CNC Mill Conversion

Over the summer of 2001, my main shop project was converting the Grizzly Mini Mill from manual to computer control, or CNC (old industry term standing for Computer Numerical Control).  This was one of the most enjoyable projects I've ever done, and with the help of shared experiences and tips from fellow members of the Yahoo CAD_CAM_EDM_DRO list, the project went pretty well.

As can be seen from the pictures on my shop page, the Grizzly Mill is a small benchtop milling machine.  It comes standard with manually driven milling table, with handwheels on the X and Y axis leadscrews.  The milling head raises and lowers by means of a rack and pinion gear setup, operated by hand crank for coarse feed and a manual worm gear drive for fine feed on the Y-axis.  The X-axis controls movements left-right, the Y-axis forward-back, and the Z-axis up and down.

The objective of the project was to mount stepper motors to each of the X, Y and Z axes, build a power supply/motor controller to drive the motors, and connect the motor control to a PC running CNC software to complete the system.  One of my goals was to make all modifications without destroying any part of the Grizzly Mini Mill or drilling/tapping new holes into the castings or table, so that I could return the mill to stock condition should I decide to upgrade to a larger machine later.  To accomplish this I used only existing mounting holes for my motor mounts.  This was a low-budget project.  I spent about $600 including everything except the mill itself, with the larger expenditures being the software and driver kits.  Most other components, including the motors and power supply components came from the surplus market, most of it unused or little used.

CNC Specifications.

Software: CNC Pro by Yeager Automation. 

Computer: 486DX2 66MHz, 40 MB ram, DOS mode.

Drive Motor, X-axis: Superior Electric MO-92, 325 oz-in

Drive Motor, Y-axis: Compumotor S83 NEMA 34, 350 oz-in (est.)

Drive Motor, Z-axis: Superior Electric MO92, 375 oz-in

Motor Drivers:  Camtronics 5 amp Bipolar Chopper Driver kits.

Power Supply:  Converted Superior Electric Translator Drive, 24 VDC, 15 amp.

X-Axis.  The drive motor is connected to the lead screw by a direct shaft coupling secured by set screw to the end of the lead screw.  A 1/2-inch aluminum mounting plate was milled to receive the size 34 motor, mounted to the existing mounting holes for the table end plate.  Since this photo was taken, I upgraded the X-motor to a more powerful 325 oz-in Superior Electric MO-92 bipolar motor.  This gave me more speed, particularly when making contoured cuts.

Y-Axis.  The Y-axis is directly coupled to the lead screw, which requires a very accurate coupling system to prevent binding during rotation, which was a challenge because of the rotational error in the Grizzly mill itself.  After a couple of tries, I got it but I had to stand the motor off the mounting plate with the nylon spacers to get the last bit of error out.  By using a double shafted motor, I was able to re-mount the hand wheel with setting circle intact for manual use when the motor power is turned off.  (But I never use the hand wheel now).

Z-Axis.  The Z-axis motor is coupled to the shaft of the fine adjustment worm (fine adjust wheel and U-joint removed).  This axis requires a higher power motor to overcome the frictional losses in the rack & pinion drive and in the gibs.  The Z-axis has a considerable amount of backlash (mine had 0.060") and I wasn't sure it would be practical to motorize the Z.  But of course I tried it anyway.  Borrowing tips recently posted to the list, I shimmed the rack out about 0.015", which helped reduce backlash but also increased friction and power requirement.  I also carefully adjusted the depth of the worm as it meshes with the worm gear, which also helped.  To eliminate most of the backlash, I  secured the worm gear to the pinion shaft by a set screw.  By doing so, I lost the ability to use the coarse feed hand crank, but the backlash reduction was well worth it, and I no longer need the hand crank anyway (unless of course the Z-motor goes down!).  Total backlash is down to .013, which to me is acceptable for a $500 mill.  After backlash compensation, Z-movements are accurate to within 0.002 inch.  This is as good as it's going to get with the rack and pinion setup, because most of this error is due to the non-linear nature of the spur gear meshing with the rack.  At least it is very repeatable.  If greater precision is required, I'd recommend a ball screw conversion.

Power Supply.  Bought an old Superior translator for $20 at a surplus warehouse, took out the translator board, kept the power supply components, added a cooling fan and a 25 pin computer cable connector.  The three driver board kits fit nicely inside the box for a single unit electronics package.

BEFORE:

AFTER:

First CNC Operation:

1st CNC Part: