and diminish the wearability of a part rather than
 extend it. "Some of the problems encountered with the prior apparatus
 described above arise as follows:--(1 ) delivery of liquid nitrogen to the
 bottom of the chamber below the payload platform often splashes or
 splatters the liquid on the payload parts causing extreme thermal shock to
 the parts that are still relatively warm; (2) the coldest gas in the
 chamber is just above the liquid and the gas does not flow upward (rise)
 to the payload parts--the cold gas does not reach the parts until just
 about all of the gas in the chamber is cold and the coldest gas will
 always be below the payload parts; (3) pre-soaking the part partially
 submersed in the liquid nitrogen causes the part to chill unevenly, as the
 portion of the part that is submersed chills much faster than the portion
 that is not submersed; and (4) any submersion of the part in the liquid
 nitrogen results in boiling heat transfer from the part at an excessive
 rate that does not allow all portions of the part to cool evenly." U.S.
 Pat. No. 5,174,122.
 The foregoing cautions about cryogenic problems are exponentially
 exacerbated when the article-under-treatment is ultra-small.
 Here, the PCB drill bits range in diameter from between about 20/10,000-ths
 of an inch (0.0020 inches) and 1/4-th of an inch (0.250 inches).
 Especially in the smaller sizes, any minute thermal irregularity which
 might not noticeably affect a drill bit measuring three (3) inches in
 diameter might just as likely render unfit for its intended use an
 ultra-small drill bit measuring 20/10,000-ths of an inch (0.0020 inches)
 in diameter. For perspective, that diameter is finer than human hair in
 most instances.
 Accordingly, what is needed is a thermal treatment which incorporates a
 cryogenic process and which provides the advantages obtained but cryogenic
 process for large articles while avoiding the hazards that endanger the
 success of cryogenic process when applied to ultra-small articles.
 These and other aspects and objects are provided according to the invention
 in a process for treating carbide tool bits used by the electronics
 industry for PCB fabrication combines a cryogenic cycle with two or more
 tempering cycles. The inventive process preferably comprises the following
 steps.
 At the start, carbide tool bits as used by the electronics industry for PCB
 fabrication resting are found at rest in an ambient environment likely
 between about 65.degree. F. and 100.degree. F. The tool bits are subjected
 to a cryogenic cycle having a ramp down phase during which from an initial
 start time the tool bits are ramped down in a dry cryogenic environment to
 about -300.degree. F. over between about six (6) and eight (8) hours,
 followed by a cryogenic hold phase during which the tool bits are held at
 about -300.degree. F. over between about twenty-four (24) and thirty-six
 (36) hours, followed by a cryogenic ramp up phase during which the tool
 bits are ramped up to about -100.degree. F. over between about six (6) and
 eight (8) hours.
 That is followed by a first tempering cycle having a ramp up phase during
 which the tool bits are ramped up in a dry tempering environment to about
 350.degree. F. over about one-half (1/2) hour, followed by a hold phase
 during which the tool bits are held at about 350.degree. F. over about two
 (2) hours, followed by a ramp down phase during which the tool bits are
 ramped down to below about 120.degree. F. but not generally all the way to
 the ambient temperature over between about two (2) and three-and-half
 (31/2) hours. A second tempering cycle follows that and it has a
 time-temperature profile fairly comparable to the first tempering cycle.
 Optionally, a third tempering cycle can be included too.
 The inventive process might have the cryogenic ramp down phase arranged
 such that it has a varying rate of descent that is more steep initially
 from ambient to about -100.degree. F. and then more gradual thereafter for
 temperatures below -100.degree. F. to about the cryogenic hold temperature
 of about -300.degree. F. The temperature descent from the start time at
 ambient temperature to the about -100.degree. F. level might be achieved
 over about the first one (1) hour after the start time. That way, the
 temperature descent from below about -100.degree. F. to about -300.degree.
