Abstract:
Apparatus for heat treating a cutting tool comprises a furnace and a tool holder within the furnace adapted to receive therein a first portion of the tool, a second portion of the tool projecting from the tool holder, the second portion of the tool being directly exposed to radiant heat from at least one radiant heating element within the furnace with the first portion of the tool being shielded from the radiant heat.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the heat treatment of cutting tools, in particular, although not necessarily exclusively, the heat treatment of cutting tools such as twist drills having a shank and a cutting portion to which it is desired to impart different hardness. 
     BACKGROUND ART 
     Cutting tools such as twist drills, milling tools, reamers, countersinks and the like include a cutting portion, formed with a number of cutting edges, and a shank by which the tool is held, for example in a collet chuck or other holder of for example a lathe, machine drill or hand drill. It is common practice to harden the cutting portion of these tools in order that they can cut efficiently. However, it is undesirable to harden the shanks to the same degree, because a relatively soft shank is required if the chuck or other holder is to grip the tool securely. 
     These cutting tools are typically manufactured from steel, most usually a high-speed steel. The process by which they are hardened is a heat treatment process, in which blanks for the tools are heated up to a temperature of about 1150-1230° C., at which temperature they are held for a sufficient length of time to ensure that the blank is heated to its core. The blank is then rapidly cooled (i.e. quenched) to effect the change in microstructure that gives the steel its hardness. Hardening of other ferrous and non-ferrous metals can be achieved in a similar manner with suitable heat treatment regimes. 
     To give the desired differential hardening (fully hardened cutting portion/soft shank), the conventional approach is to use a salt bath for the heat treatment. The cutting portion of the tool is immersed in the liquid salt, which is held at the necessary high temperature. The shank remains clear of the bath and consequently remains at a temperature which is not sufficiently high for any appreciable hardening to occur. 
     The use of a salt bath in this way can reliably produce tools having the desired hardness characteristic, and is still the most common method of hardening used today. However, the process does have drawbacks, most notably the environmental and safety concerns associated with the toxic, extremely high-temperature molten salts used in the bath, which also give rise to difficult and unpleasant working conditions for the operator of the process. 
     More recently, it has been proposed to differentially harden cutting tools by treating them in a three-stage vacuum furnace, the tools progressing in a linear fashion through three chambers in the furnace. The tools are loaded in batches into the first chamber which is closed and then evacuated. After a predetermined amount of time, the batch of tools is then moved into the second chamber, which is already under vacuum, and which is held at a high temperature in order to heat the tools to the desired hardening temperature. Having been held in the heated chamber for an appropriate amount of time, the tools are then transferred to the third chamber. Here they are quenched by pumping nitrogen gas into the chamber under high pressure. 
     To achieve the desired differential cooling, the tools are held within the chambers of the furnace in carriers, in the form of large metal blocks formed with recesses in which the tool shanks are received. The carriers shield the shanks to some degree from the heated interior of the chamber. However, the temperature of the carriers themselves will increase, particularly where the heat treatment regime dictates that the tools must be held in the heating chamber for any significant length of time, possibly resulting in some unwanted hardening of the shanks. This problem can be exacerbated if the carriers are not allowed to cool sufficiently between batches of tools. The rapid cooling by blasting the tools with nitrogen may also lead to undesirable distortion. 
     Moreover, the furnace must be sealed from its surrounding environment, and within the furnace the three chambers must be separately sealed, in order that the necessary vacuum can be maintained, leading to a relatively complex and expensive design of furnace. It is perhaps for this reason that the salt bath still predominates, despite its drawbacks mentioned above. 
     SUMMARY OF THE INVENTION 
     The present invention has as its general aim the provision of heat treatment apparatus and methods for differentially hardening two portions of a cutting tool which offers an economic and reliable alternative to the conventional, and ever less desirable, salt baths. 
     In one aspect, the invention provides apparatus for heat treating a cutting tool, comprising a furnace within which there is at least one radiant heating element and a tool holder adapted to receive and shield a first portion of the tool from the heating element whilst a second portion of the tool is directly exposed to radiant heat from said element. 
     In another aspect, the invention provides a heat treatment method for hardening a metal tool, the method comprising directly exposing a first portion of the tool to a source of radiant heat in a furnace to raise the temperature of said first portion to an elevated temperature, and shielding a second portion of the tool from said source of radiant heat to maintain it at a temperature lower than the elevated temperature of said first portion. 
