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Saturday, July 20, 2013

CNC mill conversion - Part 2

X axis stepper motor mount

Before I start making stuff today you need a little more background information so that things will make more sense.

In a CNC mill (Computer Numerical Control) a computer assumes control of the movements of the XY table, and Z axis (depth of cut) at a minimum. Additional functions might also be taken over such as spindle control, coolant activation, and many more.

To cut features into the metal the computer follows directions encoded into a cutting program written in “G code”, a common machining language. The programs can be created in three ways: 

  1. manually typed into the mill’s controller software interface,
  2. automatically generated by selecting options on “wizard programs” built into the same controller software, 
  3. feeding a CAD drawing (Computer Aided Design) into a CAM program (Computer Aided Machining) able to generate G-code tool paths for the controller software to use.

The G-code on the left represents the tool-paths necessary to mill the text on the right onto a blank.

A controller software such as Mach 3, running on the computer connected to the mill, reads the G-code, and sends commands to drivers for the electric motor, one for each axes. The drivers receive the binary low power computer commands, and turn them into much stronger and more complex electrical signals for the motors. 

Mach 3 controller interface

The motors can be stepper motors, or servo motors. Steppers are cheeper than servos, and while they have a few drawbacks, they will serve well with proper dimensioning and care, so that’s what I will buy a little further down the road.

Stepper motor

Stepper motors do not turn freely like servos do, rather they turn in finite increments, or steps. Sending an X amount of signals to a stepper motor turns the motor shaft an X amounts of steps. Common steppers take 200 steps to make a complete shaft revolution, which means that each step is worth 1.8˚ of shaft revolution. Smaller revolutions can be had through software by using micro-stepping techniques... but let’s not complicate things further right now.

A few of the items required for the CNC conversion

Obviously, the servos need to be physically positioned in such a way that they can turn the ball-screws. My conversion will employ direct drive for the X and Y axes, and belt drive for the Z axis. With a direct drive the shaft of the motor needs to be aligned with the ball-screw, with a belt drive they need to be merely parallel to each other.

Today I will focus on manufacturing the X axis motor mount out of a raw aluminum plate, and an aluminum rod. 

This was the first time I relied 100% on the mill’s DROs (Digital Read Out), so I was a little apprehensive on the final outcome, but as you will see things turned out great.

I started by parting a few big aluminum plates into smaller blanks, representing all the CNC upgrade components.

Raw aluminum before being cut

Raw aluminum pieces used for various brackets

Next, I used my lathe to cut the rough motor mount standoffs out of an aluminum rod.

Cutting the standoff blanks out of a 0.5" (12.7 mm) aluminum rod

Here’s all the rough stock required for the conversion..

All the required material

The standoffs needed some minor drilling and tapping.

So, I drilled both sides...

Drilled standoffs

... and threaded them.

Threading a standoff

Threading completed 

With the standoffs completed, I put the X axis motor mount blank on the mill, squared it, and brought it down to the required size.

Squaring the future X axis motor mount

Since I relied heavily on the DROs for locating all the features to be milled on the plate, I had to create a cartesian coordinate system, and map all the holes on my plans.

Mapped out plan of action

Here’s how it worked. I zeroed out my mill in such a way that the spindle center was EXACTLY over the lower left corner of the aluminum plate. At this point I zeroed out both my X and Y DROs, and cranked the mill table to the coordinates of every feature to be cut.

Let’s take the hole on the bottom left of the drawing, the one with coordinates (0.323, 0.323). All I had to do was to move the table to those coordinates, as read on the DROs, and start drilling. 

Spindle homed in to the first hole
First hole center-drilled

Let’s try another one. How about the hole on the bottom right of the plans? Its coordinates are (2.177, 0.323).

Spindle moved to the second hole coordinates
Second hole center-drilled

Wow, this is so simple!

You get the idea. 

To make a long story short, I drilled every hole in the plans.

All holes center-drilled

All holes drilled to final size

Now, up until this point I was not too concerned with repeatability, or how well the spindle could line up again with the same hole, given the same coordinates. I figured there might be some minor misalignments due to errors in the DROs themselves, or perhaps induced by my way of mounting them. 

I am happy to report that the precision this DROs brought to the mill is nothing less than astounding!

One way I was positively surprised, was when I went to counterbore some of the holes. The counterboring bit is very particular about being properly lined up with the hole. Getting it to slip into a hole, even while holding the bit in your hand, is difficult unless it is perfectly aligned.

Driving the mill to the coordinates used earlier though, the counterbore pilot slipped right in every hole, effortlessly.


Counterbore bit with pilot section partially inserted in hole

So, I counterbored the holes that needed it with much ease, and satisfaction.

Three holes counterbored to hide bolts

Next, I flipped the plate over, reestablished a cartesian coordinate system, and tapered the back side of some of the holes where flush bolts will reside.

Back side of plate with mounting holes tapered

Bolts sitting flush in tapered holes

The final challenge was cutting the huge center hole. This is just a pass through hole for the ball-screw.

Slowly enlarging the pass through hole with a boring bar

Putting all the parts together, you can get a better sense of how it is supposed to work.

Finished X axis bracket

This mount will fit at the end of the table, where the crank for the X axis is.

The crank wheel you see behind the mount will be eliminated

With the crank removed, the mount slipped right in place. The stepper motor will replace the crank wheel.

Bracket in place

I still have to drill and tap three holes to finish mounting it onto the mill. I will wait until all the other components are ready, including motors, drivers, computer, software, and wiring, before mounting it permanently, since there is a lot more manual work to be done on the mill.

Here is where the stepper motor will go...

Ghost motor in position 

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