Making a professional homebrew CNC mill, mechanical part, chapter 1

After lot of thinking we designed a nut that scores all the goals, it consist of a trapecial threaded nut, in bronze, cutted by the half. Two springs pushes the two halfs together, and two screws pushes the springs. In this way the nut is always tightly coupled with the screw and the backlash dissapears, but in extreme situations it can close due to wear or open in front of irregularities that can be found in the screw.
Our nut also have other important enhancements to be dinamyc but that is another story.

Choosing the motor for the machine. Steppers or DC?

Another important decission in the construction of a CNC, is the type of motors we well use.

Stepper motors

Most of homemade CNC's that I've seen use steppers. These motors make easy the electronics because the motor moves through discrete angles and this makes innecesary to develop a position system in the cnc.

These kind of motors have a lot of nuisances, though. For example, the maximum torque are in low speeds, so it won't machine very fast, as long as we don't increase the size of the screw, losing resolution.
Also they are bigger,heavier, and more expensive that their DC cousins.
The most important problem from using steppers in a CNC device, is that we can't be sure that the motor doesn't miss a step in the machining process. In the moment CNC misses a step all the following work will be defective.

DC servo motors

The cheap alternative to the steppers, is using direct current motors (DC), these are cheaper, strong and small. The cons are that the electronics and the interface with the computer will be more complex, because we will need a closed loop position system to know where the tool is in any moment.

More advanced motors

Also there is an even better alternative, the use of a brushless motor, these motors are very good, they have lots of uniform torque in all the movement, and there are no touching parts so they are very silent. The cons are that the electronics is even more difficult than DC servo motors, because the electronics must know the exact position of the shaft and must feed all the motor coils independent.
The edge technology nowadays are the linear motors, this is the technology used in the maglev train, the shaft moves in linear way with lots of acceleration and power and no friction, the electronics are almost the same as the DC brushless motors (we can consider a linear motor as a brushless motor unrolled).
Also there is no need of gears and nuts, making the mechanical system easier.
The problem is the very high price of these motors.

Because all these reasons we decided to use the second solution, the DC motors, with planetary gears reductions and optical encoders. We will need to develop special electronics for this project with dedicated circuitry to help the computer with the position system of all the motors.

Construction of the CNC machine

In the following lines I'll show the general steps that we have made to do the retrofitting. The encoders and end sensors will be explained in a posterior article dedicated to the electronics of a cnc machine.

In the following lines we'll explain the practical process of retrofitting our cheap milling machine.

retrofit fresadora.

The first I do is dismount the screws and nuts, so they can be replaced with ours.
I machine the piece that will hold the motor, the encoders, the thrust bearings and the screw. The piece is made from technical aluminium, it has low weight, doesn't rust and is strong enough for the task it is encommended.

Pieze that holds the system.

To make the nut, i needed to first make a tool internal thread trapezoidal step 3. We do this tool with a steel-cobalt bar.

Sharpening a steel threading tool.

I mill the bronze nut, ensuring that everything is with the right measures.

Milling the nut.

As the nut is square, and I do not have a dish of four claws for the lathe, I have to develop a fixture, which will allow me to pick it up in the lathe. I make some drills for threading to M6, where I will screw the machining tooling.

Threading M6 to be catched by the lathe.

The fixture is a round aluminum bar, we machine and drill it. Then we measure the fixture so as the drills are in line with the nut ones, so when it will be turning, will be perfectly aligned with the bolt.

Measuring the fixture.

We mount the fixture over the nut and tight it hard.

Mounting the fixture.

I fix the nut in the lathe and machine it so as the thread is 90 degrees with the internal side.

Machinig the nut.

Turning.

Before threading, we must put the right gears in the lathe, so as it will have a step 3 advance in the thread.

changing gears.

We machine the thread with the lathe carefully, so as we don't crash the head with the carrier, and giving little passes to avoid that the theading tool bends.

Making the nut's threading.

Finally our tool is not hard enough to do perfect threads so must make a harder tool.
We think in tungsten cutters tips that are much harder than the steel ones. We insert this tips in a carrier bar, and then we pick this with the lathe.

bar with cutter picked in the lathe.

We machine it so it will fit in the threading hole of the nut.

Machining the carrier bar.

In the machined part, we make two drills, one for the cutting tip and the other to fit a little screw that will hold the tip.

Threading the carrier bar.

I put the tungsten tip and we sharp the tooltip.

sharping the tool.

I ensure that it's correctly made with the threading template.

Measuring out threading tip.

We machine another square broze part, identical to the last one. We make the trapezial thread with our new threading tool.

Threading with the tungsten tool.

The results are much better, because the tool has not bend while making the threads.

Now we cut the nut in a half, and put the springs. This will cause the thread stick with the pressure so that there is no game and always be tight with the threaded shaft.

Nut cut.

We mount the nut in the spindle and we see that it behaves very good, with no game (no backlash) and softly.

nut mounted in the spindle.

We machine the spindle, where it will be located the bearings and the motor coupling.

Machining the spindle.

Turning.

Now we will make a part to couple the spindle with the motor shaft, we will make a Cardan joint, so as we don't must to have a perfectly aligned shaft-spindle.

Machining the cardan joint.

We assembly the system and we see that everything fits well.

System mounted.

System mounted with the nut.

We mount the system in the milling machine, we switch it on and see that all is working great.mecanicemos.

Y axis ready.

We make the X axis the same way.

X axis.

The Z axis is similar...

Z axis.

Wou! That's all for now. In the next chapter we will enhance some parts.
Having a CNC makes everything easier and the finishing will be much better. Also we must describe the electronics and the computer programs.
For now we tell you that the electronics will be a PCI card with a FPGA that will control all the motors and optical encoders in the fastest way with high high resolutions and high high speeds.
Also it will control an automatic centering tool, automatic control of cooling lubricants, final path sensors, etc..
The computer program we are going to use is the Linux CNC running in a Real Time Linux environment.

We'll see.

Greets

Ivan Reches
Eugenio Sobrino