     The term “tool” used herein is intended to include blanks and semi-finished blanks for tools as well as finished tools themselves. 
     By exposing the tools directly to a source of radiant heat it has been found possible to accurately control the differential heating of the two portions of the tool. 
     This control is enhanced when, as is preferred, the radiant heat source is arranged to lie alongside the tools when they are being heated in the furnace. In this case, it may also be arranged that the heat source, i.e. the heating element, does not extend alongside or at most extends only partially alongside the tool holder in which a portion of the tool is shielded. This further exaggerates the differential heating of the two portions of the tool. 
     Another particularly preferred measure to increase the temperature differential between the two portions of the tools, is to actively cool the tool holder. For instance air, water or some other cooling fluid may be forced through or around the tool holder or some other heat conducting element that is thermally coupled to the tool holder, whereby heat can be drawn away from the holder. 
     It is of course more economical to treat batches of tools at one time, and for this reason the furnace may be arranged such that a plurality of tools can be simultaneously exposed to the heating element. For instance, a row or two-dimensional array of tools may be held in one or more tool holders adjacent the element. To ensure a more uniform heating of the tools, two heating elements may be arranged, one either side of the tools, for example to lie parallel with a row of tools. This principle can be extended to layouts including two or more rows or arrays of tools extending parallel to one another, these rows or arrays being held in tool holders within corridors defined between opposed heating elements, e.g. three rows of tools held in three parallel corridors defined by four heating elements. 
     Where the tools are treated in batches, it is particularly preferred that each tool is directly exposed to radiant heat from at least one heating element, without being shielded or partially shielded from that element by any of the other tools of the batch. Typically, with the configuration of heating elements described above, this will mean that the tool holders should be arranged to hold at most two parallel rows of tools. Even then, it is desirable to offset the rows from one another such that the tools are fully exposed to the heating element to one side of the batch and only partially shielded from the element to the other side of the batch. 
     The furnace preferably also includes means for rapidly cooling the tool or tools subsequent to exposure to the heating element(s). Particularly preferred for this purpose are one or more cooling elements adjacent which a row or array of tools can be disposed in a tool holder, much in the same way as they are held alongside the heating element. The cooling elements, which may for example be cooled themselves by a flow of water or other cooling fluid, absorb heat radiating from the tools to help prevent the atmosphere around the tools increasing significantly in temperature, encouraging rapid cooling of the tools. 
     Similar to the heating elements, parallel rows of cooling elements may be arranged within the furnace to define one or more corridors for the tools. 
     Conveniently, the furnace may be divided into a heating zone in which the tools are heated by radiant heat and a separate cooling zone in which the tools are cooled, transport means being provided to take the tools from one zone to the other. A particularly convenient form of furnace that can be adopted for this approach is a rotary hearth furnace, in which the tools are carried by a rotating support or hearth, e.g. in their tool holder, through an annular chamber, which may be sub-divided into different temperature zones. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a part sectioned plan view of a rotary hearth furnace according to an embodiment of the present invention; 
     FIG. 2 is a section, on a slightly enlarged scale, along line II—II of FIG. 1; 
     FIG. 3 shows in cross-section, the heating zone of the furnace of FIG. 1; 
     FIG. 4 shows somewhat schematically, on an enlarged scale the central portion of the heating zone illustrated in FIG. 4; 
     FIG. 5 is a plan view of a tool carrier, on a much enlarged scale, for use in the furnace of FIG. 1; 
     FIGS. 6 a,    6   b,    7   a  and  7   b  are plan and end views of alternative heat sink blocks for the tool carrier seen in FIG. 5; 
     FIGS. 8 a  and  8   b  show hardness profiles for blanks for a 10 mm diameter “jobber drill” (twist drill) heat treated respectively by a process according to an embodiment of the present invention (FIG. 8 a ) and a molten salt bath process (FIG. 8 b ); and 
     FIG. 9 is a view similar to FIG. 1, illustrating a modification to the load/unload conveyor arrangement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a rotary hearth furnace  2  is shown along with an associated load and unload conveyor system  4 . The furnace is designed for heat treating tool blanks, in this example blanks for twist drills formed from high speed steel (HSS). 
