Basic CNC

Tony,

That was my thinking, but as I was mulling this, I was having the smaller machines in mind. With that, it seems to me that a really candidate for a hobby CNC machine would be the Super X3 due to the fact that the spindle does go both ways. I have been talking to Steve about possibly doing a CNC conversion to the machine that I have now. But reading this has me thinking that I should wait a bit, get another machine, a SX3 to take advantage of the spindle motor being able to go in both directions. But would that be a fourth axis? So in essence, if you really wanted a 4 axis machine, to mimic more of the actions of the bigger machines, you would really need a 5 axis system to complete a set up on a SX3 machine. Not only did being about to do continuious climb cutting, you would be able to do power tapping as well.

Doc
 
I have started reading this thread and a question came to mine. If a brushless motor that powers the spindle on my HiTorque mill is basicly the same as a servo motor, it brought me to thinking.

In CNC motion of a machine, why could you not use a stepper or servo to control the spindle on the mill? I mean, in my thinking, it would be prudent to have a spindle that can go both directions instead of just counter clock wise. To effect, if the spindle can go either way with the same amount of power, then in essence, you could have the cutter always in a climb cut no matter which side of the part it may be on.

Make sense?

As Tony said, on "real" machines this is the case, and on most small commercial machines, and even home built machines too. My spindle is currently not connected to the controller, but it is in the plans.

Mostly for spindles on CNC machines people use brushless DC or 3 phase induction motors with variable frequency drives. Newer VFDs give you very good control over position and low end torque. All you need is the ability to feed a control signal from your PC to the motor controller. This is usually stream of pulses sent by your controller, the frequency corresponding to the desired spindle speed.

In order to do rigid tapping, you need a spindle encoder. The signals from the encoder feed into your PC, so it knows where the spindle really is, and can adjust the frequency of the pulses it is sending to the motor controller. This is a lot like the way a servomotor works.

For the X2, I don't think the stock controller allows external inputs, but I may be wrong. You may need an add-on motor controller to be able to control the spindle speed through software.

Oh, and steppers are not used for general cutting because they have very low torque at high RPM, but they can be used for slow speed stuff like rigid tapping.

[video=youtube_share;vGTfVlsA9Ww]http://youtu.be/vGTfVlsA9Ww[/video]
 
The spindle itself is not considered an axis. The fourth axis (sometimes called the B axis) on a mill is a rotary table/superspacer type of rotating workholding device.

There is no reason you can't do this on a smaller machines. I was only citing typical operations on a full scale, factory built CNC machine.


Doc, I don't know anything about Mach3 software, or setting it up with stepper motors. There are some here that do, however, so they will probably chime in.
 
The spindle isn't considered an axis in general. The method used for rigid tapping or for single point threading on a CNC lathe is called "spindle synchronization". Basically it is a mode you enter that tells the cutter to track the spindle position. You don't need a 4th axis for this.
 
That is what I am saying, the SX3 already has a controller so to speak to reverse the motor that drives the spindle, so all it would need then is to be connected to the breakout board so that it can be controlled by software.. am I right in my thinking?
 
ok, so I miss placed my thinking in terms.. but what I am getting to is being able to control the spindle and have it go both ways. I will be wanting to set the machine up so that it will have a true fouth axis when I do this.
 
Doc

I have mach3 running machstdmill from calypso ventures pro. and it definatly has reverse spindal control.. you can also controle the spindal speed ect.
 
Just a lil note, sometimes I think way outside the box. So far out, it would seem that I don't have a grip on what is going on. And sometimes that is true too.
 
A few points as to the sizing of motors and microstepping. This is a bit rambling but here is a description of the factors involved in the calculations.

There are no set rules or formula that you can choose the "perfect solution." The reason being that *it depends*. When you set up a system, you have to look at how you are connecting things mechanically. You are connecting a rotational movement to a linear movement. They mesh together different ways. All of them will create some backlash because they have to move against each other relatively freely. The tighter you couple them, the less backlash will be present but also you are increasing friction as you tighten them together. You need stronger components and more power to overcome this friction. Let's deal with them individually.

