Page 17 - CNC Robotics

Chapter 1 / Design

Motor Drivers

At this point in the project 1 had already pu rchased motors, so I
looked at simple driver solutions. The best solution was found in
the form of the 1297 and 1298 integrated circuits manufactured by
ST Microelectronics. Their Web site ha s all the information need-
ed to build a bipol ar stepper motor driver using these two inte-
grated circuits, which are often referred to as "chips." A driver built
from these ch ips can ea sily provide the voltage and amperage
needed by the step syn motors.

Acme Screw

The question of how to move the gantry and axes slides was a lso
resolved by cost. I had originally conside red using ball screws, but
afte r compar ing the cost of the ball screw with that of an acme
screw, it didn 't make se nse to spe nd three times as much on ba ll
screws. The ad vantages to usin g ball screws are tha t a smaller
motor can be used to move a given load, and with a preload ed nut,
the re is very little backla sh in the system. As mentioned earlier in
this chapter, this is a machine that will not speed through its
assign ed jobs so we can compensate for backla sh in the software.
This means the project can be built using less expensive acme
screws.
I also had to decide how many turns per inch to put on the acme
screw. My experiments with ready rod proved that too man y turns
made for annoyingly slow movement and too few turns redu ces
the quality of resolution that allows the machine to make small,
precise movements. 1settled on a 1/2-in ch acme screw with eight
turns per inch, and a 6-foot length with a nut at a cost of $ 135
Canadian. '

Deciding on the Dimensions

of the Machine

Earlier in the cha pter 1 explained that I made the decision of
ma chine footprint size based on the area in my workshop.

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