making a cnc

Submitted by euge on Mon, 2007-07-23 09:41. CNC

Milling machine with motors installed

Automatizando una fresadora

The next project that we are going to make is retrofitting a milling machine.
Retrofitting means replacing all the manual elements in the machine for automatic ones. These consist of electric motors, electronic sensors, etc... In this article we are introducing to the mechanical system of these devices.
This computer controlled milling machine s, we will have a CNC device (Computer Numerically Controlled).
Through some years of experience machining with a CNC device, i have seen how easily we can make incredible pieces in metal or more materials, in little time, and how the difficulty of machining has moved from the machine itself to the computer programs that designs the pieces. These are the CAD-CAM programs.

A CNC device is for me, one of the most important machines we can have in the yard, and I think it's a must for the professional and the hobbyst.
With this, we can machine all the pieces we need for the projects, fast and with sufficient quality.
With a non automated milling machine, some people are able to make complex things, but with much more work, phisical and mental, and the results usually are not on par with the CNC made pieces.
Also there are shops that can machine the pieces for you, but in the long way it's very expensive for our little pockets. There is also the time line, having a cnc in your bag makes you very fast and responsive. So, I think it's clear that I believe that it's cool to have your own cnc to machine your stuff.

The relevance of having no backlash

One of the most important things of a CNC machine, is that no axis can have backlash. Backlash is due to a little gap located between the screw and the nut. These makes the machine to vibrate, inducing wear to the tools, breaks and bad finishes in the machined pieces. The backlash also affects the maximum resolution we can get:

If we put a radial encoder (axis position sensor), every time that we change the motor direction, there is a fraction of time where the screw turns but the nuts doesn't move.
These gives measurement errors, and they grow with time because of wear, making difficult compensating the errors via computer software. A solution can be a lineal encoder, because it is directed positioned in the line of the nut movement, but these encoders are more expensive and are more difficult to place. Backlash behaves different upon the direction of cutting of the tool. In one direction the tool makes force in the same vector of the direction of the nut. In the other direction the tool makes the oposite force of the direction of the nut.

Cutting direction.

Usually it's better to work in oposition, in this manner the tool tends to escape from the nut, the finishes in this way are very good. In oposition, the tool pushes the piece helping the movement of the carriage, so it avoids forces to the machine. The tool, drops the shavings behind, so it doesn't recut them, giving longer life to the tools.
With any backlash, we can't work in oposition, the nut is not always pushed with the screw, giving lots of vibrations and bad finishes. This situation forces us to work in favor, where the force of the tool is the oposite of the force of the nut, so the screw always pushes the nut, but the effects are reversed of the commented in the oposite direction.

We can conclude that it's a must to have no backlash if we want good finishes and resolutions.

Designing a nut!!

The market has multiple solutions to reach the elimination of backlash in transmision systems. One of the most popular are ballscrews, but the cost is high, so we decided to design an alternative way.
Our solution must reach certain targets, like these:

We need no backlash

The nut must be dinamyc so when there is wear in the nut it must autoregulate to the new geometry

The nut can't have too much friction, so the heat

The nut can't be too expensive

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.

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. 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.

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.

Because all these reasons we decided to use 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.

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 oxidize and is strong enough for the task it is encommended.

Pieze that holds the system.

Para fabricar la tuerca, necesito hacerme una herramienta de rosca trapecial paso 3 interior. Primero hago una herramienta de acero rapido al cobalto.

afilando una herramienta de acero rapido rosca trapecial.

Freso la tuerca de bronce, procurando que todo me quede bien a escuadra.

fresando tuerca.

Como la tuerca es cuadrada, y no tengo un plato de cuatro garras para el torno, tengo que fabricar un utillaje, que me permitira cogerla en el torno.Le hago unos taladros para roscar a M6, donde atornillare el utillaje mecanizado.

roscando M6 para coger utillaje.

El utillaje es un redondo de aluminio, lo torneo y lo taladro. Comparo el utillaje para que los taladros me queden centrados con el de la tuerca, asi cuando gire, estara perfectamente alineado con la tuerca.

comparando utillaje.

Monto el utillaje sobre la tuerca y aprieto bien fuerte.

montando utillaje.

Cojo la tuerca en el torno y la refrento para que la rosca me quede perfectamente a 90 grados con la cara.

refrentando tuerca.

girando.

Antes de roscar, hay que poner la combinacion de engranajes en el torno, que nos permita un avance de paso 3.

cambiando engranajes.

Roscamos la pieza en el torno con cuidado de que no choque el carro con el cabezal y dando pasadas pequeñas para evitar que la herramienta flexe. Vamos dando pasadas y comprobando con el husillo la rosca hasta que este entre.

roscando tuerca.

Como la herramienta ha flexado, no ha acabado de quedar bien del todo. A continuacion voy a probar a hacer una herramienta de widia que es mucho mas rígida.
Se trata de coger una plaquita de widia e insertarla en una barra de hierro. Primero corto la barra y luego la cojo en el torno.

barra cogida en el torno.

Le torneo una mecha para que no me toque en las paredes de la rosca cuando vaya a mecanizarla.

torneando barra.

En la parte torneada, le hago dos taladros, uno donde ira la plaquita metida y otro que roscare para poner el tornillo que sujetara la plaquita.

roscando barra.

Coloco la plaquita de widia y la afilo.

afilando herramienta.

Compruebo que me quede bien con la plantilla de roscas.

comprobando herramienta.

Mecanizo otro cuadrado de bronce igual al de antes y con el mismo utillaje. Le hago la rosca trapecial con la nueva herramienta de widia.

roscando con herramienta de widia.

Ha quedado mejor, debido a que a flexado mucho menos.

Ahora corto la tuerca por la mitad y le pongo los muelles. Esto hará que la rosca quede con la presión justa para que no haya juego y que conforme se vaya desgastando siempre este apretada.