Stronger components.

As you get into larger and larger motors, the frame gets heavier, the shaft gets thicker and the coils get larger. This is pretty straight forward. There are some drawbacks with getting bigger motors. One is that they are heavier. Part of the load you are moving (depending on configuration) is the motor itself, say for a z-axis on a mill. The heavier weight means you have to have more power to move it but more importantly, you have to overcome the inertia of the extra mass to start and stop it. This requires more power. The larger coils also use more power (and weigh more) and make the cogging stronger so you get a more pronounced step in the motion rather than smooth motion. You can smooth part of it out with microstepping but you are also pushing and pulling at the same time which means that you don't have as much power available in the direction you want to move. So you get a bigger motor to overcome the drag of microstepping. See where this is going? Same thing happens when you beef up the drive components. Get a stronger leadscrew that is bigger and heavier, you have more inertia to start it and stop it. Needs more power.

Getting more power.

Ok, we have different ways to get more power. One is from pure mechanical advantage. As Archimedes was famous for saying, "Give me a long enough lever and I can move the Earth." Your options are most commonly a screw or a gear train. The finer the pitch of the leadscrew or the larger the gear ratio, the more power you can deliver to move something. Of course, there is no free lunch. The greater the mechanical advantage, the more rotation you have to put in to get the same movement. This is fine if you don't care about how fast you move but generally it is an important part of the overall decision. You want to be able to move fast enough to effectively cut things. Move too slow and you are not getting enough chip load and the friction will kill your cutters. Move too fast and you can't get chips to clear fast enough and you put too high of a chip load on the cutter and it breaks. Beyond that, you have to look at the differences in the motor types. Stepper motors are wonderful at slow speed. They deliver a huge amount of torque for their size. As they move faster though, they lose that power. On the other end, other types of motors such as servos, have more power at a higher speed but lose power as you slow down. You can also increase the power by putting more electricity through the motor. You can increase the voltage or the amperage to get more power.

Now, here are all the things that bite you in the rear.

Lets say you have a leadscrew. You want to move fast, the leadscrew has to turn faster or the pitch of the leadscrew has to give more motion for each rotation. Move too fast, and the screw can start whipping around and at the least create vibration and heat from friction on the nuts. At the worst, it can rip your drive system right out of the machine. Increase the pitch so it doesn't have to turn as fast, and you lose mechanical advantage, it has more torque on the screw which can bend it and make whipping worse, and there is more pressure on the nuts. Go with a gear train and each component of the train has it's own backlash that adds up to the overall backlash. Each component also has it's own inertia that will decrease the effective power transmission. Finer pitch on the gear train will give smoother motion but more friction. Coarser will give rougher motion. Go with stepper motors to get more low speed torque or servos to get more high speed torque. Gear up a stepper to get more speed, you get more cogging and rougher finishes. Gear down a servo to get the low end torque and you can't move as fast overall. Increase the voltage, you need to specify electrical components that have higher voltage ratings. Increase the amperage, you need to increase wire sizes and dissipate more heat.

What you really end up doing.

You can't just plug the numbers into a program and get the sizing you need for a system. All this can get pretty theoretical and looks like you need a degree in Electrical Engineering and Mechanical Engineering but the answer is that a lot comes by trial and error. You look at other systems and what they used and how well it worked. You also make decisions based on what you are going to be cutting, how much mass you are moving around, the power of the cutter, and how fast you need the system to move. Most of the factors can be accounted for. Backlash can be compensated for in software and it can be minimized by adjustments to the relationship of the components. You can control microstepping and go with fine steps when you want precision and use single steps for fast motion. You can do prediction of movement to slow motion down gradually instead of slamming on the brakes. Same for acceleration.
 
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