 F. is achieved over between about five (5) and seven (7) hours.
 The inventive process might have the cryogenic ramp up phase arranged such
 that it has a varying rate of ascent that corresponds to an exponential
 decay of the cryogenic hold temperature from the about -300.degree. F. to
 about -100.degree. F. over between the about six (6) and eight (8) hours
 therefor. The exponential decay of the cryogenic hold temperature from the
 about -300.degree. F. to about -100.degree. F. might transpire such that a
 temperature of about -200.degree. F. is not reached from the base hold
 temperature of -300.degree. F. until six (6) hours into the cryogenic ramp
 up phase, the remaining decay up to -100.degree. F. occurring over a next
 two (2) hours. Alternatively, the exponential decay of the cryogenic hold
 temperature from the about -300.degree. F. to about -100.degree. F. might
 be arranged to transpire such that a temperature of about -200.degree. F.
 is not reached from the base hold temperature of -300.degree. F. until
 five-and-half (51/2) hours into the cryogenic ramp up phase, the remaining
 decay up to -100.degree. F. occurring over a next half (1/2) hour.
 Optionally, the cryogenic environment is provided by a Dewar chamber. The
 tempering environment might be provided by a convection oven. Accordingly,
 the transition between the cryogenic cycle and first tempering cycle would
 thus entail physical transfer of the tool bits from Dewar chamber to the
 convection oven.
 A number of additional features and objects will be apparent in connection
 with the following discussion of preferred embodiments and examples.

DETAILED DESCRIPTION OF THE INVENTION
 The cryogenic tempering process in accordance with the invention involves a
 controlled thermal profile (vis-a-vis ramp-down, hold, and ramp-up phases
 &c.) for treating the ultra-small carbide tool or drill bits used by the
 electronics industry in printed circuit board ("PCB") fabrication. While
 the steps and values of the process, particularly as applied to PCB tool
 bits, are unique, the deep cryogenic freeze as well as the heat tempering
 equipment used in the process are known to those skilled in the art and
 will not be described in detail in the interests of clarity.
 PCB tool bits are called on to provide bore holes or machined edges at
 great precision as measured in respect of bore diameter, depth of bore, as
 well as axial straightness. The dulling or wearing down of a given tool
 bit is likely caused by the heat generated by repeated cutting and the
 erosive wear of the bit as it works on the abrasive matrix of the
 glass-phenolic composite. As the tool bit heats up from extended use in
 cutting/drilling strokes, it may anneal and hence soften or may thereafter
 experience quenching and hence embrittle; it may even warp off axis due to
 residual stresses in the crystalline or grain micro-structure. But by far
 the worst is that the tool bit softens and erodes or embrittles and chips
 in localized places. These deleterious effects compound themselves after
 extended time. Experience to date throughout the industry finds that
 conventional PCB drill bits have a use life of between about 500 and 2,000
 cycles out of each drill bit before it is so dull it is spent. Spent drill
 bits are typically replaced with fresh ones and discarded after being
 sharpened three times. Replacement of spent drill bits costs resources
 both in terms of labor as well as fabrication line down-time.
 However, PCB drill bits treated with the controlled thermal profile of the
 process in accordance with the invention have demonstrated that they have
 increased wear resistance. Trials show that this treatment in accordance
 with the invention extends the wear life of the untreated drill bit by
 about 11/2x factor (eg., from about 500 cycles to 1,250 cycles). This
 provides economy in workers' time spent attending to the switch-out
 process of spent drill bits, as well as the associated down-time in the
 PCB fabrication line while the switch-out transpires. Certainly, the
 cost-savings realized in reduced consumption of drill bits is a
 significant cost-savings to the industry. Even though individual the drill
 bits are relatively affordable (between .about.$1.50 and .about.$5.00),
 the savings can be substantial when considering the quantities used
 (nowadays a modest sized enterprise in the PCB fabrication industry might
 run through $10,000/week in such tool bits). More surprisingly,
 significantly further savings will be realized from the diminished time of
 skilled labor and fabrication line down-time saved for every occasion an
 array of spent drill bits are not switched out as often as previously.
