Cryogenic tempering process for PCB drill bits

Details Cryogenic tempering process for PCB drill bits Cryogenic tempering process for PCB drill bits

2000-09-15

2001-11-13

Doerrler, William (Department: 3744)

Refrigeration

Processes

Treating an article

C148S577000

Reexamination Certificate

active

06314743

ABSTRACT:

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 {fraction (20/10,000)}-ths of an inch (0.0020 inches) and ¼-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 ¼-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° F. (ie., 300° 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 invent

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