Biomedical Engineering Reference
In-Depth Information
where: h
ff
= x/f, h
fm
= x/M, h
Mf
= θ/f, h
MM
= θ/M, Gij - transfer functions of the
assembly, x - translational displacement of beams, θ - rotational displacement
of beams.
It has been suggested that there is a 3% difference between the
experimental and receptance coupling results, which is considered to be
acceptable for chatter prediction purposes. It has been confirmed that adding
the effect of the shank, taper and tip bending modes can be ignored for chatter
prediction purposes because the additional lobes did not have any effect until
the speed reached 120000 rpm. At that depth the allowed axial depth of cut
exceeded 100 microns, which would break the tool. The lobes did reduce the
depth of cut to 38 microns at speeds in excess of 180000rpm, which are
beyond the speed ranges of available spindles. Due to this analysis, it has been
concluded that only the primary bending mode of the tool must be evaluated.
This paper only covers the case where the feed rate is greater than the critical
chip thickness, which means that it is only applicable to soft materials.
Therefore, the RC technique for chatter prediction is applicable only to
shearing-dominant cutting mechanisms, which occur only in machining soft
materials. The ploughing would bring non-linearity effects into the system,
making RC less suitable to use.
Paper [43] presents a review of the state of the art in micromachining with
micro milling being mostly covered. A comparison between micro and macro
scales has been done, pointing out how the knowledge from one scale can be
applied to the other. There are several problematic issues when dealing with
micromachining. First, the accurate measurement of forces is extremely
challenging due to the size of tools and small values of the forces themselves.
Noise if present, greatly disturbs the signals and gives distorted results. The
issue of applying the Merchant's cutting model that is used for macro cutting
processes is problematic to say the least. This is due to the effects of edge
radius that will result in high negative rake angle and elastoplastic effects.
Presence of high negative rake angles will also affect the magnitude of
shearing and ploughing resulting in a substantial increase in the axial force and
increased friction on the rake face of the tool. The shearing and ploughing
effects would result in a relatively large volume of material having to become
fully plastic for a relatively small amount of material to be removed, which
would cause significant increase in the specific energy.
Apart from the aforementioned problems, there is also a challenge
regarding the interaction between tool and the workpiece. In micro cutting the
depth of cut needs to be larger than the critical chip thickness in order for
chips to be formed due to the elastic effect present when machining such small