 With reference to FIG. 1, one embodiment of a cryogenic tempering process
 in accordance with the invention comprises both a cryogenic cycle in
 combination with a set of two or more tempering cycles (eg., three shown
 in FIG. 1, but see alternatively FIG. 2). The cryogenic cycle of the
 process generally involves the gradual ramping down, holding, and then
 ramping up of the temperature of the PCB tool bits to cryogenic
 temperatures of -300.degree. F. (-185.degree. C.) or lower. The tempering
 cycles involve plural like cycles up to about 350.degree. F. (177.degree.
 C.) (again, three cycles shown in FIG. 1).
 This cryogenic tempering process in accordance with the invention is
 accomplished with deep cryogenic freezing and heat treating equipment. The
 PCB tool bits are placed in a treatment chamber which is connected to a
 pressurized Dewar and metered feed-line and/or other supply of cryogenic
 fluid such as liquid nitrogen or the like; liquid nitrogen is preferred.
 Exposure of the chamber of the cryogenic cooling system lowers the
 temperature of the PCB tool bits until the desired temperature or
 temperatures is/are achieved. Control devices of a common nature are
 employed to ensure that the cooling is gradual as desired. The cooling is
 intentionally very gradual to avoid stressing the ultra-small diameter
 payload in the chamber. As stated, the equipment relied on for carrying
 out the process in accordance with the invention is generally known to
 those skilled in the art. The tempering of the PCB tool bits can likewise
 be accomplished in any well-known conventional manner,
 With renewed interest in the FIG. 1 cryogenic cycle, FIG. 1 shows that the
 ramp-down phase is accomplished very gradually and with an intermediate
 shelf before the bottom is reached, and in accordance with a very specific
 set of parameters of temperature and time. At the frame of reference of
 initial time (or arbitrarily, time=zero), the PCB drill bits are resting
 at equilibrium in room temperature, or about 72.degree. F. (22.degree.
 C.). The following table correlates the target times and temperatures for
 the process in accordance with the invention. By way of background, a
 control system is programmed with these parameters. Its temperature
 measurement for the system is taken from a sensor or probe in the lid of
 the cryogenic chamber.

Ramp down phase of Cryogenic cycle
 Hour(s) after start Temperature Rate (.degree. F./hrs)
 1 -100.degree. F. 175
 3 -220.degree. F. 60
 4 -220.degree. F. 0
 5 -250.degree. F. 30
 6 -290.degree. F. 40
 between 7-8 -300.degree. F. .about.5-10
 Following the ramp down phase is a "hold phase" in which PCB tool bits are
 exposed in the deep cryogenic temperatures for an extended period of time.
 FIG. 1 shows that the duration of the preferred "hold phase" is preferably
 no less than about twenty-four (24), and more preferentially might be
 extended up to thirty-six (36) hours and more.
 Some of the prior art cryogenic processes in accordance with the prior art
 literature call this a "soaking" phase, which is certainly technically
 correct in cases where the payload is immersed in liquid nitrogen. The
 process in accordance with invention utilizes a dry process. Here the
 payload is never immersed. Any boiling heat transfer environment which
 comes with immersion would be too damaging to the delicate PCB tool bits.
 The entire cryogenic cycle of the process in accordance with the invention
 can be characterized as "gentle":--gently down, gently hold and gently
 back up, especially very gently back up.
 The liquid nitrogen is introduced into the chamber by means of a nozzle. In
 fact, in the preferred set up, the supply of the cryogenic fluid comprises
 a pressurized Dewar of liquid nitrogen. The feed nozzle for feeding the
 liquid nitrogen into the cryogenic chamber comprises a nozzle mounted in
 the chamber. The metering device comprises a processor-controlled solenoid
 valve in the feed line.
 By the foregoing means the payload is held at about -300.degree. F. for
 between about twenty-four (24) and thirty-six (36) hours. During this
 "hold phase" the metal certainly thermally contracts. It is assumed that
 the metal's microstructure re-organizes itself to become more spatially
 uniform. Regardless, trials with the drill bits after completion of the
 treatment prove that something advantageous happens to them.
