Figure 70. The exaggerated hole errors caused by an incorrect drill point geometry and the manufacturing techniques for its subsequent correction . Drilling and Associated Technologies Figure 71. Reaming can correct an assymetrically drilled hole – when correctly adjusted. Chapter large harmonic variation in the ‘plots’ is depicted, as is the case when a ‘Floating reamer’ with roller drive has been used inappropriately. Floating Reaming Solid machine reamers can be ‘oated-down’ 59 a pre- drilled hole, to produce a much straighter reamed hole, than would otherwise be the case. When ‘oat- ing’ reamers within their specially-located toolhold- ers, two techniques are used to ‘oat reamers’ (i.e see Fig. 72), these are: 1. Radial play – where the machine reamer has lim- ited movement laterally with respect to the princi- pal axis, 2. Composed radial and pendulum play – this has both radial play, together with a degree of limited angular movement (i.e. this motion is similar to that of a Grandfather clock’s timing mechanism, via its pendulum motion). NB is latter ‘oating’ technique has the potential for a combination of both radial and pendulum motions to the machine reamer. ese unrestrained kinematic motions gives it free motion without lateral and angu- lar constraint, to simply follow the ‘line of least resis- tance’ along the spindle axis, as the reamer progres- sively feeds down through the predrilled workpiece. .. Radially-Adjustable Machine Reamers Special-purpose machine drill/reamers (Fig. 74a) are oen utilised in high-volume production envi- ronments such as in the automotive sector, for util- ity engines which can account for >55,000 complex- reaming operations per week. Conversely, for defence vehicle engines the production volumes are quite low, accounting for <300 operations per month. Typical operations on such automotive components, using a machining centre include the reaming of: 59 ‘Floated-reaming’ , relates to the reamer’s ability to have some degree of lateral compliance, namely limited motion, allowing it some ‘play’ to follow the hole’s path, but still correcting for any previous ‘helical wandering’ by the drill. • Cylinder head tappet rail drill-reaming – in a sin- gle operation, • Cylinder head valve seats and guides – machining both features, in the parent bore and nish machin- ing, Figure 72. Solid machine reamers can be ‘oated-down’ a pre-drilled hole, by two distinct ‘oating techniques’: (I) radial play, (II) composed radial pendulum play. [Courtesy of Guhring Ltd.] . Drilling and Associated Technologies • Engine block and crank bores and cheek faces – nish machining, with this latter feature requiring controlled ‘radial infeed’ of the cutting/reaming in- sert. NB e special-purpose ‘radial-infeed’ tooling neces- sary for the satisfactory machining of the cheek faces of this latter low-volume production engine block, will now be briey discussed. Case-Study of Engine Block Bore Features In this novel, but interesting automotive ‘case-study’ , all of the challenges facing such special-purpose reamers are present. Here, the machining application consisted of the following: a six-cylinder diesel cast iron engine block for an armoured personnel car- rier, reaming at 70 m min –1 , requiring a bore straight- ness of 0.02 mm/m, tolerance on the bore diameter of 0.025 mm, with >0.003 mm tolerance between the individual journals. e solution to this demanding industrial problem, was the machining with two tools and three operations of the crank bore and the genera- tion of two cheek faces – this latter operation was nec- essary to minimise the tted cranksha’s end oat. is particular special-purpose reamer had a ra- dial feed-out/retract cutting insert requirement for the nal-machining of the cheek faces. erefore, the base-tool holder contained a thrust and feed-out mechanism, in addition to the whole tooling assem- bly ‘running-true’ , so that it could be ‘datum-out’ and precisely and axially-set with respect to its potential engine block machining features. e radial mecha- nism would incorporate an actuator sha mechanism which can be pulled-/pushed-back, thereby resulting in either a radial infeed, or retraction, respectively, of the cutting insert. is bi-directional control of the feed-out/-in of the radial mechanism is achieved in conjunction with the CNC feed spindle of the machin- ing centre. In general, these special purpose reamers, have two guide pads and a blade (Fig. 74a) with the reaming blade set with a back-taper, producing the well-known characteristic ‘saw-toothed prole’ to the reamed sur- face (Fig. 74b – right). Such reamed surface texture to- pography has been highly magnied in the schematic diagram (Fig. 74b – right) and, requires very high vertical magnication of the surface topography (i.e. x50,000) to see any trace prole details at all! e posi- tion of the cemented carbide guide pads, with respect to the blade is critical to the reamer’s performance, as is the residual stiness of the whole cantilevered tool- ing assembly. For many automotive industrial reaming applica- tions, the components are oen cast from high-silicon aluminium materials, as the addition of the element silicon, creates a micro-grained and harder cast struc- ture, than would otherwise be the case. However, the disadvantage from a machining viewpoint, is that the resultant cast matrix is highly abrasive to the cutting edge. Under these circumstances, the reamer’s blade Figure 73. Reamers in action, reaming automotive parts. [Courtesy of Shefcut Tool & Eng’g Ltd.] . Chapter 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 oen 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 oering 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 oen 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 specic tooling to nish o the hole-making activities. e most popular of these are briey men- tioned below, but this is by no means an exhaustive account of the many oen 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, oers 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, sacricing 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) Insucient 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 modication. [Courtesy of Guhring Ltd] . Drilling and Associated Technologies Figure 75. Some alternative hole modication 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. 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