     The annular interior of the furnace  2  is divided into ten equally sized zones  6  around its circumference. Likewise, the rotary hearth  8  of the furnace  2  is sub-divided into ten equal segments  10 , each segment  10  being adapted for transporting a batch of tool blanks  12  sequentially through the zones  6  of the furnace in a tool carrier  14  as the hearth is indexed through ten corresponding positions. 
     The furnace is operated at or very near ambient atmospheric pressure. That is to say it is not evacuated. In this preferred embodiment, the furnace atmosphere (i.e. the atmosphere within the furnace) is nitrogen gas. This helps prevent discolouration of the blanks, and possible de-carburisation of the steel which might occur if they were exposed to oxygen at the high temperatures at which the furnace operates (1150-1230° C.). 
     In use, tools are loaded in batches into the carriers  14 , which then travel along the load conveyor  16  to arrive one at a time at transfer table  18 . From here, the carrier  14  is loaded into the furnace  2 , onto a segment  10  of the hearth  8  in a load/unload zone  20  of the furnace  2 . The hearth is then indexed by the length of one segment  10 , in the anti-clockwise direction as indicated by arrows A in FIG. 1, taking the just loaded carrier  14   a  into the first of two pre-heat zones  22 ,  24 , and bringing another carrier  14   b  from the last of five cooling zones  26 - 30  into the load/unload zone  20 . The carrier  14   b  is then extracted from the furnace  2  onto the transfer table  18 , from where it travels along the unload conveyor  34 , which runs parallel with but in the opposite direction to the load conveyor  16 . The now heat treated, hardened tool blanks are then removed for further processing (e.g. flute grinding, etc.). 
     In subsequent indexing steps, the carrier  14   a  and the segment  10  of the hearth on which it sits are taken sequentially through the second pre-heat zone  24 , two high temperature heating zones  36 ,  38  and the five cooling zones  26 , to return to the load/unload zone  20 . The pre-heat zones  22 ,  24  serve to bring the temperature of the blanks up to about 900° C., prior to their being exposed to the very high temperatures in the heating zones  36 ,  38 . This avoids very rapid heating of the blanks  12 , which might lead to undesirable distortion. The time spent in the two heated zones  36 ,  38 , in which the tool blanks  12  are elevated to a temperature of about 1200° C., is sufficient to ensure that the blanks  12  are heated through to their cores. The blanks are then rapidly cooled as they enter the first cooling zone  26 , very quickly cooling to a temperature of about 600° C. As they pass through the remaining four cooling zones  27 - 30 , the blanks  12  then cool down to around ambient temperature before being discharged from the furnace  2 . 
     Insulation ‘bridges’ (not shown)—that is to say insulating members which span the width of the furnace interior, but which do not encroach on the passage of the tool blanks—are located between the second high temperature zone and the first cooling zone and between the cooling zones themselves. It is notable that this arrangement of the zones, with the tool blanks being loaded and unloaded to and from a cool zone, which is separated from the heated zones not only by the insulating bridges, but also by the two pre-heat zones, leads to only very little loss of heat from the furnace to the surrounding environment. 
     Each time the hearth  8  is indexed, one carrier  14  holding treated tool blanks is unloaded from the load/unload zone  20 , to be replaced with a carrier holding new blanks ready for treatment. In this way, the process can operate continuously in a very efficient manner, with both loading and unloading of the carriers taking place at the same location. Advantageously, the rotary hearth design of furnace  2  also takes up a relatively small amount of floor space, particularly when compared with the known vacuum furnaces. 
     Turning to FIGS. 2,  3  and  4 , the construction of the furnace will now be explained in more detail. As seen best in FIG. 2, which shows a section through one of the heating zones  38  on the right and one of the cooling zones  30  on the left, the hearth  8  of the furnace is mounted for rotation within a housing  40 . An opening (not shown) is formed in the housing  40  adjacent the load/unload zone  20 , through which the tool carriers  14  can be introduced and removed. A pit  42  below the furnace houses a motor (not shown) to drive a rotor  44  to which the hearth  8  is mounted and by which it is driven to move the segments  10  of the hearth  8  step-wise through the zones  6  of the furnace  2 . Any of a variety of indexing mechanisms may be used for this drive, including for example a globoidal cam indexing mechanism. Such a mechanism is particularly preferred because, although it is simple in construction, it can very accurately index the hearth  8  (e.g. within ±1.0 mm). 
     Mounted on each hearth segment  10  is a base plate  50  of mild steel (MS). These plates  50  are water cooled, water being pumped (e.g. at about 3-4 bar) through channels provided in the plate for this purpose. The coolant is supplied under pressure to each base plate  50  from a common supply via the hub of the hearth  8 , from where the coolant is transferred to the plates  50  through flexible pipework. A fitting at the hub allows for relative rotation between a stationary supply pipe and the pipework rotating with the hearth, whilst maintaining a flow of coolant from one to the other. 
     The tool carrier  14 , the structure of which is described further below, stands on the base plate  50 , such that it is in thermal communication with the base plate to be cooled by it. 
     The heating zones  36 ,  38 , as well as the pre-heat zones  22 ,  24  are enclosed at their sides and top by a thick layer of an insulating material  52 , to help maintain the necessary elevated temperature in these zones. The insulation  52   a  across the top of the heated zones  36 ,  38 , and the second pre-heat zone  24  is broken to allow an array of heating elements  54 , in this example four side by side in each zone, to protrude through the insulation from above into the interior of the furnace. The elements are preferably electrically conducting elements which rely on resistance heating, allowing their temperature to be accurately and rapidly controlled. Silicon carbide elements have been found to be particularly suitable. 
     The first pre-heat zone  22  does not contain any heating elements in this example, instead being heated by radiated and/or convected heat from the second pre-heat zone  24 . 
     The heating elements  54  are equally spaced from one another across the width of the heated zone  38  to define between them three circumferentially extending passages  56  of equal width along which the tool blanks  12  travel as the hearth  8  is indexed. This arrangement, along with the design of the tool carrier  14  (described below) ensures that all of the blanks  12  are uniformly heated by radiant heat from the elements  54 . It is to be noted in particular that, unlike the known vacuum furnace described above, the elements  54  are arranged to be very closely spaced from the tool blanks  12 , allowing very accurate control of the heating of the blanks  12 . Typically, the spacing between an element and an adjacent tool will be about 50 mm or less, although the precise spacing for any particular batch of tools can be selected dependent on the heat treatment regime they require, by adjusting the position of the blanks in their carrier  14 . 
     Further control is effected by monitoring the temperature in the high temperature heating zones  36 ,  38  of the furnace and the second pre-heat zone  24 , for example using standard thermocouples, and controlling the power to the heating elements to maintain the desired temperatures in these zones. In a typical set up, six thermocouples in each of these three zones would be adequate to give the desired control. The three zones are preferably independently controlled. By way of example, typical temperatures in the three controlled zones would be about 1000° C. in the second pre-heat zone  24 , about 1200° C. in the first high temperature heating zone  36 , and about 1230° C. in the second high temperature zone  38 . Actual values may be varied dependent on factors such as the desired heat treatment regime and the material of the tools being treated. 
     In the cooling zones  26 - 30 , which are not insulated, cooling elements  60  depend downwardly from a roof member  62  in a similar array-like fashion to the heating elements  54 , defining continuations  56   a  of the passages  56  defined between those elements  54 . The cooling elements are aluminium blocks, which similarly to the base plates  50 , are formed with channels through which cooling water is pumped, in this example at about 3-4 bar pressure. This arrangement can provide for very rapid, yet controlled cooling of the blanks  12 , which is less harsh than the nitrogen quench of the known furnace, resulting in minimal if any distortion. 
     As already noted, the blanks are carried through the furnace  2  in tool carriers  14 . Referring to FIGS. 3 and 4, each of these carriers has an MS base  70  on which are mounted three MS heat sinks  72 , which are equally spaced across the width of the base and extend for the full length of the base  70 . The spaces between the heat sinks  72  are filled with an insulating refractory material  74 . 
     As seen in FIG. 5, the heat sinks  72  are each formed from two MS blocks  72   a,    72   b,  joined mid-way along the length of the base  70 , which are offset at a small angle to one another so that the line of each heatsink  72  approximates to the curvature of the hearth  8  on which they are carried. The base  70  is similarly shaped. The positions of the heatsinks  72  across the width of the base  70  is such that they coincide with the passages  56 ,  56   a  defined by the heating and cooling elements  54 ,  60 . 
