Drilling and Associated Cutting Tool Technology Industrial Handbook_7 docx

Figure 74. High-performance reamers, having the ability for radial infeed (i.e. ‘feed-out inserts’) – when tted. [Courtesy of Cogs- dill Tool & Eng’g Ltd.] . Drilling and Associated Technologies  is oen produced from an abrasive-resistant mate- rial such as PCD, in order to maintain and extend the tool’s life and holding a good cutting edge over many machined parts. .. Reaming – Problems and Their Remedies For any resultant reamed surface, its form, accuracy and surface quality are tremendously improved by dividing the machining process into either, roughing, or nishing reaming operations. Low cutting speed together with high feedrates, in association with good lubrication agents oering adequate cooling poten- tial, provide the basis for optimum reaming practice. While, observing these ‘rules’ , improves both the reamed surface quality and its individual tolerance. It is worth restating, that a reamer only follows the pre- drilled hole, consequently it cannot correct for any previous alignment errors that might be present (i.e. see the schematic diagram in Fig. 70). Although er- rors between the spindle’s axis and the axis of the pre- drilled hole, can be adjusted with the aid of ‘oating reamer’ toolholders (Fig. 72). In Table 6, the following fault-nding chart may be useful in tracing the pos- sible causes of some common reaming problems. 3.4 Other Hole-Modification Processes Once the hole has either been: cast, core-drilled, or drilled into solid workpiece material, it oen requires a further post hole-making operation to complete the job, for example, a tapping operation. ere are a num- ber of these pre- and post-drilling hole operations that require specic tooling to nish o the hole-making activities. e most popular of these are briey men- tioned below, but this is by no means an exhaustive account of the many oen hybrid operations that are available to the potential designer, or machinist. Countersinks ere are several reasons why a countersink tool might be employed when machining features on a compo- nent, ranging from: •   Countersinking  a  countersunk-headed  screw  – for ‘ush-tting’ to the surface (Fig. 75a), •   Short  tapers – can be adequately machined on a component, •   Providing a lead – for a soon-to-be-tapped hole, •   Deburring  operation – on a previously drilled hole. Countersinks are available with a range of included taper angles and come in a variety of dimensional sizes, the most popular being either: 60°, 90°, or 120°, or indeed ‘specials’ can be ground to suit any angular and diametral workpiece features, of varying lengths. Countersinks are available from simply HSS, through to a coated cemented carbide matrix. Counter-Boring Counter-bored tooling (Fig. 75b) is available as either a solid tool, or is designed to be modular in construc- tion. is latter modular counter-boring tooling, oers a range of exibility to machine a wide assortment of component features, by simply changing the ‘pilot‘, or cutting element’s diameter. e ‘pilot’ as its name im- plies, follows a pre-drilled hole and guides the counter- bored cutting element enabling it to remain concentric with the hole’s axis. is is important for any cap-head bolts that require to be recessed either ush to a part’s surface, or sunk below its outer face. Counter-boring is also employed to machined a clearance face in the female part feature allowing for a stepped bar to have a ush face to locate against, or simply to provide clear- ance for such a workpiece feature. Again, as with most of these tool materials, they are produced from HSS, through to coated cemented carbides. Spot-Facing Spot-facing tooling is normally utilised to produce a consistent and uniform seating on for example, a cast, or forged component, allowing a washer, or bolt-head to be ush across its contact face. Spot-faced tools (Fig. 75c), are available as either a solid, or modu- lar constructional design – the latter version, giving greater exibility across a wider range of features to that of the former counterparts. Materials for these tools are similar to those mentioned for other post- drilling tooling, namely, HSS through to coated ce- mented carbides.  Chapter  Table 6: Potential reaming problems and their possible causes, with some remedies Reaming problem: Possible causes and some remedies: Holes to large i) Concentricity error of either: machine spindle, toolholder, or tool. (ii) Damaged t between tool and toolholder (i.e. taper, chuck, or collet). (iii) Bevel lead on tooling incorrect. (iv) Cutting speed, or feedrate too high. (v) If problem is the result of workpiece material, eliminate it by using a weaker coolant medium (i.e. by increasing its cooling potential, sacricing some of the lubricating abilities). Hole too small i) Tool tolerance incorrect. (ii) Ductile material that contracts after reaming – possibly eliminated by using a quick spiral reamer. (iii) Excessive heating during the reaming process: perhaps by the hole expanding, then subsequently contracting. (iv) Reamer blunt. (v) Cutting speed, or feedrate too low. (vi) Insucient stock left on for reaming: tool seizes in the hole. (vii) In most cases, eliminate problems using a more concentrated soluble oil mixture (e.g. 1:15 to 1:10, alternatively use cutting oil). Conical, non-circular and other hole malfunctions (i) Machine spindle not concentric. (ii) Bevel lead not correct. (iii) Axis of pre-drilled hole and reamer not in alignment – eliminate by using a ‘oating’ toolholder. Unsatisfactory surface texture of hole i) Reamer blunt. (ii) BUE on edges, caused by ‘cold welding’ , eliminate by using high concentration coolant, possibly cut - ting oil, or by a reduction in reamer’s land width – to almost zero. (iii) Cutting speed too high, feedrate too low. (iv) Stock removal allowance too small – caused by the pre-drilled hole being too large. (v) Incorrect bevel length. Reamer seizes and breaks (i) Reamer blunt. (ii) Too high a cutting data employed (i.e. speed and/or feed). (iii) Pre-drilled hole too small. (iv) Poor coolant mixture – lubrication too dilute. (vii) Reamer geometry requires modication. [Courtesy of Guhring Ltd] . Drilling and Associated Technologies  Figure 75. Some alternative hole modication machining tooling. [Courtesy of Guhring Ltd.].  Chapter  Back Spot-Facing Back Spot-faced tools (Fig. 75d), are usually employed in ush-facing an internal hole’s face on either a cast- ing, forging, or wrought stock. e Back Spot-facing operation, enables a bolt-head, or nut and its washer to be accurately seated. In some instances, it is possible to generate, the back-face, rather than to form it, via specially-modied tools that are fed to the other side of the part, then circular interpolation techniques are used to create the required back-face. NB  With most of these post-drilling operations, the cutting data is restricted and calculated to the outer di- ametral dimension of the part feature to be machined. Solid post-drilling tooling can usually be operated at higher cutting data to that of their modular tooling counterparts. References Journal and Conference Papers Agapiou, J.S. and DeVries, M.F. On the Determination of ermal Phenomena during Drilling – Part I. 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