allow the screw to move
several times faster than a
standard threaded rod. The
ACME nut is often spring
loaded to help keep backlash
to a minimum.
The ball screws shown in
Figure 14 are the ones used
on the KRmc01 milling
They have small grooves in
the threads to allow a series of
small ball bearings to be
inserted between the nut and
screw. Because the ball bearings need to move as they spin,
a small tube is used to allow the balls to circulate as the
Ball screws are much more expensive than ACME
screws. The price goes up considerably as the length is
increased. For this reason, they are rarely used on larger
Stepper motors are attached to the various screws or
rack and pinion drives with couplers or pulleys. The stepper
motor receives instructions from a controller that is
connected to your PC. The PC talks to the controller via
controller software like Mach 3.
SERVO 04.2014 63
As stated earlier, we will be converting a MF70 mill.
The MFC70 mill has your basic dove tail ways with
adjustable shims (Figure 15). Unlike a larger mill which has
cast iron ways, the MF70 ways are aluminum. While not
perfect, they are okay on a machine this size.
The MF70 mill utilizes a high speed direct drive spindle
head (Figure 16). While at first glance it may seem like a
rotary grinder, it’s far from it. The motor is 1/8 HP. It has
low run out and runs at 5,000-20,000 RPM. While I
normally like a slower spindle on a milling machine, these
speeds are adequate for the smaller bits that this machine
The machine comes with 1/32”, 1/16”, 5/64”, 3/32”,
7/64”, and 1/8” collets shown in Figure 17. With the 1/8”
collet, you can use many small end mills and drill bits that
are readily available from various sources.
The machine has an X-Y travel of 5-17/64” and
1-13/16”. It has a Z travel of 2-3/4”.
We only dipped our toe into the
CNC waters, but I can assure you, if you
build this CNC machine it won’t be your
last. Once you catch the CNC bug, it’s
Next time, we will go over all the
components needed to complete the
conversion. I will supply a complete
BOM with links so you can order your
I have created a SERVO Magazine forum. If you have any questions, please post
them at http://forum.nutsvolts.com/viewtopic.php?f=49&t=17107.
If you want more information on my KRMx02 and KRmc01 CNC books, you can
find them at www.kronosrobotics.com.
I also have a free download for an early CNC router book that I wrote. You can
download it at www.kronosrobotics.com/krmx01/index.shtml.
I have started a MF70 CNC conversion page. It’s at www.krcnc.com/mf70.