 Following the "hold phase," there is a correspondingly gradual "ramp up"
 phase. The cold of the chamber is allowed to decay in accordance with
 exponential decay such that the temperature ramps up from -300.degree. F.
 to -100.degree. F. in eight (8) hours. By a straight line method of
 reckoning the rate of ascent, the rate of ascent would measure as
 25.degree. F. or warming each hour. However, as said, the temperature
 ascends in accordance with an exponential decay curve. The temperature of
 level of -200.degree. F. is not reached from the base of -300.degree. F.
 until six (6) hours into the start of the ramp up phase; the remaining
 warming up to -100.degree. F. occurs over the next two (2) hours. Hence,
 again by a straight line reckoning method, the warming rate for the first
 six (6) hours of the ramp up phase measures about 17.degree. F. each hour.
 For the last two hours, it goes at 50.degree. F. each hour.
 It is believed that the rate of ascent plays a singularly substantive role
 in the measured success of the process in accordance with the invention.
 It is during this portion of the ramp up phase which all thermal
 irregularities such as convection currents and the like, are more
 preferably eliminated than the majority of other times.
 The temperature level of -100.degree. F. marks the end of the ramp up phase
 for the cryogenic cycle. Whereas the temperature continues to ascend, it
 is reckoned that the next-described ascent belongs to the first (of two or
 more) ramp up phases of the tempering cycle. In contrast with the
 cryogenic cycle, where the temperature changes were controlled down to a
 slow almost snail's pace, there is much quicker movement with the
 tempering cycle(s).
 To begin with, in the physical world, the payload of tool bits is
 physically transferred out of the cryogenic chest. That is, the payload is
 loaded into a convection oven provided with a circulating fan. This
 transfer occurs at the rate of a worker lifting the payload racks out of
 the chest and placing them in the oven as fast as he or she can in a
 moderate hurry. As soon as the oven door is shut, the heat and fan start
 right away. The controller is programmed to ramp up the oven to
 350.degree. F. (ie., above zero) in 1/2 (one half) hour. Again, the
 temperature measurement which the controller works off of is a probe or
 sensor mounted inside the oven.
 Observations record that frost forms immediately on the tool bits, which
 cooks off in about ten (10) minutes). Then after 350.degree. F. is
 reached, the controller then begins to count off a "hold phase" of two (2)
 hours. Following that, the oven is shut down and the heat is allowed to
 leak or "decay" away until the temperature in the oven approaches room
 temperature. In practice, it so happens that the oven used requires two
 (2) hours or so to fall all the way back to about room temperature.
 Arbitrarily, the inventor has chosen the value 100.degree. F. to mark the
 end of the ramp down or cool down phase for each of the plural tempering
 cycles. Hence, when the temperature measured in the oven falls to
 100.degree. F. or below, the controller cycles the oven for another
 tempering cycle. Again, the heat is pulsed up to 350.degree. F. in about
 1/2 (one half) hour. The temperature is held at 350.degree. F. for a hold
 phase of two (2) hours or so duration. Then the oven is switched off and
 the heat is allowed to decay away to about 100.degree. F. in about another
 two (2) hours or so. And that completes tempering cycle number 2.
 If a third tempering cycle is chosen, then the tempering cycle number 3
 follows immediately. The processor is controlled with the same values for
 cycle number 3 as for number 2, except that at the end of cycle number 3,
 when the temperature has cooled down to below 100.degree. F., the
 controller idles itself.
 The process in accordance with the invention is complete. The tool bits are
 ready for retrieval from the oven and thereafter deployment by the end
 user(s) thereof.
 Trials have established that a given superior grade of PCB drill bits which
 were giving 500 drill strokes untreated before dulling, persisted for
 about 1,250 cycles after treatment by the process in accordance with the
 invention. These drill bits cost about $2.00 apiece. They were processed
 in mass arrays of multiple trays, each tray holding 500 bits apiece, so
 that a thousand or more were processed as a unit. This accomplishes the
 necessary economy. The cost investment measured in terms of liquid
 nitrogen and electric power for the oven only, averages out to a modest
 amount for each drill bit. Certainly the cost of treatment did not drive
 up the costs in each drill bit a manifold factor.
 As previously stated, some end users are known to have a present budget of
 $10,000 a week or so for replacement tool bits alone; and these are just
 modest sized enterprises in the industry. Therefore, the modest extra cost
 or investment involved with processing drill bits through the treatment
 process in accordance with the invention promises to highly likely
 substantially cost justify itself to the industry.
 The inventor hereof has applied a pair of processes in accordance with the
 prior art to PCB drill bits to test the efficacy of the invention. The
 U.S. Patent of Voorhees, No. 4,482,005, discloses a cryogenic cycle having
 ramp down and ramp up phases flanking a wet or immersion "soaking" phase.
 The Voorhees disclosure also asserts that for "tool steel" drill bits, the
 wet process got a seventeen (17) fold improvement in number of holes
 between re-sharpening. Applicant finds that its ultra-small, carbide PCB
 drill bits must be substantially different articles of manufacture than
 "tool steel" drill bits practiced on by Voorhees. Wet or immersion
 processes simply prove to be incompatible with the ultra-small, carbide
 PCB drill bits of the PCB fabrication industry. The quality between one
 another after wet treatment is too uneven for industry standards. One
 wet-treated PCB drill bit might have a weak spot where it breaks on a
 first use. Another wet-treated PCB drill bit might not even reach the
 use-life level of its untreated counterparts.
 The above-referenced U.S. patent to Nu-Bit, Inc., Pat. No.
 5,259,200--Kamody discloses a quenching process in which a four-inch
 diameter steel (not carbide) drill bit is essentially dropped into a
 liquid nitrogen bath, and let set there for the ten (10) minutes it takes
 for the liquid nitrogen to boil away. After the bath the drill bits are
 brought back to room temperature by a jet stream of room-temperature air.
 This disclosure asserts that, in forty minutes start to finish (including
 the 10 minute bath), this quick dip method gains up to a fifty fold (50x)
 improvement in drill bits (again, which may be of a four inch diameter).
 Applicant has found that submerging ultra-small carbide PCB drill bits in
 a liquid nitrogen bath, and then directing a jet of air on them after that
 as disclosed and claimed by Kamody, plainly destroys them.
 Whereas applicant 11/2 fold improvement factor may at first blush be
 relatively modest in light of the asserted accomplishments of the prior
 art, it stands up to measuring as substantial in the use environment in
 which the work pieces comprise the ultra-small, carbide drill or tool bits
 of rotary tools used by the electronics industry in printed circuit board
 (eg., "PCB") fabrication.
 To turn now to FIG. 2, it shows an alternate time-temperature profile in
 accordance with the invention for cryogenic tempering of the ultra-small
 carbide tool bits used by the electronics industry in PCB fabrication. In
 FIG. 2, the ramp-down phase is accomplished in two stages which--unlike
 the FIG. 1 time-temperature profile--are not separated by a shelf. At the
 frame of reference of initial time (eg., time=zero), the PCB drill bits
 are assumed resting at equilibrium in room temperature, or about
 72.degree. F. (22.degree. C.). The following table correlates the target
 times and temperatures for the FIG. 2 version of the process in accordance
 with the invention.***

Ramp down phase of Cryogenic cycle
 Hour(s) after start Temperature Rate (.degree. F./hrs)
 1 -100.degree. F. 175
 then thru hour 6 -300.degree. F. .about.40
 Following the ramp down phase is a "hold phase" in which PCB tool bits are
 exposed in the deep cryogenic temperatures for an extended period of time.