     In the top surface of each block  72   a,    72   b  of the heat sink  72 , there is formed an elongate recess  75 , extending for the full length of the block. Received snugly in this recess is a tool holder  76 , also of MS, in the top surface of which are formed a uniformly spaced series of holes  78  sized to accept the shank ends  80  of the tool blanks  12  to be treated. When received in the holders  76 , the tool blanks protrude upwardly so that their cutting portions lie between the heating elements  54  as they travel through the heating zones  56 ,  58  of the furnace  2 . In this way, the cutting portions are exposed to the radiant heat from the elements  54 , whilst the shanks are shielded within the holders, which are themselves disposed below the level of the heating elements (see FIGS.  3  and  4 ). 
     The tool holders  76 , which are themselves cooled by the water-cooled base plate  50  through the heatsinks  72 , also serve to conduct heat away from the shank  80  when it is in the furnace  2 . This, together with the shielding they provide, ensures that the temperature of the shanks  80  is kept below about 800° C., so they are not hardened to any significant degree. 
     The division between the soft shank end  80  of the tool blank  12  and the hardened cutting portion  82  can be controlled by the depth of the holes  78  in the tool holder, the deeper the holes the longer the soft shank  80 . The transition between the hardened and soft portions of the blank will not coincide precisely with the depth of the hole, due to the effects of conduction of heat through the blank itself, but it is a matter of simple experimentation to determine the relationship between hole depth and the location of the transition for any particular design of tool. 
     The degree of hardening will also be influenced significantly by the spacing between the tool blanks  12  and the heating elements  54  in the furnace  2 . This can be controlled by appropriate positioning of the holes  78  in the tool holders  76 . Different diameter tool blanks will also require different hole arrangements to ensure that they are uniformly heated. The tool holders  76  seen in FIG. 5, having two staggered rows of holes  78  in each holder, would be appropriate, for example, for tools having a diameter of about 8-10 mm. For larger diameter tools, a single row of holes, as seen for example in FIGS. 6 a  and  6   b  would be more appropriate, whereas smaller diameter tools can be packed more tightly (FIGS. 7 a  and  7   b ). 
     Advantageously, this approach to accommodating different size tools means that only the tool holders  76  need be changed for different tool batches. A further advantage is that a great degree of control is given over the hardening process by the variables in the described furnace structure, including the position of the heat sinks, the flow of cooling water, the amount of insulation between the heat sink blocks, and the spacing and depth of the holes in the tool holders, the particular optimum parameters for any form of tool, taking into account also the temperatures and time spent in the furnace, being deducible by experimentation. This in turn means that the furnace operating parameters need not necessarily be altered for different forms of tools, the characteristics of the heat treatment process instead being controlled through an appropriate selection of the heat sinks and holders. This has the great advantage that different forms of tool can follow one another through the furnace without any significant time loss. 
     FIGS. 8 a  and  8   b  illustrate the effectiveness of the heat treatment process possible using the furnace described above. Specifically, if one compares the hardness characteristic of two identical tool blanks (in this example blanks for 10 mm diameter HSS twist drills), one treated in a rotary hearth furnace in accordance with the invention (FIG. 8 a ) and the other in a conventional salt bath (FIG. 8 b ), it can be seen that similar hardness of the cutting portions (i.e. “flute length”) is achieved by both processes, whereas the shank of the blank treated in accordance with the present invention is, if anything, softer than that arrived at conventionally. Moreover, tests have shown that this approach produces very consistent final hardness figures, attributable to the re-produceable heating and cooling profiles that can be achieved for each cycle of work. 
     As will be appreciated, the specific example described above is intended to be illustrative, and many modifications to the apparatus described can be made without departing from the invention. For instance, as illustrated in FIG. 9, additional cooling may be provided by cooling fans  90  positioned above the unload conveyor  34 . This figure also illustrates vacuum locks  92  which are provided in this example to stop the ingress of oxygen into the furnace during loading and unloading of the tools. During loading, the tools enter the vacuum lock chamber  92   a  at the end of the load conveyor  16 . Doors on either side of the chamber seal the chamber, and the gas within the chamber is pumped down to approximately 1×10 −2  m bar. The chamber is then back-filled with N 2  gas from the furnace. The tools are then loaded into the furnace through the inner chamber door (ie. the one that opens to the furnace load zone). This scheme substantially prevents any oxygen entering the furnace. 
     Vacuum lock  92   b  operates in a similar way when the tools are unloaded from the furnace onto the unload conveyor  34 .