06-05-2013, 04:07 PM
RETROFITTING THE X3 MILLING MACHINE (1)
RETROFITTING THE X3.pdf (Size: 1.28 MB / Downloads: 62)
Rationale
Readers, who have followed my articles on
aspects of CNC, will be aware that I have a
Wabeco CNC mill in my workshop. These
readers will now be wondering why I am
retrofitting a Chinese mill when I already
have a perfectly satisfactory CNC machine.
There are a number of reasons why I am
embarking on the retrofit. First and
foremost I like to keep myself busy. I
generally have a project on the go most of
the time particularly in the winter when I
cannot go fishing. Secondly there are a
number of ideas I want to try out with the
retrofit. Thirdly model engineers are
showing an ever increasing interest in CNC
and would like to retrofit an existing mill
but are unsure of how to begin what may
seem a daunting project. Though this
series will be principally concerned with
the X3 mill, the underlying principles
should be applicable, with modification, to
the retrofit of any mill.
Selecting the ball screws
The ball screws are the first item that
needs to be selected. The pitch of the
screws will determine the size of stepper
motor required. I searched the Web for
suitable screws with little luck. There were
lots of screws available; unfortunately the
ones available were either prohibitively
expensive or the wrong length. The only
manufacturer that was able to supply 12
mm diameter 2 mm pitch screw was THK
(MBF 1202 nut 500LC7 rolled screw). The
maximum length of screw rod they were
able to supply was fortunately 500 mm,
exactly what I needed. When I fitted ball
screws to my Wabeco I used THK screws
and found them absolutely excellent. The
standard C7 grade rolled 12 mm 2 mm
pitch ball screw has a maximum error of
50 microns for a 300 mm travel. THK have
a new design of ball nut for this screw,
which features replaceable nylon wipers
conforming to the profile of the screw
thread to prevent the ingress of all but
the very finest dust. The nuts are also
self- lubricating. The only lubricant
required is a light oil, applied to the
screw. If any reader is intending to
purchase these or any equivalent ball
screws he should not pay more than
£145.00 for a single screw and a nut.
Selecting the stepper motors
Having decided on the screws for the
machine the stepper motors can now be
selected. Many model engineers I have
spoken to on the subject of stepper motors
for CNC are of the opinion that one should
fit motors as powerful as possible. The
consequence of this may be that the
motors, and hence the associated drivers
and power supply are all larger (and more
expensive) than necessary. A complete
approach to selecting motors, including
accounting for resonance effects is really
outwith the scope of this article, but I will
describe in detail the steps I followed in
selecting the motors for the X3 retrofit. I
should note that as my work generally
uses small cutters and fine feeds, I have
simplified things somewhat and calculated
the torque required firstly to move the
inertial mass of the table and secondly to
overcome the frictional forces generated
at the slideways. As my work invariably
uses small cutters, I chose to ignore
cutting forces.
Slideway Friction
In addition to accelerating the mass of the
table assembly the stepper motors have to
overcome the slideway friction. There is no
method of reliably calculating the
magnitude of the slideway friction. The
only simple way to obtain a value for the
friction is to actually measure it. I used a
spring balance to measure the slideway
friction on the X3. There are two
components to the friction, the initial
starting value as the table accelerates from
rest and the value during motion at
constant velocity. The initial friction is
significantly larger than the friction at
constant velocity. For the table assembly
(both X and Y) of the X3 the initial friction
force was approximately 10 K gm
dropping to about 5 K gm once the table
commenced motion. When selecting the
stepper motor it is the initial friction that is
important. The frictional load seen by the
motor is reduced by the mechanical
advantage of the lead screws. A further
factor that determines the frictional load
seen by the motor is the efficiency of the
screw. For Acme screws the efficiency is of
the order of 35%, while for ball screws the
efficiency is in excess of 85%.