3.3.3 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 eit
Trang 1Figure 74 High-performance reamers, having the ability for radial infeed (i.e ‘feed-out inserts’) – when fitted [Courtesy of
Cogs-dill Tool & Eng’g Ltd.]
Trang 2
is often 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
3.3.3 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 finishing reaming operations Low cutting speed
together with high feedrates, in association with good
lubrication agents offering 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 ‘floating
reamer’ toolholders (Fig 72) In Table 6, the following
fault-finding 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 often requires
a further post hole-making operation to complete the
job, for example, a tapping operation There are a
num-ber of these pre- and post-drilling hole operations that
require specific tooling to finish off the hole-making
activities The most popular of these are briefly
men-tioned below, but this is by no means an exhaustive
account of the many often hybrid operations that are
available to the potential designer, or machinist
Countersinks
There 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 ‘flush-fitting’ 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 This latter modular counter-boring tooling, offers
a range of flexibility to machine a wide assortment of component features, by simply changing the ‘pilot‘, or cutting element’s diameter The ‘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 This is important for any cap-head bolts that require to be recessed either flush 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 flush 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 flush 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 flexibility 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
Trang 3Table 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 fit 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, sacrificing 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) Insufficient 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 ‘floating’ 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 modification.
[Courtesy of Guhring Ltd]
Trang 4
Figure 75 Some alternative hole modification machining tooling [Courtesy of Guhring Ltd.]
Trang 5
Back Spot-Facing
Back Spot-faced tools (Fig 75d), are usually employed
in flush-facing an internal hole’s face on either a
cast-ing, forgcast-ing, or wrought stock The 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-modified 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
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