 FIG. 2 shows that the duration of the preferred "hold phase" is preferably
 as extensive as about thirty (3) hours, as between no less than about
 twenty-four (24), and more preferentially might be extended up to
 thirty-six (36) hours and more. Again, this "hold phase" is a dry process.
 The payload is never immersed.
 Following the "hold phase," there is a correspondingly gradual "ramp up"
 phase. The cold of the chamber is allowed to decay in accordance with
 exponential decay such that the temperature ramps up from -300.degree. F.
 to -100.degree. F. in six (6) hours. By a straight line method of
 reckoning the rate of ascent, the rate of ascent would measure as
 33.degree. F. or warming each hour. However, as said, the temperature
 ascends in accordance with an exponential decay curve. The temperature of
 level of -200.degree. F. is not reached from the base of -300.degree. F.
 until five-and-half (51/2) hours into the start of the ramp up phase; the
 remaining warming up to -100.degree. F. occurs over the next half (1/2)
 hour. Hence, again by a straight line reckoning method, the warming rate
 for the first five-and-half (51/2) hours of the ramp up phase measures
 about 18.degree. F. each hour. For the last half (1/2) hour, it ramps up
 corresponding to about 200.degree. F. per hour.
 It is believed that the rate of ascent--particularly for the first half of
 the ramp up phase (eg., below and up to the -200.degree. F. level)--plays
 a singularly substantive role in the measured success of the process in
 accordance with the invention. It is during this portion of the ramp up
 phase which all thermal irregularities such as convection currents and the
 like, are more preferably eliminated than the majority of other times.
 The temperature level of -100.degree. F. marks the end of the ramp up phase
 for the cryogenic cycle. Whereas the temperature continues to ascend, it
 is reckoned that the next-described ascent belongs to the first (of two or
 more) ramp up phases of the tempering cycle. In contrast with the
 cryogenic cycle, where the temperature changes were controlled down to a
 slow almost snail's pace, there is much quicker movement with the
 tempering cycle(s).
 To begin with, in the physical world, the payload of tool bits is
 physically transferred out of the cryogenic chest and loaded into a
 convection oven provided with a circulating fan. As soon as loaded, the
 heat and fan start right away. The controller is programmed to ramp up the
 oven to 350.degree. F. (ie., above zero) in 1/2 (one half) hour. Again,
 the temperature measurement which the controller works off of is a probe
 or sensor mounted inside the oven.
 After 350.degree. F. is reached, the controller then begins to count off a
 "hold phase" of two (2) hours. Following that, the oven is shut down and
 the heat is allowed to leak or "decay" away until the temperature in the
 oven approaches room temperature. The oven is controlled so that after
 three-and-half (31/2) hours, or so the temperature is allowed to decay way
 down to about a warm temperature of 120.degree. F. or so.
 Arbitrarily, the inventor has chosen the value 120 F. to mark the end of
 the ramp down or cool down phase for the initial one of the plural
 tempering cycles. Hence, when the temperature measured in the oven falls
 to 120.degree. F., the controller cycles the oven for another tempering
 cycle. Again, the heat is pulsed up to 350.degree. F. in about 1/2 (one
 half) hour. The temperature is held at 350.degree. F. for a hold phase of
 two (2) hours or so duration. Then the oven is controlled to have the heat
 decay away all the way down to about 100.degree. F. in about another
 three-and-half (31/2) hours or so. And that completes tempering cycle
 number 2 of the FIG. 2 version of the invention.
 If a third tempering cycle is chosen (not shown in FIG. 2), then the
 tempering cycle number 3 follows immediately, wherein cycle number 3
 follows with the same values as for number 2, except that cool down to
 below 100.degree. F. finally marks the end.
 The invention having been disclosed in connection with the foregoing
 variations and examples, additional variations will now be apparent to
 persons skilled in the art. The invention is not intended to be limited to
 the variations specifically mentioned, and accordingly reference should be
 made to the appended claims rather than the foregoing discussion of
 preferred examples, to assess the scope of the invention in which
 exclusive rights are claimed.